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Exterior Roof Insulation Question (another one)

josephny | Posted in Expert Exchange Q&A on

I have a 100 year old 24’x24′ cottage in zone 4 that I have upgraded several times, including with closed-cell spray foam insulation in the attic on the underside of the roof deck, underneath the floor in the crawl space between house and dirt (house is on piers), and in the walls with little holes (which I know for a fact nowhere near fills the cavities 100%).
The cottage has a 18×24 addition that is mostly glass walls and a very tall cathedral ceiling.  The underneath side was sprayed with foam (same time as the main 24×24 portion), and the small amount of walls have a little bit of foam in them, but the ceiling/roof has no insulation at all.
It is super expensive to heat (heat pump with electric baseboard), uncomfortably cold, and terribly wasteful (un-green).
I would like to install exterior insulation on the roof.
I’ve read a bunch of articles, here and elsewhere, and understand that rigid board (ISO or XPS) is recommended over mineral wool for a pitched roof, and that because 100% of the insulation will be on the exterior, I don’t have to worry about the “ratio”-dew-point issue.
My question is how to proceed.
I see here:
that primitelamps put down ply, ice-water, 3″ iso, 3″ EPS, 2×4, ply, ice-water.
I can rip the shingles off and put down 6″ of foam board, 2×4, ply, ice-water, shingles.


1)  6″ of board will yield an R of approximately 30 (which, I know is less than 49).  Raising the roof even 6″ will be a little challenging.  Any more, quite difficult.  Any feeling for what I’d be sacrificing at 30 that I’d have at 40 (for example)?
2)  Why 2 layers of ice-water (first and last)?  I’ve seen so many warnings of not making unbreathable sections in a roof.  Should I just put ice-water on the top layer of ply?  And, even then, over the entire sheathing or just the edges?
3)  I believe this would be a non-vented roof assembly.  No problem with this, right?
Thank you!

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  1. AndyBower | | #1

    Note that the shingles themselves are effectively vapor impermeable, so you won't get significant drying through them anyways.

    1. josephny | | #3

      Sorry for the confusion but I don't understand the significance of the lack of drying through the shingles. These shingles would be the one and only top-most layer, right? So my concern is that if I put a full layer of ice-water on the sheathing right below this top-most layer of shingles, and another one between the original sheathing and the rigid board insulation, will I be creating an area that will trap moisture?


  2. gusfhb | | #2 mostly glass walls.....

    and will be uncomfortable no matter how much you insulate the roof

    Not saying you shouldn't insulate the roof

    Doing even a beer bag heatloss will show you how much a bunch of poor glass will affect the room

    1. josephny | | #4

      I googled "beer bag heatloss" and I only see references to brewing beer. I suspect you don't mean that the test is to see if I feel cold after drinking a bag full of home brewed beer (;-).

      Attached is a picture of the room -- with all its glass.

      So does it make sense (i.e., will it help noticeably) to insulate this roof with all this glass?

      Thank you.

      1. Expert Member
        DCcontrarian | | #11

        What way does it face? I bet you get significant solar gain. With south-facing windows and good glass, in most of North America windows "break even" -- they let in as much heat as they lose over the course of the day.

  3. gusfhb | | #5

    beer bag
    back of envelope
    rough estimation
    I see somewhere around 200 sq ft of glass that if it is at r2[basic thermopane not low e]

    Not at all saying don't insulate the roof, but with that much glass the room will not be comfortable in the cold and you may be disappointed

  4. maine_tyler | | #6

    If you're heating (or cooling) the room, it's worth insulating the ceiling.

    In your clinate zone (4) I wouldn't worry about going from R 30 ish to 49, especially given that you are talking continuous which gives a lower total assembly U value (higher effective R value).

    In zone 4, I would probably use all Polyiso-- the highest R per inch. 6 inches is a bit more than R 30.

    The lower ice and water membrane is just to establish an airtight layer over the existing roof plane. This is very important. Ice and water is not the only way, and note that it may smell on the inside. But you want to tighten the shell at this layer and connect it with whatever component of your walls is the main air barrier as best as possible.

    I see no reason to use all ice and water on the top layer of sheathing. Typical detail around here is a strip or two along the eaves then a roofing underlayment. This is mostly for ice dam 'symptom' treatment. The foam overroof goes a long way towards ice dam 'cause' treatment.

    If you are laying 2x4 strapping over your 6" of foam, this typically means you are providing a 1.5" vent space, so there is no double vapor barrier going on. If you don't provide a vent space and fill in between the strapping with foam (which is more work), then you may have a double vapor barrier, which is not necessarily a problem but for some builders does represent a less than ideal stack up.

  5. Expert Member
    Akos | | #7

    We can put some numbers to this. Assuming that 18x24 addition is about 9' ceiling height and low-e glass.

