Fiberglass batts are a poor choice of insulation for most applications, yet this still seems to be the insulation of choice for most handy homeowners. I'm been complaining about fiberglass batts for a while now, and for good reason.
Fiberglass batts are more expensive. I stopped by the Maple Grove Home Depot and compared the prices of unfaced fiberglass batts, loose fill fiberglass, and cellulose. Here's what I found:
$0.93 / square foot for R38 Fiberglass Batts
$0.56 / square foot for R38 Loose Fill Fiberglass
$0.30 / square foot for R38 Cellulose
Fiberglass batts take more time to install. To be installed properly, fiberglass batts need to be painstakingly cut to fit the exact size of the space that they're supposed to fill. Here are a few examples:
- A 2x6 wall that has electrical wire running perpendicular to the studs; should you push the insulation in front of or behind the wire, leaving a void in the insulation? No. You would need to cut the batt so it gets installed above and below the wire. The images below show the original photo on the left, and the photo with a thermal image overlay on the right. As you might guess, the red areas are warm.
- A 2x6 wall with electrical outlet boxes; should you stuff the insulation around it, leaving nasty gaps around the box? No. You would need to cut out a rectangle in the batt so it fits around the outlet box without being compressed, and you would need to install a thin piece of fiberglass behind the outlet box.
- An attic with a bazillion electrical boxes, wires, pipes, truss chords, etc...; should you stuff the insulation around all of these obstructions, significantly reducing the insulating value? No. You would need to make a bazillion cuts and fits in the fiberglass batts to fill all of the gaps and voids.
The alternative to making all of these cuts and fits is to use something that fills in every little gap and void, such as loose fill fiberglass or cellulose in the attic, dense pack cellulose in walls, and spray foam at rim joists. These methods take far less time.
Fiberglass batts are extremely difficult to install properly. As you read through the examples above, you were probably wondering who would ever take the time to actually do all of these things. My experience has told me no one. I can't say fiberglass batts are impossible to install properly... but I have yet to find fiberglass batts installed properly in an attic. All of the little voids that are left in fiberglass insulation equate to an exponential level of heat loss. The photos below came from a five-year-old custom built home in Edina that I inspected.
