Adding insulation to existing walls?

[JackOfDiamonds]

The bedroom above my garage has walls that **** against the attic space. So they are unconditioned garage below, unconditioned attic space on two sides, and unconditioned attic space above. Consequently this room is usually an oven in the summer.

I already have the TOP of the room (attic) insulated with blow-in. And there's not much I can do about the garage below unless I put in a drop ceiling in the garage or something. I want to attack the side walls.

They are 2x4 walls with fiberglass insulation between the studs. There is no facing or vapor barrier visible, just raw fiberglass, and there are a few gaps visible too where you can see directly to the sheetrock.

I could rip out the insulation and put in new, faced insulation to seal the wall cavity better, but I would rather ADD insulation than replace.

A quick fix would seem to be nail up the blue or pink foam insulation boards over top the studs, and tape the seams. That would seal the envelope and add R value at the same time. Or you could do the same thing with that Reflectix foil-covered bubble wrap stuff. Is this a bad idea because it needs to be breathable? I live in a very dry Idaho climate and don't usually worry about moisture.

 

[Bert_]

You want the vapor barrier on the warm side. In northern states that would be against the sheetrock

 

[rsanter]

The best is to remove fiberglass and use spray foam.

Don't want to do that?Fibergalsss needs 6 sided contact and enclosed walls to work.
Fiberglass needs still air or it does not work to rating.
If you have fiberglass and can see it, then you need to cover that up.
So you want to add? Then add 2" of rigid foam. You will be adding a layer of insulation and closing up what is there

 

[Bert_]

You can add foam on the exterior but it needs to be enough to prevent condensation.

You could also just cover the exposed insulation with house wrap.

Yes fiberglass does not perform when exposed to air movement

 

[JackOfDiamonds]

Well I just thought that I can't get any sheets of material into the attic because there's only a 2 foot square access hole. It will have to be something on a roll or cut into strips.
How thick/r-value does foam have to be to eliminate condensation risk?

Is tyvek house wrap breathable enough to eliminate the condensation concern?

 

[billconner]

I think I'd go for foam board, especially if attic space is vented. 2" is usually enough to prevent condensation. Some foam sheets are pre scored so easy to snap into 16" x 8' pieces. Gap them a 1/4" and use foam in a can to seal seams.

 

[Imatk]

I had the same issue, our game room/theater room is directly above the garage and it gets super hot in the summer even with the A/C on (I live in Texas).
I was able to reduce the heat in the garage a bit with insulated doors which DID help. But it was still a fight.
Now I've got a mini-split in there and don't have that issue. I run it a little during the day to cool it off and it stays pretty cool for quite a while which makes the room above able to cool off.