Does insulating the slab in climates where cooling dominates over heating make sense?
Foot note from Martin’s https://www.greenbuildingadvisor.com/blogs/dept/musings/wolfgang-feist-defends-thick-insulation article- 1. After I published last week’s blog, I had a discussion on Twitter about this issue with Steven Toomey. Along with Anne Decker, Toomey has built a Passivhaus in Bolton, Connecticut that they call the Hayfield House. Toomey kindly supplied energy budget information from two iterations of PHPP for his house in Bolton, which is a two-story house built on a raft foundation. The raft slab has an area of 1,248 square feet. The foundation is insulated with a horizontal layer of R-35 rigid foam, as PHPP required. Using R-35 rigid foam saves 68 kWh/year of site energy (worth about $10.35 if electricity costs $0.15/kWh) compared to using R-20 rigid foam. Upgrading from the R-20 foam to R-35 foam cost about $1,872 (assuming that the foam cost about 10 cents per R per square foot). So the extra foam has a simple payback period of 180 years. If the same $1,872 had been invested in a PV system costing $4/watt, Toomey could have bought a 0.468 kW PV system producing 541 kWh/year, worth about $81. So a investment in PV would yield more than 7 times more energy than the investment in extra foam.
In light of the simple financial facts Martin points out for buildings where heating is the greatest concern, what justification is there for isolating the slab in cooling environments?
For example, we design homes in Zone 3 right on the dividing line between humid and dry. In fact the county we office in is ‘dry’ and the county we live in is ‘humid’. In winter we have a good chance for a freeze with ice storms not uncommon. But true winter really lasts only weeks here at about 2,000 above sea level, not 3 months. We have extended falls and springs. My experience is that by the time our slabs ‘warm’ we are ready to turn off the heat.
My intuition suggests we be better to gain a slight advantage during our longer cooling season from an uninsulated slab’s ‘heat sink’ than the slight winter advantage of an isolated slab.
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Replies
Bruce,
In your climate zone, it makes sense to insulate the perimeter of the slab with vertical insulation, but to skip the horizontal insulation under the slab.
As you argued successfully, spending money on the insulation does not make sense to save energy in your climate. In some damp climates, a cold slab floor might be a potential place to get condensation and mold. Insulating the slab can then be useful for avoiding mold, rather than saving energy. A low thermal mass alternative to slab might also be useful in that case.
Martin, that's the way I see it. In my view the greenest answers are the most resource efficient-