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Will 1″ EPS baffle withstand pressure of dense pack cellulose?

RyanDG | Posted in Energy Efficiency and Durability on

“(If you use rigid foam for your baffles, it probably makes more sense to choose thin EPS or XPS rather than foil-faced polyisocyanurate, to allow a bit of outward drying, however slow, by diffusion. A thin layer of EPS or XPS is somewhat vapor-permeable, while foil facing is a vapor barrier.)”

Will 1″ of EPS be strong enough to withstand the pressure of 12″ of dense pack cellulose ? The plan is to be as airtight as possible in the ceiling, but if moisture does escape, I’m using EPS as a vapor permeable board that also contributes insulation. Would 1.5″ or 2″ EPS still allow vapor to move through but be stronger to withstand the dense pack?

We’ve built a timbrframe that will have cathedral ceilings and an exposed ridge. From the inside out we’re planning air sealed drywall (taped and gasketed), 1×4 strapping, 2×12 rafters 24″ OC, dense packed cellulose, a baffle of eps that is taped and caulked, 2×2 sticks at the top corners of the baffle, 5/8 cdx plywoood, 30 # felt, galvanized metal roof.

I know plywood is an option for the baffle but I would prefer to gain insulate value if I’m creating he baffle in the rafter bay and loosing the cellulose there.

Thank you!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    User-6890577,
    Different brands (and different types) of EPS vary widely in quality and structural characteristics. Inexpensive EPS has visible beads and snaps easily. Higher quality (denser) types of EPS have no visible beads and are much harder to snap.

    So there is no rule here -- just common sense. Buy a piece of EPS that you are considering using, and test it. Cut it into a 15-inch-wide rectangle and bend it. How stiff does it feel? You should be able to judge the suitability of the EPS with common sense.

    I look forward to reading any comments from GBA readers who have created site-built vent baffles out of 1-inch EPS.

  2. RyanDG | | #2

    Ok. Thank you Martin!

    The alternative someone had recommended was 2x12 rafters, taped Triflex membrane, 2x2 running on directly on top and parallel with rafters, then plywood etc. But I could not figure out how to install this easily...

    In your articles you mention that sometime venting make actually cause moisture in the roof from the outside in the summer, that condenses with radiational cooling. Is there anyway to prevent this?

  3. GBA Editor
    Martin Holladay | | #3

    User-6890577,
    Q. "In your articles you mention that sometimes venting may actually cause moisture in the roof from the outside in the summer, that condenses with radiational cooling. Is there anyway to prevent this?"

    A. I'm not sure which of my articles you are referring to. Before I can address your concerns, it would be good to know the climate zone or the geographical area where you are building.

    A vent channel between the top of fluffy insulation and the underside of the roof sheathing is a code requirement if you are insulating a sloped roof assembly (cathedral ceiling), unless you follow the requirements for an unvented assembly. This applies to all climates. Properly detailed, this type of ventilation channel won't introduce moisture into your roof assembly.

    That said, there are all kinds of ways to mess up the details and create a disaster. For more information on the needed details, see this article: How to Build an Insulated Cathedral Ceiling.

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