Extending Window Openings for a Deep Energy Retrofit

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).
 

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 Norwood windows with a high-solar-gain low-e coating. They are made in New Brunswick, Canada, reasonably affordable, and — by most standards — energy efficient. But the center-of-glass R-value is only about a third of what we achieved with our high-tech, quad-glazed, triple-low-e coated, Alpen windows.

We decided to install these windows in the plane with the wall sheathing (Zip sheathing from Huber Engineered Woods serves as the wall system’s air barrier) 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 Guardian Glass Industries. 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 gain.

Both of those glazing properties were important to us: the visible light because we want our house to have as much daylighting 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 Guardian’s ClimaGuard 75/68 glass (PDF file), and in our double-glazed configuration with a half-inch gap filled with argon, the windows provide 75% visible light transmittance, a solar heat gain coefficient (SHGC) of 0.684, and a U-factor of 0.275 (R-3.64).

We were willing to accept the relatively low R-value (3.6 is a far cry from 12.2 that we achieve with the Alpen Windows), 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.
 

 

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 475 High Performance Building Supply, a specialized product distributor targeting the Passive House movement) and a pre-formed metal sill cladding.

Next-up: the tricky installation our exterior cork insulation.  Exteriors family,” says Barr.