Need help with insulating my garage..we are making a bedroom in it.?

i don't see an acurate answer so far. normally R-15 is the max you can get for a 3 1/2 wall cavity. if you compress anything else into the space you defeat the purpose. insulation works because of the air spaces between the glass fiber. compress the air out and you have glass and you know how glass transfers cold.

yes you could "fur out" the studs but i don't think you need it. once it is insulated and sealed up it should be plenty warm. there is no reason to use more than R-15 in the wall. i'm in NE Ohio and that is more than enough here. But, you should be sure to put as much in the ceiling as you can. they make an R-38-C that is made for 2x12's.

if R-15 isn't enough you do not have enough heat going into the room. also be aware that you do need a cold air return or the heat ducts won't push enough heat in.

You cannot put R-20 in a wall that will only take R-15, you can Squish it in, but this destroys the R-value. You either have to see if you can get a different type of insuilation that will fit in the 3 1/2 " wall that has a higher R factor, or make the wall thicker as well.Check out, solid foam or spray foam(I am not sure if these have a better R value)

After the insulation you will need a 6 mil vapor barrier on the walls and ceiling before drywalling. You are making a "Cold Space" into a "warm Space" These are construction terms that you can look up. And this is IMPORTANT.

You will need heat in this room as well, The easiest to install would probably be Electric Baseboard heating.

You have several issues: 1) how is the room heated & cooled, 2) pre-insulation seal, 3) ceiling insulation, and 4) wall insulation. #1 -- If you're adding air from the existing home's hvac unit, then the unit has to be sized accordingly, which it probably isn't. Adding the additional space will put a strain on the existing unit, especially in extreme conditions. #2 -- seal all penetrations at the walls and ceilings where air can leak, and seal the bottom plate at the foundation too. #3 -- You should have at least an R-38 insulation system in place. Spray-in is better then batts as it gives better coverage without voids and gaps at batt seams. #4 -- Install the r-15 batts or better yet go with a spray-in fiberglass with the netting in place (Optima Brand comes to mind). The spray-in encapsulates itself around pipes, wiring, etc. and leaves no voids for a complete R-factor.

If you use blown in foam insulation, you can get the R value up to about 18-20, depending on if you use open cell or closed cell. I think you would be better spending the extra money on additional ceiling insulation. As the previous answerer mentioned, most of your heat loss will be through windows, and the ceiling.

2x4 walls are not an issue.....your biggest heat loss is throught the roof and around windows and doors......and believe it or not where the roof meets the top of the walls.....

make sure you get good seals and insulation around your openings...and that you insulate the attic well and i believe you will be fine

R38 insulation should be the right size, It's they also sell it with a thin barrier over it so that you don't have ot worry about itching. (encapsulated)

Also for this bedroom to be legal it need to have a window or another door for fire escape route purposes.

try styrafome

say what?