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Community and Q&A

Best insulation and sealing for a tucked-under-garage ceiling?

Geoffrey Simon | Posted in GBA Pro Help on

We are purchasing a circa 1966 Cape in the Philadelphia are with the garage tucked under the master and guest bedrooms and two baths. The garage ceiling is currently covered with Homasote, but there are small areas where we can see an underlying plaster ceiling with missing chunks. The joists are 2x10s and there appears to be some fiberglass insulation batts in place.The HVAC ducts run below the joists and are mostly covered in Homasote, but several feet of ducts near the overhead door opening are exposed and uninsulated, but with remnants of asbestos.

We suspect that the rooms over the garage have been uncomfortably cold in winter and hot in summer, and know that the current owners utility bills are very high. We are also concerned about the safety hazards of the asbestos plus the currently damaged fire-break and carbon monoxide risks.

Since we plan to use the first floor master bedroom, our first project will be to tear out the garage ceiling, remediate the asbestos, seal and insulate the HVAC ducts, then re-insualate the ceiling joists and cover with 5/8″ drywall. We thought that blown-in foam would give us the best insulation plus a good air and carbon monoxide barrier, but a contractor we consulted thought that new fiberglass batts followed by plastic film and the drywall would be adequate and far cheaper. What do you think? Since the garage ceiling will also need re-wiring for lights and door openers, would you run the new electrical wiring under or over the drywall?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Geoffrey,
    Run the wiring between the joists, so that it is not exposed.

    It's true that spray foam insulation is expensive and unnecessary. If you decide to install fiberglass batts, (a) Don't install polyethylene on the garage side -- that's the cold-in-winter side of the assembly, so putting polyethylene there is a big no-no, and (b) I advise you to install a continuous layer of rigid foam on the underside of the floor joists before installing the drywall ceiling. The rigid foam will improve the R-value of the assembly, reduce air leakage (if detailed correctly), and greatly reduce thermal bridging through the joists.

    For more information, see How to Insulate a Cold Floor.

  2. Geoffrey Simon | | #2

    Thank you, Martin. This is just the information that I need. Greatly appreciated!

  3. Geoffrey Simon | | #3

    Martin: It seems like every answer generates some new questions. Have read the above reference and additional materials from FHB. Also aware that there are always cost vs quality trade-offs and that the details greatly affect the finished result.
    I don't understand how fiberglass batts will stay tight to the subfloor, even if carefully installed that way. It would seem that time, gravity, and the vibration of walking on the floor will work against this. Is there a way to adhere the batts up, or should we consider a more expensive product like open cell spray foam?
    If we use the batts, we would need to carefully air seal any perforations into the joist spaces and the rim joists first. It seems that spray foam would accomplish this and eliminate a step.
    If we use batts, would you seal the rim joists with rigid foam blocks, can-foamed in place?
    Which type of rigid foam, and what thickness, would you recommend to use below the joists? Does the rigid foam get temporarily nailed or glued to the joists until the dry wall is installed? Can we screw the drywall directly to the joists through the rigid foam?
    The HVAC ducts are currently below the garage ceiling. Once the duct joints are properly sealed, do you recommend that we carefully insulate them with rigid foam? Which type? How thick? Can we use adhesive to attach the foam to the ducts? What about the dry wall over the foam? Will we need to build out a wooden frame for the dry wall, or is there a good way to attach it otherwise?
    It appears that the current owners of this house use one of the second floor bedrooms as their master, even though they are in their 70's. I strongly suspect that is because the first floor master bedroom and bath are so poorly heated and air conditioned. If we do this project the least expensive way and the master bedroom is not comfortable we will have wasted the cost and effort. We would rather do it right the first time!
    Appreciate your expert advice.
    Jeff

  4. GBA Editor
    Martin Holladay | | #4

    Geoffrey,
    You have a lot of questions. You may be more comfortable hiring a contractor; that might be easier than getting up to speed on residential construction skills.

