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Community and Q&A

Roof replacement in unique house

djsina2 | Posted in General Questions on

Region 5
1960s Residential
Roof deck/structure is 2.75″ thick T&G decking
Interior is all cathedral ceilings 
Interior finished ceiling is the T&G decking
All insulation has to be on the exterior side
There are limits to how much the roof can be built up due to windows, etc
Right now the house has minimal, if any, insulation in the roof

The plan is to strip the roof back to the T&G decking and start fresh with 2″ of Genflex PolyISO insulation board, then furring strips on that, then 1/2″ OSB sheathing.  The hope is adding some venting helps with condensation and ice dams.

The question is, should there be some sort of vapor barrier?  If so, where?  I don’t think there is anything now.

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  1. Expert Member
    Akos | | #1

    Unless you are in very heavy snow area, top venting will not make any difference with ice dams. Getting your ceiling air tight will though. The only way to properly seal T&G is either with a peel and stick membrane or spray foam.

    Your best bet is to install a peel and stick membrane over your T&G as your air and vapor barrier, then the foam over that. The extra extra space for the furring strips is better used for thicker insulation, that will help more with ice dams than the top venting.

  2. djsina2 | | #2

    The house is in Michigan.

    That was the big question all along, use that extra 1" for more insulation or put in the vented air gap. Different people (contractors, manufacturers) have said different things. The contractor I want to use is suggesting the air gap to help with any possible condensation issues, at the suggestion of my city's inspector.

    In the attached picture my roof would look like #4.

    1. Jon R | | #8

      A vent will certainly reduce the chances of OSB moisture problems as compared to an extra inch of foam thickness.

      50% more insulation vs a vent gap in terms of ice dams? Let's say that a roof ventilates at .5 CFM/sq ft (I calculated this, but it could be off). I get 1.1 btu/sqft/hr heat removal with the vent and .8 btu/sqft/hr heat blocking with more insulation. But the vent isn't even (colder at the bottom, warmer at the ridge), so not clear which is better. Either one helps, probably neither is enough.

      In terms of heat loss, more insulation is clearly superior.

      1. djsina2 | | #9

        I spoke with one manufacturer who ran the numbers and says 1" air gap would not even be enough to vent it since the run is over 20' long. So now I question if it's worth doing because of that. To go to a 1.5" gap, which he said would be needed for good flow, would really limit my insulation.

        1. Jon R | | #10

          "enough" for moisture control is 100% dependent on how much moisture is leaking through - air seal well and a 1" vent will definitely be enough. But with blower door verified air sealing and foil faced iso, foam alone should also be enough.

          "enough" for ice dam prevention - ask him to explain his data. There are some unknowable assumptions needed.

          "enough" for code compliance - 1" gap suffices and there is no hope for R value compliance.

          1. djsina2 | | #13

            I should have been more clear. We were talking about ice dams when he provided the 1.5" vent number. We did not discuss the condensation issue.

  3. Expert Member
    Akos | | #3

    Not sure where condensation would come from. Remember, the most important part is to keep interior air out of your roof (this the peel and stick over the T&G), after that the only source for condensation is night time dew if you have a metal roof. Even that is so small that gaps in the roofing panels dry it up in no time.

    I'm just a bit north of you on the other side of the lake and had similar issues with exterior high limits. I went with all rigid foam over peel and stick + metal roof and no venting any kind (#2 on your picture), has been working great for 6 years.

    1. djsina2 | | #6

      Akos - Right now there is no venting. Solid roof with minimal insulation. Likely no vapor barrier? I had a roofer do a patch repair a couple years ago and he said all the nails are badly rusted, to the point he was surprised the shingles are staying on. He suspected this is due to condensation issues.

      How much insulation do you have on your deck? You have none on the interior?

      1. Expert Member
        Akos | | #11

        There is a lot of confusing information out there about roofs and venting.

        In most roofs, venting is used to clear moisture, typically this comes from interior air leaks. There is a way of designing a top vented roof to accomplish this but that is not your setup.

        Top vent a roof deck is for introducing cold air under the deck to keep snow from melting. This is only in an issue in high snow country, snow itself is an insulator, you get a couple of feet on the roof and the bottom layer will get hot enough to melt and turn into ice. So if this is your issue, top vent away.

        The nails are rusted because the roof deck is cold, interior air leaks easy make it between the gaps of the T&G and condense.

        So first step is to seal this air leak. This is why you need a layer of peel and stick over the T&G. No air leaks, no moisture, no condensation.

        The rest of the insulation is to limit heat flow. With your roof stackup 2" of plyiso gets you to an overall R17 roof. Bumping that up to 3" brings it up to R23.

        Assuming Detriot weather, Feb average low of 20F.

        Snow is around R1/inch, so if you have a foot of snow (R12) with inside of 70F, outside 20F, with the R23 roof, you get around 37F on the bottom. So that is a bit too close for comfort.

        If you can bump up the roof to 4" of insulation, then there is no chance of snow melting, so no need for top venting.

        Your call. I rather shoot for more insulation to solve the ice dams than top venting. The extra insulation saves you money and increases comfort, to me that is a much better deal.

        P.S. My roof is 2.5" polyiso with R28 batts on the inside.

        1. djsina2 | | #12

          Thanks for all this info! I need to get up on the roof to actually measure how much clearance I have from deck to a long run of windows. I hope to do that in the next few days. So it sounds like no matter what I do I have to have the peel and stick vapor barrier applied directly to my T&G deck. That was one of the questions I had. Right now my ice dams are severe. I have to rake the roof with any snowfall and it's not an easy job. I'm hoping to fix this if possible but I doubt it will happen. Second priority is to save on the heating bill.

  4. Joel Cheely | | #4

    I have seen a similar roof that had problems after it was reroofed. Rigid foam was used but only in one layer. The roof developed condensation under a poly vapor retarder and dripped through the ceiling, probably due to gaps in the insulation board. You need to provide a good continuous insulation layer. Why are you limited to 2" insulation? What are roof slopes?

    1. djsina2 | | #7

      Joel - The limitation of 2" is due to a lot of windows which are not far off the deck. The roof is all 12/12.

  5. Mike Kolder | | #5

    I removed all the cellulose, sealed all wire holes and found a chase that needed sealed. Built a wooden box around the pull down stairs and a lid made from 2" foam with sides. Installed insulation baffles each space. Replaced all the plywood soffit with aluminum vented. Blew in 24" of fiberglass. Ridge vent the entire length. I still need to remove the gutter heat tapes, because I don't need them anymore.

  6. Expert Member
    Peter Engle | | #14

    You have mentioned a couple of times that the thickness of roof insulation is governed by windows near the roof level. Can you provide a sketch or a photo? If the roof dead-ends at a vertical wall, then getting decent ventilation with air channels is a challenge, especially if there are windows sitting nearly flush to the roof. So is flashing the roof so it doesn't leak. if this is the case, an all-insulation solution will be much better.

    FWIW, I remediated condensation and ice damming issues on a house like yours close to 10 years ago. Zone 4, so not as difficuly weather, but similar issues. We used 3" of XPS with taped seams, with caulk and foam around penetrations. Peel & Stick underlayment was not really yet a thing around here and the owner didn't want to spring for full Ice&Water shield. We went with 30# felt (per the house manufacturer's instructions). Not a perfect air or vapor barrier, but no problems so far. Your all-insulation solution should work, though more insulation would be better.

    1. djsina2 | | #15

      Peter - The way you describe it is correct. The roof dead ends to a vertical wall, which is I'm guessing about 50' in width and full of windows. It's hard to tell exactly how far off the deck these windows are because it's about 22' up the roof. It's not a lot though. I think adding ~3" to the deck is the best I could do. I will be up there this week to actually measure it.

      I'm a little uneasy about putting peal and stick over all the decking, but it sounds like this needs to happen. I guess I'm concerned because this is going onto the structure of the house. Once it's on there is no removing it and no replacing the T&G wood it is attached to. Should I be concerned about this at all?

      1. Expert Member
        Akos | | #18

        You don't need to run the insulation at full height all the way up to the windows. What you can do is bevel the last 6"-8" to keep it bellow the windows. You can then get the window flashing bent up with a longer lower piece and cover of the this section. With such steep roof slope, you probably won't even visually notice this.

