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Q&A Spotlight

Should Historic Preservation Trump Energy Performance?

What to do when the interests of energy efficiency and historical preservation collide

Save or replace?

Old single-pane windows can represent significant energy losses, but whether that warrants replacement in historic homes is a matter of opinion.

Windows are often a dominant architectural feature in old houses. A six-over-six sash with wavy, bubbled glass has a charm that modern windows can only aspire to, more than offsetting their less-than-stellar energy performance.

Or so many local historical preservation committees would argue. And, as Mike Keesee has discovered, that’s a frustrating problem for builders and homeowners who want to make energy-efficient windows part of a renovation.

Writing in a Q&A post at GreenBuildingAdvisor, Keesee says his local historic preservation committee just doesn’t get it.

“Our local historic preservation committee will not allow retrofits/replacement of so called historic windows — the aging, single-pane, wood frame with ‘wavy’ glass,” Keesee writes.

“Some in the historic community claim that the existing windows can be retrofitted with films, weatherstripping, insulated shades or the like and match the performance of a high-performance (e.g., Energy Star rated) replacement window. Unfortunately, they don’t respond well to arguments based on the laws of physics.”

Keesee’s appeal for studies that quantify the benefits of replacing old windows with new ones is the subject of this week’s Q&A Spotlight.

Energy losses are easy to prove

Matthew Nolette has been researching exactly this question and has facts in hand that would support Keesee’s point of view.

“As it turns out, I just spent the last couple of hours trolling NFRC [National Fenestration Rating Council] data for specifics on the performance of a single-glazed wood window with a combination storm window applied,” Nolette says.

A double-hung window with 1/8-in. thick clear glass separated by a roughly 2-in. air gap from a single low-e combination storm has a U-factor of 0.35, Nolette writes. That U-factor is 25% worse than the U-factor of a new, double-pane low-e insert with a U-factor of 0.28. “The data I’m seeing says that the folks who think the retrofit will meet current standards are incorrect,” he says.

Rhaud MacDonald thinks Nolette and Keesee are both on the right track.

Marvin inserts are available in a range of sizes and configurations, clad and unclad, MacDonald writes, and they “easily outperform” the historic preservation committee’s window-and-storm configuration.

“The man-hour and materials costs significantly decreased and offset the increased costs,” MacDonald says. “They range from approximately 0.21 to 0.45 U-factor and have SHGCs [solar heat gain coefficients] from 0.16 to 0.56.”

The historic preservation folks are right

Not everyone is on the new-window bandwagon.

“I’m going to give a counterargument here and suggest the ‘historic types’ are not all full of hooey,” James Morgan replies. “The laws of physics, when consulted on the big energy picture, tell us that for a historic building (with usually rather limited fenestration), glazing U-factors are a relatively minor consideration compared to all the other usual energy retrofit suspects.”

There are other arguments for keeping old windows, he says, including the avoided manufacturing and transportation costs associated with installing new windows. “And finally, the laws of economics consistently tell us that window replacement is about the least cost-effective way of improving energy performance in an old building, with a payback period generally exceeding the projected lifetime of the unit,” Morgan says.

Although old windows require more maintenance, he adds, the same could be said of owning an old house in the first place. Instead of looking at old windows as a hindrance, try viewing them as a “precious and increasingly rare resource… There’s a good case for leaving your irreplaceable historic windows as part of that whole fabric.”

Old isn’t necessarily significant

To Keith Gustafson, preventing the replacement of historic windows with “cheapo vinyl” is a good thing, but “allowing self-appointed historical committees to trump energy code is a travesty.” He’d argue with anyone who claims there isn’t much of an energy penalty with a house-full of single-pane windows.

There are ways to work around no-replacement edicts from historic preservation committees, he says, but the larger issue may be that these groups assume that old age automatically equals historical significance. That’s not always the case. “In my area, pre-revolutionary houses are quite literally a dime a dozen,” Gustafson writes. “Most were built by Ed the barrel-maker and George Washington never slept within a day’s ride of them. They should be saved. But preserved?”

