insulating in crawlspace under finished wood floors


Old 01-22-02, 12:16 PM
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Question insulating in crawlspace under finished wood floors

I'd like to place some insulation under the wood floors of this 100 year old house. They are over a crawl space with a dirt floor, and there is no subflooring between the polyurethane finished pine wood flooring above (this is hard old pine, which in those days was far different from pine used today) and the crawlspace below - that is, the top of the flooring is finished and the bottom is exposed directly to the crawlspace. Actually, I should add that I was told pine is what was used for flooring in these old houses around here. I myself cant tell, other than I know its not oak hardwood, which when used around here, was placed over the pine(?) flooring I have. I do know poplar (again, rock hard old growth poplar) was used for the regular lumber in the house, so it could be that for all I know.

Crawl space is around 25' x 32' with two vents on each side and a big opening on the end where you gain access from a cellar (which is where the furnace is). House sits on a small hill in an elevated part of town in Louisville, Ky. So drainage is pretty good. Outside the crawlspace, three sides are protected by a covered porch, covered deck and cement walkway. The other side has dirt and landscaping, though I've tried to see to it that the earth is sloped away from the house, and all downspouts are diverted to drain away from the house in the back yard. So there shouldn't be any problem with any soggy earth in the crawlspace.

What concerns me is changing the amount of moisture that gets to the flooring. I'd hate to screw up. I read a post by someone who's floors dried up and shrank after they put in a vapor barrier and insulation below, causing them to creak. Yikes. Hate to screw up a house that's been doing well for so long. There is currently no vapor barrier or insulation between the flooring and crawlspace. I would like to insulate (floors are *cold* , but avoid any change in the moisture level exposed to the flooring that would be adverse. Is it a valid concern in an old house like this? I would just put unfaced insulation up, but then I might need to be concerned about the insulation becoming damp. Any thoughts on how I might go about determining the proper action to take down there?

I should add a reminder that the 3 coat polyurethane acts as a vapor barrier to the air inside the house already. The moisture concerns will definitely be from below.
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Old 01-25-02, 12:01 PM
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insulating in crawlspace under finished wood

One other question.

If I were to staple the craft paper backing to the bottom of the joists facing the dirt (about 3 feet below), it would mean having about 4 inches of pocketed air between the floor and the top of the pink insulation. I say pocketed because at the ends of the joist I would push the insulation up to the floor to seal of the ends.

Being an old house, the joists are large and extend 10" down from the bottom of the floor. So if the insulation is 6", that leaves 4" of air. Is this not good? Should I shove the insulation up until it touches the floor? (assuming I end up putting kraft paper facing the dirt)

Old 01-25-02, 02:02 PM
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The U.S. Dept. of Energy states that areas where they experience 8,000 degree days or more the vapor barrier should be between the insulation and the home, Louisville does not. In areas where the climate is very dry, no vapor barrier is recommended. And in areas where they experience a lot of heat and humidity, the vapor barrier should be between the outside and the insulation. Which is the way you described in your second post. I am assuming that is the way it is done in Louisville, Ky. Creating the dead air space between the insulation and the floor is a good idea. You're lucky because most home that vintage does not have 10" floor joists.

Covering the dirt floor with a moisture barrier is also a good idea. What it will do for you is prohibit evaporation. If you're wondering where will the moisture go if it doesn't evaporate? The answer is the outside. The ground is like a sponge, the moisture barrier will force the evaporation to occur outside the home, instead of under it.
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