        I wouldn't worry too much about the peel and stick. If anything has to get replaced, you can just cut it at seams of the T&G and pop it out. You definitely not getting the peel and stick off the wood though.

        P.S. Its always nice to see unique architecture!

        1. djsina2 | | #19

          The contractor did mention something like what are you referring to (I couldn't quite visualize it exactly). He didn't think the flashing would be an issue though as they would be custom bending all of it for at the upper windows. I guess I am just overly concerned about getting the whole project right since it's not a standard design.

          With the peel and stick I was more worried about it going on, then causing some issues, and as you said there is no taking it off!

          This seems similar to what I am considering although I don't quite understand the layers as it is spelled out there -

        2. Jon R | | #28

          +1 on putting more thickness (insulation and venting or just insulation) on the majority of the roof and thinning it down in limited areas for windows. Under shingle vents allow venting when windows block the path to the ridge and/or there are no soffits.

          With your steep roof, I expect that metal roofing would also help substantially with ice dams.

    2. djsina2 | | #16

      Here's a picture...

  7. djsina2 | | #17

    This sounds similar to what I am proposing but I don't quite understand the layers as spelled out here:

    1. Expert Member
      Peter Engle | | #23

      Coincidentally, the architect who wrote that article is a friend of mine.

      You should note that he was discussing a roof in Tennessee, a far different climate than yours. Also, he had clear chases from the eaves to the ridge and was able to implement ridge and soffit venting. You don't have that luxury.

      I would also hesitate to use a peel and stick that has an asphalt or bitumen base. Some of them weep asphalt and that can be accelerated by pine sap. Asphalt weeping from your ceiling would suck. I would use one of the synthetic peel & stick roofing underlayments instead.

      1. djsina2 | | #25

        Thanks for the advice on the synthetic. It would not be good to have it weeping asphalt onto the carpet below.

  8. djsina2 | | #20

    The contractor did mention something like what are you referring to (I couldn't quite visualize it exactly). He didn't think the flashing would be an issue though as they would be custom bending all of it for at the upper windows. I guess I am just overly concerned about getting the whole project right since it's not a standard design.

    With the peel and stick I was more worried about it going on, then causing some issues, and as you said there is no taking it off!

    1. Mike Kolder | | #21

      Considering you wont remove the interior T&G, and the windows are very close to the roof deck exacerbated by the steep pitch. You've literally painted yourself into a corner. I would say the only way to add exterior foam would be to replace the windows with shorter ones or maybe window sills over the foam. Or remove the interior T&G and work from inside in my opinion.

      The windows would be warm and melt snow though.

      1. djsina2 | | #22

        There is no separate interior T&G. What you see is on the inside finished ceiling is the roof structure, it's all one piece. Likewise, to shorten all those windows would be cost prohibitive. I don't want to rebuild the house. It would be easier to move at that point. I just want to work with what I have and do the best I can.

  9. Expert Member
    Peter Engle | | #24

    Tapered foam could be a solution. If you are buying new foam, there are manufacturers who will provide tapered foam as turnkey solutions. Each piece of foam is numbered and you get a map along with the foam. Tapers as small as 1/8" per foot are common, even less is possible. At that rate, you would probably not notice the taper, but most of your roof would get the 4"+ of foam that you would prefer.

    1. djsina2 | | #26

      I will look into this. First goal is to get the actual measurements on how high those windows are above the deck. I now have some gear to climb up there.

      1. Expert Member
        Akos | | #27

        I've done what I've suggested above for a low window on a 12:2 roof to gain height (around 3/4" in that case). Pretty straight forward, no need for tapering the entire roof surface.

        With a 12/12 it is even less work. You would only have to bevel a couple of inches of the foam near the window and stop the new roof deck maybe 3" away from the window base. The beveled foam can go all the way up to the bottom of the window.

        Things to watch for is that the existing rough opening sill flashing needs to be extended to drain above the roof surface and the new window flashing to have a slope for proper drainage.

      2. Joel Cheely | | #29

        Easiest way I have found to get up on a steep roof such as yours is to place a long extension ladder on the ground and extend it up the roof. I've even backed a pickup truck up to the house and placed the ladder in it to gain more extension.

