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Community and Q&A

Insulating Cape-Style Roof With Rigid Foam

pmarsh4 | Posted in Energy Efficiency and Durability on

Details :
– 1940 1.5 story Cape in zone 5
– Attic had new fiberglass batts to R-49 depth after removal of old insulation 3 years ago
– Gable vents on on the sides of the house and at some point they cut out a couple additional roof passive vents
– Sloped ceilings have useless old insulation or no insulation
– Ice dams build up, some rooms get cold/hot
– Roof needs replacing

What I am thinking is since we will be tearing up the roof is to take the opportunity to put down rigid foam (sealing up the roof vents, leaving the gables) to insulate the sloped areas. The question is how thick given that the attic already has new bats to R-49 levels.

Any thoughts/shared experiences are most welcome!

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  1. PAUL KUENN | | #1

    It will only work if the entire attic is sealed tight and everything, including the gable ends are insulated as well as the roof sheathing.

    1. pmarsh4 | | #5

      Paul do you mean close up the gables themselves or just make sure the walls around them are insulated?

      Thanks for replying!

  2. andy_ | | #2

    I don't really see the point of what you want to do here. You have a ventilated roof, so adding foam on top won't really do anything.
    The ice dams and inconsistent temperatures sound more like failures of air sealing between the living space and the attic. Could also be some insulation missing in key spots or poorly installed, but probably the air sealing would be the main issue.

    1. pmarsh4 | | #4

      Thanks for the reply!

      What I'm trying to do is insulate the lower half of the roof that creates the sloped ceilings on the second floor.

      Those slopes are either empty or full of old insulation I do not want to touch.

      I could tear down the plaster & lathe inside, but since the roof needs replacing I am trying to take advantage of that.

      Maybe I am wrong but I was thinking that putting rigid foam on top of just that lower portion would not work because it would be a few inches higher than the un-insulated portion above it to the ridge.

  3. GBA Editor
    Kiley Jacques | | #3

    Here are two articles you should probably read: Five Cathedral Ceilings That Work and How to Build an Insulated Cathedral Ceiling.

    1. pmarsh4 | | #6

      Thanks! I've been reading at much as I can but of course haven't found anything that quite fits the situation of vented attic but insulated floor and then applying rigid foam on the roof sheathing. I might be the first idiot to want to do so.

      Thanks for taking the time to dig them up.

  4. maine_tyler | | #7


    I think it's pretty uncommon to insulate on the exterior while leaving the attic insulation in-tact. Typically you move the entire thermal (and usually air) control layer to the roof line. Moving the air control layer to that point is really one of the biggest benefits in old capes because it's a plane that you have easy access to while the roofing is off.

    If you leave the attic portion, you create some complicated transitions and some funky thermal dynamics where the insulation splits between the attic floor and the roof line.

    How much sqft. of attic floor (second story ceiling) is there?

    As far as your gable vents: if you are moving any or all of your thermal/air control layer to the roof plane, the gable sections become part of the envelope. That means no giant air holes (vents).

    1. pmarsh4 | | #8


      Thank you this makes complete sense and what I was looking for. It's about 900-1000 sq feet of attic floor.

      So I kind of shot myself in the foot by putting new insulation up in the attic a few years ago not realizing that when I redid the roof I would have the opportunity to fix the issue.

      Just have to decide if it is worth the sunk cost + the additional cost of rigid foam. Or just suck it up and deal with the issues created by the poorly insulated slopes.

      1. andy_ | | #10

        I still think it's mostly an air sealing issue. You should look at a cross section of the house and see where the control layers really are. Where are you blocking bulk water? Air movement? Thermal movement (insulation)?
        Do the old pencil test on that cross section to make sure it's continuous. You should be able to trace the outline of those control layers without lifting the pencil.

        1. pmarsh4 | | #12

          Well I learned what the pencil test was today. Thanks :)

          Have a bit more learning to do obviously as control layers is kind of a new term as well.

          Thanks for bringing them both up.

  5. maine_tyler | | #9

    Weighing that sort of thing is tough. I live in an old 1910's 1.75 story and am working on a project for work that will likely involve exterior wood-fiber on a cape (1.5 story), but it's hard to know what the right thing to do is. Weighing performance with cost is tricky on such projects. Still not sure how best to think about it.

    Part of it for me is how much living area is sloping vs how much is flat attic. I definitely won't be adding ext. insulation on my 1.75 story bungalow (especially since the roof is fine). The 1.5 story has a much smaller attic and the roof needs replacement so...

    If you don't go through with the ext. foam, you may still want to take the opportunity to blow cellulose into the sloping bays during reroof since that can be done from the exterior. It may also be an opportunity to do some air-sealing in strange places if they're particularly bad, like around the top-plate area (easier if it's board sheathing, just pull up the bottom couple boards).

    1. pmarsh4 | | #13

      Yup, cellulose in the bays during the reroof is my plan B to do more research into since it is board sheathing.


      I am not going to have this completed right away, but I will try and remember to come back and post what I ended up doing.

  6. PAUL KUENN | | #11

    I think these folks have answered your question about the gables. Yes, everything must be closed up like the living space to make the roof exterior insulation work correctly.

    1. pmarsh4 | | #14

      Yes they did, very helpful!

      Now just have to decide the best course of action. Anyone want to win the lotto for me to make this easier? lol

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