I'm in the process of creating a mudroom. I live in a wet climate in western North Carolina. I wanted to go for a more natural ground insulation, so I chose perlite. Currently, I have 3 ½ inches of gravel against the ground. I was planning to put a little sand on top and then put a 6ml poly vapor barrier. Then, I was going to put on 3 inches of perlite and then an earthen subfloor that is mostly comprised of road base, clay slip, and lime. I was going to do a low moist mixture to tamp it and then allow it to dry fast.
I know that perlite can wick water, but what about pulling in moisture from the air. Will that be a problem if it's directly below the earthen subfloor and be able to grab moisture from the air through the earthen floor? Will it compromise the lime subfloor? FYI, eventually, I will put a finish earthen floor oiled and sealed on the top.
I guess I'm having a hard time imagining it to be a problem, since clay can absorb moisture from the air and we use that for plasters and such. In some ways, these earthen materials help to regulate the humidity in the air. Curious to hear any significant concerns if there are any.
Another option could be to put the perlite on top of the gravel and put the vapor barrier on top of the perlite, but I worry about the tamping and how that might create tears in the 6 ml plastic.
I used 8” of perlite underneath 6” of road base, with 2” earthen floor topping that sealed with linseed. I wouldn’t worry about any wicking. It will eventually reach homeostasis and moisture won’t be an issue. You definitely want the perlite on the dry side of your vapor barrier. Keep in mind that the perlite will compact more than you think, which means more gravel than you think. It’s fluffy stuff.
I used StegoHome 10 mil vapor barrier. Would recommend. I understand the resistance to using plastic in a natural home, as I had the same feeling, but an under slab vapor barrier is not a place to cheap out as it can never be replaced. This product was extremely durable and easy to use, has a very long lifespan. We did not get a single rip during install. I am somewhat chemically sensitive and have an obnoxiously sensitive nose, and I couldn’t smell this plastic, which is rare. That should mean it’s a stable plastic and doesn’t offgas or degrade “much”.
Thank you so much for sharing your experience. I did a quick test sample and it seemed like if I used about 3" of perlite, it compressed to 2 ½". I'm technically going to use a hair over 3 ⅛" of perlite, so I think it will compress to that and if it's a little more, then I guess I will do a thicker finish floor (I doubt it would compress more than 1", but who knows after you put all the weight of the subfloor on there). Also, I already have 6ml plastic (therefore, don't have to buy more) and hoping that would be good enough, though 10ml sounds pretty stout.