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Foam insulate only portion of roof

Jthayer | Posted in Energy Efficiency and Durability on

Hi All,
I have a 1918 Bungalow. It has a finished attic that we are renovating.  I would like to insulate it with close cell foam but I can only access about 70% of it.  Would it be a problem if only 70% of it was foam but the other is fiberglass batt? 
Thanks

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Replies

  1. Expert Member
    Akos | | #1

    It really depends on where that 30% is and how much drying potential it has. A sketch of the cross sections indicating where the fiberglass and SPF will be and maybe some pictures would help.

  2. Jthayer | | #2

    Thanks for getting back to me. The red portions in these pictures would be where the fiberglass batt would stay. Everywhere else would be closed cell foam. Any thoughts?

    1. Expert Member
      Akos | | #3

      The area where most of the these 1 1/2 storie houses leak the most air is the area you highlighted. If there was one area you want to spray foam it would be that, after that the sloped ceiling section. If you want to save a bit of cost, the top attic area can be kept vented with a thick layer of loose fill insulation provided you don't have ductwork or air handler up there.

      I would look at cutting a couple of access holes into the knee walls so the spray foam installer could get in there. Patching these should not add a lot of extra cost and you'll get a much more comfortable and energy efficient house down the road.

      If getting in there to spray foam is really not an option, you can add gable vents and keep this section vented. I would still try to get in there and do a bit of air sealing by the floor joists with rigid insulation and canned foam.

      1. Jthayer | | #4

        Thanks. Really appreciate it. I just finished a custom built in closet in that space and getting access to it would involve tearing out the whole thing which isn't going to happen. Oh well. Thanks though.

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