Product for Sealing Joist Bottoms in a Ventilated Crawlspace
Let me start off by saying I don’t like ventilated crawlspaces, but sometimes you have to do what you have to do. I’m working on a house in Washington, DC with a crawlspace. My go-to guide is this article by Joe Lstiburek: https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces
Note that the “icky crawlspace” in his article is also in Washington, DC. So this is the climate I’m dealing with — hot and very humid in the summer. We are rebuilding this crawlspace, and the old one pretty much looked like that picture — insulation between the joists with no vapor barrier below, moldy and rotted joists. It was also rodent-infested so we have to protect it better.
Dr. Joe’s recommendation is to put foil-faced insulation below the joists, to act as both a vapor barrier and insulation. The assumption is that the air under the house will be near 100% relative humidity, the crawlspace will be cooler than the interior of the house year-round, and most of the time and there will be condensation on the underside of the house. He also notes that “rigid insulation needs to be airtight so that requires tiny people with good workmanship,” which is the problem I’m running into. The clearance under the joists is 18″ and I don’t think we can get sheet goods in.
The joists are I-joists, so the contractor has suggested dropping rips of plywood onto the bottom flanges of the joists from above. They would then be sealed with caulk and the joist bays insulated. There would need to be a vapor barrier at the bottom, which could be foil-faced insulation, poly, or even a vapor barrier paint on the plywood. I’m confident we can make that assembly rodent-tight as well.
But this leaves the bottom of the plywood and the bottom flanges of the joists exposed to high humidity and condensation. As Dr. Joe notes in his article, joists can rot from the bottom up in this situation.
The question is whether there is something we can paint on the bottom of the joists and plywood to give them protection. Would regular exterior housepaint work? Or would it not stand up to continuous moisture. What about something like RedGard?
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