Wouldn't maybe spray foam be a better option / less invasive? I would call some insulation companies and ask their recommendations on the art of the possible before agreeing to anything with the contractor.

And sorry you are going through this - the fire was terrible enough. Hugs.

They still use blow in insulation? I'm not sure, but doesn't it compact over time leaving gaps at the top. I would also use spray foam. Maybe the GC don't want to use it because it cost more. My daughter had her older home sprayed foamed, I believe they did it from the outside.

What does your contract say about insulation?

Did he forget to get a permit or forget to get inspections?

Blown in cellulose is a better product than fiberglass batts so that should work. If you can negotiate it, spray foam would be the best choice for you, but not the contractor due to cost.

Your permit may require an installation certification also

Get a written guarantee that there will be 100% coverage of the insulation with no gaps or voids at all. Perhaps a performance bond.

Thanks for all the advice so far. Let me see if I can answer:
Contract does state insulation, and we were told it was installed. Some areas it was. It was a case of a greedy contractor taking on too many jobs and leaving an inexperienced crew to do the job. Lots of work in my area due to the fires, so everyone is having troubles with awful work quality right now unfortunately. We had to live in another town during the repairs, so not always there daily unfortunately.
Permits - well . . Here is where a construction ignorant owner with fire PTSD did not realize that a permit was necessary. GC clearly didn’t pull one. We are not a complete rebuild, if that matters? We had to pull all drywall and insulation out due to damage.
Blow in vs foam - my exact concern was settling with the blow in. Foam does seem to be the way to go if we need to. Seems a shame to cut a bunch of holes in brand new walls but all new Hardie siding just installed (our old vinyl all melted in the fire).
How do I go about getting a performance bond? Sorry for the ignorant questions! We got thrown into construction without much time to research.

Drywall has to be taken off before you can use spray foam insulation.

blown in cellulose is actually an upgrade over fiberglass batts and works quite well as long as there are no stud cavities under 3 inches wide which can’t be blown. Unless using special equipment sprayfoam isn't a good idea as it can blow out the sheetrock.

You can check for gaps in the insulation through the drywall with an infrared camera as long as the temperature inside is differant than the temperature outside. If you insist on being there when the missed areas are checked you can see for yourself if any areas got missed as they will be a different color.

Wall cavities can be insulated after drywall is installed using injection foam insulation. It expands differently than spray foam insulation and applies less force to the backside of the drywall than spray foam would. It can be installed from the exterior after removing a few courses of siding and drilling holes in the sheathing which is the route I'd recommend. It's not a job for an amateur. Spend time vetting the insulation contractor and don't settle for some fly-by-night outfit. A post installation quality check should consist of a visual inspection of the drywall to ensure it isn't "bulging" and an IR camera to show any spots where insulation is missing. Those can then be treated on an exception basis.

Can injection foam be injected from holes drilled in the drywall instead or is this not advised? We just had all siding newly replaced with Hardie and I wou look d like to not disrupt if possible.
One whole 2 story wall is shared with our uninsulated garage. Does any sort of a barrier need to be applied to keep garage fumes out? The rest of house has new house wrap under the siding, but not that shared wall. Wondering if foam would be a good enough barrier?

Thank you for everyone’s help. I can’t tell you how much I appreciate it. We’ve moved from one nightmare to the next with this rebuild.

Installing injection foam from the interior of your home will require more patching/repair than removing a couple of courses of siding and reinstalling them afterwards, but it's certainly a viable option.

Installing foam insulation in the wall between the garage and living space will reduce the potential for air exchange between the home and garage.

We just discovered that our contractor “forgot” to put batt insulation into the walls before replacing drywall on many of our exterior walls.

When you say contractor, do you mean the drywall contractor, insulation contractor, or the General Contractor?

I am sorry to hear about your terrible ordeal and wish there was an easy solution to offer. It was not your fault that a contractor "forgot" to do a very important step in the renovation process. You have every right to tell the contractor you want sheet rock removed and the insulation installed per code in your area. You should also ask the contractor to apply for permits and have inspections done for all the work that has been done. You should not do a major remodel without permits and risk having problems if you ever have to file an insurance claim or sell your house.

Our GC’s own crew did the work including insulation (or lack thereof) and drywall so not a subcontracted drywall or insulation contractor. Probably the first problem right there. We asked early on if any of the interior work needed permitting and was told no. Learning to not be so trusting at this point . . . They did get proper permits/inspections for exterior siding and for the electrical panel that needed replacement, so we assumed they knew what permits were required. We will be having a difficult conversation this week with the GC to rectify all of this. I like the idea of injection foam as it seems a good product, but it seems ridiculous to chop up my new siding or drywall to fix his mistakes. I do think we will request that he pull the drywall out, get permits, use an insulation contractor, drywall contractor. Just unfortunate how much further delayed it makes the project. Family of 4 now in an RV waiting for our home . . Ugh. Thank you so much for your advice.

If the GC's crew were inexperienced and poorly supervised, what else was done incorrectly? What else was "forgotten"?

I wouldn't budge an inch. It's his problem, not yours. Insist on the drywall being taken off in its entirety and whatever permits that were "overlooked" be pulled and inspections take place.

Having spent good money for a standard of work specified in a contract, you shouldn't need to compromise on the GC's flippant work practices and attitude. Time to dig in heels. Hire a real estate lawyer, not one who's quick on the draw to file a lawsuit but one with ideas on how to work to the conclusion you expect. Including, when it's all over, filing a complaint with California's Contractors State Licensing Board. Scoundrels don't deserve to have license with conduct like this and suggesting band-aid compromises for less expensive (for him), half vast remediation steps.

Good luck, It's unfortunate but I think you've got a mess on your hands.

So sorry you are going thru this! We had to rebuild our condo after the harvey flood here in tx. I dont know much about this type stuff but can the insulation/foam be blown down from the attic into the walls.

As others have said:

Cellulous insulation can be blown in from the inside and it's very effective. You can also blow in low-expanding foam insulation. Don't mess with your hardiplank once it's up.

Sure, there should have been permits and supervision and... and...

Now you need to get it done.

Nobody who is giving you advise about lawsuits and such are living in an RV with a family of 4.

Be prepared for the GC to walk off the job if you take a hard line now. The hard line should have been taken in the beginning, and you wouldn’t gave hired this guy.

Best to gently ask him about his failure to insulate, and how he is going to handle that. You may still part ways eventually. But its best to start out way below Defcon 4 as it gives you options to escalate.