If the craftsmanship and charm of your old windows is quickly being eroded by cold drafts and frost on the panes, it may be time to consider doing something about them. 

Installing modern replacements is an option to be considered. But you give up a lot when you replace historic windows. Few modern windows are built with the workmanship that went into your old windows, and the cost of those few can be astronomical.

Can This Window Be Saved?
Old WindotIt may look ‘way beyond fixing, but this old window can be restored and made as energy efficient as any standard thermal replace­ment window.We replace a lot of windows. We probably replace nine old windows for every old window we restore and save. The fact is that most windows made in the past 60 years are not good windows. The post-war push to build a lot of new housing quickly virtually eliminated the traditional wood window. It took too much time to build and install the window, and a high level of craftsmanship that just was not available. Builders opted for steel and aluminum windows, and factory made, self-contained wood window units that did not need a lot of site preparation and wall modification. Just put them in the opening and nail them up. It was faster, it was cheaper, and builders then, as now, were for anything faster and cheaper. (For more on the post-war housing boom, see Postwar Styles: Cape Cod, Colonial and Ranch.)

Photo: Andersen Windows. Nearly all window manufacturers market windows designed to look like heritage windows. This Andersen window looks like an original Craftsman window. But most replacement windows last at best 40 years while original windows were built to last generations with proper care. This window starts at about $600.00 — not including installation. Restoring the original window is about half that and since the window is already installed, no installation is required.But the sad consequence is that most of the windows installed since the World War are not worth saving even if they could be saved, and most cannot be. Manufacturers that are not out of business don’t make those windows any longer, so parts just are not available. The only option, then, to fixing your window problems is to replace the windows.

But most pre-War housing, and some better housing built since the World War have shop-crafted wood windows. These most often can be saved, and as for parts — if you have a glass company, hardware store and lumber yard in your town, then you have all the parts you are likely to need.

OK, so old windows can be saved, but can they be made as energy efficient as modern windows? The answer is “yes”.

Most heritage windows can be restored and upgraded to rival the performance of a standard replacement window, and usually at a fraction of the cost. And there are other, nearly as important, advantages of repairing rather than replacing. You not only save on your own heating a cooling costs, which reduces waste and your carbon footprint on the planet, you also save the resources and energy cost required to manufacture new windows — which considering what new windows are made out of, is not an inconsiderable savings.

You also preserve, not just wonderful old-time workmanship, but the superb old growth wood from which your windows were made. We can’t build windows like that any more. It’s not that our craftsmen do not have the skill and experience. Any of our master carpenters or cabinetmakers could build a traditional window. But we can’t get that dense, heavy old growth wood, and the new wood is … well, we’re pretty sure it’s wood, but it’s not very good window wood.

Are Replacement Windows a Good Investment?

Click to enlarge


Jonathan Poore’s classic “Anatomy of a Double Hung Window” published in Old House Journal in 1932. This is how windows were made for hundreds of years. Every one of these parts is a single piece of wood that can be removed and replaced without replacing the whole window.Before 1996, primarily as a consequence of the pervasive and unceasing marketing of replacement windows after the energy “crisis” of the 1970’s (when gasoline prices jumped to an astounding $.80/gal. — oh, for the good ol’ days), it was nearly universally thought that replacement windows were vastly superior energy performers. But then the State of Vermont and the U.S. Army joined together to actually test the performance of restored heritage wood windows, not by using computer models and a laboratory setting, but with an actual field test.

They repaired and restored 150 windows all over Vermont, then tested them against replacement windows in similar homes. What they discovered was completely unexpected. They found that

  • The energy savings difference between restored old windows and new replacement windows amounted to just a few dollars a year, and
  • When a storm window was added to a restored wood window, the window/storm window combination performed at least as well as most new thermal windows.

No one was looking for a result like this. The researchers had assumed, like everyone else, that thermal replacement window would be more efficient. The question they were really looking to answer was how much more efficient. The last thing they expected to find was that new thermal replacement windows were not necessarily more efficient.

After this, other researches were prompted to start examining the issue. Their findings amply support the conclusions of the Vermont study, which is still on-going, by the way. It has been extended several times, and is now due to end in 2016. The important thing to note about all of these field studies is not a single one has found that replacement thermal windows were much better than restored old windows.

How Long to Pay Back Your Window Investment? Haberern’s Findings in Brief

Keith Haberern calculated the annual savings and payback period of several alternatives for improving a single pane wood window’s thermal performance. The most cost-effective option, with a payback in 4.5 years, was simply adding a good storm window to the existing single pane window. The worst option is replacing a single-pane window that already has a storm window with a new double-pane thermal window. This option had a payback of 240 years. The most interesting finding was that a single panel/storm window combination with a U-value of 0.50 (R-Value=2) slightly outperformed the basic double-pane thermal window with a U-value of 0.58 (R-value = 1.72).


