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Community and Q&A

Adding insulation to attached garage

kwoolfsm | Posted in Energy Efficiency and Durability on

Good morning GBA,
    I have just taken possession of a new build home.  The attached garage is roughed in for hydronic heat.  From outside-in the wall assembly would be vinyl siding, osb, 2×6 with r-22, 6mil vapor barrier with acoustic sealant, 5/8′ drywall, with one coat of mud.  I was thinking of adding 1.5″ eps over the drywall, and then 2×4 walls and using r14 in that cavity, and drywalling for finish walls.  IS there any vapor risk by adding these layers? Should I add 6mil poly before my final drywall layer?  Location is Calgary AB, Canada. Thanks

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  1. Expert Member


    My one worry is the location of the existing poly vapour-barrier. It may turn out to be too far from the warm side of the wall, and have moisture condense on it. I suspect it may be fine due to the rigid foam - I just don't know. The safest plan would be to remove it.

    It would be worth doing some rudimentary energy modelling of the space before deciding how much insulation it makes sense to add to the walls. The losses through the doors, and edge and bottom of the slab will offset much of the gains you make by improving the walls too much.

  2. Expert Member

    Are you converting this space into a 'livable' area?

    If not, I question the motivation. Can you tell us a little more about what you're doing?

  3. Expert Member
    BILL WICHERS | | #3

    If your goal here is to keep using this as a garage, with an overhead door for a car, then there isn't much point in going all out to insulate the walls -- the overhead door is going to be a big air leak in almost all cases, which will negate your efforts to a large extent.

    Your proposed assembly would be about R42.3. That's very high for a wall, and wouldn't make much difference in terms of energy usage in a garage with the usual relatively poorly insulated and poorly air sealed overhead door. If you want to go for a little more insulation, I would use 1" polyiso, with the perimeter sealed to the existing wall to help minimize the chances of moisture being an issue with the poly layer. Put new drywall right over hte poly. I would not bother with a second studwall and batts. 1" of polyiso would add R6 to the wall, for about R28 total, and it would be continuous insulation. With the polyiso on the interior (warm) side, no derating issues would come into play. You'd end up with a wall close the R30, which is usually considered to be around the sweet spot for R value in most walls.


  4. mr_reference_Hugh | | #4

    I agree with others that it would be good to know why you would want to increase insulation from the existing R22. I agree with Bill that the garage door is a weak point and adding any insulation/drywall is going to be very expensive with little to gain.

    I would also expect that the slab of the floor extends under the garage door into the exterior environment. Heating the slab would mean that a ton of heat would be lost in that section of the slab that extends to the exterior, much more that will ever escape through an R22 wall - IMO.

    If you do what you plan, I strongly advise against using an additional polyetheline. The existing polyetheline is the the vapour barrier - per code. With 2 vapour barriers you would theoretically be creating a location within your wall that cannot dry if water (e.g. rain water) gets in. In Ontario, you can place the vapour barrier behind some of the insulation, but it cannot be too close to the exterior sheeting.

    Malcolm wrote this piece on GBA...

    On my house, we have the followign assembly starting from the outside:
    -Wood furring
    -Wood fibre insulation (vapour open)
    -2x8 with cellulosse insulation
    -Huber ZIP OSB as a vapour barrier
    -a 2x4 structural wall with Rockwool mineral wool batt insulation

    Maybe that is why I am Ok with having the poly further into the wall. Malcolm's piece he wrote has references to the building code for you to verify how "deep" into the wall you can place your vapour barrier.

  5. kwoolfsm | | #5

    Folks, fair question and comments. There is a bedroom for my son above the garage, the slab is also insulated underneath. There is no plan to change from this being a usual garage. My intention was nothing more than trying to hold more heat inside the garage, simply to limit heat loss, retain more heat from the hydronic, and limit and potential for "cold floor" for my son upstairs. Thanks for the feedback!

    1. Expert Member
      BILL WICHERS | | #6

      If you have a cold floor for the room over the garage, the easiest way to deal with that would be some rigid foam under the floor joists, which would act the same as "exterior" rigid foam continuous insulation would work on walls. I would probably use 2" polyiso. You would put 5/8" drywall right under (between the polyiso and the open air inside the garage) the polyiso in this case to act as the ceiling of the garage. Be sure to air seal things too.


