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Correct way to insulate an attic with HVAC?

bwsct | Posted in General Questions on

Hi,
I’m a new homeowner looking for advice.

I bought an older home with HVAC supply ducts in the attic but no insulation present.  The only venting are 2 gable walls vents.

I live in climate zone 5.

What is the correct way to insulate an attic of this type?
Should the vents be sealed?

Any advice is appreciated.

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bwsct,
    First of all, can you tell us your name? (I'm Martin.)

    Here is a link to an article that should answer your questions: "Creating a Conditioned Attic."

  2. bwsct | | #2

    Hi Martin,
    I thought I had typed my name at the end which is Brad.
    Thanks for the reply.

    I've read the article and see you mention you don't recommend closed cell spray foam for a conditioned attic. What would you recommend in my case?

    I will also need to replace the roof once spring or summer comes.

    Thanks
    Brad

    1. GBA Editor
      Martin Holladay | | #3

      Brad,
      Read the linked article again. You got it backwards. Here's what I wrote:

      "Recent research suggests that open-cell spray foam may be risky in all climate zones, so the safest spray foam insulation to use is closed-cell spray foam. For more information, see 'Open-Cell Spray Foam and Damp Roof Sheathing.'"

      Even better: Install rigid foam on the exterior side of your roof sheathing, or (assuming your attic has the right roof configuration for a vented approach) create a vented cathedral ceiling above your attic. These approaches are explained in my article.

      1. bwsct | | #4

        Martin,
        This is what I read in the article and why I thought you didn't recommend closed cell in the rafters.

        "If you insulate between the rafters
        Although I don’t recommend it, it is possible to install all of your sloped-roof insulation between the rafters."

        I'm currently going through a kitchen remodel and the kitchen has been gutted. I planned to have closed cell done in that area but needed to choose another location to get to the insulators minimum day charge which is why I was considering the attic.

        I'll have to read closer to see if the rigid foam would be a good option for my roof type.

        Brad

        1. GBA Editor
          Martin Holladay | | #5

          Brad,
          I apologize if my writing was unclear. My intended meaning: If you are going to create an unvented roof assembly, installing rigid foam above the roof sheathing is better than filling the rafters with spray foam.

          But if you end up using spray foam, remember that closed-cell spray foam is less risky than open-cell spray foam.

  3. bwsct | | #6

    Martin,
    Does anyone ever used closed cell on the rafters and rigid foam above the sheathing?

    How would that look?

    Shingles, rigid foam, plywood, closed cell foam?

    Thanks

    1. GBA Editor
      Martin Holladay | | #7

      Brad,
      If you plan to install rigid foam above the sheathing, make sure that the rigid foam meets minimum R-value requirements for your climate zone (so it keeps the sheathing above the dew point for most of the winter).

      If you want to install additional insulation between the rafters, the best insulation choices are vapor-permeable (cellulose, mineral wool, fiberglass, or open-cell spray foam), so that the sheathing can dry to the interior if it ever gets wet.

      For more information on this issue, see "Combining Exterior Rigid Foam With Fluffy Insulation."

    2. Expert Member
      Dana Dorsett | | #8

      That's done, but it's not always the safest or greenest approach. As long at the rigid foam is sufficient for dew point control at the roof deck for the amount of insulation between the rafters, it's safer/better/greener to use fiber insulation under the roof deck, rigid foam above the roof deck.

      In zone 5 that would be R20 minimum rigid foam out of R49 total, (40% +) with painted gypsum board on the interior as the vapor retarder. That could be done with 4" of roofing polyiso (R22-R23) and R30 rock wool between 2x8 rafters with a gypsum board ceiling with at least a primer coat on it on the interior. There are other stackups that work.

