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Crawl Space Pony Wall Foaming

Last Post 16 Mar 2010 02:59 PM by Dana1. 3 Replies.

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G. Parkinson User is Offline

New Member

Posts:11

07 Mar 2010 02:33 AM
We asked a foam company for a quote on foaming under our lower story floor above a crawl space. Instead they suggested we foaming our crawl space walls to prevent the plumbing from freezing in the crawl space (12" high perimeter foundation with 12" 2x4 pony wall). The crawl space is vented now and has dirt with some poly sheets laid down rather casually around the stone pillars supporting the heavy beams (1913 house in North Vancouver, BC) and is moderately dry as the soil is sandy. The estimator from the foam company suggested that we run the plastic up against the concrete so that they could seal against it with the foam (to prevent moisture leakage into the air around the perimeter?). I am having a hard time visualizing this and understanding whether the foam would adhere to the poly sheeting. How will the foam attach the poly sheeting to the old concrete? Is some material used to stick the poly sheeting to the concreete, and foam goes on top? Unless the plastic is attached to the concrete pretty firmly, one couldn't crawl around in that part of the crawl space again without fear of disconnecting things. What is normal practice for foaming perimeter walls? Thanks, Graham and Laurie

aardvarcus User is Offline

Basic Member

Posts:226

13 Mar 2010 01:05 PM
You should use furring strips to attach the poly sheathing to the concrete wall. Let the poly lap up the concrete wall and past about a foot, then wrap the poly around a strip of wood several times, starting at the top and wrapping down. Then, screw the wood into the other wood, basically sealing the concrete out. This will keep moisture coming through your concrete and getting into your foam. Also use another furring strip at the bottom of the concrete close to the dirt, screwing it through the poly. This will hold the poly in place at the bottom. Since you are going with an unventilated crawl space, there can't be anything casual about your poly sheathing. You need to keep the moisture out, because now it can't just leave through the vents. Put a whole new layer of poly down, tape the joints, and attach it to the beams with some good tape(like window flashing tape) or by the same way you did on your foundation. Once you are satisfied that no moisture is getting in from anywhere, you can spray foam the outside perimeter walls. Lastly, the icing on the cake is making an insulated door to go inside the normal crawl space door. On a superinsulated house I did this last summer, which had a similar crawl space, except insulated with board foam, I took a piece of the board foam cut a little small, and used pipe insulation to let it slip into the door frame and seal nicely. If you don't do this step, in the summer when your crawl space is cool from the AC above, your door will get a lot of condensation on it.

Matt G

New Member

Posts:93

14 Mar 2010 08:58 AM
Good advice above. Some things to add: If there is any possibility of termites, don't use any wood in your install. When you re-do the poly in the crawl, you really need to upgrade to something better than the regular 6 mil stuff. There are products especially made for this or building supplies (not home centers) often sell 12 mil poly or similar. The idea is that you need to be sure the CS is bone dry. Depending on the condition of the concrete foundation you may be able to use Great Stuff foam in a can (or similar) to glue the vapor barrier to the crawl walls. Also, something to keep in mind is that when they spray the foam on that stuff will stick to nearly anything. Depending on what type of foam it is, that stuff is a direct relative of gorilla glue - if you are familiar with that. Another thought is that now adays, many insulation companies offer sealed crawl spaces as a package in which they will do the floor covering, and either spray or apply rigid foam to the walls. If there are any areas where water pools by the exterior of the foundation this must be dealt with; make sure water runs away from the house due to proper grading. If you hae gutters and downspouts, the downspouts need to direct the water well away from the foundation, via putting extensions on the down spout pipes. If radon is a possibility in your area you need to first check that. Radon is a naturally occurring poisonous soil gas that is particularly common in rocky areas. You can get a test kit at a home center. The idea is that if you enclose your CS and there is radon, the radon will no longer be partially mitigated by the CS vents. This is another reason that you need the CS poly job to be installed exceptionally tight. Lastly, even though you will be blocking the foundation vents and any other air leakage, you still need some source of air. If there are air ducts down there, they will possibly have enough leakage to give some air circulation. If not an air duct should be installed to provide air to the area.

Dana1

Senior Member

Posts:6991

16 Mar 2010 02:59 PM
Not much radon is going to get through a poly vapor retarder on the crawlspace floor, but I s'pose it doesn't hurt to test. Dry sandy soil also helps, since it's somewhat self-ventilating below the vapor-retarder. Duct mastic is sufficient for sealing the poly to the concrete, and spray foam sticks to ANYTHING. The poly doesn't need to be more than a foot from floor, but if the top of the concrete is only a foot from the floor anyway, stapling the ply to the sill to hold it in place prior to foaming to hold it in place is fine. Separate vapor retarder sheets overlap at least 12" on the floor they need to be mastic-sealed at all overlaps- make it as gas-tight as possible at every interface. I wouldn't worry too much about condensation issues on the exterior crawlspace door/hatch during the summer in N. Vancouver, where both latent & sensible loads are low. (Do many people even HAVE air conditioning in your neighborhood?) With the crawlspace walls insulated and vapor sealed against the ground, the temp and humidity of the crawlspace will track that of the conditioned space above, not the outdoors. Dew points above 20C are the exception, not the rule there (the monthly average temps for the summer aren't even 20C!) and the crawlspace will be above 20C on the hotter days- it simply can't condense unless the exterior door is brought below the dew point of the outside air. On the other hand, insulating the hatch keeps it warmer in winter. Monitor the relative humidity in the crawls space- if it stays above 60% for weeks on end a small dehumidifier might be used. At 75% RH the mold hazard takes off considerably. But my guess is that in that climate zone the RH of conditioned space air usually stays between 30-60% year round, and the smallest amount of air communication between the crawl space and the house above is enough to keep moisture from building up to mold-levels.