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Community and Q&A

Air seal the ceiling or the attic?

Domenico Perrella | Posted in Lakesideca Techniques on

I live in the East Bay part of the San Francisco Bay Area. Maps show me in Climate Zone 3B, but I’d say I’m a good deal cooler than that suggests. I’m on the east side of the Coastal Range/Berkeley HIlls, but still in the hills, not the hotter flat area further east. I’m also at the bottom of a valley near a creek with some tall trees and a big hill to the west and the house gets very little direct sunshine on all but one of its windows during the warmer months. We heat a lot in the winter and cool very little in the summer.

The house has no air sealing whatsoever and is only insulated at the ceiling and on one wall that was recently replaced. The attic insulation is about 4″ of blown in celulose. We have (somewhat) insulated flexible supply ducts in our HVAC system that are mostly in the basement crawl space with a few in the attic crawl space. Our return ducts are in the attic and totally uninsulated. I suspect the original asbestos insulation was removed during the abatement before we moved in and not replaced by the contractor that replaced all the supply ducts that were ripped out during the abatement.

With that long intro, I would like to air seal the top of my house at some point and the question is whether to try to seal the ceiling or the roof. Moving all the ducts out of the attic would be virtually impossible, although insulating the return lines would be doable. Sealing at the ceiling plane would be hard and I think would be almost impossible wihtout removing the blown-in celulose first. Also, adding thick insulation on top of the ceiling after doing the sealing work would probably cause code/safety problems with our electric wiring, most of which passes slightly below or just above the blown-in insulation. The more insulation surrounds a wire, the less current it can safely carry without overheating. 12 gauge wire is good for a 20 amp circuit up to a certain length, but not if it’s under 6 inches of insulation. For that, you need thicker wire.

So, I’m obviously marshalling my arguments in favor of sealing and insulating the roof when I have the roof replaced. At the gables, I was thinking of something like plastic sheets or sealed drywall inside of the batt insulation (probably rock wool), or a thin layer of spray foam outside of the batt insulation.

For the soffits and roof deck, I was thinking of a thin layer of rigid insulation outside of the decking, taped seams in the plywood deck, and a 1″ layer of closed-cell foam inside of the deck, with batts between the roof joists. And I’m inclined to remove the celulose in the ceiling.

So, the two questions are:

1. Would I be better off sealing at the ceiling, despite all of the issues I raise?; and
2. If I seal and insulate the roof, does the method of sealing and insulating it I mention sound close to the right way of doing that?

Thanks for reading my lenghty disertation.

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  1. Malcolm Taylor | | #1


    i'm not familiar with your electrical codes, but this is the first time I have heard of wire being de-rated by surrounding insulation to the extent that it becomes a factor in how one would choose to deal with an attic. We routinely run the majority of our wiring, both 14 and 12 ga, on the bottom chords of our trusses, and then cover them with blown cellulose insulation. We also run them in stud spaces where they are embedded in spray-foam insulation.I'll be interested to hear the experiences of other posters dealing with this problem.

  2. T Carlson | | #2

    The wiring thing isnt a problem, Knob and tube yes, NM, even the old stuff no. Only time it *maybe* could heat up is if the electrician zip tied a good sized bundle of home runs that were being used to capacity and that was covered by insulation. Due to code it seems like I can easily fill a 42 breaker panel and a lot of those runs are hardly ever used or have very light load on them as it is.

  3. Domenico Perrella | | #3

    Malcolm an T,
    Let me say two things about the derating of electrical cables that are embedded in thermal insulation.

    First, and probably most important, I did mention that I was looking for excuses to seal and insulate the roof rather than the ceiling,right?

    Second, the derating of NM cables that are embedded in thermal insulation is a real thing, but it may primarily be an issue with long runs of 12 ga wire on a 20 amp circuit (or 14 ga and 15A) that are already close to the longest runs permissible without going to a larger conductor. I can't be sure without looking it up because, while I've seen a rule on that, I'm neither an electrical code expert, nor an electrician. If it is only an issue with long runs, and I think I ran into the rule while looking at the maximum run for a certain guage ofnwire to supply an electric car charger, that would suggest that the rule is based more on voltage drops than any actual fire danger.

    So, please excuse that particular excuse for doing the insulation at the roof and just look at it from the perspective of a house with large uninsulated returns in the attic space and a homeowner who likes the simplicity, albeit at a higher cost of spraying foam all over the bottom of the roof and where the roof meets the exterior walls rather then the less expensive but fussy process of trying to find and plug every plumbing penetration, hole for electrical cable to go into walls, electrical box for a light fixture, etc.

    Are the costs and other disadvantages so extreme that it's irrational to insulate at the roof plane in a retrofit situation like mine? And, if not, is the way I Asuggested doing it generally reasonable?

    So, let's go ahead

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Q. "Would I be better off sealing at the ceiling, despite all of the issues I raise?"