Action Resulting
Cost* Annual Energy Savings (BTU) Annual Savings per Window Payback Period (Years)
Add a storm window to a single-pane window 0.50/2.00 $50.00 722,218 $13.50 Best4.5
Replace a single-pane window with double-pane thermal window 0.58/1.72 $450.00 625,922 $11.07 40.5
Replace a single-pane window with double-pane thermal window, low-e glass 0.35/2.86 $550.00 902,772 $15.10 34.0
Replace single-pane window/storm window combination with double-pane thermal window, low-e glass 0.35/2.86 $550.00 132,407 $2.29 Worst240.0
* Costs include installation and are based on costs in New Jersey, including heating costs, at the time of the study.

For example, Keith Haberern, a professional engineer, in an study of New Jersey homes published in 2007 found that the annual energy savings of a modern double-pane replacement thermal window over an old wood window was insignificant. In fact, an old wood window with a good storm window outperformed a typical new thermal window, and was a lot cheaper with a short 4.5 year payback period. What Others Say
Tom Silva“… [i]f your windows are single-paned, look into the cost of adding storm windows for a fraction of the cost of new windows.”

Tom Silva
General Contractor
This Old House

“…While the thermal perfor­mance of a refurbished single-glazed window Renovating Old Houses Coverfitted with a tight storm can never quite equal that of the best factory-made double-glazed windows, the difference is not so great as to merit the replacement of old windows solely for reasons of improved energy efficiency…”

John Leeke
Save America’s Windows

“…Homeowners tell me they know something is wrong with rippng out all their old windows and throwing them away, Save American's Windows coverbut they don’t quite know what the alternative might be. They cannot find tradespeople to do the work…. If you are not a do-it-yourselfer don’t worry. More and more savvy tradespeople are recognizing this new marked for traditional window maintenance and repair services.

John Leeke
Save America’s WindowsOn th other hand, if you already had a wood window/storm combination, replacing it with a thermal window resulted in very little energy savings. So little, in fact, that the payback period was 240 years.

Taking a slightly different approach, researcher and energy consultant Michael Blasnik looked at the energy bills of houses in upstate New York both before and after replacement windows were installed and found the actual average annual savings per household was just $40.00 — not per window — but per house. Based on these findings, he calculated that it would take 250 years for the cost of the replacement windows to be repaid from energy cost savings alone.

Obviously there are reasons other than economic reasons to buy replacement windows. New window sashes tilt out for easy cleaning, they have several lock positions so the window can be opened slightly and still be secure. Old windows often don’t have these features (although most can be added). Or your old windows may have deteriorated to the point where they really can’t be fixed — although this is very unlikely. But if it is your intent to reduce your cost of heating and cooling to pay for your windows out of energy cost savings, think again. Unless you have actually found the fountain of youth, it won’t happen in your lifetime.

How Heat Loss Happens
How is it then that replacement window manufacturers can advertise huge energy savings from replacement thermal windows? Are the manufacturers just lying?

Not really (well, maybe just a little). They don’t do actual field studies, but rely entirely on computer models to predict how much energy will be saved. The problem is that their computer models are incomplete and, therefore, inaccurate. Heat is not actually lost the way the computer models say it is.

Reader Comments
Finally, an article on replacement windows that makes sense…. I’ve known for years that modern replacement windows were not a good fit for period homes, but I could never find information about repairing old windows that I could share with customers. How do I get reprints?

Carlos L., Denver, Colorado Restoration Remodeler

There is so much GREAT information in this article!!! I would not change a thing, …. it is the BEST article I’ve seen on making the case for window restoration and what I’ve been preaching to clients for years!

Jack A., Albany, New York ArchitectAs we discussed in our article on house insulation (Insulating Your Old House), heat moves in three ways, radiation, conduction and convection.

Radiation is how the sun’s heat gets to Earth in the form of electromagnetic waves. In hot, sunny climates like Arizona, radiation is a prime source of heat gain in the summer. In colder climates, like Nebraska, it is not nearly as important since while a lot of heat can radiate into your house, especially in direct sunlight, very little heat radiates out of your house in winter. But some does, and window manufacturer’s combat this form of heat loss through low-e (for “low-emissivity”) coatings that block heat while allowing light to pass through.

