GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Spray foam the ceiling joists vs. the back of the roof sheathing?

CMRICHA | Posted in Energy Efficiency and Durability on

Is there ever a time when an attic space is so large that it is better to spray the ceiling joists rather the backing of the roof sheathing?

(No cathedral or vaulted ceilings…)

The home is ~6000 sq ft under roof (including garages) the builder is thinking that it is better to spray the ceiling joist since the attic space is so large.

(There are areas were the ridge row is ~15+ ft above the ceiling joists.)

His logic is why  do I want to cool the attic and that my electric bill will be higher since I’m heating/cooling a larger area.


I feel he is missing the advantages of the HVAC not working as hard since you don’t have the extreme heat/cold in the attic…


  1. Expert Member
    Dana Dorsett | | #1

    Where are you located? (Local climate matters, in terms of getting to the most rational solution.)

    Are the cooling ducts and air handler going to be located in the attic? (And if yes, why?)

  2. GBA Editor
    Martin Holladay | | #2

    This is the third question you have posted today on this topic. Please post follow-up questions on the same thread you started -- without creating more and more threads. Thanks.

  3. CMRICHA | | #3

    Houston, Tx... Yes, because there is no other place to put them.. There are no basements in the Deep South... Also at this time the HVAC installation is 50% complete... No turning back...

    1. Damiann | | #11

      Hi Cynthia,

      Of course, you could or should have sprayed the ceiling joists ONLY(The back to the sheetrock) instead of the roof shedding.

      99% of your neighbors don't have conditioned attics and their HVAC systems are in their pathetic attics. If this is a new home, I would NEVER design a house with HVAC anything in the attic. I WOULD NEVER DESIGN AN ATTIC SPACE.


      99% of your neighbors do just fine with their insulated ducts in the attic. Know that hurricanes will destroy homes with vented or non-vented attics (this is a selling point for many).

      Although there is a lot of logic in their hurricane argument, in Houston Tx, this is almost never a concern.

      Yes, you should spray foam your ceiling joists only (The back to the sheetrock), and if you were too paranoid and concerned about not having your HVAC system in a ("conditioned space", WHICH IT IS NOT even if you sprayed the roof shedding, ) you could spray foam your ducting too.

      Spraying under the shedding comes with a lot of issues too. The R-value of the foam is better at the ceiling than at the roof shedding.

      I GUARANTEE and I am willing to bet that you will NEVER go into your attic while you're alive.
      However, there is a chance that your kind and wonderful soul might float up to your attic when you are no longer among the living.

      Attics are evil, and if you're unlucky and cursed for having one, don't waste your money on unnecessary foam that will NOT give you more benefit in the short or long term.

      1. Expert Member
        MALCOLM TAYLOR | | #12


        Cynthia posted her question six years ago. I suspect she has already made a decision on how to proceed.

        1. Damiann | | #13

          Mmm, how would you know, Malcolm?? Maybe she is a time traveler who wants to do things correctly without wasting money on spray foam shenanigans.

  4. GBA Editor
    Martin Holladay | | #4

    As this point, you have a couple of choices:

    1. You can install spray foam on the underside of the roof sheathing to create an unvented conditioned attic. Even though your attic is large, this makes sense -- because you don't want to install your ducts or HVAC equipment outdoors.

    2. You can install collar ties (ceiling joists) and drywall to lower the ceiling of your attic, so that the volume of the attic is less than it is now. This approach would create a new attic above the new ceiling; the new attic could remain unconditioned.

    I'm not sure whether the expense of installing these new ceiling joists is worth it, frankly. But it's an option.

    You certainly don't want to accept any option that leaves your ducts in an unconditioned attic.

  5. Expert Member
    Dana Dorsett | | #5

    I once had a friend in Lubbock living in a house a full basement, and I've seen other houses in TX with crawlspace foundations, and many gulf-coast houses with pier foundations suitable for routing the mechanicals other than in a hot attic. They exist, even if slab on grade is the norm for new construction. But that's clearly not your house.

    By foaming the roof deck and sealing the attic you are cooling a larger area/volume, true, but your cooling bills will be lower and your comfort higher due to much lower outdoor air infiltration. In hot-sticky Houston the latent loads of that outdoor air can even equal the sensible load of the house, and a vented attic will bring in more moisture than it ever purges. Even though vented attics are the norm in Houston, unvented attics will work better overall, with or without ducts & air handlers in the attic.

    To hit the IRC 2015 code-min R38 for climate zone 2 requires about 11" of half-pound foam, which has to be applied in two lifts of ~5.5" for both fire safety during the hours after installation as it cures, and for a quality installation without excessive thin areas due to uneven expansion & shrinkage. If local codes only demand IRC 2009's R30 it requires ~8-8.5", but it still has to be applied in two lifts of not more than 6". In my area (which is probably more expensive than yours), R38 open cell foam runs $3-3.50 per square foot. If the foam installers are hungrier in your neighborhood you might be able to bring that down to $2.50 per square foot, but probably not any lower than that.

