Getting R-30 in an attic suite with 2x6 ceiling rafters

Better than spray foam (thermal break and relatively consistent R-30+). Here's how to use 3 layers of 2 inch polyiso foam board to meet code and keep your new upstairs cozy.
I used a 2 inch rigid polyiso foam (GAF Energyguard) I got from a local roofing supply company (ABC Roofing). It is really the type of foam they use on commercial roofs beneath a membrane. I believe mine was made by GAF, and ran me about $18 per 4x8' sheet. I cut two layers to fit in the rafter bays (so that's 4 inches), spaced 1&1/2" away from the roof-deck with scraps of 2x4s (which maxed out my 5 1/2" inch rafters). Then I put another layer across the rafters, which made a total of 6 inches of polyiso. Each inch was R-5.4, so the total R value of all 3 layers was 32.4. Wherever there were gaps, I squirted some Great Stuff in, then shaved the overflow back with a flush cut hand-saw.-- Yes, it passed insulation inspection.

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[glenndent]

This is a good way to go. From what I'm hearing lately there's a lot of finance companies that won't do a loan on property with spray foam.

[cheeseheadfiddle]

I did a project similar to this. A problem that occurred with it was that our vapor barrier got punctured allowing bathroom moisture into the insulation cavity. One wiring penetration wasn’t tightened up. 5 years later there was moisture in that insulation space and some rot in the roof deck. Roofer found it. We went back and fixed it. You can’t be too careful when you insulate in a space like that, especially if there is a shower in a bathroom or a kitchen. Venting moisture sources is so important.

[nholt]

Pretty ingenious. And I like how you left the all important gap between the roof decking and foam. And the third layer of foam stops the thermal bridging.

[corey]

Five years ago I did a large addition on my house. Scissor trusses with a vaulted ceiling. Local inspector required an R-value up there that was virtually impossible without ruining the venting at the eaves. I had been in construction for quite some time, and I knew the inspector well. While discussing the issue I asked him how R-value was measured. He said it was by the thickness of the insulation. I asked about measuring it vertically, when the ceiling is sloped? By that measure, because of the ceiling pitch, I just made the thickness requirement by code. He laughed, and OK'd the inspection.

[berkeyfamily]

TIP for working with insulation and fiberglass or polycarbonate translucent/transparent panels: Reverse your saw blade. It still cuts like butter through the materials but it does not kick out so much stuff all over the place.

[scottyellis]

I like how the 3rd layer covers the roof rafters, makes it a little harder on the drywall hanger but it does save a lot of energy. It's amazing just how much energy you lose from wooden studs.

[travelingkaspersworld]

Your video is great, it is drawing out a plethora of comments from which we can overthink everything. We think you did a great job and credit you for pioneering this system. A friend did well with a cold roof design 20 years ago. (Your 1 1/2" spacing kept his shingles strong). Our plan was one layer of foam board followed by spray but you showed this to be more economical.

[zschudrowitz]

That’s exactly the idea I’ve been batting around in my head for years. Thanks for the vid.

[MegsCarpentry-lovedogs]

Hi Dave, thanks for this vid. Last Nov 2019 I applied your technique in the porch ceiling of my home and made it into a cathedral ceiling! I want to thank you for caring to prepare and edit and share your video. Throughout the layering of insulation I incorporated two different layers of bubble foil for radiant heat effect. :) You rock! Imitation is the best form of flattery. This proverb imitation is the sincerest form of flattery means that one imitates someone else because one admires that person or values what that person is doing. Cheers from Atlantic Canada during Covid pandemic :)

[rocketsurgery]

Bosch T313AW3 jig saw blade is designed to cut foam and will do so with little or no dust. 3 blades for $15 on Amazon.

[gumboot]

I use a big shop vac hooked to the table saw for all my long rips. It cuts down on the foam board dust. Lots of dust like you show.

[wx]

Looks good; depending on how how hot your summer gets; a 10" auto shutter fan on one or both ends of the ridge would be a good addition; I had ridge vents across the entire length; and the heat still hangs without any prime mover.

Peace

[chrisevans]

I'm in commercial roofing and very familiar with this material. Only thing I would have changed would have been to Isolate the air cavity with poly film. The facer of this insulation is organic and depending on location could result in unanticipated condensation and mold growth potential.

[bigk]

Polyiso foam cuts easily using a snap-off knife extended. Usually 2 passes with the knife will cut the foam cleanly. Plus this is quiet and no foam flying everywhere.

[SweetCaroline]

Thank you!! I’m a bit of a DIYer - redoing a master bedroom in a bungalow up in MA - 4x6 rafters. I was going to go closed cell spray foam because I didn’t want to lose headspace, but it’s so $$$! This video has completely changed my direction. Found R13 2” polyiso board - 3 layers will get me up to a compliant R 39 in my area and it will save me THOUSANDS. You are the man!

[JK-gkje]

I had the same issue. We need only 1" of air gap above the insulation here in CA. Sister 1x2's to the rafters to give yourself 8" of space. Then take two layers of r-15, which will be 7" thick. Install normally. Prego.

[chris]

hint for working with fibreglass.
stay as cool as possible, and don't sweat. if this means planning your work for cool days, do so!!!
Take a full, cool shower immediately after working, THEN heat yourself back up after you're fully scrubbed with the cold water.

This helps BIG TIME for the itchiness.

[davehoover]

Forget the worm drive or hand saw. Us an electric Turkey carving knife. Works awesome!! Thanks for the video.

[jean-claudearsenault]

Because the portion of the wall is sloped, in Canada, it's considered a wall, as far as R-Value requirements. However, an air space of 2 1/2 inch is required. This is required to prevent choking air circulation between other compartments.

[nunyabusiness]

Cant thank you enough for posting this. I'm considering a diy for my attic and the visual of your thought process is enormously helpful. Thanks dude!