Conduction is more important in a net heating climate, like Nebraska. It is the movement of heat on a microscopic level from molecule to molecule within a material. Heat one end of a steel bar with a propane torch and soon the other end is too hot to touch. Heat moved through the metal through conduction until it reached the other end of the bar. To combat heat loss Infrared photograph of heat loss. Yellow and red show places where heat is escaping. Even after replacing old windows with new thermal windows, most heat loss from the house is through the windows. But a significant amount is also being lost through the wood framing. Wood is a better conductor of heat than the surrounding insulation.through conduction, window manufacturers use low-conductivity materials in window construction where possible. The problem is, the essential part of any window is glass, and glass is not a low conductive material. If it were, then the simple solution to window insulation would be to install thicker glass to slow down heat movement. But, unfortunately, glass conducts heat very well. One solution would be to create a vacuum between two panes of glass. We know that vacuum is an excellent insulator. The problem with this approach is that so far no one has been able to create a reliable glazing system that works in an extreme climate like ours in Nebraska. The temperature differences between the two panes of glass causes the rigid glass welds along the edge of the unit to fail through stress.

So the more usual solution is to fill the gap between two panes of glass with a transparent gas. The gas most commonly used is plain, ordinary, dry air. The problem with air, however, is that it supports convection currents, and convection also transfers heat.

Understanding Room Air Convection
Let’s say you decide to follow our advice and insulate your outside walls with blow-in fiberglass or cellulose insulation (see Insulating Your Old House). You call in your insulation contractor and tell him to insulate the entire wall except a 3′ wide by 4′ high space right in the middle. After he stops sputtering, he will explain, hopefully in a nice way, that for insulation to work properly every nook a cranny in your wall must be filled with insulation. Otherwise all your heat will escape through the uninsulated gaps.

Ok, this makes sense. It’s pretty clear that a 12 square foot gap in your insulation is not a good idea. But now lets replace the 3′ wide by 4′ high gap with a dual-pane thermal window. The result is the same — a 12 square foot gap in your insulation. In fact, it’s worse. Your uninsulated wall had an R-value of about R-3.22. (the siding, sheathing, interior plaster and other components of your wall have some insulating value, see The Insulation Value of Common Construction Materials). Your new window has an R-value of about R-2.2.

As far as heat is concerned, your window is just a weak point in your wall insulation, and like water, heat seeks the path of least resistance out of any container. Very little heat actually escapes through the walls or roof of a well-insulated house. Most heat gets out through the windows. This is due to a natural physical process by which windows tend to draw heated room air to themselves. This process is room air convection.

Room Air ConvectionRoom Air Convection DiagramThis diagram shows a vastly simplified room air convection process. Actual room air convection may involve hundreds, even thousands, of air currents, big and small, all tending to move heated room air toward the windows.

Convection is a natural physical process that occurs in any room that contains air. Heated air rises, cold air falls. As heated air comes in contact with the room’s windows, the air gives up some of its heat to the glass. Now cooler and, therefore, denser, it starts to fall. As it falls, still pressed against the glass, it gives up yet more heat, getting ever colder and denser, and falling even faster. This creates a vacuum which other warm room air rushes to fill. This new air is in turn cooled and also falls, continuing the cycle. Until someone figures out how to repeal the physical laws of thermodynamics, it is completely unstoppable.

Room air convection would not have much conse­quence if just the little bit of air directly in front of the window was the only air affected. But the vacuum formed by the falling cooled air draws heated air not just from above, but from both sides of the window, forming convection currents that very quickly reach to every corner of the room, directing almost all the room’s air to the window through one convection current or another.

Unfortunately the computer models used to predict heat loss do not take into account room air convection. In all fairness, convection is very complex and very hard to model. Climate scientists have been trying for decades to develop an adequate model of atmospheric convection. It takes a long time and a super-computer to produce even a rudimentary model.

To simplify measurement, window engineers simply ignore room air convection. Room heat loss models assume that heat tries equally hard to escape in all directions. So if only 10% of your room walls are windows, then the assumption is that only 10% of the room heat is trying to work its way through the windows. This assumption ensures that any computer model woefully underestimates the rate of heat loss through windows, which may be as hight as 90% in the real world.

So, the next obvious question is, since all the heat will eventually get out through the windows no matter what you do, why bother to insulate at all? The answer is that we want heat to escape as slowly as possible. We cannot keep heat from escaping. Eventually it will get out. The object of insulation is not to keep it from getting out, but to slow it down. The more it is alowed, the less often you have to add heat to make up for the heat loss.

Let’s fill a box with water. Now take a hammer and knock down one of the sides. All of the water will immediately spill out. Fill another box with water and drill a hole near the bottom of one side. All the water will run out, but it will take a lot longer. Insulation works much the same way. If you don’t insulate your walls, the room will empty of heat very quickly — not just through the windows, but through all parts of the wall. By restricting heat to escaping only through the windows, the process is considerably slowed. That’s why we insulate walls despite the big holes in the insulation occupied by your windows.

For much more information on how insulation works, see Insulating Your Old House.Convection is the movement of molecules within fluids (i.e. liquids, gases). Convection does not take place in solids because solids (with the exception of some unusual materials called rheids) do not flow, nor in vacuum, because there are no molecules in a vacuum to move around. But in gases such as air, convection is one of the major modes of heat transfer.

Air, if it is dry and still, is a surprisingly good insulator. But, unfortunately, air between two glass panes is almost never still. It flows inside the glass. The air touching the warm inside pane picks up some heat and starts to rise — warm air, as you know, rises. It soon reaches the top of the window sash where it cannot rise any further. Eventually it is pushed against the colder outside pane and gives up some heat. Now colder, it starts to fall and as it falls it continues to give up heat until it reaches the bottom of the cavity. There it is soon pushed up against the warm inside pane again, draws in some heat and start to rise once more. This cycle repeats continuously and has the effect of efficiently conveying heat from inside to outside where it is lost. If the heat difference between the inside of the house and the outside is large, the convection current moves faster, conveying more heat. If the difference is slight, it moves slower, but it never completely stops moving.

Measuring Heat Transfer in Windows
NFRC LogoThermal resistance in windows is measured in U-value. The lower the U-value, the better the window unit resists heat transfer. U-Value sounds a little arcane and mysterious until you realize that it is just the reciprocal of the more common R-value rating. To translate U-value into R-value, just divide 1 by the U-value. A good triple pane window with a U-value of .16 has an R-Value of 1/.16 or R 6.25 — not very impressive when you consider the minimum acceptable for walls in this part of the country is R-13. Even less impressive is the average thermal window which has an R-value of abut 2.2.

U-Value is determined by testing the window using a process established by the National Fenestration Rating Council. The NFRC does not do any testing itself, but establishes the testing process and certifies the laboratories in which the actual testing is done. According to the NFRC “[t]he rating system employs computer simulation and physical testing by NFRC-accredited laboratories to establish performance ratings for fenestration products and product lines.”

To test for U-value, the window provided by the manufacturer is placed between a hot plate and a cold plate inside a tightly sealed, environmental controlled chamber, and heat flow between the two plates is measured with a heat flux sensor. The unit’s U-value is calculated from how long it takes to heat to transfer from the hot to cold plates. The NFRC procedure has at least one advantage over older testing processes, and those still used in Europe — the entire window, including frame, is tested, not just the glass.

The problem with this testing procedure is that the rigidly controlled, virtually airless, environment is not a very realistic simulation of the real world environment windows actually inhabit. It tests only two of the three heat transfer methods: conduction and, to a much lesser extent, radiation. It ignores convection. But convection is how most heat loss occurs. The consequence is that the tests results are meaningless in the real world — unless, of course your world has no air.

If your windows are installed in a room that has no convection currents and in an environment that had no air movement outside the house, then the U-value results would be a reasonably accurate predictor of how the windows would perform installed in your walls. But there is no such place. All rooms have convection currents — a window itself creates convection currents in an insulated room — and there is no such thing as completely still outside air. If there was such a thing, then a window itself, again, would create its own outside air currents.

So, while U-value comparisons between window units may help you decide which of two windows is the better thermal performer in the abstract, it does not tell you much about how either window will perform in your house. For the most part this necessary research has not been done. Window manufacturers are not the least bit interested in showing that their windows do not perform as well as advertised, and no government agency seems to have been aroused enough to do a comprehensive formal field study — even though going green is now officially the government’s policy.

From the limited field studies that have been done, however, we know that actual thermal window performance is well below that predicted by U-value ratings. There is plenty of evidence that properly restored old wood windows with storms perform at least as well as new thermal windows, and in the long run, as seals start to leak and the fills and coatings that temporarily boost new window thermal performance start to degrade, restored old windows may perform better.So, in trying to combat heat loss through conduction by using double- and triple-pane insulated glass units, manufacturers have introduced the third major source of heat loss, convection. Now the problem is to control the effects of convection, and that’s very difficult to do. In fact, as of now, there are no really good solutions.

The one most often adopted is to replace the air between the glass panes with a heavier gas such as Argon or Krypton. Heavy gases are more viscous and flow more slowly, thus slowing down heat transfer. These gases can temporarily raise a window’s resistance to heat movement considerably. Unfortunately they are not permanent. They leak out over time, and every manufacturer that uses them admits that they leak. No one warranties in-fill gases against leakage. After 10 years they are all but gone and have been replaced by, you got it, plain ol’ air. So that extra $5,000 you spent for

Fill Gas is Guaranteed by the Manufacturer to Leak Away
No manufacturer warranties in-fill gas from leaking, because they know full well it will leak out over time. This warranty language from Milgard Windows is typical:“For Milgard Products with argon or krypton gas-filled insulating glass, Milgard injects the gas at the time of manufacture. The gradual dissipation of the gas may occur naturally over time and is not a defect. Other than gas loss due to seal failures, this warranty does not cover the gradual dissipation of inert gas or the amount of inert gas remaining in the Milgard Products at any time after manufacture.”

a house full of Krypton-filled windows just went adios; and the environment got a nice dose of Krypton that it did not really need. We think temporary measures such as films that degrade and gases that leak should not be allowed in determining a window’s thermal resistance. And we suggest that if you are contemplating buying a gas-filled window, you pay attention only to the U-value rating without the gas — eventually you will be gasless — and there is no way to get a gas refill.

