Designing a superinsulated house can be tough. How much insulation should you install under a slab? Should your walls be sheathed with rigid foam, or should you go with double-stud walls? Could SIP walls save you money? Does the added cost of triple glazing make sense?
The answers depend on your climate, your performance goals, and your budget. Coming up with an optimized design requires careful heat-loss calculations, multiple energy simulations, and construction cost estimates. For those who haven’t yet struggled with these calculations, it can be instructive to compare the conclusions of thoughtful designers who have gone through the exercises.
Energy-efficient homes in Massachusetts
Two recent magazine articles describe cold-climate superinsulated homes designed by experienced professionals who did their homework. Their conclusions are instructive.
The first article, “High-Performance Homes on a Budget” by John Abrams, appeared in the January 2011 issue of the Journal of Light Construction. In his excellent article, Abrams provides the specifications for a cluster of superinsulated homes built on the island of Martha’s Vineyard in Massachusetts. The designs were perfected with the help of energy consultant Marc Rosenbaum. (For a photo of one of the homes, see Airtight Wall and Roof Sheathing.)
Abrams built his walls with double rows of 2×4 studs. He explains the decision this way: “After years of experimentation with a variety of approaches, we have settled on cost-effective wall and roof assemblies that meet our insulation standards … and minimize thermal bridging — and that our carpenters and other tradespeople are comfortable with.”
Minimizing residential energy use in Colorado
The second article, “Heading for Zero: Smart Strategies for Home Design” by Jim Riggins, appeared in the February / March 2011 issue of Home Power. Riggins describes the multiple calculations and simulations he used to design a superinsulated house in Monument, Colorado.
When Riggins began the design process, he guessed that his walls and roofs would be made with SIPs. After running the numbers, however, he settled on double-stud walls. “Overall, SIP exterior wall construction ran 14% higher than the double-wall, and the 2×6 with exterior foam ran 5% higher. .. The double-wall technique uses more lumber but goes up more quickly than a single wall with two layers of exterior foam plus more intricate sealing and trim work around the windows.”
Here’s a table showing the specs of the two homes:
|â—||Massachusetts house||Colorado house|
|Foundation type||Full basement||Slab on grade w/ perimeter frost walls|
|Sub-slab insulation||4″ XPS (R-20)||4″ closed-cell spray foam (R-20)|
|Basement wall / frost wall insulation||3″ polyiso (R-20) on interior||2″ XPS (R-10) on exterior|
|Wall construction||Double 2×4 walls, 9.5″ thick, w/ Zip System sheathing||Double walls, 2×6 outer wall and 2×4 inner wall, 12″ thick, w/ plywood sheathing|
|Wall insulation||9.5″ cellulose (R-31)||3″ closed-cell spray foam and 9″ blown fiberglass (R-49)|
|Rafters||14″ deep I-joists||18″ deep parallel trusses|
|Sloped roof insulation||14″ cellulose in unvented rafter bays (R-49)||3″ closed-cell spray foam and 15″ blown fiberglass (R-66) in unvented rafter bays|
|Window manufacturer||Thermotech Fiberglass||Accurate Dorwin|
|Glazing||Low-e argon-filled triple glazing||Low-e argon-filled triple glazing|
|Heating system||Ductless minisplit (Daikin RXS24DVJU) plus electric resistance ceiling panels||Ductless minisplit (Mitsubishi Mr. Slim MSZ-FE09NA)|
|Mechanical ventilation||Fantech SH704 HRV||UltimateAir RecoupAerater ERV|
Designers of cold-climate homes will find both articles worth reading in full; click on the links below to access the articles and learn more about the homes’ design and construction:
Last week’s blog: “Are Energy Codes Working?”