    You would have about 18*2*24*9'=500 sqft of glass and about 18x24x1.3(guessing at the slope here)=560 sqft of roof.

    Assuming an outdoor design temp of 20F and indoor at 70F you losses are:

    50f*500sqft/R4=6200btu through R4 windows, 2x that if clear glass

    50*560sqft/R3=9300btu through R3 uninsulated roof.

    Bump up the roof to R30, the roof loss would be


    So that is about 3/4 ton less heat loss, so definitely something but also not a whole lot better overall. The room will still be cold and expensive to heat.

    1. maine_tyler | | #8

      "not a whole lot better overall."

      I'm not sure I'd say that based on your numbers. It's cutting losses by more than half. Yes it's still not 'efficient' but it's 'more efficient' by a good bit.

      1. Expert Member
        DCcontrarian | | #10

        Yeah, I get 15,500 before and 7130 after. That's a 54% reduction. You could increase the interior temperature to 75F and still save over half on your heating bill.

        1. Expert Member
          Akos | | #12

          There is two things here. Does it make much difference in heating cost and in comfort.

          Heating cost is a bit of math. Assuming 4000HDD the delta is 179therm VS 17.9therms per year in heat loss through the roof. So that is 5200kWh vs 520kWh, at my hydro cost of $.16/kWh that is $830 Vs $83. That is definitely something but adding the rigid is probably a 10-20k job so ROI is pretty long, heating with heat pump cuts that cost by about 1/3 so ROI would be never.

          The comfort one is easier to answer. The comfort issue is the cold air downdraft from the windows, insulating the roof won't change that one bit. The only way to fix it is to get better windows or add heat right under each window.

          1. maine_tyler | | #15

            What I'm hearing you say is that the only reason to insulate a ceiling in zone 4 (at least if overfoam is the only option) is for comfort?

    2. Expert Member
      DCcontrarian | | #9

      Note that the math for the roof is completely independent of the math for the walls. Regardless of the makeup of the walls, insulating the roof saves 3/4 ton of heating. The only question is whether that savings is justified for the cost.

  6. josephny | | #13

    OMG! What an eye opener and education! Thank you all so much!

    Each double window is 36 sq-ft. We have 6.5 double windows for 234 sq-ft. Door is 18 sq-ft. Fixed glass is 48 sq-ft

    Total (clear) glass = ~300 sq-ft

    Roof peak is 16'. Roof at walls is 11'. 24' wide room, drop of 5' over 12' of run. Roof (excluding the eaves overhang) is 500 sqft.

    My 99% heating temp (which I think is the design temp) is 3*F, for a delta of 67*



    Total: 21,260 btu

    With R30 roof (1116btu), total is 11,166

    As to the benefit: Cost should be cut in about half, which is great, and would have an ROI of about 4 years.

    But, what I hadn't mentioned (but you experts clearly already knew) is that my floors are always (uncomfortably) cold. I've been slathering (okay, spraying) on the foam from underneath and have been confused about what I'm missing for years. I wonder if the reason the floors are cold is because of the cold-air-downdrafts from all the clear glass?

    The guy who sold me one high wall mounted mini-split head in the corner of this room really either didn't know or didn't care.

    And now I understand why the recommended practice is to put heaters under windows!

    It sounds like lining the room with baseboard radiators (as opposed to panels) would be more effective.

    I was thinking of installing an LP combi boiler for the whole cottage, having had such bad experience with heat pumps (in my 3* design temp location).

    Thank you all so much. I am in awe and grateful.

  7. gusfhb | | #14

    cold floors:
    air leaks
    lots of glass.
    While an uninsulated roof costs you more to heat, 'bad glass' creates rivers of cold air that make you feel uncomfortable.
    Don't give up on the mini split, once you finish your improvements, it may be just fine.

  8. josephny | | #16

    How does this look for a plan?

    1. Expert Member
      Akos | | #17

      That is a good plan. Careful with the lower ice and water, you generally don't want regular I&W with modified bitumen adhesive near the interior of the house as it can smell, I would use something with either acrylic or butyl adhesive.

      3/4 ply for a roof is a bit overkill but won't hurt.

      This will solve some of the energy efficiency issues but won't do much for comfort. It also sounds like the extension is on piers which makes this much more challenging. To get reasonable comfort with pier foundation you need a layer of continuous rigid under the floor joist and air seal as much as possible. If over a crawl/foundation, you really need to encapsulate and insulate the foundation walls and again air seal especially at the rim joists.

      There is nothing wrong with a wall mount in the place, you can use it to deliver the bulk of the the space needs for much lower cost than resistance heaters.

      The issue is, as it sits, the structure has too many problems to get decent comfort with a point heat source, so you need to supplement with either baseboards or floor heat around the perimeter. I have a cottage in much colder climate that is heated with a single wall mount without issues but it still doesn't compete with the heat and comfort from the wood stove.

