I am puting a pre-fab house on a basement. The basement has poured walls
and tar on the outside of the walls. I live in northern michigan and would like to insulate the basement walls on the outside. The man I contracted to pour the basement walls said I should use blue board foam 4×8 sheets. I’m not sure of this. Will the blue board hold up in the dirt and sand for many years or should I go about this a different way. Please help. -nailbender
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Extruded polystyrene is what you want. Blue, I think, is the Dow color. Ours was pink, Amoco. Comes in differing compression ratings, but you probably don't care.
I buried a truckload here, protected with nothing more than 6mil sheet poly. We were careful with rocks. Carpet scraps work great if you're throwing rocks at it. Every carpet store will be delighted if you empty their dumpster for them.
The only issue is bugs, which haven't been a problem here. Borax treated is available if you're worried.
Now to throw something different at you. How about putting your insulation almost horizontal (slight slope) with 6mil poly above and below? That would approximate our insulation umbrella. Gives your basement greater mass for more summer heat storage and, assuming your water table's low, gives you much greater probability of a dry basement. Likely your foundation guy's never heard of it. Not unlike a frost-protected shallow foundation, but for full-sized walls. Works very well.
PAHS Designer/Builder- Bury it!