But convection does not occur just between panes of glass in a window. It also occurs in the great outdoors, and is a major source of changes in our weather. There is always some air movement outside. It may be just a mild breeze, or it may be a tornado. Even if you can’t feel it on those hot, muggy summery days, it’s still there.

Inside your house, the story is the same. Air is moving constantly, and, unfortunately, the air convection inside your house is an efficient heat conveyor that constantly moves warm inside air to the weak spots in the room’s insulation where it loses its heat to the great outdoors. The weak spots in the insulation of most rooms are its windows.

We already know that warm air rises and cool air falls. The warm room air next to a cooler window glass gives up heat to the glass and falls. This creates a slight vacuum which other warm room air rushes to fill. This new air is in turn cooled and falls. This would not have much effect if just the little bit of air directly in front of the window was the only air affected. But the vacuum formed by the falling cooled air draws heated air not just from above, but from both sides of the window forming convection currents that very quickly reach to every corner of the room directing almost all the room’s air to the window through one convection current or another.

This is the process by which almost all of the heated air in the room is lost, not through the well-insulated walls, but through the relatively poorly insulated windows. The windows, in effect, draw heat to themselves through convection currents and dispatch it to the outside. It is an unstoppable physical process as long as the house contains air.

Testing Window Efficiency
Unfortunately the standardized tests used by window manufacturers to rate windows do not take into account room air convection. In fact, room air convection is eliminated as much as possible from the normal, static tests because it is too hard to standardize and no one can agree on a dynamic test that includes room air convection (See Sidebar: Where to Find Window Parts
AA Window Parts & Hardware

All About Doors & Windows

Blaine Window Hardware

Phelps Company

Pickens Window Parts

Strybuc Industries

http://swisco.com/“Measuring Heat Transfer in Windows”). So in the tests that assign R-values to windows (actually U-values which are just the reciprocal of R-values. See the sidebar R-value, U-value… What Do They Mean? for more information), convection is ignored. This makes the tests wildly inaccurate because it is room convection more than any other heat transfer method, that produces the most heat loss. So, while it sounds impressive that a new, double pane, window has an R-value twice that of an older, single pane window, this does not translate into “The unfortunate consequence of static testing is that window manufacturers tend to build windows that score high on tests, but do not necessarily perform best in the real-world environment.”a window that is twice as energy efficient. It may be somewhat more efficient, but we don’t know precisely how much since there are, as of today, no accurate tests of real world window heat loss.

The unfortunate consequence of static testing is that window manufacturers tend to build windows that score high on tests, but do not necessarily perform best in the real-world environment. Most potential window customers know little about windows, but do know that a high R-value is better than a lower R-value, so to sell windows, window makers strive for as high an R-value rating as possible in independent static R-value tests.

So, while many replacement window manufacturers claim reductions in household energy use of up to 35% and even 40% after installing their replacement windows, most admit that these projections are based on static testing in laboratories and hypothetical computer models, not actual field testing. When these claims have been field tested, little or no actual savings have been found. Where an old wood window is restored and equipped with a good storm window, the old window system has been repeatedly to perform about as well as a double-pane thermal replacement window. And it costs much a lot less to restore an old wood window than it does to replace it with even an average, run-of-the-mill, thermal window.

Modern Window Design and Materials
In former times windows were engineered for longevity. Now, they are engineered for energy efficiency, with longevity being a lesser consideration. This focus has drastically changed how windows are designed and manufactured, and how long they last. Most modern windows have a expected lifespan of less than 30 years — some of the best may reach 40 years — but 10-30 years is more likely. The problem is not that the windows are poorly made. Today’s windows are usually very well made. The problem is the engineering and the materials used. They just don’t last like old wood windows. Complex Spring Balances
One type of modern tension spring sash balance. Compare to a simple iron weight hanging from a cord, and it is easy to see how many more pieces there are to malfunction in this mechanism.

Mechanical BalancesA comparison of old and new sash balances, for example, illustrates the difference. If you raise the sash of an old double hung window, it stays in place at whatever position you leave it. This is possible because the weight of the window is counter-balanced by two iron weights attached to the sash by ropes that ride in pockets built into the wall alongside the window. The mechanism is simple, and works by gravity. There is nothing to break but the ropes, which can be easily replaced and once replaced last between 50 and 100 years, or even 200 years if bronze chain is used in place of rope.

