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Extending Window Openings for a Deep Energy Retrofit

For weather-tight window installations, we prefabricated the window surrounds and used advanced European tapes

Posted on Mar 14 2013 by Alex Wilson

A few weeks ago I reported on the amazing, high-tech Alpen, R-12 (center-of-glass) windows that we installed on the north and west facades of our farmhouse in Dummerston, Vermont. At that time I promised to report on the other windows we were installing on the south and east facades (windows 2.0 if you will).

First, some context

With our new home, we are creating a demonstration with dozens of cutting-edge energy-saving and green building features and products that one can include in a new or existing home. As someone who has written about such products for several decades now, this is a lot of fun — though the decision-making often remains a challenge, since there are so many great products and materials to select from.

With our house — the rebuild of a 200-year-old Vermont Cape — we wanted to demonstrate what one might do to dramatically improve the energy performance of existing windows if those windows are in good enough shape that one can’t justify replacement. So that’s what we set out to do on the south and east facades — only we installed new windows, because what had been there (installed in the 1970s I suspect) were small and didn’t serve our needs.

In our product research, we were looking for was a solidly built wood window that would look great in an historic home, not cost too much, and offer reasonable performance.

Good quality, honest wood windows

The new windows we installed on the south and east walls are wood, double-hung with a high-solar-gain low-e coatingVery thin metallic coating on glass or plastic window glazing that reduces heat loss through the window; the coating emits less radiant energy (heat radiation), which makes it, in effect, reflective to that heat; boosts a window’s R-value and reduces its U-factor. . They are made in New Brunswick, Canada, reasonably affordable, and — by most standards — energy efficient. But the center-of-glass R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. is only about a third of what we achieved with our high-tech, quad-glazed, triple-low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. coated, Alpen windows.

We decided to install these windows in the plane with the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ( from Huber Engineered Woods serves as the wall system’s air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.) and then build window surrounds to frame the six inches of exterior insulation to be installed on the walls. This will be a fairly common need with existing houses if we are to carry out “deep-energy retrofits” that rely on exterior insulation.

Our Norwood windows use a specialized low-e coating from . It is a sputtered coating (like most low-e coatings being used today), but it has very high transmissivity. In other words, it is highly transparent, both to visible light and solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss..

Both of those glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. properties were important to us: the visible light because we want our house to have as much daylightingUse of sunlight for daytime lighting needs. Daylighting strategies include solar orientation of windows as well as the use of skylights, clerestory windows, solar tubes, reflective surfaces, and interior glazing to allow light to move through a structure. as possible with unimpeded views of the gorgeous surroundings; and the high solar heat gain because, on the south, we want to benefit from passive solar heating.

The windows use (PDF file), and in our double-glazed configuration with a half-inch gap filled with argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. , the windows provide 75% visible light transmittance, a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) of 0.684, and a U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.275 (R-3.64).

Ready for storm windows

We were willing to accept the relatively low R-value (3.6 is a far cry from 12.2 that we achieve with the ), because we’re planning to add high-performance storm windows toward the outside of the window surrounds. We haven’t figure out exactly what type of storm window we will add, but our designer-builder, Eli Gould, designed the window system with an added storm in mind.

Eli refers to our window surround system, which can accept storm windows, as the WindowPLUS™ system. Functions include extending the wall out to the plane of the exterior insulation, providing a framework for the sophisticated system of air-sealing and weather-protection components, providing a thermal break at the window edge, housing the high-performance storm windows, and potentially providing a space to house a hidden, roll-down screens or shades.

Our hope with the storm window is to work with some leading manufacturers to envision and build the ideal storm window for deep-energy retrofits. It will be highly durable with a frame made of either aluminum or fiberglass, and it will include low-e glass. We’re trying to figure out whether it will include an integral screen with an operable glass panel, or whether — like old-fashioned storm windows — require seasonal removal. The more durable storm window will also offer protection of the wood-framed prime windows..

With our application we are trying to determine whether having two low-e coatings — one on the prime window and one on the storm — will cause the temperature between the two windows to get too high. This may inform the type of low-e coating we use or other material decisions. With older prime windows that don’t include low-e glass, this wouldn’t be a problem. In fact, we would like to see a storm window developed that could be configured with insulated, low-e glass for an even higher level of performance.

Splayed window openings

Another great feature of Eli’s WindowPLUS system — one that took some real figuring — was to splay the openings so that more light will enter and the view out will be less restricted. Our total wall thickness will be about 15 inches, and without the splayed openings it might seem that one is looking out through tunnels.

Eli developed a system that allowed these splayed window frames to be prefabricated and installed with lapped weather barriers (high-tech German products that we got from , a specialized product distributor targeting the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. movement) and a pre-formed metal sill claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. .

Next-up: the tricky installation our exterior cork insulation.

Alex is founder of . and executive editor of . In 2012 he founded the . To keep up with Alex’s latest articles and musings, you can .

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  1. Alex Wilson

Mar 16, 2013 11:00 AM ET

looking ahead
by JoeW N GA Zone 3A

Alex, I'm really looking forward to hearing how you deal with the storm windows ... it would be nice to find "ready made" that would work on an historical house, but I've come up with zilch. I'm currently looking over the options from Concord New Hampshire Window Repair and those from TLC ttraditional storm windows. They each offer low-e glass options and I'm anticipating $400-$500/window ... I hope to do better and, of course, it would be nice to get something closer to home.

Mar 16, 2013 4:26 PM ET

Window replacement
by David Meiland

It's hard for me to tell how embedded the window unit itself will be in this wall assembly, but shouldn't we be concerned about the long term, when presumably a carpenter will have to extract and replace the window units as they wear out? Peeling back and possibly destroying layers of waterproofing and insulation would add quite a bit to the cost. What would one have to remove in order to get a new window into the wall and correctly waterproofed?

Mar 18, 2013 12:22 PM ET

Window replacement
by Joshua Lloyd

David, I was wondering the same thing. How will a future homeowner install replacement windows in the future. However I assume the windows may last longer since they will be recessed and be a little more protected from the elements. As long as everything was flashed properly of course. I mean if you go into some of these older homes with the original windows, they operate almost like new when properly maintained, it is just their efficiency is not up to par.

Mar 20, 2013 5:51 PM ET

Hi performance storm windows
by Brian Sipes

Alex, The Center for Resource Conservation commissioned a study titled "The effects of energy efficient treatments on historic windows" that included testing of several types of high performance storm windows. The study can be found here: Perhaps the findings of this study will help inform your decisions about the ultimate possible performance of your windows.

Mar 20, 2013 6:04 PM ET

Replacing windows down-the-road
by Alex Wilson

David and Joshua,
Being able to remove and repair or replace windows was a high priority of mine. The system is designed to allow removal to the inside. Splayed interior openings will facilitate that. (Both the interior and exterior window surrounds are splayed.) In fact, the entire window surrounds could even be removed to the interior should that ever be needed, though that would be more involved. The windows were ordered without flanges and were installed with "jamb mounting," as is common with European windows.

Jun 29, 2013 11:09 PM ET

Experience with two layers of Low-E glass.
by Richard McClintock

A couple of years ago, deep energy retrofitting our old (Circa 1820) Greek Revival house was a problem as most of the heat would be lost through ordinary Low-E windows compared to the R-67 in the roof assembly and R-47 in the walls.

Really high performance windows did not seem to be obtainable at the time. I solved the problem by installing Marvin Integra windows (Fiberglass outside, pine inside.) using ordinary high heat gain Low-E argon filled glazing on the sheathing attached to the frame of the old house. Then on the sheathing over the foam insulation, I installed an additional Marvin Integra window using ordinary high heat gain Low-E argon filled glazing. There are no problems with thermal runaway between the windows. In winter, there are no cold drafts coming off the bottom of the windows.

In the west where extra heat gain was not wanted, I installed heat gain blocking Low-E windows on the outside. (Be very careful, they look the same, read the etching on the glass.) Again, no problems.

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