I'm currently in the process of insulating the floor over my
crawlspace to increase the comfort level in the house during winter.
This is our second winter in the house and from experience, the floors
were cold, extremely cold. We live in Eastern Canada, so minus 35
Celsius is not out of the ordinary.
So far, I have been in the crawlspace and have laid down a vapor
barrier over the dirt floor (full coverage, taped seams etc.) but I am
concerned about the fragile fiberglass insulation that I will be
installing between the floor joists. From experience, I know that a
vapor barrier always faces the warm side of the building to prevent
moisture from migrating into the insulation and condensing. This makes
sense as there is more moisture inside in winter than out, however, I
have a gut feeling that a crawlspace will always contain more moisture
regardless of steps taken to alleviate it. So, I am contemplating
placing a second vapor barrier under the insulation (attached to the
joists) to protect it from the moisture in the crawl.
My questions is, how do I protect the insulation? How much moisture
would realistically flow from inside the house into the crawlspace
through the floors trapping condensation in the fibreglass? I can't
imagine the flow would be anywhere near that of walls and ceilings.
Also, my subfloor has two layers of 6 mil poly underneath it, plus my
flooring is 3/4" tongue and groove pine with three coats of Varathane.
Those two layers of poly will make a good vapor barrier. You need not
worry about the moisture getting into the fiberglass.
Consider this. The moisture will be something at or below 100% in the
ventilated space. All else being equal as air warms it can absorb more
moisture so the humidity goes down. So as the air might move through the
fiberglass it will also be warmed and as it warms it will have a lower
humidity, so it will always be less than 100%.
The problem comes from the other way. The warm air in the house may be
60% humidity but as it travels out through the insulation and cools to
say -10C it may now be super saturated and condensing on whatever is cold
like your wood and fiberglass. It's like the moisture that forms on you
glass of iced tea and what you are going to have is a cup of hot tea, so
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