How to insulate uninsulated finished walls

The walls in my attached garage aren't insulated. Short of removing drywall; any options for insulating the walls?

Blown in can be done, but you'll have to cut small holes and if there are any horizontal framing members the cavity won't be fully filled.

^that..

Cut holes in the portion of the wall blow it in. Lowes will let you use the machine for free that has the tip to blow it in the wall with. HD may also but we dont have a HD around here.

You could use a good stud finder to see if you have horizontal 'fire breakers.'

When we insulated our first home, we installed drywall first so we would blow the insulation in.

The rental machine that most home stores have are not for blowing cavities nor do I recommend DIY doing so. Blowing cavities requires adjusting air and material rates. Only the larger pro machines can do this. Also I takes experience and feel and even then you will end up with gaps/voids and if not packed tight enough settling. To much can blow the drywall out.

I have dense packed to structures with a pro machine but was using mesh where you can see an feel voids and density. There was a big learning curve that could not be done it one could see or feel it.

You may want to check with some insulation installers, it may be cheaper than you think.
My brother had his multifamily done for little more than buying the materials would have been if I did it, and they did it from the outside, removing clapboards where needed, then reinstalling them when done. I did my house years ago using a Home Depot machine, they
did not have the wall adapter so I made my own, worked fine. It made so much difference that when I built my garage I sheetrocked it first, then drilled holes and blew in the insulation. I feel blown in cellulose gives a better job than fiberglass batts.

I had mine done about two years ago. I believe it was around 45-48 feet linear, 10 ft ceilings, 2x6 framing. I went with foam injection but wish I had gone with a blown in product (more on this at the end).

I got quotes from several companies for dense cellulose, one company for blown fiberglass, and one for the foam injection.

The problem that I found was that most companies had a minimum job size that they would do - ie they would want paid for a full days job or a minimum $ amount to do a small job like garage walls that would take a half day or less.

What I ended up doing was getting good measurements of the square footage of walls that needed filled. I called a bunch of companies, explained what I wanted done (that it was a small job), that it probably didn't require them sending out someone to do an estimate, I could be flexible on scheduling if it helped them fill a hole in their schedule etc., give them the wall square footage and 2x6 framing info, and ask if they could provide a quote. Using this approach, I believe every company I spoke with did end up giving me a quote without coming out.

As with anything get a bunch of quotes. I got quotes as low as $800 ish to $1,600+ for cellulose (all but one were over $1,000). The fiberglass company wouldn't do it for under $1,300 or $1,400 but they were going to do the back half of the attic above the garage (I have a room above part of the garage) too in that price (they would not do the walls alone for less $). The foam injection guy had done some work for my dad and came in at $1,000 or $1,100.

I ended up going with the foam because I though it would be a better product/do a better job insulating. I did some research on the specific foam product and could not find much recent issues with it. In the past when it was a new product 15-20+ years ago there were issues with major shrinkage (inches) when it dried but supposedly this was attributed to installers jumping into the field not knowing what they were doing and improper mix ratios. The guy I used has been doing it for over 20 years and assured me that this was no longer an issue an it would shrink less than a credit card thickness. The guy that did my install was the company owner (runs a other couple crews) and supposedly the state or regional distributor for the foam liquids - distributing to the other companies that install.

On my side wall I had them cut holes on the inside (due to brick veneer on outside) and I patched the holes after it was dry, I was planning on painting anyway. On the back wall they went through the outside (vinyl siding). There was a major chemical odor and burned my eyes in the garage for about a week maybe a little more.

Since having it installed, a few months later I was doing some electrical work and looking around the edge of a wall box could see there was no foam near the box. He came out and touched up that bay claimed something blocked the flow all the way down. He looked at everything with an IR camera and claimed he did not see any other voids but he seemed a little close at hand with the camera.

I have since cut a couple holes all the was through the walls in two spots - one for a mini split and one for an exhaust vent. In both bays, it was clear that the foam had shrunk maybe 3/8 - 1/2 inch side to side, as the foam block could be moved about that much left to right in the bay. On bay, it was short of the top by 6-9 inches (prob not due to shrinkage but not filling all the way - could happen with any method). But if it shrinks away from the studs say by 1/4 inch or so on each side I would say it is not providing much in the way of air sealing.

So I don't feel that the foam injection was a good product for this application and wish I had gone with a blown in product. I don't know if the installer was full of BS about the product/shrinkage or just does not understand it. He seems like a good guy and a straight shooter. Years after my dad's work was done, he had a section of water damaged stucco and thought they found some areas that the foam guy missed. He came out and inspected it, they figured out it was actually ok - years after the job was done - he came out for free. I guess in older houses where the cavities to fill are much smaller/narrower (not 2x6) the % of shrinkage may not be as much of an issue.

Take down the drywall.
It will give you the chance to add electrical outlets and put up some wall covering intended for a garage.

I did the exterior side wall in my attached garage at my last house. The other side wall and back wall were adjoining walls to the house so they were already insulated. Didn't bother with the wall with the roll up because the wall sections were so narrow on either side of the door. Garage wasn't terribly large.

I pulled the sheetrock down. Put in the batts. Re-sheated with plywood. Was very nice. Made hanging things on the wall very easy. Whacked it a few times accidentally with heavy items and it took it rather than punching a hole in sheet rock. Definitly more cost and effort but I found it worth it.

If I were to go the DIY blown in route then I would cut a 1' strip all the way across the top of the wall. Easier to patch - tape two horizontal joints rather than a mess of holes. Will also be able to look down inside the wall and see what you are doing.