Sub-Slab Insulation in the Southwest
Location: Northern N.M
Any references for foundation performance with vs. without below slab insulation in an area where we are much more concerned about cooling rather than heating loads? Small home will be heavily insulated above grade, quality glazing and exterior shading of windows, but will be a flat EPDM roof and no natural shade. Home will not have AC for the summer months, relying on passive house levels of above grade insulation and window shades. Heat will be solar gain and wood- not concerned about the heating loads at all. Will we run much cooler in the summer with only perimeter insulation and nothing beneath the interior of the slab?
Thank you.
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Perimeter insulation would still be a good idea, even if you decide not to use under-slab insulation. Even as low as Zone 3, under-slab insulation is recommended by BSC (see the BSC article referenced in this post: https://www.greenbuildingadvisor.com/article/r-value-advice-from-building-science-corporation).
But the big difference between that recommendation and your situation is that you do not have air conditioning. In that case, I think going without sub-slab insulation would help keep your home cooler during summer months. The downside though is that it will make your home colder during winter and across the entire year you will have higher loads without sub slab insulation than with it. But since you would prefer to shift loads from summertime to wintertime, it might work out better for you to skip the sub-slab insulation.
Make a beopt model
Lots of variables and nuance to ground and perimeter heat gain. Example, if your slab is exposed then solar gain to it and indirectly yo surrounding ground will result in heat gain through uninsulated slab. Soil thermal conductivity and albedo will matter here too... You'd have to mess with the EPlus slab inputs to tune that outsid of beopt.
If you have good diurnal temperatures in summer, then just keep it simple and insulate slab and then run a ventilation cooling fan at night to cool the building's mass.
Beopt simulates that