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Community and Q&A

Insulating wall of attached garage

kevek101 | Posted in Energy Efficiency and Durability on

Hi I just wanted to double check my thinking here with the pros. Got a 50s prefab cape in VT. An addition was built ooff back of house connecting existing garage to house. They now share a wall. On the house side it’s 2x4framing with batt insulation and a vapor barrier on conditioned side of house. On the garage side it’s now 3/4 sheathing (yup 3/4) and 2×6 framing. Looks like the garage sheathing between house may have house wrap on it.

I’m thinking of adding 5_1/2 rockwool between studs in garage. Garage sill plate is on open (non filled) CMU and the garage generally gets pretty humid when it rains. I’m a little concerned about trapping humidity in this wall system since its essentially a double stud wall with sheathing between the two, and vapor barriers on one of the walls. Could adding insulation on garage side prevent framing and sheathing from drying out.

Overall the house is pretty effecoent so I don’t necessarily need the insulation between garage and house, I’m just looking for sound dampening qualities.


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  1. Expert Member
    Dana Dorsett | | #1

    Adding insulation to the 2x6 side of the assembly will keep the 3/4" sheathing warmer in winter, for a much lower moisture content.

    As long as you don't put anything too vapor tight on the garage side (latex paint on wallboard is fine) it won't be a problem.

    For optimal sound dampening first carefully air seal any penetrations of the 3/4" sheathing, and caulk the perimeter of each stud bay to the sheathing. Then rather than installing a full cavity fill install some 3.5" thick R15s in the middle, with an inch of space between the batt & sheathing, and an inch of space between the garage-side gypsum, making sure that the batt makes good contact with the framing- no gaps, and minimal or no contact with the plywood or gypsum. Air-seal any electrical boxes and wiring penetrations of the framing, and air seal the boxes to the gypsum board. Taking it a step further, caulking the gypsum to the framing with Green Glue™ can also make a difference.

  2. kevek101 | | #2

    Thanks Dana. That makes sense. The insulation on the garage side would moderate the cooler temps from the garage before they hit the warmer surfaces of the framing on the house. I just wasn’t sure if the house wrap between the garage and the house would have any effect.

    With regard to soundproofing I’ve got a mix of 2x4 and 2x6 studs in the wall. Might be difficult to get insulation right in the middle of the two different sized studs, unless I use a rigid fiberglass or rock wool board...I think they tend to only be two inches thick so it could sit flush with the edge of the 2x4s and be right in the middle of the wall, leaving an air barrier on both sides. Is the thinking that the two air cavities will dampen some more than just insulation alone?

  3. GBA Editor
    Martin Holladay | | #3

    Dana answered your insulation question. But I'm concerned with another issue you mentioned: "The garage generally gets pretty humid when it rains."

    I'm guessing that your garage has insufficient roof overhangs, or that the grading around your garage is flawed. To address these issues, you may need to adjust the grade on the three exposed sides of your garage so that the grade slopes away from the building. If there is an "uphill side," you may need to install a swale to interrupt water that comes down the slope.

    Finally, if you are insulating the walls of your garage, remember that you will need to install vertical rigid foam at the slab perimeter. Two inches of rigid foam (ideally, EPS) works fine. The foam should be at least 24 inches wide, and it should extend from the top of the slab into the soil. (You'll have to dig a trench around your garage to install this foam.) Remember that you need to protect the above-grade foam with a durable material to prevent physical abuse or degradation from sunlight. You'll also need metal Z-flashing at the horizontal joint between the top of the foam and the lowest course of siding.

    This rigid foam will help retain heat during the winter. It will also help prevent your slab from absorbing moisture during rainstorms, which will help address the humidity problem in the garage.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "I’ve got a mix of 2x4 and 2x6 studs in the wall. Might be difficult to get insulation right in the middle of the two different sized studs."

    A. On walls with this problem, install vertical strips, 2 inches by 1.5 inch and as long as the stud, on the edges of the 2x4s to make a co-planar surface for your drywall. Then the stud bays can be insulated with batts that are 5.5 inches thick.

    The vertical strips I describe can be make of softwood lumber -- run some framing scraps through your table saw -- or rigid foam. If you use rigid foam, you'll need long drywall screws for those padded-out 2x4s.

  5. kevek101 | | #5

    I think the humidity in garage is partially due to grade, but I’m not at bottom of a hill and the soil is sandy so any damp from slab drys quickly. However it could also be from condensation, but that’s getting into a completely different topic. I know how I may address grade so there is more positive drainage, but I wasn’t sure about rigid in ground on outside of block wall. I should add the block only sticks out of ground by a few inches. There isn’t much of a stem wall. Also I only have access to two sides of garage foundation. The third side is covered completely by driveway, and forth side is shared with house. Any value in insulating just two sides of foundation when most of it is below grade?

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