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Foam Board insulation between attic joists?

Hello, First thanks for providing this site, and the Q/A forum. I spent the last week reading through all of the blogs, and think I have a much better plan for beginning to insulate/ air seal my 1930's craftsman/ cottage house.

I have the ability to get free / very cheap 4x8 foam board insulation. ( not styrofoam, either pink or blue iso board) I'm not exactly sure on the type(s) at this time.

Currently, my attic only has about 10-12 inches of blown in insulation, is not sealed and leaks air. The front half of the attic has a regular slope, and the back half is a low slope roof, so space is limited.

Please advise if this is a feasible plan:

1. remove all blown in insulation
2. Cut foam board to fit in between joists. Use spray foam to seal the foam between the joists, this will both insulate and create my air barrier for this portion of the attic.
3. Place More foam board pieces perpendicular on top of the joists.

I think with his method I can achieve 6-8 inches of foam. And any future projects (wiring, ect) in the attic will not cause me to kick around blown in insulation.

Any problems with this plan? I'm concerned I might need a fire barrier to meet code? but since it's is separated from the living area downstairs by the drywall ceiling is that sufficient?


Asked by Casey M
Posted Jul 29, 2011 11:28 AM ET
Edited Jul 29, 2011 11:42 AM ET


7 Answers

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Assuming that you are talking about insulating between the rafters (not the joists), there is no reason to remove the existing insulation to proceed with your plan. Leave what you have in place.

Your plan can work, but it is a time-consuming and fussy way to insulate a sloping roof.

If I were doing this work, I would leave a ventilation channel between the top of the rigid foam insulation and the underside of the roof sheathing. This can be done by putting 1"x1" sticks in the corners of the rafter bays, up against the roof sheathing.

Answered by Martin Holladay
Posted Jul 29, 2011 11:34 AM ET


Sorry, Got my terminology wrong. I'm talking about insulating between the ceiling joists, not the attic rafters.

I edited my original question to now be correct.

Answered by Casey M
Posted Jul 29, 2011 11:41 AM ET
Edited Jul 29, 2011 11:42 AM ET.


You can do it, but it's still time-consuming and fussy.

If I were you, I would sell the rigid foam on eBay. If you want to remove your existing insulation and perform air-sealing work, that's fine. But once you're done with the air sealing work, do yourself a favor can call up a cellulose contractor.

Answered by Martin Holladay
Posted Jul 29, 2011 12:29 PM ET


Time consuming and fussy is OK, as I work for free. Trying to keep the project cost low, but I want good results.

The house has had 2-3 additions over the years, so I am anticipating lots of leaks once I start removing the existing insulation. I am think that it might be just as easy to cut / seal around the foam board that it will be to plug each small hole that I find.

Answered by Casey M
Posted Jul 29, 2011 12:38 PM ET


Hi Casey

I wanted to do the same. Did you actually go ahead with this project?
I am planning to cut blue/pink boards and do the same for an unvented cathedral ceiling. I did this job in the basement and it made a huge difference.

Answered by Don J.
Posted Jun 22, 2012 8:33 AM ET


Hi, Don.

My basement currently has exposed foam batts that I want to replace with something safer for breathing. Would you mind telling me the specifics of how you did your basement job, or point me to a link of instructions that you followed? I'm hoping to do the work myself. Thanks!

Answered by Anne Clark
Posted Nov 17, 2012 4:02 PM ET


Here are links to two articles with information on insulating basement walls:

How to Insulate a Basement Wall

Marc Rosenbaum: Basement Insulation

Answered by Martin Holladay
Posted Nov 18, 2012 6:28 AM ET

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