Image Credit: Chris Stratton and Wen Lee Not much to start with except undersized rafters and a cavity without any insulation. After the retrofit, the roof is 17 inches thick and has three distinct layers of insulation. The R-value of the assembly is 46. The pieces of 3-inch-thick polyiso insulation are installed between rafters against strips of foam measuring 2 inches square. This creates a 2-inch gap between the sheathing and insulation for ventilation. Cutting back the sheathing at the ridge creates an opening for air to exit the roof assembly. The slot is covered by a ridge vent, which keeps water out but allows air to exit. Fiberglass batts went in after the first layer of rigid foam. The batts are nominally 6 inches thick. The roof now has three layers of insulation, plus a ventilation gap. Chris used acoustical sealant and canned foam to seal the seams of the second layer of rigid foam insulation. Chris used 2x3s to create a service cavity below the insulation.
Editor’s Note: This post is one of a series by Chris Stratton and Wen Lee, a husband-and-wife team living in the Los Angeles area who are turning their 1963 suburban house into an all-electric, zero-net energy home. They chronicle their attempts at a low-carbon, low-cost, and joyful lifestyle on their blog Frugal Happy. This post was written by Wen.
After spending months painstakingly insulating our walls, Chris turned his attention to insulating the vaulted ceiling.
You may be wondering, “Is this guy a glutton for punishment?” Valid question.
In many ways, insulating the ceiling was even harder than insulating the walls. The assembly is thicker and more complex. Chris had to install the insulation while standing atop a ladder, which made the job awkward and dangerous. Moreover, much of the insulation work took place during the hot summer months, when the air temperature at ceiling height often neared 100°F. It was laborious and slow-going. In the end, the process lasted more than four months.
So yes, maybe Chris is a bit of a glutton for punishment. But for the end product of a foam-filled, air-sealed, attractive cathedral ceiling with an insulation value of R-46, there was no question in Chris’s mind he wanted to do it. The process helped create a comfortable home with steady indoor temperatures and low energy bills year-’round.
Since we converted our flat ceiling to a vaulted ceiling, the ceiling and roof became one and the same. So our roof went from zero insulation to superinsulation. The two drawings at the bottom of this post show the roof assembly before and after the work (see Images #2 and #3 below).
That’s a big change! By the time Chris has completed the retrofit, our ceiling/roof will have 10 distinct layers and be 17 inches thick. Wowza. Let’s walk through the insulation phases one by one.
Phase 1: Rigid foam and ventilation
Chris wanted to ensure that the finished ceiling/roof assembly was properly ventilated so that any moisture that gets into the assembly (e.g. from rain) is able to evaporate. He did this by constructing long ventilation baffles, or chutes, from rigid insulation. The chutes allow air to flow from soffit vents at the eaves to the ridge vent at the top of the roof. (For more information on this step, see “Site-Built Ventilation Baffles for Roofs.”)
Often people install plastic vent baffles for this purpose, but since Chris was planning to insulate using polyisocyanurate rigid foam, he decided to use the foam itself to serve as the chutes (see this fun video from Fine Homebuilding). Two birds with one stone!
Each panel of foam is pushed up against 2-inch-square “sticks” of foam glued into the upper corners of each rafter bay with Loctite PL 300 foamboard latex construction adhesive. This creates a 2-inch gap above the insulation for air to flow (see Image #4 below). As you probably guessed, what you see in the photo is just half a chute — when it’s complete it will extend all the way up to the ridge.
For air to flow successfully through each chute, there needs to be a way for air to 1) enter, and 2) exit. In the photo, you can see that Chris created openings at the base of the rafters (where daylight is shining through along the top of the wall). Later, he installed plastic soffit vents.
But where will the air exit? At the ridge, the highest point of the roof. Chris cut out openings along the ridge too (see Image #5 below). Then he climbed on top of the roof and installed a ridge vent, carefully covering it with ridge shingles (see Image #6 below). That way air from inside can flow outside, but when it rains, water will not leak into the house.
Another benefit of ventilating the roof in this manner is that on very warm days, hot air (which rises) will flow out the top vents, drawing cooler air into the chutes from the bottom — a passive way of cooling the house down.
