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Should I correct the ductwork or the un-insulated walls or both?

kipstiggak | Posted in Energy Efficiency and Durability on


We live in Hudson, Quebec between Montreal & Ottawa. My plan is to insulate walls of 2 rooms, from the inside, using 2″ EPS Type I insulation, covered with a reflective vapor barrier directly on the existing drywall and 1″ furring strips and 1/2 drywall. These rooms have 2 “exterior walls” each and tend to be more humid and cold than the other rooms.

The exterior walls are concrete block covered with traditional stucco. There is currently 2″ of air space behind the concrete blocks, then 2 layers of ½” drywall. In addition, there is a loose black sheet of some kind of paper material behind the drywall. I am not sure if this is a form of vapour barrier or not. The house was built in 1959.
I am worried about potentially creating a moisture problem somewhere within the assembly.

Also, we have forced air heating and the ductwork is in the attic. I insulated the ductwork three years ago, with R30 fiberglass insulation. However, the heating vents are at the top of the walls close to the ceiling and the returns are at the bottom, either on the walls or in the floor. The duct work for the returns are in the crawl space which is about 2 to 3 feet high. The crawl space walls are insulated with blue sprayed insulation. There isn’t insulation beneath the floors and floor of crawl space is sand with vapour barrier over it. Our floors tend to be cold almost all year round. There is a return in every room except for the kitchen.

So the question is, should I be more worried about correcting the ductwork or the un-insulated walls?

We hope to live here for a long time – say at least 5 to hopefully 20+ years. Note: As it is, the house stays cool during summer – so no need for air conditioning.


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  1. Expert Member
    Dana Dorsett | | #1

    You really need to worry about both, but fixing the uninsulated walls will make a much bigger difference in comfort.

    Where is the furnace/air handler located, and what is the fuel? One story, or two?

    If it's a 1-story it would be far better to put all the ducts and the furnace too (if possible) in the crawl space, then air seal the ceiling below the attic, and it might not be a super-huge project.

    The black sheet material is likely to be a vapor retarder, to protect the interior finish wall from the high moisture drives of the block & stucco. The foil facers on the EPS are true vapor barriers, and would perform that function as well or better.

    But 2" of continuous EPS is not much insulation for your zone 6 type climate, WAY below IRC 2012 code min, which would be R13 studwall + R10 continuous insulation. (see: With 2" of EPS you don't have quite the R10 continuous!

    Since you're planning to live there long term, if you can afford the space it's worth installing a non-structural 2x4 24" o.c. studwall with R15 batts in the cavities inside your 2" EPS which would bring it close to the IRC, and it would be sufficient exterior R for dew-point control at the foam/fiber boundary to not need an interior vapor retarder other than standard latex paint on wallboard. If the code inspectors insist on an interior side vapor barrier, use a smart vapor retarder such as MemBrain or Intello Plus, not 6-mil polyethylene, since polyethylene would form a moisture trap with the foam layers.

    If that eats into the space too much, installing installing 2" of EPS and 1.5-2" of foil-faced polyiso on the interior side of the EPS would be almost as good, if not quite up to IRC 2012 standards. With R8 EPS on the outside of R9-R13 polyiso the polyiso will perform at it's full rated R, and the EPS will outperform it's rated R in winter, but don't flip the stackup- the EPS works better in the cold side of the sandwich. Adding even 1" of polyiso to the stackup would make a difference here.

    How thick is the blue foam in the crawlspace?

    Do you have a fuel-use history on the heating this place? (For estimating the peak heating load.)

  2. kipstiggak | | #2

    I’m so glad to have stumbled onto your site after all the conflicting information I came across on the internet. Can’t thank you enough for your help, Dana.
    Ok, answers to your questions and a few more from my end.

    The forced-air electric furnace is in the utility room between the kitchen & the garage. House is one story with standard 8’ high ceilings.
    Hard to say how thick the blue foam is, it was there when we bought the house 3 years ago. All I can say is it looks very thick and very well done. Walls are totally sealed from top to bottom, including beams. Forgot to mention in previous email that crawl space has vents which are replaced with tightly fitted wood blocks. I used to put in the screened vents for the summer season, but I don’t anymore since I read that keeping them closed prevents moisture problems. I found conflicting information about this, as well, so and advice on the matter would be greatly appreciated.

    Are you saying removing and installing the duct work in the crawl space can be a DIY job?
    How about the vents and returns? Should they be reversed; vents at the bottom of the wall, intakes at the top?

    Sorry for being such a noob . Just want to be sure I understand you correctly; you’re saying to add the studwall with R15 batts against the existing drywall, not against the concrete blocks, right? If so, I can afford the space in the living room but not in the guest room(9’11” x 14’2”). For the guest room I can use the 2nd method with the 1” polysio.
    Again, just want to be sure I understand you. I attach the 2” EPS to the existing drywall and the 1”polysio against the 2” EPS, then furring strips and drywall, right.
    And it’s okay to have two (EPS and Polysio) foil faced boards, in the sandwich, facing the interior?
    BTW, the 2”x 4’x 8’ EPS product I bought states it has 11-R value

    Thanks again,

  3. charlie_sullivan | | #3

    The R-11 quoted for that 2" foil-faced EPS is taking credit for the R-value of an air gap between it and the next layer. The R-value of that air gap is enhanced by the foil. But if you are space limited, you are better off skipping the air gap and filling the space with polyiso as Dana suggests. Then you are back down the the actual inherent R-value of the foam, which is more like R-8. The foil is no longer needed--you can then by cheaper EPS without the foil.

  4. GBA Editor
    Martin Holladay | | #4

    1. If you don't need air conditioning, I don't know why you have a forced-air electric furnace. If you are heating with electric-resistance heat -- usually quite expensive, but somewhat affordable in Quebec -- why not just get rid of the furnace and the ductwork, and install electric resistance baseboard instead?

    Q. "Are you saying removing and installing the duct work in the crawl space can be a DIY job?"

    A. The answer depends on the skills of the homeowner. Considering all of the questions you are asking, I would advise you to hire an HVAC contractor to move around your ductwork if you decide to keep your furnace.

    Q. "You’re saying to add the studwall with R15 batts against the existing drywall, not against the concrete blocks, right?"

    A. If you follow Dana's advice about the new stud wall, you should install a layer of rigid foam on the interior side of the concrete block wall, followed by the new stud wall on the interior side of the rigid foam.

  5. Expert Member
    Dana Dorsett | | #5

    Charlie is correct 2" of EPS with foil facers an air gaps BOTH sides would be ~R11, but without the air gaps it's between R8-R8.4when the average temp through the foam is 75F/24C but when the average temp through the foam is 5C it would be about R9, and when the average temp through the foam is -5C it's close to R10. (The colder it gets, the better it performs.)

    The average temp through the foam is a function of the outdoor air temp, the indoor air temp, and whether or not you have other insulation between the foam and exterior or interior (and how much other insulation.)

    Either way, Martin is correct that you need the foam between any fiber insulation and block wall to install at 2x4 studwall. The absolute minimum R-value for the foam in your climate needs to be R7.5 to avoid wintertime moisture accumulation in R13-R15 batts without an interior side vapor retarder. So 2" of EPS is enough, but 1" would not be.

    In your climate it's better to have both the supply & return ducts near the floor. Warm air naturally rises to the top, cold air sinks to the floor. Placing the supply registers near the exterior walls (usually by a window, to counteract the draft of cold air sinking down the face of the window), with the return ducts on the other side of the room at floor level usually works quite well for heating.

    Drill an exporatory hole into the blue spray foam until it hit's concrete, and use wire or toothpick or something to determine it's depth. It may or may not be worth adding more insulation in the crawlspace, depending on the depth. Here too, with R7.5 or higher on the foam it's fine to install a studwall with R15 batts, but if it's only R5-R7 you'd need a smart vapor retarder to control wintertime moisture accumulation. If it's already R15-R20 (3" thick closed cell foam) there isn't much point of adding more insulation on the crawlspace walls.

  6. kipstiggak | | #6

    Hi Dana,

    I understood and appreciate Charlie & Martin’s clarifications and advice. Thanks guys!

    Dana, hopefully this will be the last time I bother you about this, but I don’t want to make a mistake. Keep in mind, I’ve already bought the foil faced EPS boards.

    Questions for EPS board & 2x4 studwall method.
    Question #1: Is having the EPS foil behind the fiber insulation okay?
    Question #2: R-15 fiber insulation comes to 3.5 inches, so that would leave me with a small air gap behind the drywall, is that okay (2x4 not being quite 4”)?
    Question #3: I know you said no vapour retarder on the interior but no vapour barrier behind drywall either?

    Questions for EPS & Polysio board method.
    Question #1: Would I need to remove the foil from the EPS boards, if I attach foil face polyiso boards to the EPS?
    Question # 2: Would I leave an air gap between polyiso & drywall?

    Thanks again for all your help.

  7. Expert Member
    Dana Dorsett | | #7

    EPS board & 2x4 studwall:

    #1: Yes, it's fine for the fiber to be in contact with a foil facer of the EPS.

    #2: A standard 2x4 is 3.5" deep, which is why they manufacture R15 batts to be about 3.75" thick ( for a compression fit.) Where is the additional depth coming from?

    #3: You may use a "smart" variable permeance vapor retarder on the interior side of the insulation, but not a true vapor barrier, such as polyethylene sheeting or foil. But as long as there is at least R7.5 of foam on the exterior side of the R15 fiber you don't really need the smart vapor retarder.

    EPS & Polysio board:

    #1: No, there is no need to remove the foil facers when applying multiple layers of foil-faced foam. Tape the seams with a foil tape for ALL layers, for better air tightness.

    #2: It's fine to leave an air gap between the drywall and the foil-faced foam. In some locations local fire codes would require fire-blocking in the air gap to limit the vertical spread of fire in that air gap space, but if you install 1x furring horizontally instead of vertically on which to hang the drywall it would usually meet those requirements.

  8. kipstiggak | | #8

    #2: A standard 2x4 is 3.5" deep, which is why they manufacture R15 batts to be about 3.75" thick ( for a compression fit.) Where is the additional depth coming from?

    I assumed it was the other way around; 2x4 being closer to 3.75 and I read somewhere the R15 comes to 3.5”.
    So 2x4 is 3.5” deep & insulation 3.75 pressing against drywall – so no air gap?

  9. Expert Member
    Dana Dorsett | | #9

    Nominal lumber dimensions (in both inches & mm):

    Batts designed to fit into standard dimension framing are typically slightly thicker than the lumber dimensions when manufactured, then compressed or rolled more tightly for shipment. High density batts like R15s are usually tested for R-value compressed to the standard lumber dimensions, but some low-density batts are tested at a somewhat higher loft. Low density R19 batts are usually tested at 6.25" thickness, and only perform at R18 when compressed to the 5.5" nominal depth of at 2x6 stud. Similarly, low density R22s are tested and labeled at 6.75", and only perform at R19 compressed to 5.5":

    When installing a batt (high density or not), it is important to tuck it in at the edges and corners to make sure that you don't leave small voids there, then lightly tug it out decompressing it until it projects slightly more into the room than the stud edges. That way it gets compressed into place when installing the drywall, with no voids.

    So yes, R15 batts are 3.5", which is the nominal 2x4 dept, but only if you install them in such a way that they have that much loft, and are not still compressed to less than that by the shipping bags or by handling. Since they are manufactured somewhat thicker than 3.5", it doesn't damage them to pull it out to slightly deeper than 3.5".

  10. kipstiggak | | #10

    Got it. Thanks for all your help, Dana.

    Sorry, I have one more question. Is there a way to attach an electrical outlet to 1" furring with 1/2 inch drywall?

  11. GBA Editor
    Martin Holladay | | #11


  12. GBA Editor
    Martin Holladay | | #12

    Manufacturers of electrical boxes make boxes that are only 1 1/8 inch or 1 1/4 inch deep, and these boxes work for some but not all purposes. It depends what the boxes are used for.

    Of course you can carve out the rigid foam to make room for an electrical box if you need to. Repair a sloppy carving job with canned spray foam.

    You can buy electrical boxes with a variety of flanges, or without flanges, designed to be attached to framing or strapping in many different ways.

    And by the way -- I'm just guessing here -- you might want to consider hiring an electrician for this aspect of the work.

  13. kipstiggak | | #13

    Thanks so much for your time & help, Martin

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