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I am building a new 16 x 16 addition with a pier foundation well off the ground.

EHkFWT9ZvU | Posted in Energy Efficiency and Durability on

I’m questioning the best way to insulate and close off the floor. I live in northern Maine. I am insulating the rest of the house with closed-cell spray foam and an exterior layer of iso foam will be applied.

Can I spray up into the floor joists and around the rim joists to seal this area and then close off the area with the iso and exterior plywood? My foam contractor would not be filling the floor joist cavity so there would be an air space. Or should I fill the air space with additional insulation?

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  1. SLSTech | | #1

    The first part on the CC Foam is a great option and something I have done previously

    Leaving an air space is not a good option & I would recomend filling that space in with Open Cell or dense packed cellulose, then your foam board & protective covering

  2. Riversong | | #2

    You didn't complete your description: insulating with closed cell foam and exterior? Rigid foam?

    If so, you've created a double vapor barrier and are inviting moisture problems, in spite of the hype of the foam industry.

    And your explanation of the floor insulation is not clear. You say your insulator is not going to fill the joist cavities, but you're asking about spraying up into the joists? And leaving an air space?

  3. Allan Rennie | | #3

    To finish my last sentence in the first section, " exterior layer of iso foam will be applied.) I was also concerned with the air cavity and was looking at what to use to fill the area as filling it with foam is $$. I am intending to have 4" of foam put in the 9 1/2" joists covering the floor and along all rim joists.

  4. Riversong | | #4

    If you sandwich your sheathing between two impermeable layers, particularly something as closed as foil-faced foam board, you're asking for trouble. Every wall must be able to breath in at least one direction and ideally two.

    You'll have a similar potential problem area in the floor assembly if parts of the joists are between CC foam and foil-faced polyiso. You would be better off filling the joist cavities with some kind of insulation and closing the bottom with just the plywood. Blue jean batts with borate fire retardant (which is also an insecticide) would be far better than fiberglass.

  5. Kevin Dickson, MSME | | #5

    A method worth considering is to use a 12" SIP as the entire floor assembly. Instant subfloor!

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