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Insulating an attic ceiling

Nathan_PA | Posted in Green Building Techniques on

Hello all,

As mentioned in a prior post, I am looking at getting a heat pump installed in my house. Although I have been trying to get the heat pump contractors to put the air handler/ducts inside the finished space (we have a centrally located closet we can sacrifice), the vendors we have had out so far are only interested in putting them in the attic. We have another heat pump vendor coming out this week, but assuming he is singing the same tune as the last guy, I am thinking about what it would take to insulate the attic rafters and gable walls so as to keep the hvac equipment inside the building envelope. Also, I could see us maybe wanting to finish the attic to add two bedrooms sometime in the future, so if we did that, we’d have to do this insulation at that point anyways. 

I live in eastern PA, right at the border of climate zones 4a and 5a.

From doing some reading on this site over the weekend, it seems like the basic options are:
1. Leave a vented channel from the soffit to ridge vent, then put fiberglass or cellulose insulation beneath that.
2. Spray foam the rafters and underside of the roof, with additional fiberglass/cellulose insulation underneath that.

I had an insulation contractor at my house Friday who offered to put fiberglass batts in the rafters, but he didn’t mention anything about a vented channel. Also, even if I did have a vented channel, with fiberglass underneath it, I have a hard time believing that would prevent air infiltration into the attic, without some type of membrane or other barrier at the bottom of the vented channel. Does that seem like a reasonable concern?

Regarding spray foam, I am turned off by the high gwp of cc foams, plus the general toxicity level of them. 

In this application, it sounds like I could use open cell foam, followed by a fire barrier, followed by additional fiberglass/cellulose insulation? Does that seem reasonable?

Are there other options that don’t involve spray foam but are easier to install correctly (for air infiltration prevention) than fiberglass batts? 

Could I just put polyiso or eps boards inbetween the rafters, or is the thought that you will never get that close enough to the underside of the roof to prevent there being a space for air/moisture to get trapped?

Could I do the boards between the rafters approach but leave a vented channel?

I have another insulation contractor coming tomorrow – any thoughts are appreciated.


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  1. user-2310254 | | #1


    How deep are your roof rafters? Is it a simple roof that would allow uninterrupted venting from the soffit to the peak in each bay? Is the attic tall enough to allow you to create more room for insulation if that was necessary?

  2. Nathan_PA | | #2

    Thanks for the response, Steve. The rafters are 2x6's, so 5.5" deep. 16" on centers. The attic is fairly simple, just one long room. There are 5' tall 2x4 studs every 16" rising to support a 2x4 cross brace going across the bottom of the rafters (eg framing for where a knee wall would go if I ever did finish the attic). A vent could go above that cross brace no problem. The chimney is in the middle of the attic. There is some blocking on the sides of the chimney that would block a channel there from getting to the ridge. That all being said, looking at it more closely, while I can obviously see a soffit vent at the edges, I don't see a vent at the ridge, just a 2x8 that the rafters attach to. So maybe the vented channel approach isn't an option. There are vented triangular panels in each of the end walls.

  3. Nathan_PA | | #3

    Additional picture

  4. Nathan_PA | | #4

    And another

  5. Nathan_PA | | #5

    Picture #4

  6. Nathan_PA | | #6

    Picture #5

  7. Nathan_PA | | #7

    Picture #6

  8. Nathan_PA | | #8

    Picture #7

  9. Patrick_OSullivan | | #9

    Your options are basically highlighted here:

    > In this application, it sounds like I could use open cell foam, followed by a fire barrier, followed by additional fiberglass/cellulose insulation? Does that seem reasonable?

    Open cell is likely not appropriate in your climate zone.

    > Regarding spray foam, I am turned off by the high gwp of cc foams, plus the general toxicity level of them.

    Some closed cell brands (e.g. Demilec Heatlok HFO) have switched to a blowing agent with a GWP of 1. Confirm this with your installer and have it on your contract. The guys doing my application were happy to have me jump in the truck and take a look at the drums.

    If you're going to do spray foam, make sure you're happy with the roof deck you have and that you have no plans to strip it.

    1. Nathan_PA | | #10

      Thanks Patrick

  10. 1910duplex | | #11

    I used Demilec Heatlok HFO, and rockwool batts under the foam and drywall on eave walls. Doing this is really not cheap, though, so you might want to see if you can find anyone willing to do your original idea. It did make a big difference in the temperature differential between second floor and attic. :)

  11. Expert Member
    Dana Dorsett | | #12

    It looks like there is enough headroom to install 4" of foam board on the underside of the rafters. With 2x6 rafters one could go with 3.5" of rock wool batts (R15) leaving 2" of vent space between the batts and roof deck. Then with 4" under the rafters the center-cavity R would be ~R40, but since the polyiso layer isn't thermally bridged by rafters it would just make code min (on a U-factor basis), and would outperform 5.5" of thermally bridged HFO blown closed cell foam (~R38) between the rafters by quite a bit.

    The code max found in TABLE N1102.1.2 is U-0.026:

    1/U0.026 = R38.5 "whole assembly" so...

    At 4" foil faced polyiso is good for about R24 (some would be R26)
    The 2x6/R15 layer is good for at least R12
    The vent channel and indoor air films add another R1
    The roof deck is good for another R1
    Half inch gypsum on the inside would be about R0.5
    Shingles + exterior air film adds another R0.3.

