GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Heated Floor Above unheated Garage

Julison | Posted in General Questions on


Here’s a twist to the “How do I prevent a cold floor in the heated room above an unheated garage” line of questions.

I am building a garage in Ottawa Ontario and my builder has proposed an option to insulate and heat that sounds attractive but is apparently difficult to implement.

The ceiling is 16″ open web truss system on 16″ centers.
There is 5/8 T&G plywood as the top floor substrate.
There are uninsulated 5″ ducts flush against the underside of the plywood floor feeding the heated space above.

The plan was to create an heated space ~ 5″ high just below the floor and to cover first with friction fit Rigid styrofoam followed by enough clsoed cell Sprayfoam to get to the minimum of R31 and beyond. Great attention will be spent on ensuring the rim joist area is well sprayed.
The thought is that the top will always be warm due to the duct work feeding both that space and the room above and that the spray foam once cured will produce both an effective air barrier and vapour barrier. I need to put up drywall, tapped and mudded to finish of meeting the code requirements to keep potential vehicle exhaust gas out of the living space.

I haven’t seen this described anywhere so I thought I would enquire here to see if there are issues that I am overlooking. The HVAC guy runnning the duct work thought there might be issues with the sprayfoam off gassing and getting into the ductwork and contaminating the airspace above. I did some searching and it seems there are both concerns with that and that the foam often shrinks overtime leaving cold air gaps.

Any help is greatly appreciated as it is time to insulate and it is getting cold.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    As long as you do a good job of air sealing, your plan sounds good to me.

  2. Expert Member
    ARMANDO COBO | | #2

    In addition, I have specified a couple of different ways that have worked really well:
    1. 2” of CC sprayed foam under the floor decking and around ducts, with cellulose filled cavity.
    2. Installing 2” CC rigid foam under the trusses with cellulose filled cavity.
    Either way you choose, air sealing detail is a must.

  3. Julison | | #3

    Here is a drawing of what I mean as I think I may have left the impression that I want to spray foam against the underside of the floor and the duct work. Infact, I do not want to do that. I want to create a plenum of sorts (defined by the rigid foam on the bottom, the plywood floor on the top and sprayfoam on the outside perimeter when the space meets the outside walls or the common wall with the house) that will contain heated air from the Propane furnace.

    thanks again all,

  4. Expert Member
    ARMANDO COBO | | #4

    Your detail will work as long as your air sealing is good; however, I'm always leery of too much CC foam. I would rather have the rigid foam attached under the TJIs and fill the cavity with cellulose. Also it's a lot easier to install w/o all that cut-n-fit-n-seal labor.

  5. Julison | | #5

    Thanks al.
    Armando, I too am a little leary of using too much foam - really would rather restrict it to the rim and gables where I anticipate having difficulty insulating and providing an air / vapour barrier. I like the idea of using high density cellulose but cannot figure out how to get a vapour barrier mid way up the web joist. Remember, I am trying to create an air channel from the underside of the plywood floor to just beyond the underside of the 5" ductwork. I am under the impression that as this is where the warm zone ends that I need to have a vapour barrier here. If I use the rigid foam divider with the spray foam underneathe it I will have accomplished both a vapour barrier at that side and a air barrier at the intersection of the sprayfoam and the cold zone in the garage. With the spray foam way, I don't think I need to cut-fit-n-seal as the spray foam should expand to fill the gaps along the areas where the rigid foam does not sit tight. Am I missing something here. Won't the CC foam make it easier to create a tight air and vapour seal.

    thanks again

  6. GBA Editor
    Martin Holladay | | #6

    Here's the building science answer: if you have an adequate thickness of rigid foam under your joists, on the cold side of the floor assembly, then you don't need to worry about condensation, and you don't need a vapor retarder on the top side of your cellulose.

    As I said, that's the building science answer. However, it's probably not the code compliance answer.

  7. Julison | | #7

    Well, the inspector came out today and said my plan was not possible as the code specifies that you cannot have isocyanate insulation uncovered in a conditioned space . He gave me two options. 1) Forget the warm floor idea and just spray foam onto the subfloor covering the ducts, go to beyond R 31. 2) keep the warm floor by building a false ceiling 11" below the existing one - fill with cellulose. Neither sounds good to me as they both have significant loses. In 1) I assur myself that the floor will only ever be as warm as the cold air running along the floor to the furnace cold air return. #2) I add more material cost, more labour cost and lose headroom. I am leaning towards #2) as I have been speaking with many people that have had spray foam and have issues with shrinkage. Can you imagine spending all that money and then find you have 1" air gaps at each perimeter junction with framing!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |