Basement retrofit–is a waterproof membrane necessary between foundation and sill?
In “BSI-041 Rubble Foundations note” Dr. Joe describes a recipe for fixing old leaky basements. Among other things, he recommends jacking up the foundation and inserting a waterproof membrane between the sill and the foundation in order to break capillary moisture transport. Which makes sense to me. But in a different recent publication from BSC titled “RR-1107 Final Retrofit Pilot Community Evaluation Report,” only one of the six deep energy retrofits described has that feature. The other 5 projects leave out that step but follow roughly the same recipe: rigid spray foam on the wall down to the floor, perimeter drainage, a membrane across the floor etc. I also saw a comment elsewhere to the effect that someone heard Joe say in a recent lecture that he would skip that step. So is this overkill or good practice or it depends on local conditions or? On a typical project several thousand bucks are at stake.
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Bennett,
Your question is an excellent one. I think honest building scientists would answer, "We need more data on this question."
For the time being, this is a judgment call. Clearly, if there are signs of dampness in the basement, soils, or existing wall, prudence would dictate installing a waterproof membrane at the top of the foundation wall. If everything looks dry and the existing sill is in good shape, the savings may justify the risk of skipping this step.
BSC's current guidance on this topic is in this report, completed under the Department of Energy's Building America Program:
RR-1108: Hybrid Foundation Insulation Retrofits: Measure Guidelines
http://www.buildingscience.com/documents/reports/rr-1108-hybrid-foundations-retrofits-measure-guideline/view
See Chapter 6 "Sill Beam/Rim Joist Durability Issues."
Great, many thanks to you both. It's not often that you get an 85 page answer to your question published on the day after you ask it! I am extremely grateful for BSC's work.