      Used roofing polyiso is cheap and ubiquitous and well worth considering here. Running this search on your local craigslist might turn up a more local supplier, but there is at least one reclaimer that will ship almost anywhere in the continental US:

      https://boston.craigslist.org/search/sss?query=rigid+insulation

      https://buffalo.craigslist.org/search/sss?query=rigid+insulation

      https://cleveland.craigslist.org/search/sss?query=rigid+insulation

      https://lansing.craigslist.org/search/sss?query=rigid+insulation

      https://chicago.craigslist.org/search/sss?query=rigid+insulation

      https://quadcities.craigslist.org/search/sss?query=rigid+insulation

      https://desmoines.craigslist.org/search/sss?query=rigid+insulation

      etc etc etc

  4. bwsct | | #9

    Thanks Martin and Dana

    Its all quite confusing which is the best route to go for my scenario.

  5. Expert Member
    Dana Dorsett | | #10

    How deep are your rafters?

    How many hips, valleys and dormers are there on your roof?

    What are the first three digits of your ZIP code?

    1. bwsct | | #12

      HI Dana,
      Did you have additional information to add for my situation?

      Thanks
      Brad

  6. bwsct | | #11

    Rafters are 6" deep.

    There are no hips, valleys or dormer.

    Zip 066

    1. GBA Editor
      Martin Holladay | | #13

      Brad,
      If your roof has no hips, valleys, or dormers, it's a good candidate for the vented approach. More details are available in these three articles:

      "How to Build an Insulated Cathedral Ceiling"

      "Creating a Conditioned Attic"

      "Site-Built Ventilation Baffles for Roofs"

    2. Expert Member
      Dana Dorsett | | #15

      Bridgeport CT is on the warm edge of US climate zone 5A. The IRC prescriptive is for R20 on the exterior foam of an R49 total R-value, or about 41% of the total R, but you can safely cheat that a bit. Across the water on Long Island it only needs to be R15 out of R49.

      Are the rafters a true 6" deep, or are they 5.5" deep x 1.5" wide milled 2x6?

      Assuming it's milled 2x6...

      With 4" of reclaimed roofing polyiso (~R23) and R23 rock wool between the rafters you'd be just shy of code-min on an R-value basis at "only" R26, but due to the exterior foam thermally breaking the rafters it would meet code on a U-factor basis, which only has to duck under U0.026, which is R38.5 "whole assembly" with all thermal bridging and the R-values of the non-insulation layers factored in, including the roof deck, nailer deck, roofing shingles, and even the interior & exterior air films.

      With a simple no-hips no-valleys no-dormers type roof it's pretty simple to do foam-overs, with 1/2" or 5/8" OSB nailer deck through-screwed to the structural roof deck. There are multple vendors of used roofing foam operating in southern New England, including these folks in Plainville selling pallets with 46 sheets of 2" polyiso for $750 (I've seen it both cheaper & more expensive than that):

      https://hartford.craigslist.org/mad/d/plainville-iso-rigid-foam-insulation/6816865813.html

      But it's not hard to find others:

      https://hartford.craigslist.org/search/sss?query=rigid+insulation

      To lower the vapor permeance to the interior and to keep the batts in place it's pretty cheap and easy to staple PERFORATED aluminized fabric type radiant barrier on the underside of the rafters, which adds another ~R1 of thermal performance. It comes in 4' wide rolls which works fine with either 16" or 24" on center rafters. Don't use bubble-pack type RB, or UN-perforated RB in this application, since that would create a moisture trap. Most perforated RB runs about 5 perms, which is comparable to a layer of latex primer on half-inch wallboard, which is an alternative (but more expensive and labor intensive) way to go.

      With 4" of foam and 5/8" of nailer deck a 1x6 facia board at the roof edges would be enough to protect the edges of the foam and provide reasonably cosmetic treatment. Sometimes wide drip edge stock used for commercial roofing can do double-duty as the end coverage too. Your house, your call.

      Re-roofing is a once in a generation opportunity moment for doing this sort of thing, but sealing up the attic is very reliable way to keep duct leaks from robbing efficiency, and using reclaimed roofing polyiso may be the most economic way to hit current code minimums for roof/attic performance. Expensive closed cell foam between the rafters takes 7-8" at $8-$10 per square foot. The 4" of exterior foam costs about $1.10 per square foot, the OSB nailer and 6" pancake head timber screws for mounting it adds another 75 cents per square foot, and even at box store pricing the R23 rock wool + RB is about $1.25 per square foot. Materials-wise it's less than half the cost of R49 closed cell foam, and most of it is DIY-able.