    A. From a building science perspective, transforming your vented unconditioned attic to an unvented conditioned attic is by far the best option. The only disadvantage is cost. For more information on this issue, see these two articles:

    Creating a Conditioned Attic

    Keeping Ducts Indoors

    Note that it is sometimes cheaper to abandon attic ductwork and install one or two ductless minisplits than it is to create a conditioned attic.

    Q. "If I seal and insulate the roof, does the method of sealing and insulating that I mention sound close to the right way of doing that? I was thinking of a thin layer of rigid insulation outside of the decking, taped seams in the plywood deck, and a 1 inch layer of closed-cell foam inside of the deck, with batts between the roof joists."

    A. If you plan to install any rigid foam on the exterior side of your roof sheathing -- a method that requires the installation of new roofing -- then you don't want to install any closed-cell spray foam on the interior. (It's best if the roof sheathing can dry in at least one direction.) I vote to install as much rigid foam on the exterior side of the the roof sheathing as you can afford, with the balance of the R-value supplied by some type of fluffy insulation (cellulose, fiberglass, or mineral wool) installed on the interior of the roof sheathing, and in direct contact with the roof sheathing.

    For more information on this issue, see How to Install Rigid Foam On Top of Roof Sheathing.

  5. Domenico Perrella | | #5



    Thanks very much for your answers. For some, probably not entirely rational reason, I like the idea of spending money to add insulation and air sealing to spending money to upgrade my antiiquated HVAC equipment to something more efficient. I acknowledge that ministries can be an efficient and cost-effective solution to many problems, especially in my mild climate.

    I guess I'm hoping to upgrade the equipment when my heating and cooling loads are lower.

    As for the mold sandwich issue with rigid foam over sheathing backed with spray foam, my extremely long question omitted one detail. For reasons of resistance to fire and sound (and not liking what I think of as Styrafoam, I was considering using the Ruxul Comfortboard product as the rigid exterior insulating. BUT, and this is a big but, I just remembered while writing this that Comfortboard is tested and approved for walls, but not roofs. I can't think of any reason that it wouldn't work on a roof, but my inspector won't care about my uninformed opinions. If it's not marketed for use on roofs, I can't use it there.

    So, I'll either have to use rigid foam in the way you describe or use a thicker, more compressible layer of Roxul's Comfortbatt product outside of the sheathing. The thinnest Comfortbatt,at 3-1/2" is more continuous insulation than I really need and I think the Comfortboard would make a better substrate to attach the roofing through, but I guess that would work.

    Thanks again for the enormously informative advice you provide in your articles and in your Q&A answers.

  6. User avatar
    Dana Dorsett | | #6

    How deep are your rafters?

    IRC 2013 calls out U0.030 for roofs/ceilings which is R33 "whole assembly" adjusted for the thermal bridging of the rafters, and credit given for the R-values of the roof deck, shingles, interior-side materials,and the interior & exterior air films.

    A milled 2x6 rafter bay with R23 rock wool batts, half-inch MDF or gypsum board on the interior side and 2" of polyiso (even R5.7/inch reclaimed roofing polyiso) on the exterior will usually get you there, assuming the typical 7% framing fraction for CA roofs.

    If they're 2x4 rafters with R15 batts it'll take just under 3" of exterior polyiso, or 2.5" of a higher R/inch product.

    If your rafters are 2x8s or deeper you can get there with 1-1.5" of exterior foam.

    If I remember correctly the currently active version of CA Title 24 isn't quite as IRC 2013.

    Polyiso has a higher ignition temperature than polystyrene, and doesn't melt, charring in place even when fully engulfed. In fire zones it's NICE to have rock wool on the exterior, but if it's hot enough on the roof to light off polyiso, it's pretty much a given that the roof is gone anyway (and maybe the whole house). A roof deck and rock wool between rafters is more than ample protection against lighting off the foam with a fire from the interior.

    If you can, use reclaimed roofing polyiso rather than virgin stock (no new polymer has been created, no new blowing agents,etc), and usually less than a third the cost of new foam. Sometimes reclaimers will advertise in venues such as this:

    If you can't find local sources, Nationwide Foam ( ) will drop ship truckload volumes to your project site from regional depots, for a price, and even with shipping a whole roof's worth it's usually substantially cheaper than buying new foam.

  7. Domenico Perrella | | #7


    Thanks very much for the information. The R-value requirements here are higher than I remembered.

    I may have been misled by basically no requirements being applied to the insulation of the wall I just had re-stucco-ed, insulated and sheathed. I think the inspector saw the transition from a totally uninsured wall to an insulated one as a win. I allowed that wall to go up with no exterior insulation because I was tired of trying to get my contractor out of his comfort zone. He's a nice guy but adding a rains reenactment under the stucco seemed likeabout as far as he was willing to go beyond slapping on new stucco over some building paper.

    Thanks again for the information and suggestions. You've given me more to think about.

    Apologies for all the autocorrect errors. I find it very hard to see what I've typed on this site when using a phone.

  8. Domenico Perrella | | #8

    Rains reenactment=rainscreen
    Autocorrect=a huge pain in my backside

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Your last comment gets the "Comment of the day" prize.

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