Fiberglass insulation hole cutter

[metric_taper]

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.

[shopandmath]

this is a good idea
will use it on my next construction job
Ray

[DIYer]

Thanks metric_taper! We've added your Fiberglass Insulation Hole Cutter to our Miscellaneous category,
as well as to your builder page: metric_taper's Homemade Tools. Your receipt:

Fiberglass Insulation Hole Cutter by metric_taper

tags: cutting tool

[metric_taper]

this is a good idea
will use it on my next construction job
Ray

It's similar to a gasket punch, but those have the ground taper on the outside of the punch.
I really was surprised this worked. I'm still hoping others have found a magic tool that does this, and comments.
I need a method of cutting through much deeper fiberglass in the ceiling of my house addition. And without compressing it much. I know that any toothed hole saws will grab the material. There the project is to get a Mini-split line set to the roof where I'll mount the condensing unit. If DOM (drawn over mandrel) seamless tubing was not so pricey, that probably would be the right material for this as it would be round and not have a weld bump to interfere with getting a consistent sharp edge.

[jdurand]

for the least diruption on the insulation it seems a heated cutter would work. Thin edge to melt through. Much caution will be required in use.

Next would be counter rotating blades like a round hair trimmer.

[metric_taper]

for the least diruption on the insulation it seems a heated cutter would work. Thin edge to melt through. Much caution will be required in use.

Next would be counter rotating blades like a round hair trimmer.

I don't think fiberglass has a low enough melt point, unless your thinking a red heat device that would reflow the glass fibers.

The idea of a nose hair trimmer might work,
Brings tears to my eyes just thinking of using them.

[gatz]

Years ago, I was faced with the problem of cutting down the width of fiberglass batting for a 2nd story bedroom.
The local hdwe store only had widths that didn't fit the spaces between studs/rafters. No big deal, right? just trim 2" off the side, right?
Scissors and shears did NOT work worth a darn as the batting would bunch up, and make it near impossible to cut.

After some serious thinking (?) a truely aha moment came.
(1st, check to make sure the wife wouldn't see)
Grabbed the electric knife from the kitchen and gave that a try. Couldn't believe it, that worked like a hot knife through butter.
Took just a few moments to trim the excess away.
The opposing blade direction is what makes it cut so easily. This is along the lines of how the "nose hair trimmer" works without pulling hairs.

Now, if someone could apply the counter-rotating principle to a hole cutter to cut fiberglass, they may be on to something.