        1. djsina2 | | #32

          Good idea. I don't have a ladder that long (would need to be about 25' to reach the windows). If my climbing idea doesn't pan out I will see about getting a ladder.

  10. Expert Member
    Dana Dorsett | | #30

    >"The plan is to strip the roof back to the T&G decking and start fresh with 2″ of Genflex PolyISO insulation board, then furring strips on that, then 1/2″ OSB sheathing. The hope is adding some venting helps with condensation and ice dams."

    With 2" polyiso as the only insulation it has RIDICULOUSLY low thermal performance roof for a zone 5 location, more than twice the U0.026 maximum U-factor, and guaranteed to have continued ice damming issues, even with a vented nailer-deck above the insulation.

    To hit IRC 2018 code-min performance (on a U-factor basis) would take 6" of rigid polyiso, three times what's proposed. A continuous 6" of polyiso with an air tight membrane under it to prevent warm air from creating melt-out points would knock back the ice damming potential to nearly- nothing. With 6" foam there would need to be to a short sloped ledge/ extended sill to make it fit with the clerestory windows (and take pains with the flashing details at that point to guarantee it drains properly), but anything substantially less than 6" would not solve that issue.

    The gap between the clerestory wall and chimney looks like both a snow trap and an ice-dam starting heat-leak point. The kick-out flashing for the wall and the chimney flashing will need to be adjusted for the new roof thickness.

    The cantilevered floor looks like a potential air and heat leak too (but that's not related to the ice damming problem.)

    1. djsina2 | | #31

      Dana - Yes, unfortunately it's a bad situation all around. I realize the ice damming issue may not be resolvable with anything short of a lot of insulation as you mention. Don't know what they were thinking in the '60's. For the aesthetics it's hard to imagine the roof being built up so high and having a big fascia board, not to mention the ledge at the upper windows. On the back side of the house I peeled back some roofing and can see the blue 1" foam board. On the front of the house I peeled back and did not see anything at all. There may be zero insulation! I wonder if 4" of PolsyISO with no vented nailer is a workable compromise which could help with the damming and while also improving on my heating bill substantially?

      You are right on the gap. It has been leaking there a while and in general that whole area by the chimney needs attention.

      The front of the house has no cantilevered floor. That long low window is at eye level from inside the house with the first floor about 5' below grade. That window was just replaced (was single pane 1/4"! with butt joints) and the overhang all spray foamed.

    2. Expert Member
      Akos | | #33

      I'm trying to figure out what the rationality be for that much insulation.

      Looking at the OPs picture, I'm guessing the roof is 25x50.

      With 4" polyiso that works out to ~R28 (taking into account the thick wood deck).

      So in the low FEB average in Detroit (20F), there 2200 BTU loss through the roof.

      Bumping that up to 6" brings it up to R41, which is 1500 BTU, 700BTU saved. The ROI on that is none existent.

      There is really no rational financial sense to go with the thicker insulation.

      As for ice dams, with such a steep roof that won't hold much snow, even a reasonable amount of insulation will keep the deck cold enough to keep snow from melting.

      1. djsina2 | | #34

        Akos - You are very close in your roof measurement guess for the front side living area (over garage is standard framing). In the 5 winters in this house, some with big snowfall, I've never seen the snow come off the roof due to the steepness. I'm about 40 miles from Detroit.

        I should mention there is a geothermal heating system. Part of the problem is without any roof insulation now (or very little) the geo has a hard time keeping up much of the time and the electric aux heat has to come on (expensive). I'm really hoping dealing with this roof insulation is going to solve that problem too.

  11. Expert Member
    Akos | | #35

    Your old roof is around R4, maybe R4.5. Under the same conditions as above the roof would loose around 15.5kBTU (4.5kW), never mind all the air leaks through the T&G. I don't know if it is enough to keep your aux coil off, but would definitely make a big difference.

    Steep slope roofs don't build up as much snow as shallow roofs, it is not that it slides off, it just doesn't build up to the same height. I have a cottage in northern Ontario with a steep roof main building and shallow roof bunkie. The bunkie usually has about 2x the snow on the roof.

  12. djsina2 | | #36

    A couple updates:

    I was able to see there is 1" blue foam on the front of the house after peeling back a different area.

