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Can closed cell spray foam be used to seal dirt basement floor?
Last Post 30 Sep 2009 09:36 AM by TomWS. 6 Replies. |
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TomWS
New Member
Posts:49
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| 18 Aug 2009 07:27 AM |
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I have an 1880's house that has a shallow basement with a dirt floor. The basement walls are a mix of stone/concrete block. The space has many piers installed over the years to provide support to the point that the space looks like a forest of trees.
My problem is that I want to provide some degree of water vapor barrier between the dirt (which is damp during the summer months) and the rest of the house. Also, being dirt, the space is messy on the rare occasion that I need to go down there (to winterize the house, for example).
I've considered plastic sheeting with pea stone on top, but, with the forest of supports, this would be extremely difficult and largely ineffective since it would be very difficult to get a good seal around each of the supports.
It occurred to me that closed cell insulation might be an alternative. My thought is that I would lay out a grid of some kind of non-rotting support material, such as Trex-like 2x4s, and then fill the space with the closed cell foam. This should do a good job sealing, provide a low permeable barrier, and, provide a clean space in which to work.
My biggest concern is that the ground temp might be too low for the first layer of spray. If I can get past this point then subsequent passes should be sufficiently insulated to cure properly.
Any thoughts?
Tom |
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rykertest
Basic Member
Posts:202
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| 27 Sep 2009 09:05 AM |
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I asked our spray foam guy that question and he said it can be done and he has done it before, but only with closed cell. It's doable but not very common. Is it not possible to spray the walls or the underside of the floor? |
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TomWS
New Member
Posts:49
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| 27 Sep 2009 09:43 AM |
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Thanks for checking into it for me. I appreciate it.
Yes, we can spray the walls and the underside of the floor. This was the original plan, but I wanted a relatively easy way to seal, insulate, and level the floor to eliminate most of the moisture source.
After doing the 'math', however, I'm now thinking the expense for this implementation may be too high and I'm back to thinking about just the walls and underside of the floor and just putting down a poly vapor retarder over the dirt. I'll have gotten most of the 'services' out of the crawlspace by then so should only need to go into the space to shut off/turn on the main water supply.
Tom |
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Dana1
Senior Member
Posts:6991
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| 28 Sep 2009 03:37 PM |
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Yeah, the cost can be kind of cost-prohibitive, since it takes 2" of 2lb foam (about $2.30-2.80 per square foot) to get to a class-II vapor retardency of less then 1.0 perms. By contrast 6 mil poly is a class-I vapor retarder (~0.05 perms), and is VERY cheap by comparison. You could cut & trim the sheeting around the supports and seal those areas with a few cans of single part foam (Great Stuff, etc) or duct mastic, and pour yourself a rat slab for less money than that, and have something left over for insulating the floor from below. Lap any seams of poly by a foot, and use mastic to seal that.
The main source of moisture may not be the ground at all, but may be warm humid air infiltration into the cool basement, raising it's relative humidity or even condensing. With fieldstone the air paths will be both through the foundation and gaps under the foundation sill & rim joist. (My basement stays roughly the same temp all year round, but the humidity tracks the outdoor dew point, and the dehumidifier never runs in winter. And I have a poured foundation, with a mostly-sealed foundation sill- the dehumidifier used to barely keep up in July/August prior to sealing the sill & rim joist. The difference was huge.) Insulating & sealing the fieldstone & concrete itself with 2" of 2lb foam may be less foam with bigger overall benefit, but still seal the floor with poly (rat slab optional, but nice & clean, and protective of the poly.) |
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TomWS
New Member
Posts:49
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| 28 Sep 2009 04:00 PM |
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Dana, thanks. I had never heard of a "Rat Slab" before. DAGS'd it and now I understand. Good advice.
Just recently I sealed the vent windows after realizing they were, in fact, a major source of water vapor. Got a good dehumidifier (90 pint @ 5.5pint/KW) and now I'm looking to seal the rest. I think your advice makes the most sense. I'm having the posts replaced with PT and placed on Concrete pads. I'll put down poly once those are in and seal around the pads, spray the walls and underside of the first floor. May not be hermetic but a darn sight better than it is today.
I'll revisit the "Rat Slab" after a year of seeing how just the poly works out.
Thanks again!
Tom |
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Dana1
Senior Member
Posts:6991
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| 30 Sep 2009 09:23 AM |
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The rat slab won't add anything to the vapor retardency, but it'll protect the poly from damage when you walk down there. From a home resale POV it also gives it a more finished look, even if it's not structural.
There are (for more money, of course) insulating non-structural concretes available for rat slabs that could give you something between R3-R5 in a 2" slab. But insulating the floor would only be worthwhile AFTER insulating & air sealing the foundation walls & sills, etc. (In most houses that age & condition insulating the basement floor is WAY down the list of efficiency enhancements, and not nearly as cost-effective as most of what precedes it.)
Sealing the footings/pads of forest of columns with a vapor retardent concrete sealer before you place the columns back on them is a good idea as well if you have a lot of 'em.
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TomWS
New Member
Posts:49
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| 30 Sep 2009 09:36 AM |
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Dana,
Thanks for the tip on sealing the pads. Good suggestion and a LOT more convenient to do BEFORE we move them into the crawlspace!
Tom |
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