Perlite or similar as wofati insulation?

Has anyone used, or considered using, perlite, vermiculite or similar materials as insulation for a wofati? It would require more depth than eps foam. Would it handle compressive loads and settling better. Would it (possibly) be cheaper?

I found this reference:

http://www.perlite.org/library-perlite-info/insulation-perlite/Perlite-underslab-insulation.pdf

in this discussion:

http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/27805/perlite-compressive-strength

expanded clay is hydroscopic and probably isn't suited for underground use.

So, vermiculite, expanded clay and shale and pumice are all at least a little hygroscopic so they are out. Expanded perlite would be the only underground option. You may need to use silicone treated expanded perlite, but I could not tell if it was specified for under-slab use.

Now, because the insulation of a wofati is sandwiched between layers of water barrier, perhaps untreated expanded perlite as well as expanded vermiculite, expanded clay or shale, or pumice could be used? There would be an element of risk, of course, but no less so than using the other materials suggested in the wofati article.

FYI: perlite and vermiculite are both expanded clay. They have some handling concerns but nothing an appropriate mask can't handle.

Too many unknowns can stall a project. Why not just focus on the underground aspect and use some conventional materials? There's enough risk involved in making a non-standard dwelling (inspection, zoning, neighbors, etc) without improvising insulation. Expanded clay, pumice, salt, and sand are more appropriate for high temperature insulation but are not particularly effective in terms of R-value. Why not stick with something that's waterproof and has appreciable R-value?

Vermiculite is a clay and can be expanded(exfoliated). Perlite is an amorphous volcanic glass which can produce an artificial zeolite, after expansion and further processing. Perlite is not hygroscopic, but the outer surface of each expanded particle has many broken hollow spherules that will absorb liquid and hold onto it. Expanded perlite can be treated with silicone to seal the individual particles, as can vermiculite, and I would suppose other hygroscopic aggregate insulation products as well.

Use of "conventional" buried insulations is not without risk. The difference is in acceptance and practice.

I brought up the possibility of using perlite (beyond a possible cost savings) to address one of the risks of using rigid insulation, that of breakage and shifting while backfilling and from settling. Perlite in bags would form a relatively plastic layer that would better conform to the curve of the mound and adjust to settling.

I trust that the perlite industry would not recommend its use as under-slab insulation if there was any doubt as to its efficacy and official acceptance -- but they could be mistaken or just plain fibbing.

In wofati and other PAHS designs, the insulation layer is sealed on both sides from water penetration so any insulation that meets the load bearing requirements could be used, with the risk that the water barrier could be compromised. Insulating foams, including closed-cell, can become waterlogged after long-term exposure to water or water vapor.

Proper siting, design and construction should significantly reduce the risk of waterlogged soil, insulation and structure.


I found another informational perlite underfloor insulation reference here:

http://www.schundler.com/underfloor.htm

Yes, there are risks all around. Even a conventional house has risks---namely, the contractor!

Thanks for the information about perlite.. I didn't realize the difference.

I don't see much difference, practically, between hydroscopic and "many broken hollow spherules that will absorb liquid and hold onto it". Either one must reduce the effectiveness of the insulation and carry some risk of infestation by molds, etc. Also, I have not heard of closed-cell insulation getting water logged. Is that documented or is it an anecdote? Either is valuable but I like to distinguish between the two.

The difference between hygroscopic and holding liquid around the outer surface is that liquid will not penetrate the particle., i.e, the material that makes up the particle is not hygroscopic. Once you make the particle small enough, it becomes clay or silt, which is how perlite can be made into a zeolite, with a lot of work.

I read about closed-cell getting waterlogged on a marine website. It takes a long time, but apparently it does happen. EPS is definitely a problem when soaked in water. It doesn't weaken the structure, necessarily, but it nullifies it as effective insulation.

Mold would not be an issue in wofati or PAHS, as it is not adjacent to the structure.

Even if your upper layers of plastic both have some damage, I can't imagine that the foam would be soaking wet for long enough to be an issue. But you're right, using conventional materials is no guarantee of success.

Such are the perils of underground construction. On the other hand some insulations may be better protected underground under the right conditions. I like the perlite idea too but wonder about the amount needed, its R per " is so crummy. For termite country, perlite might be the smarter choice as it might be more of a barrier than a tunneling medium like foam (they make bug resistant foam too). Termite areas (warmer) usually have lower R value requirements.

Instead of rigid insulation boards, I would used sprayed closed cell foam. Most amount of R per inch, conforms to any shape, doesnt need to be layered or taped (in the field, not including edge).

I agree that underground foam is not without suspicion. Mind posting the websites with the underground/water foam concerns? These issues are the main reason Andrew's advised method of "mound on top of vapor barrier" would be less risky.

Finally had time to dig through my browser history. The page that discusses water-logged foam can be found here: http://www.tinboats.net/forum/viewtopic.php?f=3&p=251458

I think the key is site location and preparation. Keep the structure and insulation away from water.

I do not contest the superiority of foam as an insulator, but it is very, very expensive (and its rigidity has some drawbacks in creating the "umbrella"). If I can get a semi-load of expanded perlite for $1,000 (for argument's sake), I won't cry over having to use 8 or 10 inches of insulation instead of 4. When I priced 1" of sprayed closed-cell insulation for my attic (7 years ago), it was over $5,000 for about 1800 sq.ft..

I am sure there are drawbacks, beyond being friable and at least somewhat hygroscopic, to using expanded perlite and I will keep researching it, off and on, until I have found more of them.