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I read it from here: https://www.homedepot.ca/en/home/ideas-how-to/home-repair-and-maintenance/how-to-choose-insulation.html

It recommends my garage "Walls Above Grade" should have R-19/ R19+R7.5 / R24+15 at best. I am not sure how can I fit in 3.5" fiberglass with 1" foam in a 2x4 wall ...

About the material, the best I could have here:

ROXUL R-14 Wool Batt https://www.lowes.ca/product/batt-insulation/rockwool-comfortbatt-r-14-mineral-wool-batt-insulation-23-in-x-47-in-305813

Owens Corning R-14 Fiberglas Batt https://www.homedepot.ca/product/owens-corning-r-14-ecotouch-pink-fiberglas-insulation-15-inch-x-47-inch-x-3-5-inch-78-3-sq-ft-/1000169644

Rockwool Stone wool Batt https://www.homedepot.ca/product/rockwool-comfortbatt-r14-insulation-batt-16-inch-for-2x4-studs/1000122327

But both of theme never close to a good R-19 from the guide of homeDepot ...

Any advise for the weather with -40 in winters and 35 in summer?

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    First question to answer is what you will doing in garage? Just driving car in or working for hours in it? Can add wood to the walls if you want more insulation. Homedepot sells stuff, the more you buy, the happier they are. – crip659 Jun 10 at 16:14
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    can you describe what your endgame objective is here? Do you have a particular use you'd like to put that space to? – Harper - Reinstate Monica Jun 10 at 18:34
  • try to maintain the temperature to around -20 in a -35 weather & keep it cool in summer if possible. I may have to install a heater if I turn it in a workshop. – Yashiharu Jun 10 at 19:22
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Home depot is telling you recommendations for conditioned space.

Assuming you are not familiar with finding/interpreting building codes:

You will need to call your local building department and ask them what the minimum code requirement for insulation is at the exterior walls of your garage (it's going to likely be "no insulation is required at your exterior walls of unconditioned space, put as much as you want or none."

However, if this is an attached garage, and you are opening up the wall between the garage and your home and this work is being permitted, expect to need to meet fire code and energy code requirements. Again, ask your building department to clarify.

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  • Thank. Sorry, didn't mention the garage is attached and already well built except insulation on 2 sides and door. I have check the builder's code, not much info or guide. btw, do you think unfaced fiberglass + vapour barrier is enough? – Yashiharu Jun 10 at 18:58
  • You generally need drywall or OSB/plywood/even just boards over the vapor barrier for fire integrity and so you don't poke holes in the vapor barrier. – Ecnerwal Jun 10 at 20:17
  • Vapor barrier is a climate specific application. You generally should not apply a value barrier at interior side of walls unless extreme humidity like American south. Unfaced fiberglass alone is fine with fire taped joints, assuming you have a weather resistive barrier underneath exterior cladding – The Ghost of Jon Jun 10 at 20:29
  • now there are (from outside): stucco > wireframe > weather resistive barrier > OSB > stud, plan to add fiberglass/wool > vapour barrier > drywall. I saw most of the YouTube video are recommending vapour barrier between drywall and stud filling, we got 60% humidity now, vapour barrier is not necessary? – Yashiharu Jun 10 at 20:55
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Insulation is basically pay now or pay continuously (assuming you are heating and/or cooling the space enclosed - if the garage is unconditioned, a bit of insulation will limit condensation from rapid temperature swings, but you don't need a lot for that.) I don't make any claim that the HD advice is correct for anything but HD selling more insulation, but you can find less biased sources of information, or run your own numbers for the cost of utilities and the climate in your area to see where the cost of more insulation starts to not save you very much more utility cost.

Current normal approaches for 2x4 walls and high R-Values are to combine cavity-fill and sheet products (normally foam, with drywall over if on the inside, &/or sheathing-grade foam on the outside) to build more insulation ONTO the wall once you've done all you can IN the wall, and also limit the effect of the thermal break from the studs. So your 3.5" goes in, and the 1" foam goes on, in your example of asking about how you fit those two in a 2x4 wall.

If you want to look at another approach "Larsen trusses" (so-called) are a scheme to build bigger cavities for more insulation and less thermal break that work with loose-fill insulation - effectively a non-structrual double-wall to create more insulation volume. A structural double-wall is another option, but not normally useful in a retrofit situation, unless the current structure is sub-standard.

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  • Sorry, didn't mention that the garage is built by the builder, I just want to insulate it before winter. Should I use the same approach for the ceiling? like: 9" + 9" filling + 1" foam + drywall? – Yashiharu Jun 10 at 19:35
  • Have you got attic up there with joists or truss bottom chords below, or is it cathedral ceiling? Normally for an attic space you want to put vent baffles on the roof decking, vapor barrier and drywall on the ceiling side, then blow cellulose in on top - most bang for the insulation Loonie. Sheet foam is expensive. – Ecnerwal Jun 10 at 20:23
  • truss attic. The vent baffles seems no use, because there are "header" in the mid way, so even with vent baffles, the air can't pass through to the top. But I saw some of my neighbour install roof vent, some didn't. I am confusing. Blow cellulose, rent a blower seems much expensive ... – Yashiharu Jun 10 at 20:29
  • Blowers are a cheap rental; or you get one free with your insulation, depending where you buy it. Quickly pays for itself because the insulation is far less expensive than batts. If the headers are above the surface of the insulation, they are not a problem. Not venting the roof is usually a problem - ice dams, etc. – Ecnerwal Jun 10 at 20:35
  • The roof is done, I am not aware if there is a "door" kind of entry point except the roof vent hole I have to open. Make a ceiling "door" to go up and blow may be a bit difficult for me. I am going to search about the way to blow cellulose. Thanks. – Yashiharu Jun 10 at 20:49

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