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Retrofitting garage to a heated shop – slab and wall details

Tyler Keniston | Posted in Green Building Techniques on

Hi all.

I am soon to be owner of a house with an attached garage, roughly 20×30. I would like to convert the garage to a heated wood shop.

I am in climate zone 6A (Maine). I will have consistent heat in the shop, probably in the 55 degree range, bringing it up to 60-64 for working temp. I am leaning towards heat pump as heat source.

The garage is a concrete slab, the details of the footings/stems I do not yet know however.

The structure is 2×4 studs sheathed with (I believe) T1-11 exterior ply. Roof trusses (not raised heel). Stud bays currently open to inside.

So my question in broad terms is what should my strategy be for insulating / air sealing.

What I am scheming is to build another 2×4 wall on the inside to create a double stud wall (insulated with dense pack and or rock wool), and then to blow in cellulose on the attic floor (and vent attic). I figure the thicker wall will help to mitigate the small triangle created by the non raised heel trusses.

The slab is where I am really scratching my head. My two thoughts currently are to add insulation over the concrete and/or to dig around the slab and install a frost protecting layer of insulation. Is insulation over the slab doable if detailed correctly (where to add the ?polyethylene?). I am also concerned about head space, but if this is generally advisable, how much and of what (eps foam?)

Thanks for any thoughts!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tyler,
    The most important detail when it comes to the slab is to install vertical rigid foam (at least 2 inches thick) around the perimeter of the slab at the exterior. In your climate zone, you want the rigid foam to extend from the top of the slab into the ground about 2 feet. So you'll have to dig a trench around the exterior of your garage.

    The above-grade portion of the rigid foam will need to be protected with metal flashing, pressure-treated plywood, or stucco-over-chicken-wire.

    Don't forget to install metal Z-flashing at the horizontal seam where the bottom course of your T-111 siding meets the top of your rigid foam.

    If your slab has vertical rigid foam at the perimeter as I've described, you can get by without any horizontal rigid foam above the slab -- especially for a shop. That way, you won't lose ceiling height.

  2. Tyler Keniston | | #2

    Thanks Martin for the response.

    "If your slab has vertical rigid foam at the perimeter as I've described, you can get by without any horizontal rigid foam above the slab -- especially for a shop."

    Is this primarily because the heating differential will be less than a house? While I don't need my shop toasty warm, I want it to be operable at very small heating costs.

    The rigid foam will effectively put the slab in the thermal envelop, correct? 2 inches 2 feet deep is enough to keep the slab from becoming a heat sink? Also the slab will, if left as is, not really be an air barrier, correct?

    If ceiling height was not a concern, would perimeter insulation still be the preferred method vs above slab insulation (does the perimeter insulation reduce the likelihood of frost heaving too)?

    If I did the perimeter insulation, would it be a bad or good idea to also lay down an air barrier and a small thickness of foam above the slab (say 1-2 inches)? I am thinking about putting a wood product floor over the concrete anyways.

  3. GBA Editor
    Martin Holladay | | #3

    Tyler,
    Q. "Is this [the suggestion of omitting horizontal rigid foam] primarily because the heating differential will be less than a house? While I don't need my shop toasty warm, I want it to be operable at very small heating costs."

    A. If you want the lowest possible heating bill, and you don't mind losing a few inches of ceiling height, you should definitely install the above-slab foam. Remember, though, that you still need the vertical rigid foam at the slab perimeter. That's where the heat loss is greatest.

    Q. "The rigid foam will effectively put the slab in the thermal envelope, correct?"

    A. Yes.

    Q. "2 inches 2 feet deep is enough to keep the slab from becoming a heat sink?"

    A. I'm not sure that I understand your question. Heat will always flow from a warm area or object to a cold area or object. The best you can hope for with insulation is to slow down the rate of heat flow.

    Q. "The slab will, if left as is, not really be an air barrier, correct?"

    A. Concrete is an effective air barrier, as long as you caulk any significant cracks.

    Q. "If ceiling height was not a concern, would perimeter insulation still be the preferred method vs above slab insulation?"

    A. Yes. Vertical insulation at the perimeter always comes first.

    Q. "Does the perimeter insulation reduce the likelihood of frost heaving too?"

    A. Possibly. But if the garage slab has been there for a few years, and it's still level, I wouldn't worry much about frost heaving. The best way to reduce the chance of frost heaving is to make sure that the exterior grade slopes away from the building (to keep the area dry).

    Q. "If I did the perimeter insulation, would it be a bad or good idea to also lay down an air barrier and a small thickness of foam above the slab (say 1-2 inches)?"

    A. You don't need an air barrier, but the horizontal rigid foam would be a good idea.

  4. Tyler Keniston | | #4

    thanks for the concise response. That is very helpful.

    "Heat will always flow from a warm area or object to a cold area or object."

    Good point; my question didn't really make sense. I suppose what I was really wondering was if the perimeter insulation (at a thickness of 2 inches and a depth of 2 ft) would slow the heat loss through the slab (into the ground or at grade) sufficiently to be 'on par' with my efforts above grade (subjective, yes. And I realize R value recommendations change from ground, to wall, to roof).

    I was thinking it would be worth doing some heat load calculations to determine how thick to build the wall (Probably not going for the 12" thick wall, maybe just a 2 inch gap or so for a total of 9" cavity).

    I see your point, though, that I need to insulate the outside of the slab to stop heat loss through its 'edges.'

    Does this detail make any sense: Lay some above slab insulation before building the inner wall and land the inner wall on that insulation, thereby creating a continuous thermal envelop from the floor into the wall assembly?

  5. GBA Editor
    Martin Holladay | | #5

    Tyler,
    Q. "Does this detail make any sense: Lay some above slab insulation before building the inner wall and land the inner wall on that insulation, thereby creating a continuous thermal envelop from the floor into the wall assembly?"

    A. Sure -- as long as you don't mind buying and installing long TapCon fasteners to secure the bottom plate of the new wall.

  6. Tyler Keniston | | #6

    After scouring the site (and other sites) for more detail on the foam above the slab, it seems most are recommending no poly below the foam. (Such as in the 'No-Mold finished basement')

    Why is it desirable to allow vapor diffusion up through the foam, which then (according to that article) needs to be taken care of with a dehumidifier? Why not just completely block it off from entering the space. It is routine to place poly UNDER the concrete to isolate the moisture below, is there some reason that moisture IN the concrete (assuming it has no poly under) can't be kept IN it.

    From my lay theoretical standpoint (no experience with it), I don't see why trapping moisture below the slab is any different than trapping it in the slab yet below the floor assembly where all the items that are adversely affected by moisture are. (Unless moist concrete in and of itself causes issues I am unaware of)

  7. GBA Editor
    Martin Holladay | | #7

    Tyler,
    Q. "After scouring the site (and other sites) for more detail on the foam above the slab, it seems most are recommending no poly below the foam."

    A. You evidently missed a GBA article called Preventing Water Entry Into a Home, in which I wrote, "The fix for an older home that lacks a layer of polyethylene under the basement slab is to install a layer of polyethylene above the slab, followed by a layer of rigid foam and a layer of plywood."

    There is nothing wrong with installing a layer of polyethylene above your concrete slab before you install a layer of horizontal rigid foam.

    The main reason that most foam-over-slab jobs don't require polyethylene is that most slabs already have polyethylene under the slab. If you are worried that your slab lacks polyethylene underneath, go ahead and install it above.

    The second reason that most foam-over-slab jobs don't require polyethylene is that rigid foam is a vapor retarder -- in some cases, close to a vapor barrier. So the rigid foam does a good job of stopping upward vapor diffusion, even without the polyethylene.

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