Can rigid foam insulation have an air space between it and rim joist?
I am insulating my rim joists with 2″ rigid foam board. Most of the rim joist is bare, and the foam fits along it nicely. Some sill boxes have some small irregularities (vent pipes, plumbing, etc) that I have worked around easily enough.
However, there are many electrical cables running along the rim joist near the circuit breaker box. For a good 10 feet in either direction, there are enough cables that it doesn’t seem practical to get the foam board flush against the rim joist. Yes, I could carefully and laboriously cut little channels for each cable in the back of the foam, but that would take a long time.
Is it okay to simply push the foam against the cables and call it good enough? This will leave some air pockets trapped between the foam and the rim joist because the foam will be off of the joist a 1/2″ or so (the width of the cables). Will this cause moisture or other problems? Seems it shouldn’t as long as I do a good job sealing around the foam board.
(p.s. I know that spray foam would be the ideal solution, but we are trying to avoid that, mostly for cost reasons. I’m also trying to go with silicone caulk instead of Great Stuff)
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We've done that for years and no problems were ever seen when re-opened to re-wire. As long as it is sealed (to stop convection), a small air gap is even more insulating as there is no possible conduction from wood to hard foam. Like the small space between new window panes. Do make sure a seal is made between rim and floor joists and foundation before pushing in the foam.
PK
Thanks, Paul. That's good to hear.
For the areas that don't have cables behind them, I have been first putting the foam in, then caulking around the edges of it. Are you suggesting I should reverse that order - caulk all edges and then insert foam but don't caulk it? Or perhaps caulk, insert foam, then caulk again?
Thanks in advance.
Susie,
There are two issues here.
Paul is talking about the problem of air leaks to the exterior -- air leaks that have nothing to do with the insulation you are about to install. He is talking about air leaks between the foundation wall and the mudsill, between the mudsill and the rim joist, and any other leaks to the exterior. These must be caulked before you begin insulating.
It is also important to seal the perimeter of the rectangle of rigid foam, to prevent air leakage into the gap behind your foam.
Thank you very much for the clarification, Martin.
I will be sure to caulk in the back between the mudsill (I'm assuming that's the horizontal wooden 2x6 that I have been thinking of as a 'sill plate') and the rim joist before inserting insulation. I didn't do that on the 10 or so sill boxes I have already completed - I assumed that caulking around the inserted foam board would be enough. Is it worth the effort to tear them out and do it over?
I have been planning all along to caulk along between the sill plate and the foundation wall, though I haven't done it yet (I can access it after putting the foam boards in). That raises another question, though:
There appears to be some sort of mortar along the top of the foundation wall. It tapers down from about 1/2 way up the sill plate to the interior edge of the wall. I'm not sure what its purpose is (in case you can't tell, I'm a homeowner doing this for the first time), and it's actually separating from the wall in a number of places. See attached image. The sill plate is, of course, behind the electric cables, and the gap you can see is between the mortar and the top of the wall.
My question is what is best to do about the mortar. I can caulk between it and the sill plate, and between it and the wall, and in all of its cracks. It almost seems easier, however, to knock it out of there and only need to caulk in once place - between the sill plate and the wall. But I don't want to do that if the mortar has some sort of purpose and removing it will be a mistake.
Thanks!
Susie,
Q. "Is it worth the effort to tear them out and do it over?"
A. No.
Q. "What is best to do about the mortar?"
A. That's a judgment call. If it is really loose and it pops off readily, you can remove it. If it is stuck in place, caulk all the cracks.
Thanks, Martin - not only for your prompt response to this question but also for the the great site and all of the information that you provide in articles and answers to questions. I really appreciate it!