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Community and Q&A

Insulating a four-season roof/ceiling.

matt2021 | Posted in General Questions on

We are planning to turn our screened, roof-covered, screened porch (approx. 22’ x 14’) into a four-season room.  I would love to hear people’s thoughts on our plans for the different areas of the project.  Yet, here, I will address only my doubts about the insulation of the ceiling.

We are in zone 5.

ANY feedback, even if not to all of my questions, would be very much appreciated!

The existing ceiling has exposed beams. Above the beams, there are wooden boards, then a gap (where some electrical wires, for the porch’s/future room’s lights, run) plywood, ice-and-rain shield for the entire surface of the roof, and then asphalt shingles.  The beams give us a depth of 5 1/2″, which we are thinking of filling with spray foam insulation.  We are also planning to apply, in addition to the spray foam, foam board panels (polyiso), which will be screwed to the beams; 1″ foam panels would be a minimum, but I am hoping we can go for 2 inches of polyiso.  (That depends on whether we have enough height, as the roof is slanted––very high on the house side, and then sloping down; as the floor will have to be raised, as per another question I had posted days ago, I fear there might be not enough height for adding a 2″ polyiso board.)  As a ceiling, we would like to go for beaded board, rather than drywall––we like the look and, also, hope that the installation would be simpler.  

I am thinking of three possible options, though I don’t know whether the third is even admissible.

1) 5 1/2″ of OPEN-CELL spray foam + 2″ faced polyiso, taped + furring running perpendicular to the beams + beaded board.  R value should be 30.9, right?

Is it OK to use polyiso boards inside (healthwise I mean)?

Should a vapor barrier be added to this assembly and, if so, would it make sense to place it between the sprayfoam layer and the polyiso board?  (I suspect I do not need an additional vapor barrier, but I might be wrong about that.)

2) Same as above, but with CLOSED-CELL spray foam.  The total R value would be, I believe 48.5.

In this case, I am sure the spray foam would constitute a vapor barrier.

I know that, in zone five, ideally roofs should have a minimum of R-49; so, this solution would be great, if more expensive (and, also, MUCH LESS environmentally friendly).  I do worry, however, on:

(a) it making ANY sense to go past 3 1/2″ of closed-cell spray foam, as I understand there is a maximum thickness for it;

(b) any health concerns with having closed-spray foam inside, possibly releasing gases within the living space;

(c) the possibility that (despite the ice-and-water shield) a roof leak could be produced, and go undetected for a long time because of the closed-cell foam.

3) Is it possible to use Zip-System Insulated R Sheathing INSIDE the house?  If that were to be the case, then I could skip the furring, and just install the beaded board on the sheathing.  Of course, if such a use of the Zip-System sheathing is allowed (if, for example, there are NO health concerns related to using it inside), it could be applied to the open-cell or the closed-cell option.

Any thoughts on pros and cons on the above options?  Thanks!

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  1. Expert Member
    NICK KEENAN | | #1

    The concern with insulating roofs is either allowing moisture to dry through ventilation, or keeping it out in the first place. It sounds like the gap between the existing ceiling and sheathing allows for ventilation, in which case any insulation is safe. One thing I would caution about is making sure your insulation is continuously thick, cold spots can allow condensation which can lead to mold and rot.

    As for the safety of foam indoors, it is allowed. Some people feel there are dangers from outgassing. I use spray foam and board foam on the inside of my homes.

    1. matt2021 | | #3


      If I were to go for spray foam, is it worth going for closed-cell, you think, or open-cell would be OK? Open-cell seems to have quite a few advantages, but it does have a lower R value.

    2. matt2021 | | #5

      One more thought, in response to what you are saying about cold spots: that seems an argument in favor of spray foam insulation vs. using other forms of insulation (in case mistakes are made during the installation); isn't that so?

  2. user-723121 | | #2

    Somehow the new ceiling will have to be attached to the existing. Describe in more detail the roof boards, air gap and plywood above. Are the roof boards below the plywood thick enough to attach the new insulation and wood ceiling to without going through the plywood and shingles. If this is the case I would forget the spray foam and attach layers (attached with plastic cap nails) of Polyiso with the last 1 1/2" having firing strips embedded in the foam attached with the right length screws to the original board ceiling. You then will have solid backing to install any ceiling finish you want. One of the first layers of Polyiso can be taped with foil tape to form an airtight ceiling.

    1. matt2021 | | #4

      Thanks, Doug!

      Starting from the outside, there are the asphalt shingles; the ice-and -rain shield; a 3/8" (I think) plywood; a 1 1/5" gap (created by 2x4's installed flat; the ceiling's wooden boards, which I am pretty sure are thicker than the plywood (I believe they might be 3/4" solid boards). The beams, which run perpendicular to the house of course, as I mentioned, are 5 1/5" deep.

      Why would you skip the spray foam altogether? Is it also that so save on costs?

      Are you suggesting stopping at the 5 1/2" thickness with the solid polyiso boards? I was thinking of going past the depth of the beams, so as to have (a) more insulation and (b) a foam layer to interrupt any thermal bridging, which otherwise would occur through the wooden beams. Or are you suggesting that, as in my original plan, I do add a board of polyiso, screwed/nailed into the wooden beams? In that case, wouldn't I need strapping/furring whether I need spray foam or not?

      I am attaching a couple of sketches, to better illustrate what Iw as thinking of doing. (In the sketch below, the darker yellow area indicates the solid polyiso board I was thinking of installing, attaching it to the wooden beams.