Some window manufacturers still make windows the old way. This Heritage Series double hung window with traditional weight and pulley balance from Kolbe and Kolbe Millwork Company, Inc. is available in a number of wood species, including oak, pine and cherry. If we did not make our own windows, this is the one we would buy.Modern, self-contained, replacement windows cannot use sash weight pockets built into the wall, so other balancing mechanisms had to be developed. These are all some form of metal spring. The tension in a spring is what holds the sash in place when the window is open. Spring ballasts, however, unlike simple iron weights, are complex mechanical devices prone to breaking. Metal springs themselves are subject to metal fatigue which can cause the spring to lose tension over time or even fail completely. We have replaced some worn out spring balances less than four years old.

Deteriorating VinylVinyl is another culprit. Vinyl deteriorates when exposed to ultra violet (UV) rays. Those of us of a certain age remember well how vinyl dash boards in the ol’ Ford used to crack and split after only a few years exposure to sunlight. The vinyl is better these days. UV inhibitors retard deterioration, but nothing can stop it entirely. Vinyl window parts, especially the thin, flexible vinyl used in balance mechanisms, will deteriorate over time, especially with repeated use. After a few years they become brittle, break and need to be replaced.

Vinyl softens at 165°, and its not that uncommon to see such temperatures in the unventilated space between window panes on a bright, sunny day. Excess heat can also cause vinyl to warp and twist. While vinyl window manufacturers say they have that problem under control, no manufacturer we know of actually warrants its vinyl windows against warping, twisting or cracking, so how under control can it be? For more information in vinyl window problems, see vinyl-windows.org for a roundup of government and other third party studies of vinyl windows.

Leaking SpacersDouble- and triple-pane glass in thermal windows are manufactured in what are called Insulated Glass Units (IGUs). Spacers between the glass panes not only separate the panes, but seal in the gas between the panes. Air between panes of glass must be very dry. If the spacer holding that two panes of glass together springs a leak, moist room air will get between the panes and condensation will form, which may lead to mold, mildew and other nasties inside your IGU where you cannot get at it. There is no cure for this problem. It cannot be repaired. The entire unit has to be replaced. Seals are much better now than they were when first IGUs were first marketed in the 1930s as Thermopane®. Still, no one has yet invented a seal that does not leak. Some leak sooner, some later, but they will all leak someday. The environment that spacers have to survive is brutal. Temperatures can be as hot as 180° in summer, plummeting to -30° in winter, or even worse. It’s very hard to come up with an adhesive that works for a long time in that kind of environment. It is common to find spacer leaks in even very good windows within 10 years of installation and in some poorly made windows within one or two years.

Window Sashes that Cannot Be RepairedLike today’s Integrated Glass Units, modern window sashes are often single units that cannot be taken apart for repair. They can only be replaced using a component provided by the original manufacturer. You have to hope the window manufacturer (1) is still in business and (2) still makes the part. And you can expect the replacement parts to cost nearly as much as a new window. The manufacturers have a monopoly on replacement parts for their own windows and are not at all bashful about charging all the market will bear.

Old Windows: Built to be Re-Built
What Others Say
National Trust for Historic Preservation
Click to Read the Report


Click to Read the Report.


Is this link broken? Please report broken links.

“…Given that an average house has between 24 and 30 windows, and the typical replacement window unit costs between $500-1,000 each, does an investment of $12,0000 or more make sense? On the flip side, the cost to restore an existing window and add storm windows (where appropriate) will generally be much less….

Many window replacement manufacturers claim greater savings than actually occur. Since windows account for at most 25% of heat loss, the payback and time to recoup your investment in terms of energy savings could take between 40 and as much as 200 years, based on various studies. A study from Vermont show the saving gained from replacement windows as opposed to a restored wooden window with a storm is only $.60. The added problem is most replacement windows will not last as long as 40 years, much less over a hundred years. And some are being replaced only after 10 year of service.”

National Park Service
Click to Read the Report


Click to Read the Report.


Is this link broken? Please report broken links.

“[We recommend] the retention and repair of original windows whenever possible. We believe that the repair and weatherization of existing wooden windows is more practical than most people realize, and that many windows are unfortunately replaced because of a lack of awareness of techniques for evaluation, repair, and weatherization.Wooden windows which are repaired and properly maintained will have greatly extended service lives while contributing to the historic character of the building. Thus, an important element of a building’s significance will have been preserved for the future.” (Emphasis supplied).Photo: Phelps Company,
Restored Weight PulleysRestored sash weight pulleys with new weight chains in place of pulley ropes. This is a 200 year repair.Unlike modern windows, old wood windows were made to last for many generations, so they were built to be repaired, over and over again, if necessary. The old-time craftsmen knew that their windows would last a good long time, but not forever. So they built windows that could be easily fixed when something finally did give way. An old wood window is put together so it can be un-put together — it can be taken apart and any of the individual parts repaired or replaced using common hardware components available at your local hardware store and ordinary wood available at your local lumber yard. There is no chance the parts won’t be available even 200 years from now, unless we simply run out of trees — which, despite the the strident alarms of the more radical wings of the environmental movement, is not all that likely.