Morgan sees his point, but adds that living near a rich stock of old houses is not a luxury everyone enjoys, and that while preservationists may see things from a narrow point of view, they’re not the only ones with that problem: “They tend to see the world from a very narrow perspective,” Morgan writes. “But so do we, and with our close focus on technical specs, we can sometimes miss the forest for the trees… Sometimes replacement is the best choice, but repair and re-use is a good green option, where the lower environmental cost can often outweigh the marginal benefit of super-low U-factor in a singular building element. When we consider that our older buildings were built to function in a pretty low-profile energy economy there should be no reason for historic authenticity and green renovation to be at loggerheads.”

Our expert’s opinion

GBA technical director Peter Yost chimes in:

There are really three very interesting questions built into this discussion:

  • Who decides the what and where and when of historic preservation?
  • Do we have quantitative, fair, energy performance comparisons of all the options for improving old windows?
  • What happens to the energy comparisons when the embodied energy of various window retrofit options is factored into the energy equation?

The first question is likely the hardest, but maybe we can all agree it is the most maddening. It is difficult if not impossible to separate out aesthetics as an element of the historic preservation question, and aesthetics is a total can of worms.

I certainly am glad that when I went to accomplish a deep energy retrofit on my own 100-year old home that it was not in an historic district. I, not a committee, made the call that recladding with wood lap siding and burying the original and authentic split-faced architectural block behind 3+ in. of closed-cell spray foam did not rip apart the character or fabric of the neighborhood, and in fact, looked better.

And when the well-meaning and hard-working Advisory Committee on our Window Attachments DOE /LBNL project was laboring over our Overview Summary table, it did not take us terribly long to agree that an attribute column for aesthetics was not only difficult, it was arrogant and inappropriate.

Comparing the value of historical character and energy performance is a no-win undertaking. There is no “correct” result.

The second question — the one about comparative energy performance — is a bit easier, but not by much. Whether or not the analysis includes the energy performance of the whole house, or only the windows but including the rough openings, or just the window unit, can make a big difference. And including options such as interior cellular shades with sealing side-tracks adds the question of adjustability and its impact on actual energy performance.

Field testing indicates that insulated cellular shades with sealing sidetracks contribute about R-4 to the window (see Interior Insulating Blinds, but only when fully deployed. (NOTE: we have just installed such shades in our home and I have to admit some winter nights go by with more than one shade stacked in its stored position with an R-value of about 0; and optimally deploying the shades on cloudy versus sunny cold winter days is more than a bit challenging, even for us folks with home offices).

But there has been significant research done on old window retrofits, comparing a variety of window retrofit options. One study down in Colorado, The Effect of Energy Efficiency Treatments on Historic Windows, included RESFEN modeling, field testing, and lab testing. The study showed that low-e storms + window repair have better energy performance than a vinyl replacement insert window.

Another study done in Scotland, Thermal Performance of Traditional Windows, involved both lab and field testing for a variety of window treatments (low-e storm, drapes, insulated cellular shade, insulated shutters, etc.). Again, a repaired and air sealed historic six-lite double-hung window with a low-e storm performed as well as, or even a bit better than, a double-glazed sash replacement system.

The third question introduces the embodied energy comparison of keeping and replacing the old windows. There really has not been much research done on this issue; the only report I could find, What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows, provides a table of the embodied energy of a variety of building materials, and correctly states that the three materials most significantly used in replacement windows — aluminum, PVC, and glass — all have among the highest embodied energy content. But this information is far from quantitative in its comparison, and ignores the fact that moving older windows to levels of replacement window performance often involves new storm windows, typically made of glass and aluminum.

Finally, the issues of durability and condensation resistance are certainly important (and related) in this comparison, and very little comparison of these attributes has been done for various window retrofit options (although stay tuned — Building Science Corporation is soon to release a report on Building America work they have been doing, “Window Repair, Rehabilitation, and Replacement,” which will address these issues).


  1. Lloyd Alter | | #1

    As past president of the Architectural Conservancy of Ontario, I can confirm that window replacement is one of the biggest issues those involved in historic preservation face. Changing windows can completely destroy the historic character and look of a building. And while the numbers show that a new window might be better, (in the example shown, U.28 instead of U.35) if you convert that to R values it becomes clear that the difference, from R 2.8 to R 3.57 is pretty negligible. You would not spend thousands of dollars and destroy the character of a building for a wall upgrade of less than R1 yet people are convinced they should do it on windows. 25% of almost nothing is still almost nothing.

    In 15 or 20 years, when the gas has leaked out of that double glazed sealed unit and the window is ready for replacement again, the old original one with the storm window will still be functioning as originally designed.

  2. Keith Gustafson | | #2

    re: Lloyd
    The likelihood of an actual historic window having a u value of .35 is pretty slim.

    You are misusing numbers there. The first 'R' is the most important one. The change from R1 to R2 will save more energy than every 'R' you add after it. While it is easy to trivialize small numbers when we are used to discussing R30 and R60 the difference between R2 and R3 is actually huge

    In 15 or 20 years, the argon in a badly designed window 'may' have leaked out, and yes, the original window will be working as designed, leaking air as if it was wide open, needing putty and paint, again

    What really amazes me is that there seems to be a dearth of products for this market, Would it not be profitable to make a true divided light "historical approved'' if you will? ought to be easy[if pricey] Especially for the 4 over 4.

    I know this will tee you off, but I feel commissions have too much power in this regard, and it actually hurts progress.

    I don't think anyone is saying the pop in replacement window is a solution in a historic district[if only because of the much smaller glass area]. but once you give one group the power to say 'no' conversation stops.

    Far more often you end up not with restored buildings, but peeling paint behind oxidized triple tracks

  3. Andrew Marani | | #3

    I live in a neighborhood full
    I live in a neighborhood full of 100 year old houses (Roland Park in Baltimore). I was on the architectural review board for the neighborhood for 4 years. Window replacement was a regular submittal. We allowed them as long as they matched the original configuration of the window (3 over 6 or whatever) and the muttons where on the exterior, between the glass and the interior. This policy has worked well over many years and I don't think the neighborhood has lost any of it's historic character.

    As far as storms go: given the choice between looking at a house with old windows behind a sheet of glass and looking at a quality, new divided light window that matches the old window, I will take the new window.

  4. Benjamin White | | #4

    Let's get real
    Most customers who want to replace original windows in historic homes either 1) buy into the premise that new windows will provide savings by improving heat/cooling efficiency, though when compared to the entire house the savings are minimal, or 2) (more likely) they want easy to clean, tilt windows and don't want to spend the $ to convert and modernize their existing windows. The results of this foolishness is great for the contractor (easier) and window salesperson, but unneeded by the building. Everyone reading this column has seen the wrong-sized vinyl windows butchered into an old home, and I've seen 24 on 24 pane windows thrown in the dumpster replaced with a vinyl picture window in a 175 old house. Let's get real - really doing the right job in rehabilitating old windows takes skill, time, and money - but it's doable and clearly preferable to replacements. As for the whining about historical commissions - no one made anyone buy a home in a historical district, and if the historical district came after you bought, you had every right to put in the effort to stop it. Some people just don't like being "told" what to do - or as others would view, living by the rules.

  5. Ed Voytovich | | #5

    Baa Humbug!
    The strips of wood that divide window sash into sections for individual panes of glass (mentioned in Andrew's remark above) are called "muntins," not "muttons."

    I'm sure it was just a typo, so he needn't feel sheepish. It is in fact refreshing not to have the dividers referred to as "mullions," since those are an entirely different animal.

    I'll keep my remarks on plastic muntins inserted between two pieces of glass to myself -- even though it panes me to do so.

  6. Mike Keesee | | #6

    It's gratifying that my initial rquest for information has stirredup the proverbially hornet's nest, but I figured that this has been a long smoldering issue. Full diclosurer I work for SMUD on building R&D issues, including deep energy retrofits so I have a dog in this fight and my bias is obvious. Also, my initial request for help came out of a deep energy retrofit project of a 5-story abandoned, historically registered multi-family bulding under going a gut rehab. Since the project was a gut rehab and the owner was willing to replace the windows, it was a natural for me as the efficiency project manager to recommend the window replacement. Little did I know that the LEED AP City historic official would trump me and the building owner. And let's get things straight --- the windows were terrible. I've got pictures. In fact in a majority of cases the owner had to rebuild the entire window unit - frames, sashes,glazing units, etc. So what did we get? Terrible windows replaced with "prettier" but still very poorly performing windows. Researching the recommendations to "retrofit" the single pane wood frame double hung windows by installing the old weights in tubes and foaming the cavity, installing weatherstripping, etc. seemed a non-started from a cost point of view; that and the fact that we just don't have a lot of peple who do that in Sacramento (we mostly tear down old buildings...or used to). So replacing the windows with similar high performance windows, such as the referred to Marvins, as part of a gut rehab were we were insulating the building, and adding new mechnical systems and solar hot water just makes a whole lot of sense to me, not only from a cost point of view, but from aethestic point and an energy performance point, too.

    By energy performance, I want to point out that windows appear to be a very big deal in a mid-rise multi-family. In the case of this project, the windows were more than 20% of the building surface. My energy analyst is re-doing the numbers and were both convinced they'll show that improving the windows, including all the thermal losses and gains associated w/the double-hung cavities, is more effective than increasing the attic insulation. (R-44 blown-in cellulose w/ cool roof was installed and were guessing that we could have got by w/ R-25).

    That being said, I think window replacements also make sense in SFD, especially in gut rehab, major additions, major rennovation type projects. Maybe not so much on an energy saved but mostly because the old windows are justt a pain to deal with and are ugly--- they're drafty and a great place to grow mold (aren't we all freaked out about mold?). I know. I lived with them in a rental once a long time ago.

    Which brings me to my final point, which is personal. I live in an old house; built in 1928, and been in it 31 years. Almost left a couple of years ago, but everything I saw convinced me that I if just upgraded it and would be great, which I did. Now I didn't replace the windows. Heck, I did that years ago w/ really nice Marvin tiltl packs, and they look great. in fact, bet no one could tell.the difference between them and the old this isn't about replacing windows. I added some mid-range vinly to my addition, in the back, where no one but me can see them.... So I'm doing my retrofit and being a utility guy, apply for our ARRA home performance incentive and lo and behold I find out about this Section 106 requirement that requires any home more than 45 years old that is doing window/roof replacement or installing PV has to go thru "historic review." Are you kidding? Let's get real about this.... we're talking about 1960s/50's vintage track homes. Is this what the preservationists wanted?

    So, I think it's time we get real about the preservation issue... I storngly support preserving structures of historic importance. And I think that when things ware out, like windows, we should replace them. It's what most of the rest of the world does. I think we have plenty of skilled folk who can figure out and have figued out how to replace old, poorly performing windows with good looking, high performance windows without scracificning the so-called character of historic structures I thought character was reserved for people, not inanimate objects) in most cases. The ARRA Section 106 requirement is just stupid and counter productive.

  7. T.C. Feick | | #7

    A couple of more points
    I sell windows for a living, and I spend many hours a year in front of architectural review boards for historic districts. The conversation is not about who gets to decide what is historical; that ship sailed when the local government set up their overlay district and inventory. Secondly, whether or not windows can be replaced is also rarely the real question, as I have not, in my twenty years in the business, ever been told that the window had to stay. The window can be replaced, but are the requirements too cumbersome?
    So this is what the bulk of my discussion and debate with such groups comes down to, and understand that I believe in historic architectural preservation. First, most of the people that sit on these boards are volunteers, and are not the ones who wrote the local law, so there is a lack of understanding of what the intent ultimately might be. I will use the example of materials. Many HARBs insist on wood divided lite windows. Why? because they borrowed this language from the National Park Service, who oversees historic preservation at the federal level (grants). So, we get harbs asking for wood, regardless of quality or aesthetic, because that is the material of the old window. The park service requirements are much more stringent, requiring an aesthetic replica of the existing window. So we have wood windows that are acceptible, and a trip around these towns' historic district shows the good, the bad, and the ugly of such a whimsical approach.
    What is truly needed is a broad based acceptance that what is important and valuable to the structure and district is the aesthetic, not the materials. Many national manufacturers make composite or clad units that look very, very, good. Further, their durability and energy performance can be top notch. A bad window is a bad window; new or old. Sometimes a window is no longer servicible, or, in the case of multi-story, may even be unsafe. Replacement is the only solution. Demanding a material made from assumably the same materials is a lousy place to hang your HARB's hopes; new wood is not old growth. Composites and cladding are the only viable option to disappointment in my opinion.
    Lastly, Using unit u-values, although important, leaves out the most important part of the equation; air infiltration. I would submit that it is difficult to get the old wood window to the air infiltration values of a better quality new window. It can be done, but in the northeast, where labor is very expensive, doing a window rehabilitation will be far more costly than replacement, and outside of very old or exceptional examples, may not yield the aesthetic benefit.

