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finished attic ceiling insulation

I'm renovating an older house in a cold climate (montana) and am trying to figure out the best / most economical way to insulate the upstairs ceiling space. The rooms have 4 foot "knee walls" which then intersect the ceiling/roof. I've got native 2x4 rafters, and not enough headroom to scab on extra framing to allow for more insulation space.

I know the best way to go would be to hire an insulation contractor to spray in foam, but the budget will not allow for this. What is my next best option?

My current thinking is to buy 3" rigid foam panels and tightly fit them between the rafters, leaving a 3/4" space between the top foam and underside of roof sheathing for venting. I'd like to use foil faced foam if I can find it for a little extra reflective help.

Does this approach make sense? Do I need the ventilation space? Is it worth paying extra for the foil facing? Is there another approach altogether that will give me a better R-value? Is there a certain type of rigid foam I should use?

Thanks for your help!


Re: finished attic ceiling insulation

Hi Jon,

Where are you in MT? I just purchased a house and am considering this same scenario. Currently, the attic/2nd story is all finished, including the areas behind the knee walls. And, currently there is old fiberglass insulation between the rafters and wallboard covering everything up. There is not ventilation built in and this is a concern of mine.

I too came to the same conclusion as you. But, I was going to put a small block spacer on each side of the rafters to pervent me from pushing the rigid foam in too far.

This is just one of many things I need to do as the house is a fixer upper. Keep in touch and good luck.

Re: finished attic ceiling insulation

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