Insulating basement with foam and moisture transfer
Hello,
I have been planning on insulating my basement with foam to make it a more usable space. The area I’m in has a high water table and the basement is very damp once the weather warms in spring until late fall. I ran across this video https://www.youtube.com/watch?v=kwn0Vjw_ji0&feature=player_detailpage#t=1619 which mentions that if the foundation is not dry, spray foam (and I assume rigid foam) will result in moisture being pushed up in the foundation wall and accumulate in the sill plate. I’ve never run across this as a potential issue before, can anyone confirm or quantify how big a problem this is? My house is almost 60 years old and the floor joists are directly on the concrete foundation. If this is an issue, what are the options for inserting a capillary break between the foundation and the joists in an existing house?
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Replies
Corey,
First of all, if your basement is "very damp once the weather warms in spring until late fall," you should probably do some work to dry out your basement before you begin installing additional insulation. For information on this type of work, see Fixing a Wet Basement.
Q. "A video mentions that if the foundation is not dry, spray foam (and I assume rigid foam) will result in moisture being pushed up in the foundation wall and accumulate in the sill plate. I've never run across this as a potential issue before; can anyone confirm or quantify how big a problem this is?"
A. Here's what really happens in this situation: rather than the moisture being "pushed up," what happens is that the spray foam reduces the wall's drying potential. The closed-cell foam is a vapor barrier. Once it is installed, there isn't as much area of exposed stone from which moisture can evaporate. So the stone stays damp longer. As a result, the sill plate or joist ends can rot.
The usual solution is to jack up the house about 1/4 inch or 1/2 inch -- just enough to give you room to slide some metal flashing or butyl rubber between the top of your stone foundation and your sill. For an account of one such job, see Joe Lstiburek's report, Rubble Foundations.
If you have good surface moisture drainage, a decent amount of roof overhang (12" per story) and 18" of exposed above grade foundation, that's more than enough drying capacity for a poured concrete foundation to be protective of the foundation sill, even if the footing is permanently below the water table.
The critical locations to manage would be where roof water gets concentrated and dumped, increasing the average moisture content of the near-surface soils (eg: a roof valley drain that directs the roof water less than 5' away from the foundation.)
If there is no foundation sill, and he joists are resting directly on the concrete it's pretty easy to jack them up 1-2 at a time to slip in some strips of EPDM or sheet metal (copper would be long-lived) as a capillary break.
As a rule stone foundations wick a lot less than CMU foundations, which in turn wick less than poured concrete.