      1. bwsct | | #21

        Dana,
        Since I'm in a time crunch to add insulation on the roof deck mainly because we are also undergoing a kitchen renovation and my wife will kill me if we don't get a kitchen soon.

        I had scheduled for closed cell to be sprayed on the roof deck and gable walls.
        Can you tell me why that isn't recommended if I wanted to add polyiso later when the roof is redone? I'd like to get to R49 and my rafters are about 5.5 inches and 2 inches wide.

  7. bwsct | | #14

    HI Marin,
    With no hips, valleys or dormers and currently 2 gable vents but no other venting which is the best approach? I was heavily leaning towards closed cell since ducts are present.

    1. GBA Editor
      Martin Holladay | | #16

      Brad,
      Your ducts don't care how you insulate your roofline. If you install R-38 insulation (for example), it will perform as R-38 insulation, whether it's spray foam or cellulose.

      It's fairly easy to add vents to a soffit or ridge. If you can't do it, you should hire a contractor to help you.

      1. bwsct | | #17

        Martin,
        If you owned my house how would you insulate the attic?

        1. GBA Editor
          Martin Holladay | | #18

          Brad,
          I don't know enough about your house to answer. Questions that affect the answer:
          Is the roofing old and in need of replacement?
          Are there any intersecting roofs which complicate the installation of exterior rigid foam?
          Will you be doing the work yourself or hiring a contractor?
          Do you have soffits that lend themselves to being replaced with vented soffit material?
          Are you an environmentalist who cares about the environmental impact of the materials you install?
          What is your R-value goal?
          What is your budget?

          I'm not asking you to answer these questions here -- just explaining all the factors that affect a decision.

          Once again, I urge you to read the articles I linked to.

          1. Expert Member
            Dana Dorsett | | #19

            >"Is the roofing old and in need of replacement?"

            He was explicit in response #2:

            "I will also need to replace the roof once spring or summer comes."

  8. bwsct | | #20

    Martin,
    Here are some answer.
    Is the roofing old and in need of replacement? Yes
    Are there any intersecting roofs which complicate the installation of exterior rigid foam? No
    Will you be doing the work yourself or hiring a contractor? Hiring a contractor
    Do you have soffits that lend themselves to being replaced with vented soffit material? Yes
    Are you an environmentalist who cares about the environmental impact of the materials you install? Not really.
    What is your R-value goal? I'd like to get to R-49
    What is your budget? Less than 10k

    I need to make a decison within the week because I'm also undergoing a kitchen remodel which is being held up by my attic/kitchen insulation.

    I've read the articles but they often leave me with many more questions.

  9. Expert Member
    Dana Dorsett | | #22

    >"What is your R-value goal? I'd like to get to R-49
    What is your budget? Less than 10k"

    Is that less than 10K for the insulation, or for the roofing + insulation combined?

    How many square feet of roof/attic?

  10. bwsct | | #23

    Less than 10k for insulation. 912 sq ft roof deck and gable walls.
    Roofing will be a separate budget.
    I have quotes of less than 4k for closed cell or open cell. I had the insulation companies give me both options.

    My goal is to have it done correctly and I find most insulation companies and contractors don't know the best way just the way they know.

  11. Expert Member
    Dana Dorsett | | #24

    With 7.25" deep 2x8 rafters 3.5-4" of closed cell foam applied directly to the underside of the roof deck with 3.5" of rock wool on the interior side of the foam will come in WELL under $10K for 912' of roof deck. That will come in around R38-ish and would meet IRC 2009 levels for zones 4 & 5.

    HFC blown closed cell foam should not be installed in more than 2" depth per pass, with a cooling/curing period between passes. The HFC blown foam can catch fire during curing if installed too thickly, and can have shrinkage/adherence defects once cured. HFO blown foam is 30-40% more expensive but 4" or more in one pass is not a problem, and it's higher R/inch. Most HFC blown foam is R24-R25 @ 4", HFO blown foam runs R27-R28- not a huge performance difference, but some.

