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Thread: Recommendation for ceiling/rafter insulation in garage

  1. #1
    Join Date
    May 2009
    Location
    SW Iowa
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    187

    Recommendation for ceiling/rafter insulation in garage

    My wife got a wonderful opportunity (new job) but it meant a dismantling of the old workshop, move to Chicago-land and now finally a rebuilding of the former workshop in the 2 car detached garage a couple years after we did the move. Basic 2 car garage, roughly 24 x 24 with a 7.5' clear height(challenge) but does have an attached Florida Room/3 season sunroom that works well for storage of some of the less frequently used items. Most walls are insulated(and already working to fix the ones that aren't) but I'm struggling with the best option for the ceiling so I can keep a reasonable temp in there and be able to continue work this winter. Key requirement is that it must be mounted close to the underside roof deck/rafters as I don't have the headroom space to loose any more height mounting to the bottom of the tie beams. I could possibly deck the top side of the tie beams and then use blown in or bats over that deck.

    What I've considered so far:

    1. Conventional fiberglass batt: Concerned that needs to be covered with drywall for fire protection and not excited about doing that, not difficult just never enjoyed hanging rock on ceilings
    2. Mineral wool/Roxul: no experience with it in an application like this but I like the product and have used in walls, rafters are on 24" centers which makes it more time consuming to put up as I've only ever seen it for 16" OC widths
    3. Spray foam: Benefits of insulation and vapor barrier, but mixed opinions on if it should be sprayed directly to the bottom of the roof deck. Brand new roof on the garage and would hate to cook my new shingles. Also not looking forward to having to get everything out of the space to do this well
    4. Rigid foam: Thinking of something like linked below as opposed to the normal polystyrene pink or blue foam board, bit cheaper and looks like it would be better for an exposed application and not require being covered by drywall

    https://www.homedepot.com/p/R-Tech-2-in-x-4-ft-x-8-ft-R-7-7-Rigid-Foam-Insulation-310891/202532856


    I'm kinda leaning towards the mineral wool/Roxul option with a separate, opaque white plastic vapor barrier or the rigid foam board..what does the group think? Any options I'm missing or thoughts or experience with a similar application? I'm not looking for a super insulated space, currently heating with propane until get a gas line run, goal is to be able to have it 50 degrees without spending a fortune on heat for the weekend and the 1 or 2 nights/week working out there, rest of time likely kept in the 40 degree range. Couple pictures of the existing condition are below

    20181112_205035.jpg20181112_205107.jpg

  2. #2
    Logan
    Fiberglass batt is typical go to insulation inexpensive and easy to procure and install. Fiberglass batt and mineral wool (Roxul) are not required to be covered by drywall by code. Neither will burn but there is some thinking that fibers will get into the air and they are not good to breath, so covering them is a good idea. Foam insulation is required to be covered by a fire barrier typically drywall. This is a code requirement because of the amount of smoke it produces when burning.

    My 2 cents would be to use paper faced Fiberglass batt. The loose fiber are possible but not a big risk in the scheme of things. Especially if it is stapled tight to the rafters.
    Also want to mention that the warranty on shingle will be void if a vented air space is not present below the sheathing.

  3. #3
    Join Date
    Mar 2003
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    SE PA - Central Bucks County
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    65,694
    Insulate with fiberglass and then put up either drywall, Homasote or a wood panel. Check local code requirements if this is an attached "garage" to your home...drywall may be required for this application. Be sure you take care of any overhead electrical needs before button things up...and I believe that the Chicago area has some very specific code requirements around that, too.

    In my shop, I recently insulated the ceiling with Fiberglas batts and have been installing surplus ceiling tiles stapled directly the the joists. The sound level reduction has been nothing short of amazing and it's containing my heat/AC well, too.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  4. #4
    Join Date
    May 2009
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    SW Iowa
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    187
    Quote Originally Posted by Jim Becker View Post
    Be sure you take care of any overhead electrical needs before button things up...and I believe that the Chicago area has some very specific code requirements around that, too.

