Towards a Net Zero Energy Home Part 2: Insulation and Windows

 by George Klassen

Last week, I outlined a number of choices we made for conserving and producing energy in our new home. Our choices were based on projected, science-based, numbers. I want to follow up with observations of what has happened in practice.

There have been non-starters, successes and “not yet” successes. 

Adding insulation is a one-time expense, returning cash benefits for the lifetime of the house, barring damage or renovation, with little or no maintenance. 

Adding ceiling insulation is straightforward. Every one installs blown-in insulation in any case, so we simply added more. Additionally, probably there is a small addition to the attic venting as well. Increasing the R value from R50 to R70 in our case had a return of almost 6% per year, a very good return. I have not poked my head up through the ceiling insulation to observe it, but I assume that the additional insulation was installed properly and that it is doing its job. I therefore consider the additional ceiling insulation a very cost effective, long-lasting success. 

Installing Styrofoam insulation on exterior walls is quite different. I believe that in some situations it could be cost effective, that is it would be a success. In our case it was not. 

I considered the possibility of applying 2” Styrofoam sheets on the outside of the normal wall sheathing, just inside of the siding. Styrofoam sheet insulation is available in a number of thicknesses, starting with fractions of an inch to several inches. It is also available with a reflective layer(s), with contentiously quoted R values. Adding styrofoam sheet insulation might require additional framing support for the windows if the windows are to be flush with the outside of the wall. 

The building contractor is a key factor in the cost effectiveness of this option. One contractor may see the additional framing as an impediment to rapid framing of the house, or may not be interested in energy conservation. Another contractor may be very interested in energy conservation and may be more than ready to do what is necessary to make this type of additional insulation happen. The quoted costs from the contractor will be affected by his assessment of the feature. 

In a retrofit situation, especially where the exterior is being redone anyway, and the home owner is willing and able to do the additional framing, the cash costs would be much lower than our additional cost. 

With our contractor, the quoted cost which included additional framing was too high to compete with other possible features. So, we rejected this conservation measure. 

Several comments about windows: 

• Window glass is available in various tints and films, and is rated according to the reflectance or transmission of sunlight heat. We decided to use heat transmitting glass on all sides of the house because we thought that in Manitoba, keeping the house warm in winter is more important than keeping the house cool in summer. 
• Windows, no matter how good, are poor insulators. If memory serves me correctly, our building code calls for R20 walls. Normally that involves a 6” stud wall with fibreglass insulation. Windows, on the other hand, have insulation ratings in the R2 to R3 range. This means that even triple pane windows are a major heat loss “weakness” in the wall system, losing much more heat than the insulated walls. A balance between energy savings and livability is required. No one wants to live in a windowless box.
• We chose triple pane windows with argon gas because of the energy savings they presented. The windows are expected to last long enough (20 to 25 years) to recover costs at present energy costs. Payback will be even better if energy costs rise over the years. These windows would also provide a more comfortable environment in the house with warmer surfaces and less window condensation.

I consider the triple pane windows a success in the sense that we have had no problems with them, and I have every reason to believe that they are doing what they are meant to do. We have not done any tests to measure their efficacy.

I plan to write about air source heat pumps and solar EV systems in future articles.