Air sealing when using batts in an open attic
skidmorebay
| Posted in Green Building Techniques on
I’ll be insulating the ceiling/attic floor in our addition using fiberglass batts. (Cellulose and other alternatives aren’t available here in rural Alaska.)
Since we have 2×6 ceiling joists, I’ll be stuffing them with R21 batts, then cross-laying R38 batts on top.
If batts truly require an air barrier on all six sides, is there a method for air sealing the top of the last layer of batts, the face exposed to the cold, vented attic space?
I’m not planning for an attic floor, other than some boardwalks to get around for inspection. On the warm side I’ll be using either poly sheeting or 1″ polyiso.
thanks much,
JS
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Replies
"If batts truly require an air barrier on all six sides, is there a method for air sealing the top of the last layer of batts, the face exposed to the cold, vented attic space?"
Is this a vented roof assembly? As in, unconditioned attic with soffit intake vents and a ridge vent? If so, then you don't need any air sealing of the batts on top. The batts are a thermal insulation and not an air seal.
It is - open attic, vented soffit to ridge. There is quite a bit of air movement up there on a windy day!
Everything you read says that batts lose R value quickly with any air movement. Curious how I might reduce losses at the top where the batts would be exposed to open air in the attic.
skidmorebay,
There is some loss of r-value due to wind washing, but it is quite dependant on location. Next to the walls you want to air-seal and provide baffles, which will afford protection. In the rest of the attic you can compensate for the loss with more insulation if you think it is worth it.
On page 56, tables #8 and #9, show the loss you should expect:
https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
Mike Guertin has a great video on the subject...
https://www.greenbuildingadvisor.com/article/attic-air-sealing-products-and-materials-overview
See this too: https://www.energystar.gov/saveathome/seal_insulate/attic_air_sealing_project
https://www.greenbuildingadvisor.com/article/air-sealing-an-attic-in-a-cold-climate
https://www.greenbuildingadvisor.com/article/air-sealing-an-attic