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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?

Yashiharu
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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.

The Ghost of Jon
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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.

Ecnerwal
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