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Community and Q&A

Zone 6 roof replacement: vent or unvented?

rjohnson056 | Posted in General Questions on

Preparing to replace my 1972 colonial roof – 8/12 pitch, Northeast Zone 6. Currently the roof creates large ice dams on both the east and west sides which eventually cause leaks. I am questioning if I should leave the roof vented or create an unvented/conditioned space in the attic by insulating the roof deck.

Current Installation: 2”x6” trusses and 2”x4” walls. Attic has 6” fiberglass insulation with 6” blown in on top. Soffit and ridge vents. No defined vent channel. There is no question why the ice dams are forming – lack of insulation and poor installation.

Below are the options I have been considering.

Vented Roof: Install synthetic underlayment on existing roof sheathing, install 1 or 2” of polyiso above original sheathing, new sheathing, ice/water shield and synthetic underlayment, new shingles. Correct the eave vent/insulation and install 1” polyiso 1” below the original roof sheathing (inside the attic) to create a defined vent space for the roof. The eave work would be done from the outside by removing the first course of original sheathing. Once roof work is complete, increase depth of blown in insulation.

Unvented Roof: Install synthetic underlayment on existing roof sheathing, 6” polyiso above original sheathing, new sheathing, ice/water shield and synthetic underlayment, new shingles. Install 3” polyiso below the roof deck. Insulate gable ends. Remove existing fiberglass and blown in insulation in attic.

Despite all the info on ice dams and roofs on GBA I still find myself questioning which I should do. If either?!

Thanks for the help in advance.

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  1. Jon_R | | #1

    Why not just improve the air sealing (house to attic) and the ventilation (attic to outside)? Spend some of the saved money on blower door testing.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi R Johnson,

    I'm with Jon. You are proposing extensive and expensive solutions to a problem that you can probably solve with some caulk, canned spray foam, and some more insulation (in fact you may not even need more insulation once the air sealing is done). Here's an article, and a video that describes the work you need to do:

    Air Sealing an Attic
    Air Sealing an Attic Video

  3. Expert Member
    Peter Engle | | #3

    +1 on the other comments. Your problem is not that you don't have enough insulation. 12" is less than we might use for new construction in zone 6, but it's not bad. Your problem is air leakage between the house and attic. Fix that and your ice dam issues should go away. If you've got HVAC equipment in the attic, that is also probably a big part of your problem. Air seal the ductwork and plenums, along with the register boots at the drywall. Insulate the ductwork to at least R-8 and consider burying it in insulation. All of this work is way cheaper than either of your suggestions.

  4. Expert Member
    Dana Dorsett | | #4

    If going with an unvented roof despite the expense, 6" of polyiso above the roof deck on it's own would meet code-minimum on a U-factor basis.

    In zone 6 long as more than 50% of the total R is on the exterior the roof deck is pretty moisture safe, but an interior side "smart" vapor retarder such as Intell0 Plus (detailed as an air barrier) to constrain the batts would be cheap insurance. Going with 4" of exterior polyiso (R24) + R21 fiberglass or R23 rock wool between the 2x6 top chords of the truss would also meet code on a U-factor basis, and would allow for a less imposing 1x6 facia board instead of 1x8 (which can sometimes look weird as a retrofit) on the roof edges.

    Under no circumstances should one install foil faced polyiso on both sides of an unvented roof deck.

    The vented approach as outlined would work from a roof deck moisture safety perspective but is a lot of work for little gain. Jon R is right- if there's enough room for R50 fluff at the attic floor, blower door directed air sealing and piling on enough cellulose on top of the existing insulation to hit R50 would be far cheaper approach than insulting at the roof assembly.

  5. rjohnson056 | | #5

    Thanks for the replies. I agree with air sealing and venting improvements, but my concern is if there is enough room at the eaves to increase the insulation enough to prevent ice dams. I don’t want to regret not taking an extra step while the roof is being replaced but I also don’t want to waste time and/or money. I have access to large amounts of seconds Dow Thermax at very good prices so that helps with cost. Am I over thinking the lack of room at the eaves?

  6. Jon_R | | #6

    > if there is enough room at the eaves to increase the insulation

    How much room is there? Where exactly are you located (effects the amount of snow)? Where exactly are your soffit vents located (with solar wall heating they can cause lower roof heating).

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