    So even for R6/inch polyiso it would be:

    R24 + R12 + R1 + R1 + R0.5+ R0.3= R39.3 (>R38.5)

    Throwing out the air film contributions it might be a hair shy, but they're legitimate to include in a U-factor calculation.

    Two layers of 2" polyiso, with the seams taped and staggered would be better than a single layer. The first polyiso layer can be cap-nailed in place and taped. It's sometimes easier to tack the second layer to the first with foam board construction adhesive then clamp it in place with horizontal 1x4 furring 16-24" o.c. through screwed to the rafters with pancake head timber screws (eg FastenMaster HeadLok or similar). With 4" foam and 1x furring the screws would need to be at least 6.25" long to penetrate the rafter by at least 1.5", strong enough to support the weight of the foam + ceiling gypsum. The half-inch gypsum (necessary for fire safety) can then be nailed or screwed to the furring. (You may be able to cheat that and go with 6" screws- most vendors skip from 6" to 8". )

    Retrofitting ridge venting isn't a big project, drilling holes on either side of the ridge board. That would draw reasonably amounts of air up from your soffit venting even if it's less than half as much cross sectional air as the soffit vents. Don't sweat the blocking around the chimney- foil faced foam board is a true vapor barrier, and those cavities can dry through the wood into the adjacent fully vented rafter bays.

    1. Nathan_PA | | #13


      Thanks for your response. Assuming I can get the ridge venting added, that sounds like a great way to go.

      If you don't mind me asking some more questions:
      1. For the vent baffles above the rockwool, I assume that I should go with something vapor permeable, so the rockwool can dry to the vented channel if it needs to. Is just some OSB good enough, or should it be a membrane like Intello plus or similar? Or a thin board of unfaced EPS?
      2. I thought I had mentioned this above, but on re-reading it looks like I forgot it - my roof is a slate roof.
      3. Regarding adding the ridge vents, I assume that the concept is to put a hole through the roof on each side of the ridge board, in each of the rafter bays? Then adding flashing and a rain cap to each of those holes? That sounds like a lot of holes to add to the roof, plus with my slate roof, a lot of slates to cut through. Any chance I can drill say 1.5" or so holes through each rafter near the ridge to connect the rafter bays, then vent at the sides and maybe one or two spots in the middle? Obviously I won't get nearly as much total air flow through the vented space that way. Not saying that it's not worth putting holes in each bay through the roof, just trying to understand exactly what I need to do.
      4. Does my slate roof itself buy me anything in terms of being able to dry to the exterior, and therefore not needing as much ridge venting? Of course, I have no clue what type of underlayment was applied between the sheathing and the slate 70 years ago when the house was built.
      5. From a fire protection standpoint, I assume that I will need to have a gap between the polyiso boards and the chimney, presumably blocked off with metal trim/fire-proof caulk/rockwool? The second insulation contractor I had out on Monday (who was only interested in quoting spray foam, and straight up declined to quote a vented assembly with 2x4's extending from the current rafters), claimed that he could spray foam right up to the chimney, since it was masonry, but that doesn't sound right to me. I assumed that I would have to do a detail around the chimney, similar to what is described in this article (,caulk%20to%20seal%20the%20joints.) Is that correct, or am I off base here in doubting the contractor?
      6. Since I have a lot of fairly good condition fiberglass batts in the attic that won't be needed if I insulate the ceiling, could I use those batts instead of the rockwool? My guess is that that is a bad idea from a mold (although it should be able to dry), rodent, and R-value perspective.


      1. Expert Member
        PETER G ENGLE PE | | #14

        1. Vapor permeable is better, but loose-fitting plastic vent channels work too (Like Accuvent)
        2. Slate roofs are good news / bad news. The good news is that they naturally ventilate through the gaps in each slate. The bad news is that they also leak through the gaps in each slate. They're also one of the most expensive roofs to work on. Your photos don't show much staining inside, so that's good news. The slates are in good shape and the underlayment (probably felt) is also well done. In your case, I am guessing that the increased drying outweighs the risk of leakage.
        3. Adding a ridge vent to a slate roof is not always easy. An alternate is to run rafter ties across the attic high up to form a small ceiling and triangular "attic." If the rafter vent channels all terminate in this attic and you have decent sized gable vents in the ends, you will probably be OK with your roof system, especially if you are attentive to good air sealing (taping) of the foam board seams and edges.
        4. Yes, see above. Pennsylvania slate is typically good for about 100 years, so yours might be aging a bit. If you know of any good slaters in your area, it would be wise to have a roof inspection and tune-up done as a part of this process.
        5. Yes, you need fireproof insulation around the chimney and spray foam isn't that. The JLC article you cite still seems pretty accurate. Sheet metal and fire caulk are still appropriate. You can then fill the gap with mineral wool.
        6. You can use fiberglass or rockwool nearly interchangeably.
        FWIW, you can often find used polyiso insulation locally on Craig's list for a fraction of the price of new. If you use 2 layers as Dana suggested, at least one of them should be foil-faced. Foil tape sticks well to the foil facings and together they make a nearly 100% moisture and air barrier.

        1. Nathan_PA | | #15

          Thanks Peter. That helps a lot.

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