  9. josephny | | #18

    Uh oh!

    I just ripped off the shingles and discovered that the shingles were nailed to 1/2" plywood.

    The 1/2" ply is nailed to 1x4 spacers (running perpendicular to the roof rafters that are visible from inside my house).

    Those 1x4 spacers are nailed to the top side of the tongue and groove pine board I see between the rafters from inside the house.

    That's a long way of saying the roof is vented, and the vented area is immediately above the 3/4" (could be 1") tongue and groove ceiling I see from inside.

    I believe I need to get rid of that vent, but I'm not sure how.

    Do I rip off the 1/2" and the 1x4 spacers and put a layer of plywood directly onto the pine ceiling (from the top)? I would be concerned about nails going right through and being visible from the inside, unless I was somewhere skilled enough to only nail into the pine exactly above the rafters (doubtful).

    I thought about maybe just gluing the polyiso to the ping ceiling but then the top layer of plywood (just under where the new shingles will be) will not have anything to anchor to.

    What should I do?

    Thank you!

  10. Expert Member
    DCcontrarian | | #19

    What about taking up the 1/2" plywood, leaving the 1x4's and cutting strips of 3/4" polyiso to go between the 1x4's? Then do your thick polyiso and sheathing with the 1x4's as nailers?

    You may even be able to reuse some of the plywood.

  11. josephny | | #20

    I’m stuck on how to secure the 2x4 that go on top of the polyiso and under the shingle sheathing.

    1. Expert Member
      DCcontrarian | | #21

      What was your plan before you found the 1x4's ? I don't see how you do it except with 8" screws.

  12. josephny | | #22

    I'm sorry I'm not explaining myself clearly. And I appreciate your help.

    I did not expect to find the vented area. That is, I expected to remove the shingles and find plywood attached directly to the Pine T&G.

    I can remove the 1/2" ply and 1x4 spacers (I'm sure there's a better term).

    The question then is: Do I put down a layer of 3/4" ply attached directly (flatly) to the Pine T&G?

    Or, do I put down a layer a water and ice across the top of the Pine T&G, and then the 6" of polyiso?

    Either way, I am faced with the question of how best to attach the assembly.

    That is, if I attach ply to the to T&G, so I just use 1-1/4" nails or screws and hope none of them go through to the inside of the house? Or, do I try to use 3" screws through the ply and T&G and into the top edge of the rafters?

    Then there's the question of how to attach the 2x4 that sit on top of the 6" of polyiso: Do I use 8" screws and sink them 1/2" below the face of the 2x4 (that would mean: 1" of 2x4 + 6" polyiso + 1" of either new ply or existing T&G), or do I use 10" screws and try to hit the rafters?

    Here's the diagram with the question of the lowest layer:

    1. Expert Member
      Akos | | #23

      There is no problems with the strapping under the plywood, what you want is to detail the plywood as your air barrier (peel and stick or tape seams) and connect it to your wall top plates for air barrier continuity. The simplest would be to rip up a strip of the plywood over your wall plates and seal it with canned foam.

  13. josephny | | #24

    I spent last week ripping off the roof (which was in pristine condition -- ugh) and discovered (perhaps, was reminded of the fact that) the T&G pine that I see from inside the room was fully exposed to the outside air. That is, the T&G had 1x4 strapping 24" OC with 1/2" ply on top of the strapping (as sheathing for the roofing). So, my entire roof assembly was about an R-1. Can't imagine why I've been cold for the past number of Winters in my zone 4 cottage (;-).

    I proceeded to put down a 2" layer of foil faced polyiso followed by a 4" layer of the same, using PL300 glue between the pine and between each layer of polyiso, with the foil faced sides up against each other (my thinking was that it probably didn't matter, but might as well make only a single full vapor barrier).

    I then screwed 2x4 from ridge to eave, flat on the polyiso using 10" structural screws going into the rafters beneath the pine.

    Then 3/4" ply, drip edge, ice-water and synthetic felt and IKO shingles.

    Surrounded the assembly with PVC molding as flashing.

    Our nights now are in the upper 40's and the room is so nice and toasty -- and the floors not cold! And that's with just about 1500 watts of electric heat (for 6000 cubic feet of indoor space).

    Thank you all very much for your help!

    If anyone would like to see pics, here's a link:

    1. paulmagnuscalabro | | #25

      Did you do anything special to up the odds of hitting rafters through 6" of insulation each time (and avoid going through the exposed T&G ceiling)? Or just careful layout?

  14. josephny | | #26

    It certainly was not easy, and I missed a bunch of times, but I followed the rafters out and up and snapped chalk lines and said a little prayer. The length (1.5” 2x4 + 6” insulation + 3/4” pine) meant that even if I started dead center, I’d the screw didn’t go in perfectly perpendicular (that is, if it went in even at a small angle left or right), the tip would like through.

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