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Garage, Bonus Room, and Basement Insulation

Renovations102 | Posted in Energy Efficiency and Durability on

Hello experts!

I am in zone 5B

I have some questions on three different areas of my home before I OK the work to be done…


I am fully insulating & going to do a two-zone mini-split to serve the garage & the bonus room above the garage. I installed R-23 RockWool in the walls, covered w/ Membrain, and then 7/16” OSB.

My new home energy company slated to do the work are proposing to spray foam the common wall between the home & garage w/ 3” of closed cell that cures to ~R-7 per inch (R-21). They also proposed spraying the garage ceiling w/ 2” of closed cell to R-14 to insulate & help isolate garage from bonus room.

My question is I’d like some input on spraying another inch or two to the common wall with the house as well as 1-3 more inches on the ceiling. I’d like to keep any heat dumped into the garage there as long as possible & 2” on the ceiling seems “thin” to me…

Bonus room above the garage:

The original plan by the first contractor was to install Accuvents w/ 5.5” of closed cell from soffits to peak. That contractor was a disaster & I had to fire them before the work was done. The next contractor pointed out that the roof vent running the length of the room actually has tar paper over it, so it does not actually communicate with the outside air. (Don’t ask me, I just shook my head). As the vent is currently useless, he recommended the same 5.5” of closed-cell BUT to block off the soffits w/ 2×8’s and then spray the entire roof line in a sealed fashion as the roof vent is useless at this point. He stated “that is the way I did the bonus room on my house.”


My unfinished basement has no water leaks & has a perimeter drain system that works great at this point, but it gets pretty cold (frost line is ~5’ down). They want to insulate the entire perimeter w/ 2” of Thermax rigid insulation. Rim joists are already closed-cell spray foamed to R-21.

My question is: They stated if there are no leaks, then there is no need to apply any type of waterproofing agent to the walls before the foam boards are glued up. Do you all agree with that?

Thank you all in advance for your help…

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  1. qofmiwok | | #1

    Is this a new house or existing house? Regarding basement insulation, foam is the best option but even with that I would wait until the house is a few years old. Concrete is very wet for the first few years.

    1. Renovations102 | | #3

      Sorry 15 years old

  2. Deleted | | #2


  3. Renovations102 | | #4

    Bumping for some input...

  4. Expert Member
    BILL WICHERS | | #5

    Why all the spray foam everywhere? That's not the only way to go, and sometimes not even the best performing.

    For the wall between the house and garage, I would use mineral wool batts in the stud cavities, then rigid foam (polyiso) on the garage side (I'd probably use 2" myself, but you could use less). I would put a bead of polyurethane sealant around the perimeter of the wall prior to putting up the polyiso up, then I'd tape all the seams. This would give you a very good air barrier between the garage and the house. I'd then put up the required 5/8" drywall over the polyiso, finished in the usual fashion. This gives you the required fire barrier, and a redundant air barrier too. This wall will also perform better than one with only spray foam between the studs.

    I would try to do the garage ceiling the same way if possible, with rigid foam underneath the joists, but it's less of an issue there. You could fill between the joists with blown cellulose too, or use batts. The advantage to using rigid foam underneat the joists is a bit more R value, and a second, redundant air barrier.

    The attic is a different story. If youl have room for venting, and can vent it properly, then building it as a vented assembly is likely the option that will give you the least potential problems down the road. You need to be able to fit sufficient insulation in there too. If you can't build it as a vented assembly, then going with a "hot roof" (unvented) assembly using closed cell spray foam is your best option here. Another reason to go with spray foam and an unvented assembly is to squeeze a little more R value into the roof without loosing ceiling height. I would try for the vented assemly if possible, even if it means trading off for a bit less R value.

    BTW, "proper" venting is really important and means continous vent channels in every rafter bay all the way from soffit vents at the eaves to a ridge vent. There are some other ways to do it, but the "soffit and ridge vent" way is the most common and usually the easiest. If you can't do that due to the way the roof comes together, then spray foam is the next best option.


    1. Renovations102 | | #6

      Hi Bill,

      This is what the contractor suggested... I expressed the "hot roof" concern. He said lots of people do it, every manufacturer of shingles now covers any potential heat damage, and that to go the other route, a roofer would need to come out, rip off the vent, cut everything away, widen the opening, and reinstall the roof vent.

