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Community and Q&A

Affordable Roof Insulation Solution for Vaulted T&G Ceiling

pgetchis | Posted in General Questions on

I have a two story house with a gambrel roof, in Hood River, Oregon.  We have winters around freezing and warm dry summers. We love the house and the vaulted ceilings have beautiful t and g with exposed beams.  I quickly realized the roof was built without insulation, just the  fir t and g and some 1/2 inch acoustic board on top.  

The house needs a new roof, so I decided to insulate from the outside.  I am doing all the work myself and am looking for a reasonable and affordable solution.  My current plan is to tear off the asphalt shingles, put a vapor barrier down to prevent warm moist air from escaping. From here I plan on adding 2-3 layers of polyiso 2” thick and spray-foaming/taping seams.  Lastly, 1/2 inch plywood, vapor barrier, and new shingles.  

I am considering this method to keep cost/labor within reason.  I am concerned about condensation and mold.    I would appreciate any insight you may offer, as I know this is a quirky project.  Thanks.  

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  1. user-2310254 | | #1

    If you are in the Hood River Valley country, that is climate zone 5B. To get to R-49, you need at least 9 inches of polyiso (as noted here:

    You may be able to save quite a lot on materials costs by sourcing reclaimed foam. I checked Craigslist and found this:

    1. pgetchis | | #3

      Thank you, Steve! I found some on Craigslist and am headed out to pick up tomorrow. I appreciate the idea and read both articles.

  2. Expert Member
    Joshua Salinger | | #2

    Steve has a good point. You may be able to reduce the amount of insulation on top of the sheathing by insulating below it in the rafter bays, but I imagine that is a huge project to remove all of the finishes. You may want to check with your local code officials as there are often pathways to reduce the insulation for retrofits. You will still want to have a min R-20 (5" of polyiso) to prevent condensation and potentially more if you insulate below the sheathing. You could strap some 2x's above the insulation to create a vented space as a way to add some resiliency into the system. Either way, puffing out the roof this much will likely affect the architecture of the home and you will have to pay attention to how you trim it out to make it look intentional. See Joe Lstiburek's entertaining article about this on the attached pdf.

    Also, be careful of the double vapor barrier. I'm not sure if your intention was to say vapor 'barrier' or vapor 'retarder' There is a difference! Don't create a vapor sandwich with two vapor barriers. The important part is to stop air leakage at the roof sheathing and this can be done with tapes (if it is a sheet good) or membranes (if it is skip sheathing). The polyiso (assuming it is foil faced) will act as the vapor barrier in this assembly.

    There is some reclaimed EPS in a lot in Portland that came off the OMSI building that could be a good option for you if you want to save some costs. I believe it is posted on craigslist.


    1. pgetchis | | #4

      Josh- Thanks for the insight. I ended up finding 5.5 inches of polyiso- 3.5 and 2 inch. The 3.5 inch is fiberglass faced and the 2 inch is foil faced. It’s what I could find n craigslist. It seems like that should be enough to keep condensation at bay in zone 5. I plan on staggering seams and at this point going the unvented route. I am a little worried about weight and creating too much of a “mushroom” thick facia look. I agree, I want it to look intentional. I need a new roof so it seems to make sense to add some insulation. It will be a huge improvement from none at all. I have a few questions-
      1) do I need a vapor retardant layer on the top and g deck? ... before the first layer of polyiso? If so, do you have a favorite product?
      2) should I put the fiberglass faced polyiso down first or the two inch foil faced first?

      I appreciate your time. This is all new to me and I’m enjoying the learning.

      Thanks, Patrick

  3. Expert Member
    Joshua Salinger | | #5


    You will want to lay down the foil faced polyiso first. One always wants a 'warm side' vapor control layer. You want the part of the assembly that is least permeable to be as warm as possible and in this case it will be the foil and you want it as close to the interior of the building as possible. This is to prevent condensation at this layer.

    You don't need a vapor retarder at the T&G deck-- the foil faced polyiso is taking care of the vapor management. The important component here is the air barrier. You have two choices to deal with it- either by laying down a membrane over the T&G to make it air tight OR using the foil faced polyiso as the AB by taping the seams. The latter is likely more cost effective. You will want to do your best to tie this AB into the walls at the edges, although admittedly on a retrofit in this scenario it will be difficult. Don't let the perfect be the enemy of the good, though. It will be important to limit moisture laden air from getting into your roof assembly and the imperfections at the edges will just have to be what they are. If you ever replace your siding, you could detail the AB so that it could be tied in at a later date.

    Hope this helps!


  4. gusfhb | | #6

    On my T&G roof I drilled a 1/4 hole through every tongue from the top and squirted spray foam into it to try to keep the wind out. My roof historically leaked badly, so I went a bit overboard on the ice and water. If you have large overhangs, as I do, make sure you use enough I&W to prevent leakage from ice dams. With airtight but less than optimal insulation, water from melted snow can form an ice dam when it hits the overhang and freezes.
    Did anyone mention staggering the seams?

  5. walta100 | | #7

    My guess is your T&G ceiling leaks air like sieve and counting on the edges of used insulation to be in good enough shape to seal air tight with tape seems unlikely.

    By default the foam is your one and only vapor barrier I think you need an air barrier you can count on as your first layer over the T&G think house wrap or membrane.

    Consider adding a vapor diffusion port at the ridge as cheap insurance.


  6. user-2310254 | | #8


    I think several of the posters are advising you to remove the T&G to properly air seal the interior or at least cover it with a smart membrane. If you skip this step, there's a risk that warm interior air will reach the roof sheathing and cause issues.

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