  8. Robert Knight | | #8

    A bit too close to the subject--it's not R and it's not vinyl.
    I'm an architect who does a lot of renovation of old houses. I'm in Maine, so there are a LOT of 19th and 18th century homes with beautiful old divided light windows. If a client with deep pockets wants to rebuild the old sashes and put OLD FASHIONED wooden storm windows on that screw in place and felt seals , fine---but that is almost never an alternative. The upkeep cost in painting alone is brutal, and putting up and taking down sash and screens is a large job twice a year. To argue to keep divided light windows with an aluminum or vinyl combination storm window is, IMHO just about the worst option from an aesthetic and practical view. Rarely have I seen anything ruin a house like combination storm/screen windows. And the energy savings is pretty illusory. The real gain from new windows is not R factor, it's reduced infiltration, and aluminum storms are so poorly done that I wouldn't even count them as doing anything for infiltration.

    Of course you don't use vinyl replacement windows with snap in muntins. A high quality "simulated divided light" window with muntins that cast a shadow on the outside as well as the inside, with matching trim profiles gets you almost everything the old window had except the wavy glass.

    If it's an historic house in an historic district, keep the old windows and eat the costs. It's appropriate.
    But if we are talking any kind of practicality I would opt for modern divided light windows with correct trim every time. 99% of the public will subconsciously see no change, but your client will have a window that doesn't need repainting every few years and a MUCH warmer house (assuming you do all the other stuff too!)

  9. Buildingwell .org | | #9

    Working Together
    The question shouldn't be should one initiative trump the other - but instead how can both initiatives push the other one to be more inventive/inclusive to meet each other's requirements. They should trump one another but work hand in hand - which sounds great theoretically, it requires getting everyone on the same page.

  10. Christopher Vlcek, Littlewolf Architecture | | #10

    window within a window
    There is another approach, that of installing new double-glazed windows on the inside of the historic ones. I believe I saw the example in a presentation by Marc Rosenbaum. Working from the interior, the casing was removed, counterweights and such were isolated, air-sealing and insulation was added, and the new windows installed inside.

  11. Bob Yapp | | #11

    Disposable Windows
    I've read all these posts with great interest. Bottom line, I am a historic property developer in the upper Midwest. In over 160 projects, small residential to huge commercial, I've never replaced a window that wasn't missing in the first place. I love working with historic preservation commissions, state historic preservation offices and the National Park Service. Their regulations are all based on The Secretary of the Interiors Standards for Rehabilitation. These guidelines recommend keeping as much original material as possible. There's nothing oppressive about this for my projects. I actually save money and get better performance as a result. I don't install disposable replacement windows for a variety of reasons:

    1) They only last on average about 15 years.

    2) From a green/environmental standpoint, throwing away a perfectly good window is environmentally irresponsible.

    3) After restoration & weatherization, the historic windows outperform most disposable replacements. This includes the entire window opening with sash weights.

    4) On average, windows only make up 10% of the energy footprint.

    5) Glass is a terrible insulator. R-1, R-2 or R-4 is all just hype by the disposable window manufacturers. Windows are there to let in light and to create ventilation, not insulation

    6) All the windows I restore and weather strip for my development projects can be easily cleaned on the exterior from the inerior of the structure.

    7) A properly weatherstripped old or historic window and general window envelope has less air infiltration than a disposable replacement..

    8) The average payback for a residential disposable replacement is about 40 years and 150 years plus for commercial. Paybacks for restored and weather stripped historic windows, that perform similarly, run about 3 to 7 years with a proper storm.

    9) Restoring and weatherizing old and historic wood windows costs less than replacing with a so-called comparable, disposable replacement.

    None of these facts are conjecture, they're are proven in study after study. I'm currently a member of the National Window Preservation Standards Collaborative. We have been conducting extensive testing and are writing the Window Preservation Standards book that will site all the objective research that backs up this information as well a detailed standards for specifiers to achieve these factual outcomes. This document will be available in the Spring of 2012 through

    As a property developer my buildings must be energy efficient and I need to make a fair profit. I can't get either if I replace original windows.

    There is nothing green or sustainable about a disposable replacement window.

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