There are no exotic materials in an old window, just wood, glass, iron, rope and a little bronze or brass for the hardware. No vinyl, no unpronounceable chemical compounds, just basic stuff available almost anywhere that has a lumber yard and a hardware store. Anyone with some basic carpentry tools, a little understanding of how windows work, and decent eye-hand coordination can restore an old window.

Restoring Window FunctionUnlike modern windows, old windows rarely break. They may stop working, but the culprit is seldom the window itself. It is accumulated layers of paint that have glued the sashes to the frame, and broken pulley ropes. These are easy fixes that usually take just a few minutes. The sash ropes we use today are not the ropes of 100 years ago. The cotton ropes used then had a life expectancy of just 25 years. The new nylon/cotton ropes last up to 100 years. Freed from decades of paint, with new pulley ropes, and with a little scraping and sanding, most old windows work like new. If you want the repair to last nearly forever, use bronze sash chain instead of rope. Some people don’t like chain because it’s noisy, but considering that a properly installed sash chain never has to be replaced again, the trade-off is, in our humble opinion, a no-brainer.

Repairing DeteriorationThe next step is to check for rot and deterioration. Water is a window’s worst enemy. Although poor design, sloppy installation, wood-loving insects and baseball-loving kids can contribute to a window’s demise, the usual culprit is wood rot caused by standing water.

We find this primarily on the sashes and stool or sill of the window. It’s not hard to fix. If the problem is minor, and exterior (which is where it usually is), then a little outdoor spackling and some new paint solve it. Photo: Gorell Windows & Doors.Correll Storm WindowsAdding a good storm window to a restored wood window increases its energy performance to the same or better than a standard replacement window.Otherwise there are special two-part epoxy fillers that are — or so their manufacturers’ say — even stronger that the wood being replaced. If the problem is even more serious we may splice in some wood or even remove and replace the deteriorated part with a new, matching, part made in our cabinet shop. If necessary, we can build and entirely new sash that duplicates the old one exactly. In 40 years we have had to do this exactly twice.

Photo: SpencerWorks SpenserWorks™ Storm WindowWood combination storm windows from SpencerWorks combine traditional appearance with up-to-date functionality and very efficient thermal performance.WeatherproofingNow we need to look at weatherproofing. Over the years the wood in your window has dried out and shrunk a little. This is the reason your sashes may be loose in their frames and sometimes rattle in the wind. Since the sash is now smaller, air can creep around the sash. The weatherstripping may also need attention. A lot of old window makers used bronze for weatherstripping, and it may be intact, but often it has come loose because the nails used in those days to attach the weatherstripping have worked themselves out — or the bronze may have been removed by some old painter too lazy to mask it off when painting. We use new spring bronze slipped around the sides of the sashes to eliminate air leaks, tighten them up in frame and provide a nice slick surface to ride on. Horsehair felt made specifically for windows, or silicon bulb weatherstripping (but not rubber or plastic which do not last) can be used where the sashes meet the frame at top and bottom and at the meeting rail to bar air infiltration.

Spring Bronze WeatherstrippingSpring bronze in the sash channel not only seals air leaks but provides a slick surface for the sash to slide up and down on.Insulating Around WindowsOnce the weatherstripping is done, we look at the insulation where the window meets the wall and in the sash weight pockets. Even if you have had your old house insulated, the insulators usually miss that small 1/2 inch or smaller gap between the window frame and the wall stud. We seal this area with low-expansion foam.

The pocket on each side of the window that holds the sash weights is also likely to be empty of any kind of insulation. Obviously we can’t fill it with foam because the sash weights need to move freely so the window will work properly. But the sash weights do not take up all of the space in the pocket, and the space they don’t take up can be insulated using a high R-value rigid board like polyisocyanurate (we don’t try to pronounce it either — its “poly-EYE-so” to us) which has a rating of R-7.2 per inch. We use 2-1/2 to 3″ of it in total together with expanding foam to arrive at a total R-value of R-18 to R-22 in the sash pocket — which is probably more than the R-13 to R-19 you have in your walls.

Replacing Single-Pane GlassWhat we never do, and don’t recommend, is replace single pane glass with dual pane glass. We used to try this, but it never worked very well. Old sashes are just not milled for the extra thickness of dual pane glass, and the sash pulley/weigh system is not designed for the increased poundage of dual pane glass. And, in the end, the gain in energy performance, if any, is negligible. A much better choice is to install a good storm window over restored sashes. Unless its cracked or broken, leave the glass alone.

How to Restore an Old Wood Window
Learn the basics of restoring old wood windows from this video by Preservation North Carolina. It won’t make you a window preservation expert, but it will get you started safely. If you have never restored a window before, you might check out our article Can I Do It Myself before you begin.

Adding Storm WindowsIf this seems like a lot of work, it is. But restoring your old window is about half the price of replacing it with a new thermal window, and wastes nothing. Old growth hardwood is saved from the landfill, and a lot of good old-time craftsmanship is preserved. A typical window can be restored for between $200 and $300. Of course it is not yet as energy efficient as a new window. For that we are going to have to add a storm window. “High-quality equivalent replacement [window] units have been shown in practice to cost as much as three times that of restoration.”

Walter Sedovic
CEO and Principal
Walter Sedovic Architects

A good quality white aluminum storm window installed will run about $80. An upscale wood combination storm window from a company like SpencerWorks will cost a bit more. If you already have storm widows, then you are just that much ahead. But assuming you don’t, your cost to repair your old wood windows and add a good storm window is about $325.00 vs. $500 and more to replace them. This is a savings of $5,500.00 in a 20-window house. For your investment you get a window that should be good for another 100 years, while a replacement window is doing very well to last 30 years. Your window performance is just as good if not slightly better and you saved 45% of the cost of installing replacement windows.

If you really want to save energy costs, assuming your attic and walls are already insulated at least to code, your heating and cooling system is already very high efficiency and all your doors are weather-stripped, go buy a high efficiency water heater ($1,000), put $4,400 in the bank against a rainy day, and treat yourself to a really lavish steak and lobster dinner ($100.00), for being “energy smart”. Just the water heater alone will save many times more energy dollars than a whole houseful of replacement windows.

Do New Windows Save Energy?
Electricity from Coal
The average thermal energy content of a ton of coal is 6,150 kilowatt hours (usually written as 6,150 kWh/ton). But not all of that energy is turned into electricity in the power plant. Most goes out as heat. Only 40% on average becomes electricity in a very efficient coal-fired plant — or about 2,460 kWh/ton.

A 500 megawatt coal fired power plant produces 3.5 billion kWh per year, burning 1.43 million tons of coal to do so. It also produces a lot of pollution, including…

  • 10,000 tons of sulfur dioxide, the main cause of acid rain,
  • 10,200 tons of nitrogen oxides which is one of the principal ingredients in smog, and also contributes to acid rain,
  • 3,7 million tons of carbon dioxide, the primary greenhouse gas associated with global warming.

And it produces small amounts of just about every other element in the periodic table, including radioactive elements. In fact, a typical coal-fired generating plant emits more radiation into the atmosphere than a nuclear power plant.”Replacement windows cannot possibly save enough energy during their lifespan to pay back even the energy invested in their manufacture.”One of the fastest ways to get tossed out of our decidedly un-pretty, over-the-warehouse offices is to start talking about how environmentally friendly your company’s replacement windows are. (Its not the fastest way. The fastest way is if Abby, our old warehouse dog, decides she doesn’t like you. She is a remarkable judge of character.)

Replacing a window means that the old window is discarded, usually in the landfill. What is thrown away with an old window? A lot of irreplaceable old growth lumber, hours of careful craftsmanship, and a little American history. It took work and energy to build the window in the first place — to fell the tree, mill the lumber, build the window and install it. That is all now in the trash.

And it takes a lot more energy to build the replacement window. Energy expert Keith Haberern estimates that building a new replacement window requires 4.3 million BTUs. of energy. If this sounds like a lot, keep in mind that it’s not just window manufacturing that uses energy. It’s producing all the components and materials that are go into the manufacturing process. Making the glass, felling the trees and sawing the lumber, producing the paint, plastics, metal fasteners, even the paper labels all involve energy expenditure. Not to mention all the shipping involved — some of it from half-way around the world. By the time the window leaves the factory the total energy investment in the window is very substantial.

How much energy is 4.3 million BTUs? It takes about 13,000 kilowatt hours of electricity to produce 4.3 million BTUs. And it takes 13.65 tons of coal to generate that much electricity (according to Department of Energy estimates). In an entire year, an average home in Nebraska uses 18,000 kilowatt hours. So one replacement window embodies almost 9 months of household power usage. That’s for just the one window. If you have the average 20 windows in your house, just building your windows will use nearly 15 years worth of household energy. So even if your replacement windows saved 25% of your electric bill (and there are no replacement windows that come even close to that), it would take 60 years to pay back just the energy used to build your new windows. The math just does not work out. The life expectancy of the windows is less than the payback period.

Which is why, if you are a window salesperson and start talking about your green replacement windows, you are going to get the bum’s rush. We hate to be impolite, but there are absolute limits to civility.

Source: www.StarCraftCustomBuilders.com