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Community and Q&A

Insulating a finished garage with unvented cathedral ceiling

user-6239952 | Posted in Energy Efficiency and Durability on

Hello –

I am hoping for any input/advice about insulating my garage, which I’m in the process of finishing. The details are:

This is a 20 x 20 garage, uninsulated concrete slab floor with an epoxy finish on it. Unvented roof which will have drywall up to the peak. The walls are ordinary 2×4, the ceiling is 2×6. The garage has several windows, all double-page argon/low-E and a insulated garage door that seals surprisingly well (but it’s still a garage door).

This is in Denver, CO zone 5.

The sound-deadening benefit of rockwool as well as a few other factors make rockwool a very tempting option. But, since the roof is completely unvented I’m asking around and getting all sorts of suggestions about how to best handle it.

Budget is tight so furring/foam, etc. are a stretch but I don’t want to underspend and then regret it. It would be easy to install a ceiling and blow it full of something, but due to the way this room is going to be used I need to have the rafters exposed.

I would be very interested in any input! thank you very much 🙂

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  1. GBA Editor
    Martin Holladay | | #1

    You have only a few options if you want to create an unvented cathedral ceiling. If you are working from the interior (without installing new roofing), your only real option is to use closed-cell spray polyurethane foam.

    For more information, see How to Build an Insulated Cathedral Ceiling.

    You'll have to install vertical rigid foam at the perimeter of your slab. The rigid foam should extend about 2 feet into the soil. Protect the rigid foam with metal flashing or pressure-treated plywood, and install Z-flashing to connect the bottom course of your siding with the protective material installed over your vertical rigid foam.

  2. user-6239952 | | #2

    Hi Martin,

    Thank you for your advice. Unfortunately I'm not sure I have budget for both the spray foam and insulating the slab. Is it really necessary to do both? I was more concerned about the roof, and I thought that having an uninsulated slab was going to make it harder to heat the place but it wouldn't really be a risk to anything.

    thanks, Dave

  3. GBA Editor
    Martin Holladay | | #3

    Q. "I'm not sure I have budget for both the spray foam and insulating the slab. Is it really necessary to do both?"

    A. No, it's not necessary (as long as you meet local code requirements). I'm just describing the right way to proceed if you want to reduce heat loss.

  4. user-6239952 | | #4

    Gotcha. Well, like many I'm trying to balance the heat loss with the budget and I have decided that improving the slab floor is a non-starter. I am going to put in an 18k mini-split heat pump, so I am hoping that the place will be plenty comfortable although the floor might be cool to bare feet.

    Today, someone came and offered to install Bayer spray-foam for .80 cents per board foot. This is the lowest price I've heard yet, and that option would allow me to do something like:

    Walls: 540 sq feet with 2 inches of closed cell = $864
    Ceiling/Roof - 520 sq feet with 3 inches of closed cell = $1,248

    Or, possibly just do the ceiling. The installer told me they use Bayer foam and it's possibly to spray 3 inches in one pass, without needed to let it cure every 2 inches.

    There would be an airspace left over in both the ceiling or the walls, which I supposed could be filled in but not sure if that's necessary. It might be cheaper to just foam the ceiling and do the walls in rockwool or fiber.

    The price per sq. foot is so much lower than other companies, and they are a 2 person shop with hardly any online presence/reviews, etc. but they seem to have tons of references and have done a lot of commercial jobs.

    I am seeing prices around $1.20 per sq foot from most companies, but they are larger companies with greater expensive so maybe this makes sense. That reduction in price is enough to let me to a definitive solution and get this garage sealed up nicely!

  5. user-2310254 | | #5

    If using flash and batt, Martin's articles states you need r-20 of closed cell in combination with r-29 of fiberglass or cellulose (at least to meet code). With the 3 inches of foam, you would be close to the first requirement but... Would your budget allow you to fur down the ceiling?

  6. user-6239952 | | #6

    Unfortunately not but to be honest I'm not all that concerned with meeting code. In the end, this is a garage conversion so I want to be realistic.

