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New roof and attic insulation and venting question

Prevch | Posted in General Questions on

Hi everyone,

My name is Ceb. I live in very northern, WI. This past winter my home suffered major ice dam damage from the historic snow, ice, and cold that we experienced. We are in the process right now of figuring out what to do so we never have this problem again! Additionally, we would like to increase the comfort and efficiency of our home and lower our carbon footprint.

At this point, we know we will be doing some form of spray foam in our attic. Our home has vaulted ceilings on either side but they flatten out in the middle so their is a small attic area in the middle of our home, not big enough to stand up in, but large enough to crawl on your knees through.

We have discussed a few options with various builders. One option would be to take out all of the existing rolled bat insulation and spray the entire attic with 6″ or 7″ of closed-cell foam. Another option would be to take out old insulation, spray the attic with 1″ of closed-cell foam and lay the attic bats back down. A third option would be take out the old insulation, spray the attic with 1″ of closed-cell foam, and then blow in cellulose. Obviously, the 6″ or 7″ of spray foam is the most costly somewhere between 15k-20k I believe with prices going down from there.

This leads me to my next question. In this case, due to the vaulting and just the situation, we are spraying the attic floor, not the roof decking. I have had mixed opinions on whether or not we should ventilate the attic. Some builders say absolutely yes while others say absolutely unnecessary. My preference would be to have an unvented attic. I live right next to the forest, so we have had problems with wildlife in the attic before, so if we could close up the attic entirely, that would be great. That being said, we have discussed installing central air, in which case to get to our bedroom, we would likely need a duct in the attic, thus making me think we should have a vented attic. Then I though, well maybe I can just spray foam the the duct in the attic so it won’t condensate in an unvented attic? I also thought, maybe I can just give up central A/C upstairs so that we can close the attic up.

The last part of this puzzle is roofing material. I can’t decided between asphalt shingles and metal. Metal is obviously much more expensive, but provides much better protection whereas we definitely prefer the look of asphalt shingles, but we have no desire to have this ice dam problem again. Essentially the metal would be twice as expensive as the shingles.

So, in summation, what, in your opinion do I do to get the best comfort, resale value, and protection in terms of insulation, ventilating, and reroofing?

Other than the potential risk of condensation, or there any other drawbacks to having an unvented roof?

P.S. I am on the border of climate zone 6 and 7.

Thanks everyone!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Is there soffit venting, with an unobstructed 1" air gap between the insulation and roof deck from the soffits all the way to the mini-attic?

    How deep are the rafters?

    Are you gutting the vaulted ceilings?

    Installing AC ducts above the insulation is going to be a comfort and efficiency disaster, which means insulating at the roof deck is likely going to be the best bet unless you decide to go ductless on the AC. (A half ton cold climate mini-split would more than cover the heating and cooling loads of most bedrooms.)

    In US climate zone 7 (northern WI) there needs to be at least 60% of the total R exterior to roof deck to go unvented. With 2x6/R20 (say, cellulose filled over pre-existing batts) would then require 6" of roofing polyiso board above the structural roof deck, which would usually be cheaper than doing it with 7" of HFO blown foam between 2x8 rafters. If you're on the zone 6 boundary you could safely cheat that a bit and go with only 4-5" of exterior polyiso with R20 in the cavities. But the amount of cavity insulation is going to dictate how much it's going to take on the exterior to be moisture-safe with an unvented assembly.

    See also Table 3 of this document, the Minneapolis and Int'l Falls rows, and the 2" ccSPF + spray fiberglass column:

    https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf

    While the roof deck might still be safe with only 2" of closed cell foam on the underside of the roof deck, that doesn't mean the fiber insulation would stay dry and perform well all winter without an air tight interior and vapor retardent latex on the ceilings.

    1. the74impala | | #5

      I have nearly and identical situation, in northern Wisconsin also. My ceiling has 2x12s with 1.5inch added to the top of them. I am going to make a sealed baffle area with 2 inch ripped 2x's, and 1/2 cdx. Then caulk. Fill with r30 and r15 rockwool. Then 2" taped polyiso foam board. Drywall over that. The plan is to allow the roof to dry to the baffle. Starting work ripping it out next weekend.

      1. Prevch | | #6

        Cool idea, Tom. Thanks for sharing. Did you also suffer from ice dams this horrific winter or is this just an upgrade for energy efficiency purposes?

        1. the74impala | | #8

          Tons of ice dams and leaks. Just bought the house in November, haven't moved in yet. Still tearing things out.

