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Venting a dense-packed cathedral ceiling

John Roy | Posted in Energy Efficiency and Durability on

I would like to dense-pack a cathedral ceiling with rock wool insulation. Rafters are 2×10. Do I need to install ProperVents along with soffit & ridge vents, or can I just close in the area and pack it in?

I do not have any ceiling penetrations to leak air in and rock wool is hydrophobic. Does this have any effect on vapor finding its way in (or is condensation occurring on the backside of the roof sheathing a totally different issue)?

The roof decking has been fitted with a Ice & Water Shield (rubber membrane) from the eaves to the ridge, and then finished with architectural fiberglass shingles.

Thanks,
jroy

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    J Roy,
    I'm not familiar with dense-packed mineral wool, so you should ask your contractor what practices are recommended.

    According to my understanding of the International building codes, you can't install an air-permeable insulation like mineral wool in a cathedral ceiling unless you first install ventilation chutes on the underside of the roof sheathing. Styrofoam ProperVents are too flimsy to withstand the pressure of dense-packed insulation, so you'll probably want to install plastic AccuVent or site-built ventilation chutes.

  2. M S | | #2

    First question will be what is your location this dictates how R Value is needed. Secondly most will say you need ventilation. You may be interested in reading these two articles

    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19878/venting-dense-pack-cathedral-ceiling#answerQuestion

    http://www.buildingscience.com/documents/insights/bsi-043-dont-be-dense/

    Although it will be more work you could always cut-in poly iso or XPS placing the board bewteen each stud (where you would put the rockwool) leaving a airspace = ventilation. Then fill the remaining cavity full with XPS or polyiso. You could then overlay the face of the studs with additional rigid board insulation and use horizontal strapping for attaching the sheetrock.

    This may not get you to the prescribed R value but will get you closer.....Of course I'm thinking more along the lines of this being appropriate for more a 1.5 story where that slant/cathedral wall is rather short in area.

    It sounds like your roof is new so overlaying with XPS on top of the decking doesn't seem practical.
    Really it comes down to furring down the 2x10 and then using the highest R Value material possible really SPF or rigid is your options depending of course on your climate. I'm sure someone else will chime in.

  3. John Roy | | #3

    I live in Massachusetts in the southeastern part of the state. I spoke with the insulation company president (they have a very good reputation and have been in the business for over 50yrs) & he said the insulation could be installed without an air chute because the insulation did not absorb water . My local building inspector said he would not have a problem with omitting vent chutes but said I should provide soffitt vents. Since Ive come to rely on GBA for good advice I want to throw the question out to those more knowledgeable on the issue.

  4. GBA Editor
    Martin Holladay | | #4

    John,
    In Massachusetts (Climate Zone 5), the 2006 IRC requires a minimum of R-38 of ceiling insulation. That's about 10 1/2 inches of mineral wool. Obviously, the insulation won't fit in 2x10 rafter bays, which are only 9 1/4 inches deep -- even if you don't install ventilation channels.

    It doesn't make any sense to install soffit vents if you don't have ventilation chutes between the top of the insulation and the bottom of the roof sheathing.

    If it were my house, I would install ventilation chutes (well sealed to prevent air leakage). I would also install 2 inches of foil-faced polyiso under the rafters, held in place with 1x4 strapping, and I would blow the mineral wool through holes in the polyiso.

  5. Matthew Amann | | #5

    Martin can you explain how to create air tight ventilation channels? I am having a hard time picturing how to bring something up to the cutback sheathing at the ridge and truly do this well. Seems to me that dense pack cellulose blown in would do a fairly good job.

  6. GBA Editor
    Martin Holladay | | #6

    Matthew,
    The most airtight ventilation chutes -- but not the fastest to install -- are site-built chutes. Usually, the builder starts by tacking up 1"x1" sticks in the upper corners of the rafter bays. Then rigid material (Celotex, thin plywood, or rigid foam) is installed against the sticks, with caulk or spray foam to make the installation airtight.

  7. John Roy | | #7

    Ive used the method described by Martin to make my own chutes...time consuming, somewhat of a pain in the backside...but compared to the flimsy stuff available making your own is far better.

  8. Matthew Amann | | #8

    I'm thinking that if Cor-a vent or equivalent should make. some baffled sheets for this application. I know the cost would be stiff, but building these on site baffles has to be pricey, any sq/ft costs worked up on this? I also wonder about a continuous honeycomb cardboard product, inventors? Seems to me that keeping a small amount of ventilation and then dense packing under that could be a "best possible approach", or the least risky one.

  9. M S | | #9

    Or make them out of sheets of polyiso. Like this... Keep in mind this is working from the exterior. So in your case you would place 1x1 against the studs then cut sheets of rigid insulation and then use canned foam to get your "air tight" channel

  10. Matthew Amann | | #10

    Good Tip M S, but I would lose like 2" if i did that. I'm also thinking that Cor-a- vent or equiv. baffle would be faster, and a lot less material involved. I'm thinking of recycled plastic or like they do in Germany, use plantation fiber trees or Hemp to make a truly "green" baffle for cathedral and heel insulation applications.

  11. M S | | #11

    Not sure how you would lose 2'' A 1x1 nominal is .75''. You said you wanted a ventilation channel from sofit to ridge. Keep in mind what you see is rigid insulation so not only does it serves as your baffle for ventilation, but your also getting R-6.5 per inch. Now as Martin suggests if you wanted to blow rock wool or cellulose vs cutting more rigid to fit BEWTEEN the studs it will be faster

    Lets say you used 2'' of polyiso + your 1x1 in your cavity which served as your baffle = 13 R value (like the pic above)
    Then blow dense packed cellulose @ R-4 per inch x 6.5'' of space left = 26 R value
    so your sitting @ R-39
    Now as Martin suggests overlay the face of your studs with rigid insulation either polyiso or XPS. Putting up another 2'' of polyiso gets you close to R-52

  12. Matthew Amann | | #12

    The Polyiso @ 1" + Batt @ 3/4"= 1.75", The insulation takes up space as well as the batt, which is too much in the 2x10 rafter space IMO.

  13. TJ Elder | | #13

    This vent chute serves partly to carry out any excess moisture from the cavity insulation, so it seems wise to make the baffle with a material that is vapor permeable. Foil faced polyiso does add R-value but it's not permeable. I'd suggest hardboard (masonite) or another fiber panel to just hold back the dense pack without impeding water vapor. The dense-pack will be only ~R-31, so you'll need to furr out below the joists or add polyiso below, as Martin suggested.

  14. GBA Editor
    Martin Holladay | | #14

    Thomas,
    You wrote, "This vent chute serves partly to carry out any excess moisture from the cavity insulation."

    No it doesn't! Heaven forbid! No one wants interior moisture migrating through the insulation to the ventilation cavity -- certainly not deliberately! You're trying your darndest to prevent any moisture from reaching the ventilation cavity.

    Ideally, your thermal boundary successfully separates the warm, humid indoor environment from the cold wall sheathing or cold roof sheathing. It is NOT desirable to encourage indoor humidity to contact cold sheathing.

    The purpose of the ventilation channel is to lower the humidity of the roof sheathing -- in case some sloppy carpenter did a bad job of caulking the ventilation chutes in place, or in case some electrician mistakenly installs wiring or a can light in your ceiling.

  15. TJ Elder | | #15

    Martin,

    Would you agree that the purpose of the vent chute is to reduce the risk of moisture accumulation? Moisture can come from a few different sources, and one could be below the chute. Why design a vent chute to intentionally exclude water vapor that just might find itself in the cavity insulation? Not that it's intentional for the insulation to be damp and need drying to the exterior, but the opportunity of drying reduces risk.

  16. GBA Editor
    Martin Holladay | | #16

    Thomas,
    Q. "Why design a vent chute to intentionally exclude water vapor that just might find itself in the cavity insulation?"

    A. There will always be water vapor on the warm side of the insulation layer (the interior in winter), and there will always be water vapor in any air-permeable insulation like fiberglass or cellulose. That's inevitable. But you certainly DON'T want to encourage that vapor to migrate toward the cold sheathing. A good roof assembly will provide one or several barriers to such flows -- usually air barriers do this job best, but in some assemblies, vapor retarders are also useful.

    The purpose of the ventilation channel is as a last-ditch method of keeping your sheathing dry in case your assembly has flaws. But you don't want to build in any paths that allow or encourage vapor to flow toward your cold sheathing.

  17. TJ Elder | | #17

    Martin,

    It sounds like you have prioritized dry sheathing, and I understand your point. My instinct here is to give the same priority to avoiding wet cavity insulation, which could result from inadequate air sealing. In either case it is essential to make the assembly airtight and to have a vapor retarder at the ceiling.

    I would still expect that it would reduce risk to have a vapor permeable vent chute, because even if moisture did migrate into the vent gap (where it could condense on the sheathing) it should get carried away by exterior air, if the vent chute is functioning.

  18. GBA Editor
    Martin Holladay | | #18

    Thomas,
    I think making it airtight is the key; I doubt the the permeance of the chute material makes much of a difference. However, I agree with you that vapor-permeable materials work fine here, as long as they are as airtight as possible.

  19. John Roy | | #19

    Martin

    In lieu of an air chute can a layer of closed cell foam be used against the underside of the roof sheathing? What should the minimum thickness be??

  20. GBA Editor
    Martin Holladay | | #20

    John,
    According to the 2009 IRC (in Section R806.4), the thickness of any "air-impermeable insulation" that is "applied in direct contact with the underside of the structural roof sheathing" shall meet the minimum R-values required for condensation control as shown in Table R806.4. These values are:
    Climate Zones 1-3, R-5
    Climate Zone 4C, R-10
    Climate Zones 4A and 4B, R-15
    Climate Zone 5, R-20
    Climate Zone 6, R-25
    Climate Zone 7, R-30
    Climate Zone 8, R-35

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