    Q. "I don't understand how fiberglass batts will stay tight to the subfloor, even if carefully installed that way."

    A. You need to either (a) choose thick batts that completely fill the joist space, or (b) choose a blown-in insulation like blown-in fiberglass or dense-packed cellulose.

    Q. "Should we consider a more expensive product like open-cell spray foam?"

    A. That's a possibility. It's your choice.

    Q. "If we use the batts, we would need to carefully air seal any perforations into the joist spaces and the rim joists first. It seems that spray foam would accomplish this and eliminate a step."

    A. On both points, you're right -- although it's possible to install spray foam and still miss some of the leaks. Proper air sealing work requires a clear head and experience.

    Q. "If we use batts, would you seal the rim joists with rigid foam blocks, can-foamed in place?"

    A. Here is a link to an article on rim joists: Insulating Rim Joists.

    Q. "Which type of rigid foam, and what thickness, would you recommend to use below the joists?"

    A. See this article: Choosing Rigid Foam.

    Thicker foam is almost always better then thinner foam. Two inches of rigid foam would be good.

    Q. "Does the rigid foam get temporarily nailed or glued to the joists until the dry wall is installed?"

    A. Rigid foam is installed with cap nails.

    Q. "Can we screw the drywall directly to the joists through the rigid foam?"

    A. You can, but it is generally easier to first strap the ceiling with 1x4s, 16 inches on center.

    Q. "The HVAC ducts are currently below the garage ceiling. Once the duct joints are properly sealed, do you recommend that we carefully insulate them with rigid foam? Which type? How thick?"

    A. The product you are asking about is called duct insulation. Here is an example:
    http://www.homedepot.com/p/60-sq-ft-R-6-Insulated-Duct-Wrap-INSWRP60/100152008

    Q. "What about the drywall over the foam? Will we need to build out a wooden frame for the drywall, or is there a good way to attach it otherwise?"

    A. If you want to hide the ducts, then you need to build a soffit or chase. Again, if all of these concepts are new to you, you should hire a contractor.

  5. Expert Member
    Dana Dorsett | | #5

    Geoffrey: Batts won't sag from gravity if it's a full cavity fill. The manufactured loft is intentionally a bit deeper than dimentioned lumber, for a "compression fit". High density batts don't even need to be a compression fit to work. R30s are designed to fit 2x10s joist bays and would exceed IRC 2015 code min (R19 for zone 4A ) even without the expense of foam board.

    In a garage in Philly it won't much matter what type & thickness the foam is on the under side of the joists. The average wintertime temperature inside the garage will be substantially warmer than the outdoor air, and there will be no moisture accumulation from moisture drives from the room above even with R30 in the joist bays and only R4 on the under side. (In Winnipeg this would need to be analyzed more carefully, but not Philadelphia.)

    Cap-nailing the foam to the under side of the joist keeps it in place while working on the rest, then installing 1x4 furring through-screwed to the joists works for foam of any arbitrary thickness. For thinner foam long-screwing the gypsum board through the foam to the joists also works, as long as the spacing and the depth of bite into the joist is correct. Long screwing through 2" foam would require some pretty long drywall screws, and it's easy to miss if there's any angle to it at all. But though 1" foam it's pretty easy.

    Rather than trying to insulate the ducts at ceiling level with rigid foam, it may be easier to frame around them to bring them into conditioned space (or at least partly so), and insulate the framed cavities. Insulating & air sealing the ducts with an inch or two of closed cell foam is usually easier than sealing the ducts followed by endless hours of cut'n'cobbled & sealed rigid foam board.

    [edited to add]

    Figures that Martin would beat me to the "Post" button on this one! :-)

  6. Geoffrey Simon | | #6

    Martin and Dana -- thanks for the detailed information. I think that this will be enough to allow me to choose the proper materials and do the task well. No, I am not a contractor, but a longtime homeowner and woodworker who likes to do projects. I will get professional help where needed.

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