My house's structure is made of wood and there are subterranean termites (non-formosan) in my yard and area. I am planning to excavate and insulate the slab perimeter in the interests of saving on heating, but am worried about creating a termite highway. I figure XPS is right out, and I'm not sure about termite-treated EPS either. The availability, price per R, and ease of installation are attractive, but since the advertised termite resistance just comes from a chemical treatment, I worry that it will fade or leach out over time.

I've considered foamglas, which would be ideal due to its many desirable qualities, but it's ridiculously expensive compared to the alternatives. What about rigid mineral wool boards? I see people online claiming it's resistant to termites, but I've been unable to locate any real confirmation or denial of this. My sense is that the mineral wool industry would be advertising this if it were true (as the foamglas industry does). So what's the skinny?

  • After the installation of perimeter insulation, the soil should be termite treated because the excavation and backfilling will have negated any previous treatment. – user23752 Aug 10 '14 at 1:54
  • Mineral wool underground/exterior is going to saturate with water and have no insulation value to speak of at that point. Foamcrete might be a better bet, but it's easy to find info about, harder to find a supplier/vendor/etc to actually make and place the material for you. For that matter I don't know for sure how it is with termites, as they are not a local concern (well, not yet, anyway.) For that matter EPS is not generally recommended for below-grade use - XPS is. – Ecnerwal Aug 10 '14 at 2:23
  • In case you were not aware, there are forms of mineral wool specifically made for sub-grade slab perimeter insulation: roxul.com/products/building+envelope/roxul+drainboard – iLikeDirt Aug 10 '14 at 3:37
  • From the brochure: * Thermal performance may vary during moisture migration in the product during drainage.* TANSTAAFL. – user23752 Aug 10 '14 at 15:01


A while back I did what you described–I dug out my stem walls down to the footings. Then I wire brushed the walls clean and painted them with a concrete sealer. Then I insulated.

I considered high density ccSPF but it's quite expensive. So I installed two 2" layers of XPS foam because it's the least absorbent. Then I installed a sticky fiberglass mesh to the XPS and troweled on an 1/8" layer of Tuff II Foundation Coating. Tuff II is primarily used above the soil to protect the foam but I went down to the footings to prevent the gravel and any tiny mouths from tasting the XPS.

Between the foam and soil I filled the trench with drainage gravel, to keep moist earth distant from my insulation and walls. Had I ever seen a termite near my home I'd have added a layer of scoria or that special sand that's supposed to deter them. I set landscaping fabric between the gravel and soil to keep the soil out of the gravel layer.

While backfilling, I also graded the ground around the house. After properly sloping the soil to drain away from the structure, I laid a 6' strip of 6 mil plastic over the grade. I covered the plastic with scoria and decorative gravel that my wife and kids picked out. The sloped plastic layer pushes water out away from the house before it can start percolating down.

It's only been one winter since I did that work but so far, so good. My basement walls are warmer, there's no evidence of unwanted biologicals invading my XPS or home, and my sump pump hasn't run in two years. I don't have a way to measure but I expect my stem walls moisture content is lower than in the past too.


I can verify boron treated EPS degrades over time. Boron is water soluble, so the treatment doesn't last through more than a couple wet seasons.

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There's apparently a new insecticidal treatment being sold under the trademark Preventol. It's unclear how many years this treatment will remain effective. In any case, I think soil treatment should be used whenever EPS is installed below grade to help long-term repel insects.

The foamglas product looks interesting. It's made of silicon instead of polymer. It would still be nice to see a long-term test, though. Until someone buries it for a few years, the claims of the manufacturer can't be verified.

  • 1
    It's made of glass. To my knowledge, there aren't any insects that eat glass. So even in the absence of such a test, it makes sense to me due to the inherent properties of the material itself. That's exactly the same thing that worries me about the treated foams: the treatment is only designed to mitigate a negative feature of their composition, but a treatment will never last as long as the structure it's designed to protect. – iLikeDirt Aug 9 '14 at 22:12

The typical reason for specifying mineral wool in building construction is for fire-stopping because other materials tend to be more cost efficient insulators.

Water-repellent does not mean water-proof. Mineral wool unlike - EPS or XPS - can hold moisture. Once wet, the water will negate any insulating properties because water is an excellent thermal conductor.

In other words, unlike closed cell foams, Wools, mineral or otherwise, are porous. The fibers may not absorb water, but a board of wool may be saturated, ie the voids between fibers filled with water. It can be saturated with water from front to back to a degree unlikely with closed cell foams. Properly installed in well drained soil it will likely meet expectations.

However, the practice of selecting materials and systems in anticipation of less than ideal conditions and installations has a proven track record. Anecdotes based on failed installations abound because ideal conditions are so rare.

The concerns over the termite resistance of treated EPS may be edge case particularly in light of the need for soil treatment and periodic retreatment. Here is a Dow study on treated EPS: http://web.ornl.gov/sci/buildings/2012/2007%20B10%20papers/153_Alcott.pdf

  • XPS has been shown to saturate with moisture over time in prolonged wetting conditions. None of this stuff is going to do well if it gets soaked and can't dry. – iLikeDirt Aug 10 '14 at 3:39
  • It is not just "homeowner theory": news.yahoo.com/… – iLikeDirt Aug 10 '14 at 13:36
  • Now granted, the study in that link was sponsored by the competing EPS industry. But that seems exactly as sensible as asking the XPS industry whether or not their product is the right choice. I wonder what they're going to say? ;) – iLikeDirt Aug 10 '14 at 13:37
  • Furthermore, I think you are mistaken regarding the moisture absorption of mineral wool. From roxul.com/files/RX-NA_EN/pdf/Brochures%20and%20Sell%20Sheets/… "Moisture can be an issue with below-grade constructions and keeping water away from the foundation is paramount. ROXUL DRAINBOARD displays hydrophobic characteristics and it will not absorb or hold water making it the perfect capillary break." – iLikeDirt Aug 10 '14 at 13:57

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