    I also was able to climb up to the windows for some measurements. I'm attaching a drawing to help explain. From the shingles it is 2.75" vertically to the top of the sill. This is the black portion in the diagram. There is then another 1.25" of metal which has weep holes drilled in it, the dark gray portion of the diagram. I guess this is part of the window, it was hard to tell.

    So it seems there is approximately 3.75" from deck to the top of the window sill.

  13. djsina2 | | #37

    I found a heat loss calculator, now I see what you mean. It would appear a sealed roof with even 2" of polyiso would make a huge improvement for my geo situation, especially after figuring in the same heat loss for the back half of the roof. The geo company has been over so much trying to tweak and troubleshoot the system and no matter how much I told them about the roof they didn't seem to think that was the issue, which baffled me before and now really does after seeing in the numbers.

  14. Cody Brown | | #38

    I have a similarly constructed home with 2x6 T&G for every floor surface and the roof deck. Peel and stick over the top of the roof will not address the biggest source of air leakage, which is horizontally at the intersection of the walls and the roof between the actual tongues and grooves. To fix this you have to cross drill a hole at each groove at the wall intersection and spray in a little plug of foam. An air barrier on top of the T&G is the final piece to address this source of leakage.

    The IR photo is from my home when we did an energy audit before we started our remodel. These leaks existed at both the floor and ceiling, aka the worst places to enable the stack effect. Before I started sealing I could stand in the stair well of my two story house and feel a draft of air flowing upstairs from the first floor and out all of those leaks. The second photo I can't remember where I found on the internet but it shows the cross drilling and foam plug I'm talking about.

    1. djsina2 | | #39

      My T&G runs vertically. Indoors where the T&G meets the ends of the roof it is contained in insulated soffits.

  15. Keith Gustafson | | #40

    I have a similar roof but low pitch without the window issue.

    I will tell you what I did, and what I would do differently

    I went from 1"[wooo hooo] of foam to 3 1/2 plus airspace, plywood, vent at peak, vent behind fascia board. I did not vent for moisture, but to protect shingles, as my personal experience is early shingle death on the south side of this style roof. I used ice and water at the edges and completely ridge to edge at any perforation. This amounted to about 2/3 of the roof covered. Twice. On the deck itself and on the plywood immediately under the shingles. I used synthetic underlayment, again, twice.
    The foam was 2 layers, staggered seams. I used scrim faced roofing foam.
    At the intersection of the wall and the roof, before insulation began, I drilled a 1/4 hole thru at every joint of every board and injected canned spray foam into the hole.
    I shingled with real lifetime shingles bought through a roofing supply house. Never buy shingles from a big box retailer, only a roofing supply house. Buy the best you can get. Read the installation instructions personally. Stand their personally and ensure that the first few rows are installed nailed as the instructions require.

    Every single professionally installed roof I have stripped has been nailed improperly. The nails are too high and do not penetrate both shingles as required, and on your roof pitch they will start falling off in 10 years.

    What I would do differently:
    screw the airspace, gimme more foam. I am more confident of my roof system and quality of my shingles. I would probably save enough fuel to pay for a reshingle in 20 years, or at least pay for the differential lifespan.
    That extra layer of foam would be foil faced to allow taping. I knew of no tape that would stick to scrim.

    I would probably drill 2 holes in the decking, one at the seam, one where the tongue ends to get a better seal.

    I would probably use standard roofing felt at least on the deck as I have come to understand it has superior moisture/vapor handling

    I would put as much foam as you can. At the windows I would build a metal pan under the window frame straight out and a small slope until you reach above your foam. Slide the foam underneath or set it on the foam or whatever. Metal flashing is cheap, and on a 12 pitch roof will only extend, what 8 inches or whatever depending on your foam depth.

    This type of roof assembly is structural for those who suggest trying to change it. It has large spans[8 feet in my case] and so removing it involves tearing the house down.

    My house has a 3 pitch and 2 foot overhangs. Ice dams? Sure we get ice dams, but the roof don't leak, unlike the 40 years prior to my re roofing it, when it leaked constantly. I met the person who built it, and she told me that the roof leaked day one and buckled her brand new hardwood floor.

    1. djsina2 | | #41

      Keith - Thanks for your insights.

      I'm starting to think this is the plan...

      Existing 2.75" T&G deck, synthetic peel and stick, 1.5" Genflex polyiso, staggered 2nd layer of 1.5" Genflex polyiso (taped), 1/2" OSB, synthetic underlayment, shingles.

      This removes the complexity and additional cost of venting. From what I gather from this thread and further reading as long as I air seal the deck with peel and stick and have R20 minimum directly on the deck then I should encounter no condensation issues? In this case then maybe go to 3.5" or 4" of polyiso with some modification at the window. Aesthetically I don't want to do anything too obvious at the windows, if possible.

      1. Jon R | | #42

        > 1/2" OSB, synthetic underlayment, shingles. ...I should encounter no condensation issues?

        With cold, unvented wood sheathing, there is always some condensation/sorption. Given this, consider plywood instead of OSB. And permeable, fully adhered underlayment. Permeable because even ~.5 perms of upwards drying through shingles is measurably better than < .05 perms. Fully adhered because it's a better barrier to water.

        1. djsina2 | | #43

          When you say fully adhered, do you mean another layer of peel and stick?

          I don't quite understand all the differences to be honest. I hear things like peel and stick, vapor barrier/retarder, ice and water guard, underlayment, etc.

          Seems like I've read plenty of other opinions saying doing our whole roof in ice and water guard is a bad idea.

          1. Expert Member
            Akos | | #46

            We have to be clear of the job of each layer.

            The base layer of peel and stick is needed as an air barrier. As a bonus it does a good job of preventing any leaks through the roof getting into the house.

            Under shingles you need a layer of underlayment. This is there so the shingles can be easily removed down the road and protect the roof deck from any condensation (this happens at colder nights with a clear sky). Felt is cheap and works well enough, most roofers do prefer a synthetic one as it is harder to rip. Most synthetic underlayments also work great.

            A 2nd layer of peel and stick doesn't buy you much in this case.

            OSB is not a good idea for shingles, it doesn't hold nails well and tends to come apart even from small water leaks, stick to plywood.

          2. Jon R | | #48

            > whole roof in ice and water guard is a bad idea.

            Agreed, it's not permeable. If you don't want to use a premium permeable and self-adhered underlayment under the shingles, then as Akos says, you can use real felt or permeable synthetic equivalents.

  16. Cody Brown | | #44

    What about nail base foam insulation for the top layer? It's what I'm planning to use when we do the roof on our house. I'm in the same boat at the moment with a whopping 2" of styrofoam insulation over the T&G.

    1. djsina2 | | #45

      Cody - That was my initial idea before I talked to any roofers. I've been through a couple roofers who were not interested in doing the job no matter what method it was. The roofer I have setup now is highly rated and not afraid of the job. When I mentioned this product for whatever reason he preferred installing each piece separately (insulation, furring strips, sheathing). Maybe just a personal preference so I wasn't going to push the issue of installing an all-in-one system.

  17. djsina2 | | #47

    Is installing black shingles a bad idea or does it not matter?

  18. djsina2 | | #49

    Is there a simple way to extend the roof a small amount (a couple of inches?) at the eaves? There is no overhang now, which causes water off the roof to wash over the windows. I'm wondering if there is a product which can be nailed to the deck to do this? It would need to be strong enough to not warp if the ice dam issues persist.

    1. Jon R | | #50

      Maybe there is something simpler for just a couple inches. But larger eaves are better.

  19. Keith Gustafson | | #51

    Since you would hopefully have plywood sheathing on the entire roof, I would think that if you are staying in overhang in the vicinity of the thickness of the foam, you are not going to have a problem. IOW extend the foam and plywood out off the roof. YOu could also treat your fascia as stuctural, Using a 2x instead of trim wood, to stiffen the edge.

    As you try to extend further or switch to OSB i think you start needing real support

    Oh, and no short lengths at the edge. Over 32"

    1. djsina2 | | #52

      Keith - That's a good idea. Since the perimeter of the house will be built up with 2x4's on the roof deck to match the height of the foam I could have another 2x4 attached to those as a structural fascia. I could even put a more finished 1" fascia board onto that which would give me the 2.25".

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