More conditioned crawlspace questions-very confused
I’m in climate zone 3A in North Carolina, about to build a conditioned crawlspace, my house built in 2005. No issues with bulk water coming through the foundation walls or the soil. I can’t reconcile the various opinions about certain constructin details, as follows:
Vapor barrier first then rigid foam isulation on the foundation walls? Around here the recent jobs I’ve seen all have FB first, then poly to the top of the FB. (Also, I’ve seen zero jobs using fire-rated FB like Thermax, but I plan to use a product rated identically to Thermax). I see valid reasons for installing the VB first, then the FB. But if the FB is a Class 1 vapor barrier itself, why run VB all the way to the top of it?
If VB first, don’t FB attachments pierce the VB in a number of places, compromising it’s utility?
Rim joist area: I have no evidence of termites, but I’ve seen photos of termite tunnels through closed cell spray foam, so I’d prefer not to use CCSF where I can use an alternative. My preferred method because it would be physically eaiser is to use a liquid-applied vapor barrier in the rim joist pocket first, then batt insulation. But I don’t see anyone doing this either online or IRL. Is this because batt insulation would still allow condensation to form aginst the interior side of the air barrier?
What is the point of running the VB so far up interior CMU piers-3′, 4′ in some photos).
Thank you. Jeff
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Replies
I like to run the vapor barrier up the wall to the edge of the mud sill, then put rigid foam over the vapor barrier. This is quicker/easier and I don't see a problem doing it that way. I know some people put the rigid insulation up against the wall and the vapor barrier over that, which is the way you'd do it with a studwall (vapor barrier on the "warm in winter" side of the insulation). The big difference in a crawl space is that you pretty much exclusively use rigid insulating materials here, none of which have the issue that batts do in terms of moisture problems -- rigid foam can tolerate moisture just fine, batts can get saturated and have mold problems.
Yes, rigid foam is usually OK as a vapor barrier, but running the poly up the wall is relatively cheap and easy to do, ensures you don't have any gaps between panels, and can double as an air barrier if you're trying to do something like seal out radon in addition to just dealing with moisture. If you're concerned about fasteners piercing the poly, you can use my trick of injecting some sealant (I usually use polyurethane sealants for this) into the hole, then stick the fastner in, twisting a bit as you go. The sealant will tend to squish around in the hole and seal things for you.
If you're in a termite area, you need to leave an inspection gap on the wall between the top of the insulation on the wall and the first part of the wood framing (usually that is the mudsill). That is code in many/most areas. The inspection strip allows for a strip of exposed masonry upon which you can easily see any termite mud tunnels with a simple visual inspection. Even if you haven't seen any signs of termite damage, if you're in a termite zone, you need to leave that inspection strip JUST IN CASE, to allow for future inspections.
You can't use batt insulation on the rim joist, because moisture will go right through it and condense out on the cold surface of the rim joist. It doesn't matter if you have a vapor barrier against the rim joist, because the moisture is coming from inside. It's possible, but very difficult, to install a good air/vapor barrier on the inside of batt insulation in this area, which is a big part of the reason why most people insulate rim joists with either spray foam or rigid foam panels.
My preferred way to insulate rim joists is to "foam in" panels of EPS rigid foam. I cut the panels to leave about a 1/4" to 1/2" gap all the way around the EPS panel, then I inject canned foam into that gap to seal the EPS panel in place. This gives a good air seal, the EPS is a vapor "barrier", but still allows for a vary slight amount of drying, and you can use enough EPS to get the R value you need. I use my homemade hot wire cutter to cut the EPS panels, and I cut them at an angle so that the force of the canned foam expanding helps to push the EPS panels tightly against the rim joist. While the canned foam cures, I use a piece of wire to hold the EPS in place. The "piece of wire" is just cut pieces of scrap 12 gauge solid copper house wire, cut slightly (~1/2" or so) longer than the distance between the top of the mudsill and the underside of the subfloor. To put the wire in, just place it between the rim joist and the mudsill near the middle of the EPS panel, then push the middle of the wire tightly against the EPS panel. The wire will bend towards the EPS panel and the ends will lock into the wood, forming a surprisingly secure temporary support for the EPS panel. Once that wire is in, I put in the canned foam. I need maybe 15-20 pieces of wire to secure a large enough number of EPS blocks that I can start "harvesting" the used pieces of wire after the foam cures, and then I reuse the wires for the next batch of EPS blocks.
Bill
Thank you, Bill. That's a clever way to hold the panels in place while the foam cures. It seems like there's no way around my using foam panels in the rim joist pocket if I don't want to use CCSF. I'm still not sure what to make of the pictures I see of termite tunnels through the CCSF. I believe Georgia was considering a code change about five years ago to require rim joist insulation to be removable (so pest inspectors can better see the rim joist pocket; this is in addition to the 3" inspection strip), which eliminates spray foam, but would seem also to eliminate foam board. I don't know if the code changes were adopted. That's not the code in my area, but it still makes some sense.
Your suggestions about sealing the FB fastener penetrations throught he VB behind also makes sense. I was thinking about a butyl strip (sticky both sides) behind the VB at the FB fastener lines. Added benefit is that it would hold the VB up until the FB was fastened over it.