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Community and Q&A

Fiberglass batts in a cathedral ceiling?

user-2179523 | Posted in GBA Pro Help on

Hi Martin,
I’ve been reading the GBA web site and have subscribed to the Fine Homebuilding magazine for several years but this is my first venture into the “green expert” arena. I could ask much but will try to narrow my questions to the two below.
My wife and I are building a small, hopefully energy efficient week-end/retirement home located approximately 50 miles east of Erie PA. Though the maps indicate Zone 5, having grown up there and experienced the winter lake effects, I’m considering it Zone 6.
The home is a rectangular ranch style (first floor 1360 SF) with an attached garage and a full basement under the home portion. It’s axis is situated north/south (due to the awesome view to the east). An old plantation of spruce and red pine creates somewhat of a buffer to the immediate south and southeast.
My wall assembly consists of 2″ of XPS insulation with taped seams over 3/4″ OSB sheathing with 2×6 studs which will be filled with R-21 high density fiberglass batts. Horizontal cement board siding will be attached to 1×4 furring strips which are fastened through the foam to the studs. A rolled screen insect barrier will be attached along the bottom edge. Tyvek house wrap with taped seams was placed over the foam.
The ceiling of the eastern half of the dwelling is vaulted and will be drywalled. The western half will be traditional unconditioned attic space with either blown-in insulation or double layers of unfaced R-30 fiberglass batts. The roof sheathing is 3/4″ OSB with sealed roofing paper and asphalt shingles.
The basement will be conditioned space. We are planning on gluing 1-2″ rigid foam to the concrete block, then furring the perimeter with 2×6’s and filling with R-21 prior to drywalling. Do you feel this is a reasonable approach?
My main question is centered around the best method(s) for insulating the vaulted ceiling. Our plans are hopefully commensurate with what I have read on your many articles and comments on the subject. We plan to install commercially available plastic baffles between the 2 x 12 ceiling joists from the eaves to the ridge vent (roof is 5/12 pitch) which will create approximately 1 1/2″ air channel. We have sistered 2 x 8’s to the 2 x 12’s creating 19″ for insulating space to accommodate one layer of unfaced R-30 fiberglass and a second layer of faced R-30 with the facing attached to the bottom of the joists. Assuming drywall is not completely airtight and we use latex paint, would the proposed assembly allow sufficient drying to the underside of the roof deck, or do you feel we will have a condensation problem on the fiberglass insulation and/or the underside of the baffle? Any advice would be most appreciated!
Thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    Your plan for the cathedral ceiling will work, although (1) I'm not a fan of fiberglass batts, and (2) Your plan fails to address thermal bridging through the rafters.

    You've probably already read the article, but here is a link to an article that will describe all of your options: How to Build an Insulated Cathedral Ceiling.

    To get your planned assembly to perform well, the most important thing to focus on is airtightness. Ideally, you'll include an air barrier between the top of the fiberglass batts and the ventilation channel. The best way to ensure you have an air barrier at that location is to use site-built ventilation baffles. For more information, see Site-Built Ventilation Baffles for Roofs.

  2. GBA Editor
    Martin Holladay | | #2

    Concerning your plan for insulating your basement walls: Your plan will work, but I prefer to use rigid foam that is thick enough to meet your R-value goals, without using any fiberglass. (Fiberglass insulation is a poor choice for a space that may get damp, especially if you ever have a flood.)

    For more information, see How to Insulate a Basement Wall.

  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "Assuming drywall is not completely airtight ..."

    Your assumption may be correct -- but please, do your best to limit air leakage through the drywall. Nothing is perfectly airtight, but you should strive to make your ceiling as airtight as you possibly can.

  4. Expert Member
    Dana Dorsett | | #4

    If it's actually zone 6 and not zone 5, you don't have sufficient exterior R on the walls for dew point control at the sheathing. The "... winter lake effects..." usually refers to snowfall amounts, but that snowfall is accompanied by warmer temperatures. US climate zones are determined by the binned hourly temperatures, not precipitation. Unless your house is a an unusually high altitude in an otherwise zone 5 county, your outdoor temperature averages (and the amount of exterior R needed for dew point control) will be in the zone 5 range.

    Since you are using the future tense "... will be..." regarding the wall stackup, it may not be too late to make the change from 2" of XPS to 2.5" (or 3") of EPS. The R5/inch labeled performance of XPS is dependent upon it's HFC blowing agents, which bleed out over time. EPS is blown with pentane, the vas majority of which is gone by the time the product leaves the factory. Despite an initially higher performance in 50 years the performance of XPS drops to that of EPS of equal density. Despite "lifetime warranty" for retaining 90% of the labeled R (so 90% would be R9 for 2" XPS), there is no reason to believe that is more than a marketing claim with an expectation that it would rarely or never be challenged. The performance of EPS is stable over time, and is cheaper per labeled-R. And the HFCs pack a powerful green house punch (more than 1000x CO2) as it escapes, whereas pentane is much tamer (~7x CO2), much of which is often recaptured at the factory and burned for process heat (turning it into CO2 and water.)

    If you are looking for Zone 6 type dew point control you would need a minimum of 2-3/4" of 1.5 lb density "Type-II" EPS (or XPS), but for zone 5 you're good at 2" .

    In the basement you need similar dew point control foam thicknesses to keep batt-insulated 2x6 framing mold-free. If there is no foam under the slab, it's also worth putting an inch of EPS under the bottom plate of the studwall as a thermal & capillary break. Summertime dew points are well above the deep subsoil temps in your area, and the bottom of an insulated studwall bottom plate will be cold enough to take on mold-growth levels moisture from the room air. With ani inch of foam under it, that won't be a problem.

    The roof would have been somewhat higher performance if the 2x8s had been perpendular to the 2x12s rather than sistered onto them, but it's not worth re-working it if that if it's already done. An interior side air barrier is still very important, (as Martin indicated), and if you have any qualms about the quality of the gypsum over time, it's worth putting a sheet of 2-mil nylon (Certainteed MemBrain) detailed as an air barrier between the sheet rock and the rest. It's a "smart" vapor retarder, that works like a vapor barrier when the air in the cavity is dry (as it should be in winter, and for most of the year), but becomes vapor open if moisture levels in the cavity reach mold-potential.

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