          1. Prevch | | #10

            Bummer! Rough winter up here this past season. Keep us updated, let us know how it goes!

  2. Prevch | | #2

    Thanks for the response and good questions.

    The attic had attic baffle vents with a 1 inch gap all the way up, but we discovered that many of them were crushed when the ceiling was put up because the insulation was stuffed in too thick. We do also have a ridge vent. The rafters are all 2x10s.

    Ductless AC was discussed as an option and may likely be the best bet in this situation.

    Thanks for the link!

    Everyone please feel free to chime in. The more information the better.

  3. Expert Member
    Dana Dorsett | | #3

    Without soffit vents the ridge vent just sucks moist air in from the conditioned space all winter, adding to the moisture accumulation load at the roof deck.

    Hopefully there is an air barrier between the ( t & g?) plank ceilings and the rafter cavities above, such as a layer of gypsum board? If not, it's a moisture and energy leak disaster that needs to be rebuilt with an air tight gypsum board layer between the ceiling and the rest.

    With 2x10 rafters you have 9.25" of space to depth to work with If you're willing to pull the ceilings you'd be able to get there with:

    4" of HFO blown closed cell foam (R28-ish) on the underside of the roof deck

    5.25" compresses R23 rock wool batt (R23-ish) or 1.8lbs density blown fiberglass (R22)

    That would be R50 total, with R28/R50= 56% of the total R exterior to the fiber insulation, which would be plenty anywhere in US climate zone 6, and OK at the warm edge of zone 7. With either using air-tight ceiling gypsum and vapor barrier latex paint, or a 2 mil nylon (Certainteed MemBrain) smart vapor retarder between the gypsum and fiber insulation it would be pretty good anywhere in zone 7.

    That may be easier and cheaper than doing massive amounts of rigid foam exterior to the roof deck, and would allow you to install any amount of exterior foam as a thermal break on the rafters. A 2" -2.5"polyiso nailbase panel above the roof deck would add another R9-R11 or so, roughly doubling the R value of the rafters, reducing the amount of melt-out heat reaching the snow to start ice dams, and very easy to install as a retrofit compared to layered 6" foam up top.

    A typical polyiso nailbase example:

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/108-h-shield-nb/file

    (there are others, Atlas, Firestone, et al.)

  4. Prevch | | #4

    Interesting ideas. To clarify a few things, the soffit is vented, I forgot to put that in my previous post. The T&G ceiling is not on gypsum, it is literally just nailed to the rafters. Granted there is a thin plastic layer between the rafters and the T&G, but I am not convinced it is doing much of anything.

    Due to the expense and disruptiveness involved in taking down the entire ceiling, the plan was to take up all of the roof sheathing, remove the aforementioned thin layer of plastic, and spray down into the rafter bays on top of the T&G Ceiling completely isolating the conditioned space.

    If I were to spray say 7" of closed cell foam and just leave a 2" gap and leave the soffit and ridge vent vented, I would think that would provide enough ventilation if it is needed and should be about R45-49. My understanding is that attic baffle vents would be needed if I used a combination of spray foam and blown in cellulose. Of course this scenario is one where I vent the roof. Maybe I could do it this way unvented as well?

    The nailbase idea is interesting. I was not aware of that product, but plan on looking in that more.

    Thoughts? Should I keep things vented?

    1. Expert Member
      AKOS TOTH | | #7

      Ceb,

      There is not much point in spray 7" of closed cell in there. Yes it would get you close to R49 center of cavity, but because of the thermal bridging of the rafters (assimg 16 oc rafters), you end up with only R35 roof.

      Would be cheaper to go with either 8" of open cell foam (I would keep the poly in this case) which would get you to an R28 roof.

      Another option would be to rip out the poly, spray 2.5 inch off closed cell to air seal over the T&G and install 5.5" of mineral wool batts on top the SPF. That would get you close to R30 whole assembly.

      Both of these options need venting and would be significantly cheaper than 7" cc SPF.

      1. Prevch | | #9

        Great feedback, thank you!

  5. Jon_R | | #11

    Everyone focuses on keeping the roof cooler - but it's also valid to say that ice dams are caused by "colder outer eaves".

    So the location of eve vents effects ice dams. Put then as far outboard as possible (eg, fascia or shingle vents), where they will provide less/more-even heating when solar heat is rising from the walls. Or more speculatively, where they they won't cool the eaves at all - under shingle vents further up or just use low gable vents (yes, this is code compliant).

    Bad (cool outer eave caused by wrong vent location) and good (retain eave heat):

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