The homeowner is weighing the cost of exterior excavation against interior spray foam for basement insulation, while also facing a dampness issue marked by efflorescence on the walls.
Cost and Insulation Strategy
The homeowner wants to excavate the foundation to apply exterior rigid foam, a method preferred for its effectiveness in keeping the rim joist and sill plate warm. However, the engineer they are working with argues that excavation is substantially more expensive than interior spray foam. A second engineer was consulted, and both insist that the homeowner will not have an issue with dampness entering the rim joist and sill plate and condensing against the cold wood because the spray foam prevents the house heat from keeping the wood warm.
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The homeowner plans to add rigid insulation on the exterior from about a foot above the rim joist to ground level, and perhaps a foot below ground level, protecting it with a faux wood finish. They are considering R-20 rigid exterior insulation as a long-term hedge against rising energy costs, though they admit their calculation of the necessary R-value is a “WAG” (wild-ass guess) for now.
Addressing Dampness and Efflorescence
Despite not seeing water mitigate through the walls into the basement, the presence of efflorescence on the basement wall has raised a concern about covering the walls with spray foam while they are damp. The homeowner asks if they should be concerned about this. One engineer suggested sealing the walls with radon seal, which has the additional benefit of preventing further efflorescence. This would be sprayed on the interior of the walls and would involve removing paint from the inside of the basement walls.
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The homeowner is considering renting a dry ice blaster for this task and asks if this sounds reasonable.
Practical Considerations
Interior solutions, while easier to install, lack the ability to vent water vapor effectively. If the soil is damp, moisture will eventually find its way into the concrete. Once trapped behind impermeable spray foam, this moisture can cause significant damage over time. The engineer’s advice to use a radon seal and remove existing paint is sound, but it is only a temporary fix.