    In my shop, I recently insulated the ceiling with Fiberglas batts and have been installing surplus ceiling tiles stapled directly the the joists. The sound level reduction has been nothing short of amazing and it's containing my heat/AC well, too.
    You're spot on about the electric needs, everything being required in conduit makes wiring more complicated but not horrible. Right now all the conduit is running on top of the rafter tie beams so this insulation project will have minimal impact there. Thank you for the comment on Homasote...that's a new product to me and the sound insulation benefits would be very nice and cost is very reasonable. I've actually got some surplus ceiling tiles....might have an option there as well

    David-My thought for the batt was to put 3.5" thick insulation in, with 2x6 rafters would then leave the 2" directly under the deck as airspace from vented soffit to roof peaks but haven't looked closely to see how many soffit vents i've actually got there right now and how many holes I'd need to add.

  5. #5
    Join Date
    Mar 2003
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    SE PA - Central Bucks County
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    Don't skimp on the insulation...R19/R21 is reasonably priced and with baffles in the 2x6 bays, you can fill them right up while still maintaining air flow for the roof deck. R-11/R-13/R-15 is really intended for walls where it works well, but it's not the best for up top where heat loss and gain is the highest.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  6. #6
    Join Date
    Sep 2016
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    Modesto, CA, USA
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    roxul does come in 24" wide batts. Fibre glass does burn and code may require covering it with drywall.
    AFAIK Roxul only comes in batts never rolls.
    Bill D.

  7. #7
    Now I live in SC and things are a little different here than up north, so anyth8g I say may not be code for your area. So, I just finished installing a ceiling 8n my workshop. Used metal, one came from the supplier of my shop building. If your room is not squared it could be cumbersome getting started as with anything else. The installer recommended that the any electrical is best to leave dangling and they will mount it easier on installation of the sheets. If you do wood same thing. I also added a platform in the rafters for attic storage or HVAC when I can afford it.

    Then added doubled layer R19 insulation bats that I bought off of craigslist. Something like 20 bags for $100. If you can afford have it blown in much easier and the. You can get more R value here you need it.

    Tom

  8. #8
    I'm in the Lowcountry of SC too and am working on how to insulate my shop as well. Waiting on a Craiglist deal on insulation too. I've decided that I can deal with summer it's the few weeks of winter I need relief from. I spend about 40 hours per 7 days week in my shop.

  9. #9
    I'm in the process of finishing up my new shop. Walls are fiberglass batts covered with 3 mil poly. For the attic/ceiling I put up drywall and I'm having 12+ inches of loose cellulose blown in. My new shop is 32 X 28 and I hired a professional to insulate it. They installed the insulation cheaper than I could buy it for. If you want to do loose fill Lowe's and HD either rent or will let you use the machine for blowing it in if you want to DIY.

  10. #10
    Join Date
    Nov 2010
    Location
    Amsterdam, NY
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    230
    If you want to keep the height, I would recommend spray foam to the deck and then gypsum wall board over that. This gives you a higher space even though you have the bracing every 4ft.
    We have been specifying this on buildings for several years now without issues. I even did my own home after removing parts of an existing ceiling to get higher ones. I actually used 6" strips of osb for the ceiling instead of drywall. (it goes with the design, I do a lot of non typical things)

    The batt insulation cannot go tight to the deck boards. It needs an airspace, which can be accomplished by proper vents. Do you have soffit and ridge venting?
    It looks like you have bracing and not trusses, so you will need to add framing to support the drywall ceiling. Then you could do loose fill cellulose (blown-in) which gives good coverage, but needs a poly vapor barrier.

  11. #11
    the 3.5" fiberglass is a good idea. the R-value is not sufficient for residential but is a vast improvement over nothing. if you go to a thicker insulation be aware that compressing the batt loses r value especially if you install baffles. I assumed this was a detached garage? if so no drywall is required. FYI i am familiar with the Chicago codes, still live in the burbs.
    if you intend to use full time then i would look at more insulation.

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