How to Install Fiberglass Insulation into Your Attic
Adding Fiberglass Insulation to Your Attic
Adding rolls of fiberglass insulation to your attic is an easy DIY project you can do in a weekend. It's relatively cheap and you can easily do-it-yourself. No helpers required. The most time consuming part of installing the insulation is cutting it and weaving it into place. If you have lots of obstructions like trusses or hvac ductwork in your attic, it's going to be a test of your patience.
Always use unfaced insulation when you're adding insulation on top of your existing insulation. The unfaced insulation doesn't have the paper vapor barrier so it won't trap moisture in between the layers of insulation.
If the insulation between the ceiling joists doesn't reach the top, like the photo above, you should add insulation to bring it up to top of the joists before you get started.
Prepwork
Before you get started hauling bundles of insulation up into your attic, do yourself a favor and do your prep work first. It's a lot harder to do with large bundles of insulation everywhere you turn. The prep work for installing fiberglass insulation falls into 5 categories:
- Air sealing
- Installing ventilation chutes
- Isolating heat sources
- Bring electrical up to code
- Safety First
Start Under the Eaves
Now that all of your prep work is done, it's time to start insulating. Shove a couple of bundles up through the attic hatch and place them around the eaves.
Slice through the packaging with a razor knife and roll them out. The goal is to lay them perpendicularly across the ceiling joists on top of the top plate on the outer wall.
As you start tucking the fluffy insulation against the roof rafters, you'll notice they get compressed under the rafters. No problem, take your razor knife and slice through the insulation so it wraps around the rafter and push the insulation back towards the outer wall.
Watch out for the nails that protrude from roof plywood over your head. Getting stuck in the head with a rusty nail is no fun.
Weave Your Blanket
Once you have the perimeter of the attic lined with insulation your ready for the easier work. Load up a few more bundles and start weaving together your blanket of fiberglass. 
Cut the bundles in the attic so they , roll them out across the joists and tuck them tightly against the previous roll. When you run into obstructions like recessed lights, plumbing stacks, or furnace flues, carefully cut around them with your knife.
Safety Note on Recessed Lights
Most recessed lights are not rated to be in direct contact with insulation. Technically referred to as "NON - IC" or Non - Insulation Contact, these lights will build up heat which can cause a fire or electrical overheating if the insulation is too close or on top of the light can. Non - IC recessed light require the insulation be 3" away from the metal light can.
Cutting Fiberglass Insulation
It always helps to compress the insulation on a piece of plywood before you cut it.
Use a board or a framing square to flatten the insulation before you cut. I've found cutting insulation with a snap blade utility knife with a long blade makes the job a lot easier, especially with thick R30+ insulation.
Inevitably, the last few pieces will need to be cut to length and some will need to be cut to width. It can be really difficult to cut long pieces to width in the attic. Sometimes there's just no room to move around. So you may want to cut some of the pieces on your garage floor and then haul them up the ladder and tuck them into place.
Stick to a Plan
You don't want to walk all over the fiberglass insulation once it's in place. So you'll need to come up with a plan to avoid "painting yourself into a corner." Try to work in an orderly fashion completing one section at a time. Work your way from the outside edges to the middle, then towards the attic hatch.
You're Almost Finished!
By the end of the job, you'll be ready to say good bye to your attic for a long time. You'll begin to appreciate fresh air like you never did before. But as you get close to the end, take your time and make sure each roll of insulation is butted snugly up against the previous one. This project will definitely take an entire Saturday and maybe encroach on your Sunday football games but you will get there. Keep reminding yourself that you only have to do this once and it will save you money for the rest of your life. It always helped me.