I've got a question that may sound stupid, but I'd rather be safe than sorry.

I feel some drafts around the baseboard trim in my master bedroom. A blower door test found this was an area with some air leakage. If I were to pop off the trim and put a small amount of low expanding spray foam in the opening, would it inhibit the wall from being able to dry? I was thinking about doing the exterior walls of the whole the house.

Also, if I use some low expanding spray foam from a can, it would be an open cell foam product (I think) which can hold moisture. If this foam is in direct contact with the wall 2x4's, can it transmit enough moisture to cause the wood to rot?

Go for it. While your concerns may be valid, the total area you'll be affecting is negligible compared to the total area of your envelope, and stopping air leaks has direct, immediate benefits.

Posted By strategery on 27 Jan 2013 10:59 PM
I've got a question that may sound stupid, but I'd rather be safe than sorry.

I feel some drafts around the baseboard trim in my master bedroom. A blower door test found this was an area with some air leakage. If I were to pop off the trim and put a small amount of low expanding spray foam in the opening, would it inhibit the wall from being able to dry? I was thinking about doing the exterior walls of the whole the house.

Also, if I use some low expanding spray foam from a can, it would be an open cell foam product (I think) which can hold moisture. If this foam is in direct contact with the wall 2x4's, can it transmit enough moisture to cause the wood to rot?

I had a house done that way back in the late 1970's.  The homeowners still live there and have had no problems.  Spray foam was also used around all of the windows and doors.  Also included all holes through the walls and ceiling. 

Do you guys think low expansion foam is the best way to go? I will be trying to insert the foam into a tiny little opening where there's a gap between the drywall and the floor (or bottom plate). Someone told me to use caulk, but I don't think I am going to get a whole lot of benefit using it (although it is cleaner).

Low expansion foam is fine, and preferred if you have gaps bigger than 1/4", but acoustic sealant type caulking is extremely effective at gaps smaller than that, and it's more flexible/pliable than low expansion can-foam. If it's all thin-crack stuff I'd favor acoustic sealant. (I've used both in my house.)

No matter what the material it's vapor permeance & wicking capacity is irrelevent, orders of magnitude behind the it's air-stopping aspects.

If the crack is too small for the spray foam nozzle to fit in, then it is a candidate for caulk.

Posted By ICFHybrid on 31 Jan 2013 09:43 PM
If the crack is too small for the spray foam nozzle to fit in, then it is a candidate for caulk.

I don't think that's going to be a problem. The spray nozzle is tiny and the opening is at least 1/4" at its smallest.

I didn't say I thought that was the problem with your particular issue. That is how I usually decide if caulk or crack sealing foam will be used. If the crack is too small for the foam nozzle, then I use caulk.

Occasionally, I run into a situation in which the crack is too small for the nozzle, but I suspect the cavity beyond is much larger. In that case, I will drill for a hole to admit the nozzle, push it through and fill the cavity with foam. Then, I come back after the foam has cured and clean the crack to a depth of 1/4" or 3/8" and use caulk to seal it up.