    Be sure to verify the air tightness with a blower door before letting the foam crew break down- getting it fully air-tight where the roof meets the walls at the soffits seems to be their weak suit, and it makes a real difference.

    If you go with a vented attic the ducts and air handler and every ceiling penetration has to be meticulously and carefully air sealed in a high-reliability fashion, since any duct leakage or room-to-room pressure imbalance generated by the air handler will drive outdoor air infiltration.

    A vented attic is also more susceptible to having the roof sheathing stripped or windows being blown in than an unvented roof during a hurricane than a sealed-conditioned attic too (not that hurricanes ever come anywhere near Houston. :-) ) The lift generated by the wind blowing over the ridge while the attic is being pressurized by wind entering at the soffits is huge, and when the first sheet of sheathing lets go the windward side windows on the first floor usually the first to follow. With an unvented attic the roof deck usually stays intact since it isn't being simultaneously pressurizes from below and lifted from above (at least until a few windows blow.)

  6. TreeceContracting | | #6

    I realize this is an old post, but I agree with the builder. I believe in foaming the ceiling deck instead of the roof deck in Central Arkansas(Zone 3), which is milder but isn't too different from Houston (Zone 4). You are correct in that there aren't many homes in the south with crawl spaces and basements as up north. We build homes in Arkansas with steeply pitched roofs that create very large attics. That seems to be the popular style currently. The extreme roof pitch can double the cubic footage under roof. We also use solar board roof decking with proper attic ventilation which aides in reducing the attic temperatures in the hot summertime. Remember, before spray foam, we commonly insulated the attic floor without a great fear of overworking the HVAC units. I think we can all agree that spray foam (in a controlled test) exceeds the fiberglass and cellulose that we commonly used to insulate our attic floors before moving the envelope became a debate. So, the spray foamed attic floor (and walls) is still a great improvement. We still insulate the duct work as we did before and we haven't seen any signs of HVAC units failing due to the insulated envelope remaining where it has been since we began insulating homes. I know this all goes against popular thought, but I'm still not convinced that moving the envelope to the roof deck is all pro and no con. Moving the envelope does make for a great sales pitch to blow more insulation to cover the (in most cases) larger area of roof deck which increases the cost greatly on a home with a steeply pitched roof. We have insulated almost every new construction home in the last 5 years with spray foam on the attic floor. Our customers have reported low electric bills with less initial investment than spraying the roof deck. I would love to see a case study with (#1- 5.5" Roof Deck Foam, no ceiling deck insulation, same home design, 10/12 roof pitch minimum) versus (#2- 5.5" Attic Floor Foam, same HVAC Unit, Same Design, Same Roof Pitch) in Climate Zone 4.

  7. GBA Editor
    Martin Holladay | | #7

    Installing ducts and HVAC equipment outside of the thermal envelope is nuts. Here is a link to an article that explains all of the different ways that you can move your ducts indoors: "Solutions to the Attic Duct Problem."

    If, for whatever reason, you have decided to install insulation on the attic floor rather than at the sloped roof, there is no need for spray foam. Just perform ordinary air sealing at the ceiling plane, and then install cellulose insulation. This approach is much cheaper than using spray foam.

  8. TreeceContracting | | #8

    I understand that the conventional setup of locating the HVAC outside the thermal envelope may be "nuts" from an idealistic standpoint. Even the referenced article and associated linked articles point to the reasons that this approach will remain to be the most popular design at least in my area. Cost and Asthetics will continue to be the driving forces in Residential Construction. We have foamed walls and foam sealed the attic floor with cellulose over it. With attics as large as the home conditioned below the ceiling, I just can't see moving the envelope to the bottom of the roofline. I would agree 100% if it were a small simple home with a 4/12 pitch roof (small attic). We build larger homes and some have up to 16/12 pitch roofs.
    Would you still recommend moving the envelope outward in that situation? Never mind the huge cost increase of the added foam. Is it more effective to move the conditioned envelope to the roof deck without increasing the HVAC tonage, when the attic has the same volume as the conditioned space below the ceiling? Would it be better to leave the envelope at the attic floor and better insulate the ducts and unit in attic? Would it be as good to increase the seer rating and furnace efficency while keeping the envelope at the attic floor? The homeowners on natural gas can get rebates on 18+ seer, 95%+ furnaces, and tankless water heaters that literally pay for the upgrades.
    Before I come across as argumentative, I want to disclose that I have a foam insulated outer envelope on my home with the HVAC units in a mechanical room and all of the ductwork exposed in the conditioned space. It is very efficient and has the industrial look that we love. However, many of my clients don't share tne same appreciation for that look. So, what do you think about some of the questions above.

  9. TreeceContracting | | #10

    Jon R,
    That's a good article and burying the ductwork on the attic floor makes so much more sense than insulating the bottom of the roof deck. Maybe foam sealing the attic floor with 1" foam and adding cellulose over while burying the ductwork would be an efficient and cost effective solution. It just may be difficult where ducts pass over other ducts, framing, etc. In the attic to maintain the coverage depth unless those sections are foamed too.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |