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Community and Q&A

How to air seal old hardwood floors before carpet

Austen G | Posted in General Questions on

Our second story is this ancient tongue and groove hardwood with massive gaps and plywood patches.  I’d like to air seal it before I put carpet down but unsure what the best material would be.  Haven’t had a lot of luck in researching this particular application.  I appreciate any assistance!

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  1. Expert Member
    Malcolm Taylor | | #1


    So we can zero in on what might be best, why are you air-sealing a second storey floor?

    1. Austen G | | #2

      Two reasons, first it’s an 1800’s farmhouse with ballon framing, so everything helps. And second, I have a mini split system in place and I’m attempting to isolate each zone as much as possible.

      I’ve already air sealed the envelope inside and out for what that’s worth. Thank you!

      1. Expert Member
        Malcolm Taylor | | #3


        Probably easiest and moist effective would be 6 mil poly below the carpet underlayment. Tape the seams and perimeter.

        1. Austen G | | #4

          Is there any concern that this would also be a vapor barrier?

          1. Expert Member
            Zephyr7 | | #5

            If that’s a concern just use housewrap instead of poly. Housewrap is vapor permeable.


          2. Expert Member
            Malcolm Taylor | | #7

            It is all within the same conditioned interior of the house. You can have impermeable materials anywhere you want, as long as they aren't on exterior walls or ceilings.

        2. Austen G | | #11

          Poly it is, thank you.

  2. John Clark | | #6

    Keep in mind you have stairs.

    Consequently, I would spend your efforts on making sure your second story ceiling and walls are air tight. The stack effect will be minimized so there will be less air moving up through the floor.

    My two cents.

    1. Austen G | | #10

      For sure, that’s all been done also. The walls are all sprayed with closed cell foam and covered in 4” of rockwool. The attic was sealed with closed cell foam all and then covered with an excessive amount cellulose.

      We all sleep with our doors closed, and I used exterior doors throughout the home. I really like being able to condition only the rooms we’re sleeping in at night, at least to the degree that we like them. No need to chill the whole house to 67 in July haha.

      1. John Clark | | #12

        Sound like you have everything covered. I don't see why you would need to worry about cold air drifting down onto the first floor through the first floor ceiling. With your levels of insulation its possible that the AC won't run all that often at all and you'll have very little temp stratification between first and second floors.

        Even with doors closed you're still going to have cool air leak out from underneath the them and travel downstairs. IJS, with stairs I think you're not going to gain much of a benefit at all. If anything, I would pull the hardwood, add subfloor and then put the hardwood back down on top, fill, sand and seal.

        1. Austen G | | #16

          Your last sentence would definitely be my first choice! Maybe down the road.

  3. Expert Member
    Akos | | #8

    Next to no airflow can move through the gaps in the subfloor once the hardwood is in. The amount of air flow is so small that it would never effect zoning. The gaps do increase sound transfer, any standard underlayment can deal with that.

    With typical balloon framed house the more important part is to seal off the floor bays from the walls as these tend to be open.

    Best is to pull up 2 or 3 rows of the subfloor near the outside walls and install blocking that is foamed into place there and re-install the sub-floor. This will make a big difference on air sealing.

    1. Austen G | | #9

      This house is ancient. The tongue and groove hardwood IS the subfloor. Below the boards are the floor joists with the first level ceiling attached directly to those. I’m going to be covering them in carpet, but just wanted to seal first. Some of these gaps are daylight big.

      But I definitely already already put in fire-stops between the first and second level floor bays. I used plywood, poly-iso, and spray foamed it all into place.

      1. Expert Member
        Akos | | #13

        With older T&G like that, I've filled big gaps with canned foam. One of those spray foam guns work great for this.

        If you can time it just right and allow the foam to skin over but not harden, you can push it flat with your fingers. Otherwise trim it flat with a knife.

        Not sure poly will work as it is too slipery and the carpet installers will make a mess of it.

        1. Expert Member
          Zephyr7 | | #14

          >”Not sure poly will work as it is too slipery and the carpet installers will make a mess of it.”

          That’s a good point. Maybe one of the rubberized roofing membranes would work better here.

          Another option might be something like 1/8” hardboard with the seams taped.


        2. Expert Member
          Deleted | | #15


        3. Austen G | | #17

          Spray foam and maybe even just some silicone for the finer seams very well might be ideal. Doing it today so will likely go that direction. Thank you!

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