GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Please help with poor man’s insulation technique?

user-956493 | Posted in GBA Pro Help on

I know closed cell spray foam would be the best for this application but I simply can’t afford it. I live in Chicago and am trying to finish my attic as a conditioned space I have a dog house dormer that has 3 1/2 ” walls and I already have some r 15 fib batt ins.that I would like to fill the vertical space with. The roof of the dormer as well as the rest of the roof will accommodate 6″ of extruded polystyrene which I also have on hand will this work? I know it isn’t the best way but do I have any potential problems?The main roof is a hip roof. Please answer.It’s getting cold here. Thanks Rich

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Yes, it's possible to install extruded polystyrene (XPS) insulation between rafters and studs. To be effective, you need to seal the perimeter of each piece of XPS. This is usually done with caulk or canned spray foam.

    If you decide to seal the rigid foam in place with canned spray foam, it makes sense to cut the XPS a little bit small. It's easier to foam a wide crack than a narrow crack.

    This is fussy, time-consuming work. But this method of insulation will cost less than hiring a spray foam contractor -- especially if you already have the XPS on hand. Good luck.

  2. user-956493 | | #2

    Thank you so much for your quick response. I have read through your forum enough to figure out how to install it. I should have been more specific about the vertical walls in the dormer. Is it ok to use the r15 fiberglass batts that I have or do I have to use extruded polystyrene there as well? Rich

  3. GBA Editor
    Martin Holladay | | #3

    The walls can be insulated either with fiberglass batts or with sealed pieces of rigid foam. No matter which insulation material you choose, the performance of the insulation will depend on how carefully it is installed.

  4. user-956493 | | #4

    Thanks again Love all the interesting topics. Keep up the good work. Rich Sorry about that double post

  5. jklingel | | #5

    Rich: Too, if you have the room, you can install the batts between the studs and then the foam on the inside. Much easier to seal the foam that way.

  6. user-956493 | | #6

    I decided to use fg bats on walls followed with 1" foam and seal with tape then drywall for the walls.A little confused after reading .I am getting a little overwhelmed at all the info.Ive read that fg should work ok as long as it has a 6 sided air infiltration barrier?Do they mean I also need to use a calk[perhaps an expandable one} or possibly silicone in the stud bays before installing the fg bats? The wall of the dormer is osb and there is no foam on the outside. Thanks again I'd like to get started on this and do not want to make any mistakes.

  7. wjrobinson | | #7

    Yes Richard, every joining of framing can be caulked to better your air barrier and aid fiberglass in insulating.

  8. GBA Editor
    Martin Holladay | | #8

    Most people who install fiberglass batts don't go to the trouble of caulking each stud bay. Here's what is meant by a "six-sided air barrier": at a minimum, you need sheathing of some kind on the exterior side of the wall, and a finish panel of some kind on the interior side of the wall, and a bottom plate and a top plate without any holes in them.

    That said, if you see any big cracks or holes in any of these surfaces, it would make a lot of sense to caulk these holes or cracks before you install the fiberglass batts.

  9. user-956493 | | #9

    I do not see any air gaps but I have what I hope is the last question . Should I be removing the craft paper from my insulation and just pressure fit it as I intend to use the 1" xps as a thermal bridge and tape it to provide a moisture barrier or does it matter and I leave the paper on? And by the way whats the difference between using a can of great stuff and the expandable calk that I've seen mentioned for inside the bay? Sorry that turned into 2 questions? Rich

  10. GBA Editor
    Martin Holladay | | #10

    Q. "Should I be removing the kraft paper from my insulation and just pressure fit it as I intend to use the 1" xps as a thermal bridge and tape it to provide a moisture barrier or does it matter and I leave the paper on?"

    A. It doesn't matter what you do with the kraft paper facing. Leave it on or take it off -- it makes no difference.

    Q. "What's the difference between using a can of Great Stuff and the expandable caulk that I've seen mentioned for inside the bay?"

    A. I'm not sure what you mean by "expandable caulk." If you are trying to seal a crack, in general you use caulk for small cracks and canned spray foam for larger cracks.

  11. user-956493 | | #11

    Thanks again I feel more confident now thanks to you as per the expandable caulk it was a product that I thought I read in your forum. I don't remember the brand name. It was billed as an alternative to spray foam' Had no real R value [meant for air infiltration],was to be sprayed along the edges of inside of the bays and then followed up with fg or any other form of insulation and was intended to be professionally installed.I couldn't see why silicone calk wouldn't do the same unless of course it was a large gap like you had mentioned. It's now time to start my project.I know this will be tedious but well worth it in the end. I wish the spray foam was more affordable. You guys are the best. Keep up the good work.

  12. user-956493 | | #12

    I have started the walls and surprisingly there are some small gaps in the bays. I have sealed them with silicone and have started to place the fg bats in the bay and will cover with the 1" polyiso board ,alum side in, and tape then follow with drywall.I have been thinking of the ceiling which is a 6" cavity in the bays after I added 1/2" scab. I am planning on putting 6" of xps along with 1" of polyiso over all of it and tape seems then drywall. I was wondering if I could get away with less foam and use fg on the underside of the roof behind the knee walls as I wont be storing anything there.Kind of like a flash and bat. Any problems with this? would it require that I trap the fg with a barrier such as 1" polyiso? also is there any benefit to me achieving a greater R value in that space as compared to the rest of the room? I can achieve approx R36 in the living area roof. No telling what I could achieve behind the knee wall. Thanks again Rich

  13. GBA Editor
    Martin Holladay | | #13

    In general, you don't want to have both exterior rigid foam and interior rigid foam with other materials sandwiched between them. However, considering the nature of your "cut-and-cobble" method, with multiple layers of sealed foam, the risk of the foam/fiberglass batt/foam sandwich you describe is very small, and I see no reason why you can't go that route if you want to.

    If you want to aim for a higher R-value behind the kneewall, go ahead. Thicker insulation reduces your heat loss, so there is no reason not to increase the R-value, as long as you have the time, inclination, and materials.

  14. user-956493 | | #14

    Would it make a difference if I where to try to do the area of the roof, behind the knee walls, the same as I've done the vertical walls in the dormers, only scabbing it out to accommodate thicker fg insulation? That would be way easier and much less expensive. I should point out, as I've said, this is supposed to be a finished, un-vented, conditioned space. The roof has osb installed over 1"x6"s, laid horizontally over the roof studs, with random spaces approx 3" wide between them, I suppose, to vent the roof, somehow, or provide some air flow. But, I can't see how as there are no vents,Seems like spray foam wood be the best choice to fill this cavity and stop the air infiltration but as I've said before to costly.Aside from the spray foam what do you think would be the best method? In the living area I was going to use xps or polyiso in the bay and follow up with the1" polyiso and tape it before the drywall Do I have any potential problems with this as I neglected to tell you about that air space. Rich

  15. GBA Editor
    Martin Holladay | | #15

    If you are insulating between the rafters to create an unvented roof, you should stick with rigid foam rather than fiberglass. (Of course, spray foam would be even better, but we have already determined that you aren't going to use spray foam.) Install as many layers of rigid foam as you possibly can between the rafters. Each layer of rigid foam should be installed in an airtight manner, using caulk or canned spray foam to seal the perimeter.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |