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Knee attic insulation in old construction

slateandall | Posted in General Questions on

I really appreciate everyone’s input on other thread. I wanted to delve into knee attic insulation because the two consultants I’m working with agreed on everything else but disagreed on this. 

First option: spray foam (open cell) on underside of roof (which happens to be slate with wood backing so the foam would not touch slate). 

Second option: insulate wall and floor of each knee attic using inexpensive rolls with hard foam backing for air sealing. 

In both cases the third floor living space will be fully within the “envelope”. 

What are the considerations for choosing one option over the other and what questions should I ask?

thank you!


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  1. markgh1 | | #1

    Open-cell foam will absorb moisture "within the envelope", no mention of vapor barrier with this option, just a point of concern.

    1. slateandall | | #3

      Is that a plus or a minus? I don’t think a vapor barrier is always a good thing so that’s why I ask. For example, if roof leaks you probably want water to move through the insulation. Also excess humidity in the attic should be allowed to exit. Right?

  2. EthanT | | #2

    Is there any existing insulation? I have a similar situation and there's lots of debris and old insulation on the floor and knee wall, which suggests maybe insulating the roof for simplicity's sake?

    1. slateandall | | #4

      Good point. Haven’t been in knee attic to check.

  3. user-2412144 | | #5

    I wouldn't have exposed spray foam against a roof in an old house if I had other choices. The second guys is being more conservative in his recommendation.

    I would use closed cell on basement walls if those walls will be finished.

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