    For the gable ends an inch of closed cell foam and compressed batts in the 2x4 stud bays would be enough. Sealing all air leaks is the most important aspect.

    With more than half the total R being foam exterior to the fiber insulation the interior side air barrier is less critical for avoiding wintertime moisture condensation at the foam/fiber boundary, but stapling a perforated aluminized fabric type radiant barrier to the rafter edges to keep the batts in place would lower that risk without creating a moisture trap while adding a modest amount of thermal performance. If you go with the radiant barrier it's important to specify PERFORATED, since most other radiant barriers are also true vapor barriers, which would create a moisture trap.

    It's possible/likely that it will take more than one contractor (or some DIY sweat-equity) to get it done that way.

    Open cell foam on it's own isn't sufficiently vapor retardent to fully protect the roof deck.

    Most closed cell foam would need an interior side thermal barrier against ignition or intumescent paint to meet fire codes. While 3.5" of rock wool + aluminized radiant barrier is not a recognized timed thermal barrier assembly, it's pretty fireproof, and would likely get a pass by the inspectors. Otherwise you might be required to install half-inch gypsum board on the interior. It's worth running the plan by the local building department before committing to any plan.

  12. bwsct | | #25

    Dana,
    Thanks for detailed explanation.

    I actually have 6" deep rafters or possibly 5.5. The insulation quotes I received were to apply 3" closed cell plus intumescent paint.

    Would you still insulate with the rock wool at that depth also?
    What could I do additionally to get to R49 when I'm ready to redo the roof?

    Thanks again.

    1. Expert Member
      Dana Dorsett | | #26

      >"Would you still insulate with the rock wool at that depth also?"

      Yes, the additional depth of rock wool would still be "worth it", and not very expensive compared to 3" of closed cell foam.

      >"What could I do additionally to get to R49 when I'm ready to redo the roof?"

      At 3" the HFC blown stuff is good for R18-R19, the HFO blown stuff about R20-R21, but the thermally bridging path through 3" of wood is about R3.5. By filling in the additional depth with R4.3/inch rock wool the losses through the rafters gets cut in half, and the losses at center cavity drop by more than a third.

      Assuming R18 for the foam, R15 for the rock wool you'd be around R33 at center cavity. To bring that up to R49 when re-roofing you could install 3" of reclaimed roofing polyiso (about R16-R17) under a half-inch or 5/8" nailer deck onto which the new roofing felt & shingles are installed. At the high end used 3" roofing foam runs $25/sheet for a 4' x 8' sheet, often available for as low as $15/sheet. You're looking at 25-30 sheets of foam- call it $750 at the high end, and a similar amount of OSB, for another $450, plus about $200-250 for the 4.5" pancake head timber screws for securing the nailer to the structural roof deck. Installing a fully adhered self healing membrane on the roof deck under the foam which will run another $750 in material cost for a roof your size.

      Add it all up and it's another ~$2500-3000 or so in material cost adder to the re-roofing project, plus the labor to install it- call it $5-6K "extra" beyond a simple re-roofing. It's not cheap, and may have a poor return on investment on mere energy savings compared to the R38-ish cavity fill, but it will pretty much eliminate the ice damming potential, and it would meet/beat current code minimums.

      1. bwsct | | #28

        Can rock wool be compressed and still retain its R value? I think it will be a tight fit with the closed cell and rock wool in a 5.5-6 rafter.

        I appreciate the ideas you've given me.

        1. Expert Member
          Dana Dorsett | | #32

          All fiber insulations lose R value with compression, but gain R/inch at the thinner layers. Taking a 3.5" batt and compressing it to 3.0" takes about R2 off the total. If the batts are a half-inch proud of the rafters and held in place by perforated radiant barrier they wouldn't be fully compressed the way it would be if held in by half-inch wallboard, taking maybe R1 of the total rather than R2, but that R1 is regained by the low emissivity of the radiant barrier.

      2. bwsct | | #30

        Not to flip this around completely but how could I get to R49 going with green insulation products other than spray foam with my 6" deep rafters?

