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Community and Q&A

Insulating walk-up attic using site-built ventilation baffles

MattB24 | Posted in GBA Pro Help on

Hi all,

I’m a DIY homeowner living near Boston, MA in a duplex townhouse with a gable roof. I’m insulating what will be a finished and conditioned walk-up attic in our 3rd floor, starting with the site-built ventilation baffles Martin Holladay described ( My initial plan was to use 2-in. foil-faced polyiso to create air baffles (leaving a 2″ gap between the roof sheathing and insulation) from soffit to ridge vent between our 2×10 rafters. Then I was going to air seal the chutes with foam sealant, add 5.5″ of mineral wool in the space remaining between the rafters, and add 2 more inches of polyiso on top of the rafters from floor to ceiling to achieve R-49 and prevent thermal bridging through the wood 2x10s. Then I’d build knee walls and screw drywall through the polyiso into the rafters to create a 12′ high cathedral ceiling. 

But I realized I might be creating a condensation problem by sandwiching the mineral wool between 2 layers of vapor impermeable polyiso. So I contacted my building department for advice, and they said I should be fine as long as I do NOT air seal the polyiso ventilation baffles and DO air seal the interior polyiso sheets that’d lay on top of the rafters. That way I’d have a vapor barrier on the interior while allowing the space between the rafters to dry to the exterior.

But NOT air sealing the polyiso chutes seems like it would defeat part of their purpose. Should I just forget about doing the interior polyiso and just stop at air-sealed polyiso chutes and mineral wool? But then my building dept. is insisting I’d still need a vapor barrier on the interior. I really like the idea of using foil-faced polyiso for the baffles for the high R-value and heat reflection, but maybe I need to use a different type of rigid foam for the baffles?

Any thoughts much appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    Air sealing the ventilation baffles is preferable, but it's probably not worth fighting your building department on this issue. If you do an excellent job air sealing the continuous layer of rigid foam on the interior side of your assembly, the fact that you haven't sealed the perimeter of each ventilation baffle won't matter too much.

    Cut the ventilation baffles for a tight friction fit, and don't worry.

    1. MattB24 | | #2

      Thanks for the reassurance, Martin! Honestly, I'd trust your advice over that of my building dept. :-) I'm tempted to just go ahead and air seal both the polyiso baffles AND the continuous interior layer of polyiso, and they'd never see it in the inspection. So from what I read in other posts, as long as I get that continuous polyiso layer up and air sealed tightly and quickly right after installing the mineral wool, the chances of having moisture issues between the baffles and the continuous layer are low, right?

  2. Expert Member
    Peter Yost | | #3

    Hi Matt -

    The soffit-to-ridge vent chutes are mainly a way to protect/moisture-manage your roof sheathing so that is why your PI "sandwich" is less of an issue in terms of reduced drying potential.


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