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).