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Community and Q&A

Insulating Against a Roof Deck

ITryToInsulate | Posted in General Questions on

I know its been covered quite a bit but I can’t find a great answer to my situation. I have a 1970s cape with sloped ceilings upstairs. The whole upstairs is finished already. I had “smart vents” and a ridge vent installed in my roof to stop ice dams from occurring. I also air sealed and re-insulated the attic to R49 using batts. I have 1.5″ XPS screwed and taped to the bottom of the rafters inside the knee walls, R30 batts in the floor joists (plywood ontop to create storage space), then R15 batts the walls of the knee walls. With all this, I still get snow melt and ice dams. I realized that the sloped ceilings have NO insulation in them and thats probably where all the heat was escaping. I don’t want to lower the ceiling as that would require me to remodel the whole upstairs. The house has 2×6 rafters and I would like to space rigid foam 1.5″ from the roof deck (to maintain airflow) which would all me 4″ for insulation. With that dimension I can only get R20 using XPS. Would that work, or is it not worth my time?

I am trying to avoid raking my roof every time it snows and balance the 8deg temperature difference between the main floor and second floor (hotter in summer/colder in winter). My neighbor has the exact same house and they don’t have ice dam problems.

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  1. Expert Member
    Dana Dorsett | | #1

    What climate zone are you in?

    Is there full in to out venting possible in the slope ceiling section, or is it blocked by framing? (Many are blocked at the kneewall.)

    Assuming it really CAN be vented, 4" of foil faced polyiso is usually good for R25 even before counting the additional R1 you legitimately get out of the foil facer adjacent to the vent space.

    If there is no way for air to freely enter the bottom and leave the top of the vent space you'll be better off using some amount of closed cell foam against the roof deck (the amount needed for moisture control depends on climate zone), even if filling in the rest with cheap fiber.

  2. ITryToInsulate | | #2

    I am in lower Wisconsin which is climate zone 6. Yes the smart vents have a clear path and do take in air up to the attic and out the ridge vent. The smart vents are maybe a foot and a half from the edge of my roof leaving that bottom edge (where the dams form) uninsulated. From my research I found that polyiso doesn't preform as well as XPS in colder climates, which is why I never used it. Is this true?

  3. ITryToInsulate | | #3

    Anyone else have any other opinions on my problem?

  4. climbing_carpenter | | #4

    Adding insulation will increase your need for shoveling, not decrease. Higher levels of insulation will help prevent heat loss- which in turn will melt less snow. This, however, shouldn’t be a consideration when making performance improvements- yes, insulate your roof.

    More on your tactic: are you going to remove the interior finish?

    1. ITryToInsulate | | #5

      I was planning on removing the interior drywall to access the bottom side of the roof deck. I'm aware the snow will pile higher, but I'm trying to get it to stop melting then refreezing on the edges.

  5. climbing_carpenter | | #6

    Not much room to work with. Best course of action is exterior insulation, either foam board or mineral wool batts. Batts would require non-structural framing.

    If that isn’t an option, you’ll need to insulate your rafter cavities from the ridge to the soffits. Yes, you have insulated your eaves. However, many tricky transitions and why I’d suggest focus on the rafter cavities, bottom to top.

    As for the type of cavity insulation: whatever you get the best pricing on. Cut and cobbled foam, as you suggested, will be expensive and laborious. Compressed fiberglass and mineral wool are also options.

    This is an application where continuous interior insulation is a good option, provided you maintain adequate venting as your roof deck will not dry to the inside.

    My course of action, provided exterior insulation is not an option, would be 2 inches of continuous polyiso, taped and strapped under the drywall. I’d use 1/4” luan to create a 1” vent, maintaining 4 1/4” for cavity insulation- compressed r23 fiberglass is a good price point option here. The whole assembly should clock in over r30 and perform reasonably well as bridging will have been eliminated too.

    Take care working around partitions and knee walls, remove any recessed lights. If you desire these, go with pucks instead.

    1. ITryToInsulate | | #8

      I do appreciate the options but I do not wish to reduce headroom on the eves if at all possible. I can take some measurements and see if Ill be able to manage it!

    2. ITryToInsulate | | #10

      So I decided to go with compressing fiber glass then over lapping the rafters with 2" xps, add drywall over that.

      From my research R30 (9" thick) fiber glass turns into R21 compressed to 5 1/2. If thats true the whole assembly clocks in at R31. My two questions now are: Do I buy the unfaced R30 (readily available) or order the faced R21? Second: for the venting my big box store sells Provent (plastic) vents to create 1" gap. In their instructions it says to leave a 1" gap between vents "for removal of trapped moisture". Does this seem logical? I'd think the air moving through the vents dries/removed the moisture.

      1. Jon_R | | #11

        > moving through the vents dries/removed the moisture

        Yes, but first the moisture needs to get into the vent channel. A 1" gap will improve this. But wouldn't be necessary with sufficiently high perms.

  6. climbing_carpenter | | #7

    Another thought-

    You can build the rafters down to accomodate r30 mineral wool. Would not perform as well as the compressed fiberglass with continuous 2” polyiso, but would be a little less expensive unless you have a distributor for reclaimed foam.

  7. climbing_carpenter | | #9

    If you do not wish to build the ceiling plane down, your best option is closed cell spray foam. 5 inches worth would clock in at r30. Your roof would not be vented and you’ll have to take care to seal the vents you have prior to spraying. Have the installers work carefully to fill the rafter cavities to a true 5”. You can then trim excess back for drywall.

    I do not advocate cut and cobbled layers of polyiso as the spray foam installation and air seal will be vastly superior. In ideal circumstances, rafters would be perfectly straight and square, where cut and cobbled foam would squeak perfectly into the cavities- that is almost never the case though, as we’re working with wood in a non-laboratory environment.

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