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Question DetailsAsked on 6/13/2017

Using closed cell spray polyurethane under flooring in a 100 year old home that always shifts and how it can effect

Concerns about Existing Plumbing and if Plumbing repair is needed in the future. The fact that it hardens and is like steel would it structurally cause a problem with house structure

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1 Answer


Yes it makes plumbing/electric repair slower - can add 1/2 hour or so to a normal repair job,, but it does not harden like steel so it would not have significant structural effect, not should it hurt the structure as it moves - will just restrain it a bit. It is readily cut with a utility knife or hot knife, though of course more risk of hitting wiring embedded in it (which is actually not legal to do because it violates the heat rating of the wire, and can also deeteriorate the insulation covering as it is curing). Tracing liquid leaks can also be tougher because they do not drip right down - can travel in the insulation before they escape.

Commonly the wiring and piping is encapsulated in fiberglass insulation, then the underside of that is sprayed with the foam-in-place insulation - or regular closed cell board insulation is put over the UNDERSIDE of the floor joists for insulation. Though exposed insulation is a temptation to termites and carpenter ants and post beetles and such - they love tunnelling and nesting in it. I trapped about 25 carpenter ants in a little foam box once - took them less than an hour to eat their way out through 1 inch of foam.

Of course it is more efficient (per inch of thickness) than fiberglass insulation, but can conceal water leaks which would otherwise quickly drop through fiberglass, so there have been cases of significant subfloor damage because of undetected water leaks. In most areas a normal unheated crawlspace or basement is warm enough that normal R-13 to R-24 batt insulation is adequate under floors - though of course if you have hard surface floors (especially stone or tile) you will notice cold flooring a lot more - but generally the floor is not a major heat loss area so ultra insulation may not pay off there, either.

I would certainly, if using foam insulation, have preventative insect protection put in - I understand DuPont or Dow (I don't remember which) now has an insect resistant additive for spray foam installations, and there is insect-resistant (borate treated) rigid foam board available - plus of course a spray protectant can be put on the foundation walls leading up to the floor and on the finished insulated subfloor to provide protection.

Answered 3 years ago by LCD

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