    My goals are really to (1) get heating and cooling in there without huge electric bills and (2) any sound reduction would be a bonus.

    What is starting to sound economical and realistic is to have the spray foam contractor do 4 inches up on the ceiling, then do an R16 of rockwool batts on the walls. This could really work well and the only sounds that I am hoping to reduce are adjacent to the building on in the alley, so rockwool in the walls would be helpful for that - the sound abatement is a secondary goal anyways.

    I'm also tempted to do 4 inches of foam on the ceiling and 3 in the walls, as suggested by the spray foam contractor. I will see how the cost compares, but the $.80/ft. price really brings spray foam into the affordable range for me.

    If I were to put rockwool in the walls, I think I would still need to put a vapor barrier between the batts and the drywall, right? The contractor said they put a plastic barrier there but I wonder if there is a specific type that might be best.

  7. GBA Editor
    Martin Holladay | | #7

    In a flash-and-batt job, you never want to install interior polyethylene. For more information on flash-and-batt jobs, see Flash-and-Batt Insulation.

    In your climate zone (Zone 5), if you intend to combine spray foam and fluffy insulation, you need to obey certain ratio rules. For walls, the spray foam R-value must represent at least 27% of the wall's total R-value. For roofs, the spray foam R-value must represent at least 41% of the roof's total R-value. For more information, see Combining Exterior Rigid Foam With Fluffy Insulation.

  8. user-2310254 | | #8

    Dave. If noise is an issue, two layers of 5/8 drywall is often a good solution. You also need to air seal any penetrations. But if you have windows in the alley-facing wall, you still may have a noise issue.

    Be sure to research your foam installer. Improperly installed foam is a stinky nightmare.

  9. user-6239952 | | #9

    It's definitely looking like closed cell foam is going to be the way to go on the ceiling, and probably rockwool on the walls.

    Reading this forum has me worried about 'getting it done right' so I don't want to go cheap on the foam. But wow I am getting some wild pricing from local vendors with good repuations. For example, the lowest price I've heard so far is $1.20 and they all seem to estimate the sq. ft of the ceiling/walls at least 10% higher than I do. I am also seeing that they don't remove any sq. foot for windows, and 2 of the windows are 48x48 which is significant. One of the estimators told me that there is so much business in Denver right now, they are all charging full price and still can't keep up with demand.

    Meanwhile, I checked some references on the cheaper installer that I met. One of the references was from one of the larger insulation companies in the city, with a great reputation, and actually the same vendor that gave me the highest bid I got - $1.60/ft. The owner of that company told me that the installer I was considering worked for him for years, and last year started his own business out of his home, with his son. He said that he was a great installer and had done all the classes and had done over 200 jobs while we worked for him. He also said that the guy was probably lowballing jobs trying to get started. Homeadvisor shows him as a vetted contractor, which really just means that he's licensed, etc.

    I called 2 other references he provided, one commercial space and they seemed happy enough. Another was a residential and they seemed happy although clearly they had no idea if the job was done well or not. They said it was very warm in their finished attic, no smell, no mess.

    I am really torn. Usually I am happy to pay for good quality services, etc. but there is something so tempting about saving this $$! It's a big ticket item, but then again if it goes wrong it will be costly to mitigate. But then again, ultimately this is a garage not a living space so that risk is limited. Then again maybe I should just pony up and get it done right. But then again, sometimes a nice deal falls from the sky.

    So, it's a tricky decision!

  10. DavisG | | #10

    Hi Dave, I am digging this thread up because I am in the exact same boat you were in. I am also in Denver, doing a 23x25 detached garage and wanting to do a cathedral ceiling. IRC does require 3" of closed cell. But I wonder whether a garage would ever have enough vapor to cause a problem if one used batts in the ceiling. I figure here in Colorado winters, relative humidity is pretty low. Plus, a garage doesn't have vapor sources except for maybe evaporation of water off your car. Plus, garage doors are leaky and thus create a high air exchange rate. It's all conjecture, I know, but I am seriously wondering if the spray foam is worth it.

    What did you end up doing, and did you learn anything to share?

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