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Old 12-20-12, 02:51 AM   #3
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Originally Posted by Exeric View Post
...I've noted elsewhere that insulating floors can create problems of their own when moisture laden air enters a crawl space in warm weather, condenses in the cooler crawl space and then causes mold and wood rot problems down there. The real problem is that by insulating between floor joists, which most people do, you keep the conditioned air out of the crawlspace and make the condensation problem worse! And because the joist themselves are not warm, even though the floor now is, you invite a perfect storm of wood rot through the floor joists that will eventually lead to moisture driving through to the floor itself. So if you have wood floors and you decide to insulate beneath the floor you will warm up the floor. But you will also cool off any crawlspace you might have. That's really bad and will likely cause problems eventually, even including cupping of the floors.

I've learned all this just a bit late because a few years ago I did exactly the wrong thing and put insulation between the floor joists. I since taken most of it back out. Luckily I haven't yet put in new wood flooring that I eventually plan on putting in. Next project (after current construction on other things) is to seal, insulate, and put in conditioned air into the crawlspace and treat it like living space (except for not living down there). An added benefit is that if I ever go ahead with my ten year plan of putting in solar hot air panels that hot air will go right into the crawlspace and act as a heat reservoir while simultaneously making the floor warm.

The main thing I'm hoping to convey from this message is that one shouldn't automatically read your article and start insulating the floor. Depending on the circumstances it could create other problems along with warming your house. Overall the article you reference is really good and useful and i'm already learning from it. I just don't want people to jump to destructive conclusions on their own. That already happens too much, as we both know.
This is very interesting information you have here. It is also in opposition to [my understanding of] the recommendations from my state energy office and also various non-profit energy groups in my area.

So, this is something I really want to get straight, because my floor insulation is pretty much non-existant. I have insulated the walls very well but I have not gotten really serious about the floor part of the house project yet.

So, I'd be interested to know your source of information on this issue.

And also, have you considered that for the moisture in the air to do its wickedness, it needs to actually get to the floor/joist area. What I mean is that if you were to, for instance, spray foam up onto the bottom of the floor, it would make an air-tight barrier.

And also, as I understand it, the moisture problem occurs when warm, moisture-bearing air is chilled below its dew point and the moisture precipitates out. This is the kind of thing that happened with a lot of fiberglass filled wall cavities... the warm indoor air leaked through the walls and dropped its moisture in the fiberglass, causing problems.

The fix for that seems to be creating a sprayed-on foam seal against the inner surface of the outside wall, about an inch thick minimum (the exact minimum thickness depends on the local temps & humidity). Then the wall is filled with an insulation of your choice. The idea is that the temperature gradient is such that the dew point is never reached in the fiberglass (or cellulose) portion of the wall, but the 'dew point' is reached in the air-tight foam layer, where the humidity-bearing air is prevented from entering.

I think a floor version of this idea would do it.

I have done a bit of my floor insulation, and I have used 2" rigid foam boards that have been cut to a bit less than the joist spaces, and I push it up against the floor and fasten it there and use one-part foam around the edges and seams. My idea is to immobilize the air inmigration.

Your thoughts?


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