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Community and Q&A

Vented Cathedral Ceiling — Affordable Insulation

aaronla | Posted in Energy Efficiency and Durability on

About a year ago, when i moved in to our new (old) home in Montreal. I had an energy audit done that really highlighted the piss poor job of insulation the previous owners had done with their attic conversion.

So, hence, we am in the process of re-doing the insulation in my cottage style 75 year old 1.5 story cathedral roofed home. There are rafters that are 2×5, that would need to be “furred” out. Also, when I moved in, we re did the old roofing shingles, installed a ridge vent, and am in the process of drilling 1″ holes in the blocking on the bottom of rafter the opens venting above the soffits.

There was previously (from sheathing down) air baffles (without venting… figure that) and about 4 inches or fiberglass, then a poly sheet, and sheetrock.

I would like, and find that the following insulation solution, would fit my budget and hopefully also my insulation needs. I have approximately 1000 sq ft of roofing.

From sheathing:
1″ home made air venting made from 1/4″ OSB with caulking to seal gaps.
5.5″ Roxul R22
1″ Extruded Polystyrene over lapping rafters for thermal bridge @ R5
Poly sheet

Can I please get some feedback from some more experienced builder out there?
How does the plan sound… sound? what about the placement of the poly, should it be above or below the polystyrene? Am i missing something?

I appreciate the help.

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  1. jklingel | | #1

    "1" home made air venting made from 1/4" OSB with caulking to seal gaps." Why worry about gaps? It's supposed to vent, right. ?? Is 1" of ventilation code and adequate?

  2. GBA Editor
    Martin Holladay | | #2

    The caulking is appropriate. It helps the mineral wool batts perform better by minimizing wind washing and convection.

    Air-permeable insulation performs best when installed in a sealed cavity with an air barrier on all six sides.

  3. GBA Editor
    Martin Holladay | | #3

    You are located in Climate Zone 6. If your house were located in the U.S., building codes would require a minimum ceiling R-value of R-49.

    I'm not sure whether the rafters are equivalent to modern 2x6s (actual depth = 5.5 inches) or are an oddball size cut by a 75-year-old sawmill (true 2x5s). If you have 5-inch-deep rafters with a 1-inch air space and a 1/4-inch layer of OSB, then you have room in your rafters for 3.75 inches of insulation. If your mineral wool batts have an R-value of R-4 per inch, that gives you about R-15 between your rafters.

    If you add a layer of R-5 foam, you are up to (at most) R-20.

    My advice: use closed-cell spray foam, or add a thicker layer of rigid foam, to improve the R-value of your proposed roof assembly.

    More information here: How to Build an Insulated Cathedral Ceiling.

  4. wjrobinson | | #4

    Arron, good plan. But forget poly. Tape rigid foam instead and also try to seal it to perimeter where rigid ends. Double up the rigid if affordable. Skip the spray foam if it is a budget buster.

  5. aaronla | | #5

    As far as I know the minimum for a "regular" attic is indeed 49, and the minimum for cathedral is less, at R30. Correct me if I'm wrong.

    My insulation proposal would give me a total of R27. R22 for the Roxul and R5 for the rigid foam. Do you think I should to with 1.5 of rigid for an extra R2.5... headroom is at a premium, and I'm also wondering about diminishing return on the increased R value... Do I value the 0.5" more than the R2.5 increase...?

    Our summers get pretty warm here, up to 100F at times, do you think I should aim more towards 1.5" of venting? I'm trying to keep as much headroom as I can.

    Do you know which type of caulking would be best? One that stays soft? mold resistant?


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