    2. matt2021 | | #7

      Sorry, Doug, I think I have another question for you: is there an easy way to explain how one embeds the furring strips into the foam? I thought that such furring strips had to sit flat on the board, and be screwed into the wooden beams, with the screws going through the 1"-2" foam board.

      1. user-723121 | | #11

        When I said embed I just meant fastening some furring strips to the last (warm side) layer of Polyiso to the original ceiling boards with some long screws being careful not to go through the roofing layers. This (furring) gives you a solid nailing area for the finished wood ceiling if that is your choice. I have used this method several times and it gives an airtight assembly with high R-value in a minimal thickness. You certainly could cover the beams with Polyiso as well to reduce thermal bridging there.

        1. matt2021 | | #12

          Thanks, Dough, for the explanation! I’m reassured that what I was thinking of doing - fastening the furring boards to the wooden beams, going through the layer of foam, is the way to h to go.

          So, really if it were you, you would not bother having a spray foam company apply closed-cell foam; you would just go for polyiso boards. Do you think it would be cheaper, or is there some other reason? One thing I thought of is that closed cell phone will never be completely flash with the wooden beams (it’s not like open cell phone, which can then be cut and can form a nicely flat surface to which to attach the polyiso board. Unfortunately, I fear that open cell is really not an option for me, as I need to reach that R38 value DCContrarian mentions below.

        2. matt2021 | | #13

          Sorry, Doug, another question - hoping that you don't mind - still in regards to using spray foam vs. using foam boards in the space between those rafters: I'm going to have 8-12 recessed lights; what is harder, you think: to apply spray foam around those fixtures, or placing rigid board insulation around them? Somehow, I think that the right foam insulation would make for an easier install. (Now I am also thinking that perhaps I should attach a 2" polyiso board to the existing ceiling, which would leave - I believe - precisely the depth needed to install the recessed light canisters, as those seem to be 5 1/2" deep; even if I were to go for closed-cell spray foam, I could have a 2" rigid foam board attached to the existing ceiling, 3 1/2" of spray foam, and a 2" rigid foam board that closes the whole ceiling.)

          1. Expert Member
            NICK KEENAN | | #14

            I would rethink the recessed lights. They make really good flush mount LED lights now that give the same effect, but you don't have to make a huge hole in your insulation. Another advantage of the furring strips is that it gives you room to run wiring and with some flush lights all the room you need to mount them.

          2. user-723121 | | #16

            I do not see the need for spray foam., all Polyiso would be much neater and cleaner. I like the flush mount LED suggestion. The best buy I could find in Polyiso was R-Max at Home Depot, I used the 1" thickness but they also show it in 2".


        3. matt2021 | | #17

          Doug, my apologies for being slow at thanking you for the advise on rigid foam vs spray foam, and adding the link to Home Depot. I am taking note of everything. I like the idea of not using spray foam for the ceiling.

          I am still wondering about using spray foam in the knee wall, the walls above the windows, and the floor. Yet, I confess that I am not sure.

  3. Expert Member
    NICK KEENAN | | #6

    Code for a ceiling is R38, which represents the thickness of a 2x10 filled with fluffy insulation. The most cost-effective thing would be to drop the ceiling another 3.75 inches to get a 9.25 inch gap, and fill that with fluffy insulation. Depending on the circumstances that could be batts -- fiberglass, rock wool, even sheep's wool -- or blown-in: fiberglass, cellulose, rock wool. Or even a combination.

    The only reason to use foam is if you can't afford the loss of 4" of ceiling height. Closed cell spray and polyiso both run about R6.5 per inch, compared to R4 per inch for anything fluffy or open cell. If that's the case, I would look into insulating right up to the bottom of the sheathing, and sealing up that air gap. You'd have to cut holes in the existing ceiling and spray foam into the gap. I count 7.5 inches between the sheathing and the bottom of the existing rafters. If you did 3.5 inches of spray foam and 4 inches of fluff that would give you a nominal R38.75.

    If you're going to eliminate the gap, you have to put closed cell spray foam against the bottom of the sheathing.

    I guess another alternative would be to fill the existing rafters with fluffy stuff, then put 2.5 inches of polyiso over everything, and leave the air gap. In order to attach beadboard you'd probably have to run 1x furring strips, attached to the rafters with 4" drywall screws. This would give R38.5 but would only save a half inch over just dropping the ceiling. It would also be substantially more expensive.

    1. matt2021 | | #8

      Thanks, DCContrarian! All things considered, I think that the following assembly would be best:

      - 5 1/2" of closed cell or polyiso, against the existing ceiling. That's a nominal R-35.75
      - 2" polyiso board, giving an additional R-13, hence a total of R-48-49

      Alternatively, a 1" polyiso board would still bring me above the R-38 mark.

      The "problem" with these assemblies might be the extra labor and cost of furring. That's why I was wondering about using the Zip-System R-Sheathing instead of the polyiso. It would be more expensive, but would give me a wooden surface to which to attach my ceiling. Does anyone ever use the Zip-System R-Sheathing INSIDE? (I fear I might be saying something absurd.)

      1. Expert Member
        NICK KEENAN | | #9

        Zip-R could be used indoors, but it's expensive. I don't see any advantage compared to just running 1x3 strips perpendicular to the joists over the foam board. It would be less work because the Zip-R boards are somewhat heavy.

        1. matt2021 | | #10

          Thanks, DCContrarian! That makes a lot of sense. Given what you say about the weight, the job would be harder. And I’m looking for ways to simplify, not complicate.

        2. matt2021 | | #15

          Thanks for the additional advice, above, in regards to the recessed lights, DCContrarian! (I am not sure why the system doesn't give me the ability to respond to that specific comment.)

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