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Yet another basement question

cmag | Posted in Green Building Techniques on

First, let me just say THANK YOU for this wonderful resource. The advice and analysis you give here is priceless and it is just a pleasure to read through the posts here.

I am planning to upgrade the insulation in my basement which, unfortunately, was remodeled less than 10 years ago in the “throw up some drywall, shove some fiberglass insulation behind it, and put some carpeting on the floor” style. The house is 100 years old and in Seattle with a slab that is in reasonable condition for its age, but presumably thin and with no vapor barrier or insulation. I think I’ve pretty much decided on my wall insulation scheme, but can’t seem to decide what to do with the floors. I’d rather not leave them bare and am hoping to put down either a cork or engineered hardwood floor. Since I don’t have a ton of headroom to play with, I can’t put down several inches of rigid foam, but would like to put something under the subfloor. My question is, how much would be needed to keep it above the dew point to prevent condensation. I know I don’t need much R value to prevent that, but haven’t been able to come up with a number.

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  1. GBA Editor
    Martin Holladay | | #1

    One inch of rigid foam will put you out of the danger zone. Even 3/4 inch will make enough of a difference to be significant.

  2. charlie_sullivan | | #2

    It depends on how high you let the humidity get, but for reasonable numbers like 75 F, 70% humidity, 3/4" would be enough. I don't know of a calculator that directly does that but a pipe insulation condensation calculation is similar:

    Use the ground temperature as the fluid temperature (50 F might be worst case, maybe 55 would be more realistic), and use a large diameter pipe since that makes it more like a flat surface calculation. Then you can play with different interior temperatures and humidities to get a feel for how robust your solution is.

    You can also cut up the slab, haul it out, insulate, and then pour a new one. I used to think that was unreasonably expensive, but the nice thing about it is that it becomes more like standard construction so your contractor is doing what they know rather than learning something new on your job.

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