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Community and Q&A

Fiberglass batts in a cathedral ceiling — Round 2

user-2179523 | Posted in GBA Pro Help on

Thanks, Martin and Dana, for your timely responses and comments [in a previous Q&A thread called “Fiberglass batts in a cathedral ceiling?”].

My wife and I read the articles, “How to build an Insulated Cathedral Ceiling” and “Site-Built Ventilation Baffles for Roofs.” The air-sealing of the site-built baffles sounded formidable and the mention of no known failures from the use of impermeable baffle materials was enough for us to consider using them.

At this point in the construction, per your advice we have the ability to modify the cathedral ceiling insulation plan and propose the following:

Currently, only the 2 x 12 ceiling joists have been installed. The plastic baffle (manufacturer says 1.75″ air space) nailed to the OSB roofing and an unfaced R-30 fiberglass batt (9″ thick) can go into this chamber. This would leave an approximate 1″ air space in the 11 1/2″ deep channel. Should we snug the R-30 against the baffle and have this small air gap at the bottom of the 2 x 12, or leave the air gap beneath the baffle?

Next, we can have the builder nail 2 x 6’s (instead of the originally planned 2 x 8’s) beneath and perpendicular to the 2 x 12’s per your suggestion. This could accommodate 5.5″ of faced R-21. We know we would be going from R-60 in our original plan to R-51, but it would help address the thermal bridging issue. We can attach the Certainteed MemBrain at the bottom of these joists prior to drywall as an air barrier/vapor retarder also per your suggestion. Should we be installing a wind-washing dam on the cathedral side as we will on the non-cathedral attic side?

As for the basement walls, we will nix the wall framing and fiberglass, and go for 3″ of rigid foam again per your suggestion.

We do plan on installing the fiberglass in a careful manner (as per, “Making Fiberglass Work”) with attention to detail and will make extra effort to seal the drywall ceilings with caulk and/or spray foam.

Dana, I expected a comment on the thickness of my exterior foam, but getting agreement on even 2″ from my builder and the building inspector (who insists we are over-building and is building his retirement home with 2 x 4’s) was a challenge. I keep reminding myself that the location is supposed to be Zone 5.

Hopefully, this re-vamp of the insulation plans is an improvement. Any advice would be appreciated!

Thank you for a great website and incredibly informative articles. I just wish all builders (and building inspectors) were required to have a lifetime membership!
Thank you again,

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    1. You don't want an air gap between the two layers of fiberglass. Ideally, you want to fill the rafter bays with insulation, without an air gap.

    2. Site-built baffles are preferred to manufactured baffles because it's easier to make site-built baffles airtight. If you take my advice and make site-built baffles, you can increase the depth of the ventilation channel to make sure that the 9-inch-thick batts fill the remaining space without an air gap.

    3. Alternatively, you can add R-13 batts to the R-30 batts. With a little bit of compression, that combination will fill the 2x12 rafter bays.

    4. What's your plan for securing the 2x6s on edge, at 90 degrees to the underside of the 2x12s? It's a solvable problem -- most people would use some type of metal bracket or Simpson clip -- but you need a plan.

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