Working by yourself can be exhausting and lonely (even when there are unlimited podcasts to listen to), not to mention s-l-o-w! One weekend, Chris organized a work party (a.k.a. bribed people with free pizza), and luckily, my brother Bin and our adventurous friends Merc and Dan showed up to help. With four people, they were able to install a whole bunch of chutes in one afternoon.
As with the wall insulation, every piece of rigid foam had to be carefully measured and custom cut for each bay, and then gently pounded into place with a block of wood and a 4-pound mini-sledge. The pieces were snug enough to stay in place with friction alone, but Chris sealed the edges with acoustical sealant.
Over the next few weeks, Chris continued to slowly but steadily install the foam on his own. As he progressed, he had to climb ever higher on the ladder. Handling an 8-foot-long piece of foam while balancing on top of a ladder (in sweltering heat) is no small feat, people.
Of course, insulation doesn’t work at its best if there are leaky seams. So Chris made sure to air seal (with acoustical sealant and canned spray foam) along all the seams of the rigid foam, creating an insulation layer that is completely airtight.
Phase 2: Fiberglass
After the rigid foam, the next layer to go in was 6-inch-thick fiberglass batts (see Image #7 below). That’s the cotton candy-looking stuff. It may look soft and fluffy, but you don’t want to roll around in it because millions of teeny tiny spiky things will become embedded in your skin, and you will be thoroughly itchy.
Thankfully these are kraft-faced batts, which means they are covered on one side by paper. This is designed to keep out moisture, but it also makes the batts easier (and less itchy) to handle and install.
Chris installed the fiberglass batts by stapling the paper facing to the rafters. This isn’t a particularly labor-intensive process, but because the batts were so long and floppy, and Chris was trying to install these things by himself so high up, it was rather awkward.
It also got challenging when he had to get into tight corners. Even so, the fiberglass batts went in much faster than the rigid foam.
Phase 3: More rigid foam
Similar to what he did with our walls, Chris next installed yet another layer of rigid foam, this time 1 inch thick, continuously across the rafters rather than between them. Why more rigid foam? To reduce thermal bridging through the rafters.
The foam sheets are not very heavy, but at 4 feet by 8 feet they are rather unwieldy for a single person to handle. Chris got creative with ways to hold up the large sheets while on a ladder (such as using another ladder).
Two layers of rigid foam, filled with fluffy pink fiberglass. That’s right, it’s an insulation sandwich (see Image #8 below)! Fast forward a few weeks, and behold! The fully insulated ceiling: 10 inches of insulation (and months of Chris’ blood, sweat, and tears) hidden behind a dull gray facade.
It looks nice and tidy in the end, but getting to this point was not. It’s all out of sight now, but we’ll always remember that it’s there. Once more, Chris meticulously air sealed the final layer of rigid foam with acoustical sealant (the white stuff) or canned spray foam (the orange stuff). Because, darn it, you can never air seal enough (see Image #9 below). The air sealing ensures that there are no air leaks in the ceiling of our giant foam box house.
Phase 4: Furring strips and a service cavity
Just as we did with the walls, we created a service cavity on the inside of the insulation for wiring before putting on the finished ceiling. To do this, Chris installed 2×3 furring strips. He used a laser level to ensure that the strips were perfectly level and co-planar (see Image #10 below). Chris can use the furring strips to anchor electrical junction boxes for lighting fixtures later on.
At long last — the furring strips are in, and the insulation is complete! We are ready for wiring, lighting, and the finished ceiling surface (which will be wood paneling).
It was long, it was laborious, it was sweaty… but it was worth it. The best thing about insulation is that if it’s done right, it only needs to be put in once. Once it’s in, it pretty much works forever (or at least for the rest of the building’s functional lifetime).
With superinsulation installed, our house now needs less than half the air conditioning it used to, and an even smaller fraction of heating. If all homes had this level of insulation, imagine how much energy (and greenhouse gas emissions) could be saved. I’ve said it before and I’ll say it again: Insulation is unsexy, but it makes a difference!