      As for the common wall, I thought about mineral wool. The trim around the door on the garage side will sit on top of 5/8" drywall. If I installed 2" of Thermax, it would stand out 2" out from the door opening / past the trim, & you would see the 2" of foam when you are in the garage & when you open the door from the does one address that?

      Garage ceiling may not have drywall for some time due to the outrageous quotes I'm getting (34'x25'), which ruled out my choice of dense-packing it... IF he foamed it, I was going to tape all advantech seams up top before the floor in the bonus room goes down for more air-sealing. In addition, it has the 1-by strapping common in the NE which would make rigid foam almost impossible to install...

      And what say you on the basement walls?

      Thanks for your input.

      1. Expert Member
        BILL WICHERS | | #7

        Installing a ridge vent goes quick, and should be cheap. You rip the slot with a circular saw, tack on the ridge vent, cover the top with some shingles to make it match. There aren't too many things that easy :-)

        You would trim around the rigid foam. I would trim out the inside of the opening in the normal way, but long enough to come out flush to the surface of the drywall, then install casing normally. It's no different than installing a door in any other thick wall. You don't need Thermax here, either, the cheaper types of polyiso will work fine here. The primary advantage to Thermax is that it is rated to be left exposed in places like basements. If you're going to drywall over it anyway, you might as well use the cheaper "regular" polyiso.

        You HAVE TO drywall the ceiling if you have living space above it! This is a fire code requirement! It has to be 5/8" drywall too. Don't skimp here.

        Your climate zone should put you in R15 territory for basement wall insulation. 2" Thermax is R13, so not quite there. Since you're starting from nothing though, R13 is a BIG improvement -- just not quite to code. Some cities will be fine with this anyway. Aside from that, I see no issue here. If you don't have a bulk water problem (water leaking through the walls after a heavy rain, for example), then you can put the polyiso up directly against the wall without worry. If you want a little extra insurance against possible future leaks, trim the polyiso to leave a small gap of 1/2 to 1 inch along the bottom of the polyiso instead of setting it directly on the floor. This way, if there is ever a leak, the polyiso won't be sitting directly in water and won't wick up that water.


  5. Expert Member
    Dana Dorsett | | #8

    >"My new home energy company slated to do the work are proposing to spray foam the common wall between the home & garage w/ 3” of closed cell that cures to ~R-7 per inch (R-21)"

    That meets a code-min R20 in a 2x4 wall, but the thermal bridging SUBTANTIALLY robs it of it's performance, reducing it to no more than a full-fill R13 using cellulose. Do the math (or read the math). Closed cell foam between studs is a waste::

    The analysis in the link was on 3.1/4" of foam, more than 3", so the thermal bridging of the proposed 3" is even slightly higher. Even at the 3.25" it's a net performance gain of less than R1 in whole-wall performance compared to an R13 batt wall, even less than an R15 rock wool batt wall. That's an EXTREMELY big upcharge for <R1 improvement!

    A continuous half inch of foil faced polyiso foam board under the interior side gypsum would deliver an R3 improvement over batts, at a much lower overall price point.

    >"They also proposed spraying the garage ceiling w/ 2” of closed cell to R-14 to insulate & help isolate garage from bonus room."

    Instead of 2" of closed cell foam (with pretty extreme thermal bridging), 8" of half pound open cell foam (R28 at center cavity , but with half the thermal bridging) would beat it on performance using the same amount of polymer, usually for less money. R30 under floors is the IRC code minimum for zone 5:

    The bonus room subfloor above is an adequate Class-II vapor retarder, even if you went ahead and used a full cavity fill of cellulose or fiberglass, with a gypsum ceiling in the garage.

    Even HFO blown closed cell foam has a substantial CO2e footprint, largely due to the high polymer weight per R. Open cell foam is blown with water, and is roughly half the CO2e footprint of closed cell foam at any given R, making it comparable to fiberglass or rock wool. See:

    1. Deleted | | #9


    2. Renovations102 | | #10

      Hi Dana,

      I see a link to NV, I'm actually in NH.

      Thank you for your input. I forgot to mention that all walls including the common wall are 2x6 & the rafters in the bonus room as well as ceiling/floor joists in the garage are I guess 2x8. Does your R30 below a floor quote apply to any floor, including above a garage?

      And what is your opinion on a non-vented bonus room? or would you insist they vent it?

      Also, do you feel that 2" of Thermax is sufficient on the basement walls or would you go thicker?

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