  6. mr_reference_Hugh | | #7

    kwoolfsm, that information is helpful.

    If you want my thoughts, here are some details on my views and these are not too far off from what Martin H says in his article (link below).

    You likely want to identify and understand the source of the cold that is affecting the bedroom. Martin's article speak to this in a clear concise fashion.

    #1 air tightness. The cold from the exterior is often getting in through air leaks in the assembly. Normally the garage ceiling should be well air sealed from the living space above to avoid CO2 getting into the livings space. This said, most GBA followers know that typical construction does not deal with air tightness all that well - no matter what the building codes says.

    #2 insulation: Insulating the floor assembly is important, but the GBA crowd knows that lots of insulation is not that valuable unless you also deal with air tightness.

    Possible solutions for consideration: Like many situations, there are lots of solutions. Myself, I would not spend money to heat the garage itself unless you have reasons beyond those mentioned.

    Option #1: Adding insulation the exterior wall with the idea of heating the entire garage - i.e. the initial plan you mentioned.
    - IMO, this would be the least effective, and possibly most expensive, option.
    - The reasons are mentioned by Malcolm and Bill above.

    Option #2 - Could you build a wall in front of the garage door to seal off that source of air leakage?
    - This is only viable if you don't use the garage to park your vehicle.
    - Sometimes the local city zoning bylaws require a functioning garage when the garage is attached to the side of the house or in front of the house (such as my own municipality).
    - You would need to check building code requirements to ensure your house insurance policy is not negatively impacted.
    - If you do it right, you would get a building permit but of course many people might "feel" that this as an optional step.

    Option #3 - Could you insulate and create a more air tight barrier on underside of the floor joist (garage ceiling) while ensuring there is a vapour barrier at the right location in the assembly?
    - This should be much less costly than Option #1. Martin Holladay's article discusses this option.
    - This would avoid having to pay to heat the garage (especially if you need to buy equipment to get the hydronic heating system working.)
    - This eliminates the most common source of cold, in existing well insulated homes, the air leakage.
    - Improving air tightness provides added protection from CO2 from car exhaust - despite any measures already taken by the builder to meet or even exceed code.
    - This will not "heat" the floor in the living space (i.e. the bedroom) but it should allow the floor to absorb heat from the bedroom and allow the floor to reach room temperature.
    - This would require having a vapour barrier in the right location in floor/garage ceiling the assembly. There are different options but I am sure that most people would call in an insulation company to have them apply spray foam to the bedroom subfloor.
    - You need to make sure (it is imperative) that any assembly that you choose that is associated with a residential garage has the appropriate fire protection per building code. The 5/8" drywall mentioned by Bill is often used as a fire barrier, but the assembly needs to be one that is "approved" under the code or by a professional stamping the drawings (but you likely will not have drawings for this).
    > Protecting the assembly from fire (a fire rated assembly) is important for at least two important reasons: A) to keep the family safe; B) to ensure that you don't render NULL AND VOID your house insurance policy.... something few people ever think about.
    - Matt Risinger ( is a big proponent of using Huber ZIP insulated panel in many assemblies. It provides continuous insulation on the cold side of the assembly and makes air sealing a bit easier. The Huber ZIP OSB is also considered a vapour barrier in my province so be aware and consider that vapour barrier component if you use this product in the assembly you choose. OSB is not typically fire rated. Apparently, LP now has similar products to compete with Huber ZIP.
    - If you do choose to go with Option 3, then you should make the least number of holes in the ceiling by, for example, placing garage ceiling light boxes on the surface of the ceiling instead of imbedding them in the ceiling. All penetrations in the ceiling from the garage would need to be well sealed, which is again a code requirement to avoid CO2 leakage.
    - You would likely need to adjust any ceiling lighting in the garage and the garage door motor installation.

    Option #3A - Create an air cavity under the bedroom floor, have a heating duct send heat into the cavity, and insulate under the cavity with the same means mentioned in option #3.
    - The only benefit is that in theory the floor would be warmer.
    - Insulating the ceiling would very likely involve more insulation below the floor joists and loosing head room in the garage.
    - Air sealing at the perimeter may be more difficult, but this depends on how you decide to create the air barrier layer that will be installed on the underside of the insulation.
    - You would need to have more significant adjustments to garage ceiling lighting and garage door motor installation.
    - Again, it requires making sure that building codes and any related fire rating requirements are respected.

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