Tough Choice: Double-Pane vs Triple-Pane Windows
For ProHOME, the double-pane choice met the net-zero target — and the cost savings paid for a PV solar array.
Windows and doors are unique in that they are an important aesthetic component of the house and also a critical part of the building envelope. And, as part of the envelope, they uniquely affect energy consumption through their insulating properties and their ability to add heat to a home through solar heat gain. Let’s tackle the performance issues first. Designer Michael Maines started with a combination of picture, casement, and awning windows to maximize energy performance. For operating windows, the hinged action of casement and awning windows closing onto the weatherstripping seals better than double hungs, where the weatherstripping between the upper and lower sashes slides past each other at the check rail. Preventing small amounts of air leaking through the window assembly pays dividends when we get to the mechanical systems and for the comfort of the person sitting near a window.
In a high-performance house, the choice between double-pane and triple-pane windows is fraught. Builders and designers typically have strong opinions on the subject. They fall either into the camp of using the highest-performing windows they can find, or the camp of using high-quality double-pane windows and directing the saved money elsewhere. Maines leans heavily toward the first camp. Mike Guertin, the builder, is firmly in the second camp.
The price differential between double-pane and triple-pane windows for the ProHOME project was roughly $7500. Using BEopt (the National Renewable Energy Laboratory’s free Building Energy Optimization modeling software package), Maines found that the triple-pane windows saved about $32 a year at current electric rates (it’s an all-electric house). Whether you evaluate increased spending on performance from a break-even or return-on-investment standpoint, the numbers aren’t going to work out at any point during the life cycle of the windows. The ProHOME is not a Passive House, so it wasn’t necessary to keep the heating load below a particular threshold. Instead, the goal is to create a pretty good thermal envelope and limit air infiltration to between 1.0 and 1.5 ACH50. The significant savings from foregoing triple-pane windows can be used on a PV solar array. Accordingly, the windows are double-pane Marvin Integrity units, which have a fiberglass frame and will stand up to the coastal weather.
Maines still contends, however, that there is a comfort argument for triple-pane windows that may sway the opinions of some clients, as the surface temperature of the interior glass can be significantly warmer than that in double-pane windows in the winter. A cold window surface chills occupants sitting near it even if the ambient air temperature at the thermostat is in the 70s.