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Using Polyiso Boards for Attic Floor Insulation

1chipster's picture

I am back again with another question.  I have a 2 family, balloon constructed house in MA, built in 1930,  and have looked at several waysto insulate my attic floor.  After reading some info on the Green Building Advisor, I am wondering if I would achieve the best insulation value by using poly iso boards.  My joist depth is 8 inches so my ability to get a high R value using cellulose is nil and FG is out of the question.  If this is recommended, what thickness foam board would I use and should it be faced or onfaced given I have BX wiring and the plaster ceiling below has a few layers of paint to act as a vapor barrier. I believe I would also seal around the edges of the foam board with spray foam in order to a good air seal.  I am also told that in MA, it is code that the foam be covered with mineral wool.  ? How much.    Thanks

Foam has to be covered with (post #207036, reply #1 of 3)

Foam has to be covered with something that takes awhile to burn.  If you are using ridgid sheets, you can cover it with drywall or plywood.

You could install furring perpendicular to your ceiling/floor joists to gain added depth for cellulose, then put plywood over that.

I would be hesitant to start applying foam in something balloon framed until you were able to attack air/vapor barriers as a compresensive whole... otherwise you risk trapping it and rotting the house.

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Thanks for replying. There is (post #207036, reply #2 of 3)

Thanks for replying. There is flooring already over the joists and I was planning to use at least 1/2" of mineral wool over foam (and underneath flooring) as a fire barrier.  The issue of moisture is a concern but I thought the plaster and painted ceilings underneath would be an adequate vapor barrier.  Maybe not.  Could you explain further?

It's not the plaster the (post #207036, reply #3 of 3)

It's not the plaster the vapor will come through - it is the unsealed top plates and wiring penetrations.  It will also come up from the side walls.

What you have to watch out for is having a cold surface for warm vapor to condense on.  Attics get cold - they are "unconditioned space"... basicly part of Outside.  Anything inside the insulation envelope is part of "Conditioned space"... I.E. you PAY to heat/cool this air.

Foam board is a nice way to get alot of R value in a short amount of space.  But if you don't have enough, the face of the foam will be cold enough for water to condense against and drip onto wood, starting mold and rot.

Think about this:  Pull up a board every 4' on the floor, then fill those joist bays with cellulose as dense as you can.  Put the boards back, then cover the wood floor with 4" of foam, then float a layer of 3/4" plywood over that.  Now you have a great thermal break, AND a high insulation.

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