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Airtight drywall approach — very uneven top plates / studs

Sfair74 | Posted in Green Building Techniques on

Hi, I have a question regarding the airtight drywall approach and very uneven top plates. We are currently gut remodeling a post-WW2 single-story Cape Cod.

One of the many issues I face is that the lumber used for the top plates is very uneven. The top plate is a full 2×4” with a 1×4” on the top. the studs are a full 4” wide but in places the top plates are only about 3.5” wide in some places. Also many of the studs require a great deal of shimming just to make the drywall flat.

When we removed the drywall there was a lot of various shims used including fiberboard, drywall, wood, etc.

I intend to use the Airtight Drywall Approach to help air-seal the house. I will be ordering Airfoil boxes for all the exterior wall boxes and ceiling fixtures. I wanted to check to see what must be done to the perimeter and top plates to use ADA. Iif shims must be added to the perimeter where gaskets will be applied what must be done to air seal between the shim and the stud / top plate??

Are there any tips or tricks to preparing uneven studs where gaskets should be placed for ADA?

Currently the attic floor joists are exposed. Would it be preferable to air seal the drywall to the top plate with cans of spray foam after the drywall is hung along with gaskets?

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  1. GBA Editor
    Martin Holladay | | #1

    There are two issues here.

    The first issue is the need to make the framing members co-planar so that the drywall can be physically fastened to the framing in a way that prevents screw pops and cracked drywall. I'm afraid that the only ways I know to achieve this are to use shims and/or a power plane. (If you use a power plane, watch out for embedded nails and screws.)

    For tapered shims, I use split cedar shingles. If a need a shim without a taper, I make it on the table saw.

    The second issue concerns achieving a good air seal (in connection with the Airtight Drywall Approach). If you have good access from the attic, it seems to me that any joints that can be addressed from above with canned spray foam should be dealt with that way, because it will be faster than creating airtight shims.

    If you have any joints that can't be reached from the attic, I advise to to attach your shims with a continuous bead of construction adhesive and staples. Then install your gasket as you would normally.

  2. Sfair74 | | #2

    Upon clearing the old poorly done shims for drywall I have noticed just how bad it will be in many locations. I feel that it may not be a good use of time to correct how badly the studs are out of plane.

    I have a couple of ideas on an alternative that should potentially be faster and yield a better result.

    I would like to build a new wall right against the existing wall. I’m torn on either using 2x? horizontal strapping or vertical strapping over the existing studs. With either method I could establish a new plane with a small gap between the new top and bottom plates. If I went with vertical strapping I could use some temporary plywood gussets on the top and bottom plates and then hold the material in place and shim behind it every 12 or 16 inches. This would save me from needing to shim the whole face of the studs. This would also have the benefit of increasing the cavity depths from a full 4” to 4” + strapping + gap. If using 2x4s ripped in half or 2x2 strapping that is a 6” to 6.5” cavity depth.

    A local hardware chain buys materials in huge bulk orders and often has prices 20-30% lower than big box stores on high volume items. I can purchase R30 unfaced fiberglass rolls for less than I can get R19 kraft faced fiberglass from a big box store per SQ FT. According to fiberglass insulation compression charts R30 will compress to R21 in a 5.5” cavity and R25 in a 7.24” cavity. I expect to get R22 to R23 in my proposed wall.

    I suspect vertical strapping will be easier to fill with fiberglass insulation.

    If I intend to use the Airtight Drywall Approach I suspect I will need to use canned foam to seal between on strapping and the existing wall in all areas when I need to apply gaskets. Do i need to worry about the small gaps between the vertical strapping and the old studs? How large of a void do i need to worry about before it create convective currents? If the void is small enough does it count as dead air space?

    Most of the house has existing hardwood flooring that we want to keep. I expect that I will need to determine where the new strapping bottom plate will be and run a chalk line and use a circular saw to remove 2-3” of hardwood flooring from the sub floor along the exterior walls.

    Another issue I am concerned with is the exterior foam. The wall currently has R5 3/4” polyiso over 7/16th OSB. I’m worried that the R5 over 2x4 will cause an issue if I increase interior R value by increasing the cavity depth and fiberglass insulation density. I am in the very southern portion of zone 6.

    I will not need to apply an interior layer of poly and I do intend to limit air movement in the wall with drywall as the air barrier. Can I get away with R21 to R23 interior with R5 foil faced on the exterior if using ADA and allowing drying to the interior?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Do I need to worry about the small gaps between the vertical strapping and the old studs? How large of a void do i need to worry about before it create convective currents?"

    A. If you decide to use fiberglass batts to save money, you have to accept the fact that the wall will never perform quite as well as it would if you installed dense-packed cellulose or blown-in fiberglass. But you are deliberately making a trade-off in order to save money, so you probably know that.

    Q. "If the void is small enough does it count as dead air space?"

    A. If the void is adjacent to a fiberglass batt, then it's not really "dead" air space. Fiberglass is air-permeable. That's why it's used to make furnace filters.

    Q. "I am in the very southern portion of zone 6. ... Can I get away with R-21 to R-23 interior [insulation] with R-5 foil faced on the exterior if using ADA and allowing drying to the interior?"

    A. No. In your climate zone, the minimum R-value for foam sheathing on a 2x6 wall is R-11.25. If your interior insulation is about R-23, you might even have to increase the R-value of the rigid foam to a value that is higher than R-11.25, since the R-23 insulation between your studs is more than is found in a typical 2x6 wall. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

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