        1. GBA Editor
          Martin Holladay | | #31

          Brad,
          If you install a fluffy insulation like cellulose -- considered "green" -- assume R-3.7 per inch. If you have 2x6 rafters that are 5.5 inches deep, and you use 1.5 inch for a ventilation channel and a baffle, that leaves you with 4 inches for insulation. That would be R-14.8, not R-49.

          If you want R-49 with cellulose, you have to deepen your rafters. More information here: "How to Build an Insulated Cathedral Ceiling."

          1. bwsct | | #33

            My goal is to get as close to R-49 as i can.

          2. GBA Editor
            Martin Holladay | | #36

            Brad,
            You asked how much "green" insulation you could install between 2x6 rafters. I answered your question -- the answer is about R-14 or R-15. If you want R-49, you need to thicken your roof assembly. The options include adding rigid foam above your existing roof sheathing -- an approach that requires a second layer of roof sheathing above the rigid foam, along with new roofing -- or thickening your roof assembly on the interior, by sistering new rafters beside your existing 2x6s. All of these options are explained in a useful article that you really should read. Here is the link: "How to Build an Insulated Cathedral Ceiling."

  13. Expert Member
    Peter Engle | | #27

    Brad mentioned above that he's planning for spray foam in the attic mostly because he's got to hit a minimum charge for the spray foam installer. So in some respects, the foam is free once he's decided to insulate the kitchen.

    Why not use closed cell in the gable walls to get him as close to code minimum as possible, and open cell in between the rafters as his vapor permeable insulation? Then, add polyiso rigid foam on top when reroofing in the spring? The month or so of cool weather isn't going to do any significant harm to the roof with just open cell on the inside.

    1. Expert Member
      Dana Dorsett | | #35

      At 6" open cell foam is about R22, so it would take R27 above the roof deck to hit code min on an R-value basis. That's about 5" of roofing polyiso, not 3", but do-able. If it's assured that there will be that much foam going up top it's cheaper to skip the open cell foam and use 5.5" rock wool (R23) instead. If it's not a sure thing, it's safer to start with the spray polyurethane + fluff in the cavity fill.

      Putting thicker layers closed cell foam in wall assemblies just to hit a code min center-cavity R value is a waste of foam, since the thermal bridging of the framing at the high framing fraction of a gable wall robs it of it's potential performance. Anything more than the minimum necessary for dew point control on the fiber insulation isn't justifiable. See:

      https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

  14. [email protected] | | #29

    Dana,

    On several occasions, I have read posts where you recommend (or at least give the okay for) a closed cell rigid foam insulation board on the the outside of a roof deck and closed cell SPF applied to the interior of the roof deck. To the untrained layperson, this sounds like something to be avoided (a closed cell foam sandwich). It would be nice to see a write up on when this is and is not advisable.
    Antonio

    1. Expert Member
      Dana Dorsett | | #34

      >"On several occasions, I have read posts where you recommend (or at least give the okay for) a closed cell rigid foam insulation board on the the outside of a roof deck and closed cell SPF applied to the interior of the roof deck. To the untrained layperson, this sounds like something to be avoided (a closed cell foam sandwich). "

      At 4" most closed cell polyurethane is still only a class-II vapor retarder, about 0.2-0.3 perms, not a true vapor barrier- there is still some drying capacity toward the interior. At ~7-8" (R49 of closed cell foam) it's getting very close to class-I territory. but still not quite there. As long as the wood's moisture content is under 18% when the foam goes up it's not very risky even at 7", but it takes a very long time for moisture to leave.

      A typical #30 felt + asphalt shingle layup is about 0.1 perms, the boundary between Class I and Class II vapor retardency, but who cares? A roof doesn't dry toward the exterior through wet or snow covered shingles anyway. A layer of Grace Ice & Water Shield is well under 0.1 perms, a true vapor barrier, at which point the vapor retardency of the foam above it is completely moot, but the 0.2-0.3 perm spray polyurethane is still a drying path.

  15. bwsct | | #37

    Thank you Martin and Dana for all of your advice.

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