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Current thoughts on insulating “bonus room” floor above garage

jadziedzic | Posted in Energy Efficiency and Durability on

We’re planning to use the “bonus room” space above an attached garage in a to-be-constructed home as a craft room for my wife.  The subfloor will extend across the entire width & depth of the area above the garage ceiling joists, with insulation running from eave to ridge with proper ventilation.  Where I’m a bit puzzled is the treatment for the area under the floor.

I’ve seen recommendations for rigid foam between the ceiling joists pressed tight to the subfloor, caulked and sealed, with fluffy insulation filling the remaining joist space, followed by a 1- or 2-inch layer of rigid foam on the bottom of the joists (thermal break), with 5/8″ drywall covering the area.  That seems to me like it would be creating an entirely sealed area between the foam “sandwich”, and I’m not sure if that’s an area for concern.

What’s the current thinking on how this ceiling/floor area should be insulated?  I want to be sure it’s nice and comfy up there even during our New Hampshire (CZ5) winters.  Would a 1- or 2-inch layer of closed-cell spray foam under the subfloor, dense-packed insulation, then rigid foam followed by drywall be OK?


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  1. Expert Member
    NICK KEENAN | | #1

    You want it to be relentlessly airtight, both for comfort and also to keep fumes from the garage out of your bedroom. In your climate the vapor barrier should be on the warm side. Ideally it's vapor-open on the cold side because the vapor drive is going to be from inside to outside.

    You really don't want plumbing or HVAC in this space.

    I think the recommendation for foam against the floor is to create a vapor barrier. Under a roof that would be problematic because it's hard to make it perfect, but here you're going to have pretty good drying potential so it should be OK. The issue is going to be whether you can get the foam in there, whether there will be wiring and bridging and even nails from hardwood flooring that get in the way. Spray foam would be an easier install but may not be necessary. Below the foam I'd just do insulation batts and drywall, you want it to dry to below so no vapor barrier. The drywall should be meticulously detailed with light boxes caulked to air seal.

    You also want to make sure the rim joists are sealed.

  2. nynick | | #2

    When I built my detached garage (unlike yours) 10 years ago, we built a second floor above it that I made into a lovely apartment. It is rarely used as such, but is handy to have for kids, relatives, friends coming to stay etc. I also have my office up there.

    We spray foamed the whole building, including the garage walls. No insulation was put in the garage ceiling/apartment floor.

    Since I had to heat/cool the apartment (your bonus room) anyway, I put a hanging "Modine" type heater in the garage that comes off the heater that heats the apartment. I work in the garage sometimes and have a shop in there. I keep the garage thermostat set to 50. We live in NY so it gets plenty cold here.

    It's great to have the garage semi-heated in the winters. 50 degrees is heaven compared to 10 or 20 outside.

    My point is, since you're in the planning stage, make provisions to warm your garage also. This eliminates any needed insulation in your bonus room floor, and keeps your cars warm too! The cost of insulation in your garage walls will be minimal.

  3. charlie_sullivan | | #4

    It needn't be that complicated. You can skip all the discussion in the BSC link above about whether it's better to have the air space above or below the insulation if you just fill the whole cavity with cellulose. And make sure you have the garage ceiling 100% airtight, for thermal, fumes, and fire reasons. If you want, you can add a layer of insulation boards below the joists, but if the joists are deep enough and you have the garage at least somewhat insulated so it's not as cold in the garage as it is outdoors, the thermal bridging of the joists won't be much of an issue--the heat loss through them would be vastly lower than the heat loss through a 2x6 in a wall, for example.

  4. Expert Member
    PETER Engle | | #5

    If you use I-joists (common for long spans in garages), the webs have far less thermal bridging than sawn lumber. For a CZ-5 floor over a garage, chances are that you will be using 12" or greater joists anyhow. Simply fill them with cellulose. Don't bother with dense packing it. If it settles a little bit, then you get the airspace the Joe L. is recommending. Just be absolutely certain that the ends of the joist bays are airtight, or you will get cold air shooting across between the insulation and flooring and that negates all of the insulation value. You may want to spray foam the ends of the joist bays just like doing the band joists at foundation level to get this finicky are well air-sealed and insulated.

  5. woobagoobaa | | #6

    Here's what I did for my bonus room. The sub-floor was up. We sprayed in 3" of closed cell foam essentially creating an air sealed bowl into which we dense packed mineral wool, then laid down the Advantech subfloor. Some thermal bridging still going on, but pretty toasty.

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