So I've searched on the internet for a method to cut a hole through fiberglass insulation that is installed in a wall cavity, between the outside sheathing, and inside sheet rock. Or in my current case, it's insulation in the 14.5" cavity between the collar joist and next floor joist. This is for installing the PVC pipes for a condensing gas furnace. I found no referenced to anything other then fiberglass RV (recreational vehicle or boat) hole drilling, but this is for solid epoxy fiberglass. There the worry is causing cracks and splitting of the surface.
I used a Morse brand hole saw to cut a 2-1/2" hole for the 2" PVC pipe, from the outside of the house. This went through the vinyl siding, and 2x8 floor joist. That went pretty good. (I should have drilled the holes about 1/2 inch higher, as I had forgotten my construction stackup that attaches the 2x8 to the 2x8 treated wood plate on the block wall).
So I thought I could take a piece of the PVC pipe and machine a sharp edge on it, and cut the fiberglass. That was a bust, except I did figure out the angle to machine the 2" electrical EMT (electrical metallic tubing) conduit piece I had laying about. My initial attempt was with a 16 degree setting, and this produced too thin a section of material that could be rolled over easy.
I set the compound of my lathe to 25 degrees, and used a boring bar to cut the inside to this taper. I then put the compound back to 0 degrees, and cut the outside (mainly to remove the paint, it also improved the knife edge). I was able to push and rotate this into the fiberglass and cut a clean hole through it. I have a photo where I tested this on 3-1/2 inch insulation in the garage that does not have sheet rock on it.
I found this piece of EMT laying along the roadside, someone had painted it green, here it is mounted in the lathe. The edge is not perfect and has a few jags, but mostly a sharp edge.
Just a photo showing the cutters length.
This is the test cut to show the idea. It did tear the kraft paper, but this insulation came to me used, and the kraft paper is very brittle.
This was where I cut the insulation for the 2 pipes for the furnace, via inserting the EMT through the drill holes from the outside. The odd garden hose with the weird clamp is for the sump pump hose discharge (a piece of scrap laying about the shop for 35 years).
Here is a photo from the basement view of where the pipes will connect to the furnace. It is a tight space to get my hands into, as the block wall is 8 inches, and then the thickness of the insulated construction is 5 inches, this is 1 inch of rigid foam insulation, and a 3-1/2 2x4 wall with 1/2" drywall (I like to over insulate). So the opening where the fiberglass insulation bat is, is tough to work on. And I could not think of any way to pierce a hole in the insulation.
It worked good enough that I don't need to get a tool post grinder out to make it sharper. The real problem with using EMT, it is not perfectly round, and certainly not tool steel.
My next project is to install the refrigeration and electrical connections for a mini-split AC condensing unit.
I'll make a 1" cutter for that.
A note to anyone who thinks you can just use a drill bit, the fiberglass will wrap around the bit and be pulled from its location in the wall. And this will leave a gap that is invisible, but a heat loss/gain.