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Attaching vapor barrier to ye olde stone foundation, oh and there’s rats

staceylamb | Posted in Energy Efficiency and Durability on

One of the things I think we don’t talk enough about is rodent-proof construction. It’s just a skip and a jump away from air-tight construction, and I think we should be marketing and selling this.

Anyway, here is the problem I’m having combining rodent-proofing strategies with air sealing and insulation strategies.

I’m dealing with a good old shallow dirt floor crawlspace with a rubble stone foundation under a 200 year old New England house (Boston). The crawlspace is under a c.1850 addition onto an 1804 house with a full basement. Yes, I’m very familiar with Joe Lstiburek’s paper on this very subject. The house spent many years abandoned and has been home to mice, rats, skunks, and raccoons. Although there don’t seem to be any active colonies present, keeping animals out is just as important as bringing the house’s building envelope into the 20th century. Yes, I will settle for 20th century.

The historic preservation carpenters who preceded me have installed a new 6″x8″ untreated white pine timber frame sill beam to replace the rotted timber that used to be there. Consistent with historical standards, the sill has no capillary break under it. (Thanks, guys.) Best I can do is treat it with borate and try to keep water away from it.

I’ll do my best to seal up the cracks between rubble foundation and sill beam with masonry and caulking, but that will likely always be a vulnerable point for rodent entry. Since I’d like to protect the vapor/WRB and insulation from the chewing of animals, the rodent layer will go first. The plan is to use 1/4″ galvanized steel hardware cloth, stapled to the interior of the sill beam, and laying 2′ down over the shallow stone foundation and onto the dirt floor of the crawl space. Overlapping that will be 6″ of gravel/aggregate on the entire floor of the crawlspace, on the recommendation of an Extension Service document about rat proof construction for barns. Apparently they won’t burrow through coarse gravel. The reason I’m not pouring concrete is because there isn’t a good access point for the concrete truck. This will be a wheelbarrow operation.

Since I happen to have a lot of EPDM roofing laying around, that will be my vapor barrier, and will lay on top of the gravel. The problem is what to seal/attach it to at the edges. The hardware cloth setup I’ve just described will create a small gap where I’d like a tight seal to be. My best idea so far is to use some Big Stretch caulking and put down a really fat bead of caulk that will go *through* the hardware cloth and adhere to the stone foundation just under the sill beam, and just stick the edges of the EPDM to the caulk bead. I don’t want to attach it directly to the sill beam, since it seems like that would pretty much direct underground vapor up toward the bottom of the untreated sill and hasten its demise.

I realize that jacking up the house and slipping my vapor barrier under the sill would be the right thing to do, but I don’t think that’s going to happen. And even if I did that, I would still have a problem of trying to combine rodent proofing with air and vapor sealing. I’m seeing my rodent barrier as one more layer of my wall and foundation assemblies that needs to stay continuous to be effective, and the rodent barrier wants to cross the vapor barrier, basically.

So does anyone have a more elegant solution than mine? Am I right that sealing the EPDM to the stone foundation is my goal, and that fastening it to the sill would be a disaster? What am I missing? How have people dealt with sealing vapor barriers to rubble foundations in general?


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  1. pv13 | | #1

    Stacey, I might be resurrecting this question from the archives but I have an interest also.
    I also live in a stone rubble foundation 130+ year old house in similar climate zone here in Ontario Canada. We have a partial full basement with partial crawl space or 'short basement' as some like to call it.
    I did some outside insulation details on the recent renovation and protected the exterior rigid with 1/4 hardiboard smooth ripped down and stuck down around the perimeter of outside foundation to keep critters out and protect the insulation and dimple drainage layer from UV.
    When I noticed a hole (beside cement board) I stuffed it with sharp stainless steel scratch coat lathe which makes for some difficult digging for those pesky buggers. I feel like separating the air and rodent control layers further to the side of the assemblies were you want either to stay is often the best approach. Happy to hear anyone elses feedback
    A chewed air barrier is just as effective as a poorly installed one. Declare war and set tonnes of traps!

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