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Air sealing an old brick building

I’ve been reading with interest the posts on weatherizing old brick buildings, but I still have some specific questions I hope someone can answer.
I have a circa 1837 brick federal with double wythe walls in central Vermont. The bricks are hard fired with tight mortar joints and moisture from the outside is relatively well controlled. Exterior walls are strapped with 5/4” planks supporting the lath and plaster. Interior walls are the same 5/4” planking with lath and plaster on both sides.
Over the years, i’ve been working on weatherizing the house. When I moved in 12 years ago I pulled up the attic floor boards, added 2” strapping to the ceiling joists and blew in 12” of cellulose. I chose to wait to air seal the tops of the wall cavities because I hadn’t yet finished the rewiring, and I wanted access to the walls to fish wires. Around the same time, I installed 6 mill poly and crushed stone on the dirt basement floor to control moisture. Now I’m ready to proceed with further weatherization measure. After reading a lot of information on the GBA website, I’ve decided against insulating the walls, but Istill have questions about how to proceed with air sealing.
We have just gone through an extended cold spell. There is frost coating the inside of the roof sheathing in the attic and evidence of frost in between the attic floor boards. There is venting in the attic via 1/4 circle windows currently, but they are not ideal as they are located close to the floor level. I plan to install gable vents in the spring.
I guess my question boils down to whether or not I should air seal all wall cavities. I do plan to foam seal the tops of the wall cavities on interior partitions, but after learning how important it is for brick walls to breathe, I’m having second thought about sealing the exterior walls. I’m even wondering if I should install proper vents along the exterior walls to allow air flow up the walls without passing through and causing dampness in the cellulose. Will the exterior walls breathe adequately horizontally with the top of the walls air sealed?, or would it be better to allow air to move vertically to ensure that the bricks avoid freeze/thaw damage and the embedded timbers stay sound?

Asked by duanegormanbuilder
Posted Jan 13, 2018 3:32 PM ET

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1 Answer

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Daune,
Considering the evidence you cite -- frost on the roof sheathing visible in the attic -- you know that there are substantial air channels linking your damp basement with your attic. It's possible that you have some type of utility chase connecting your basement and attic -- a chase that you will discover and seal when you work on air sealing your attic. It's also possible that there are air pathways in your partitions.

But the most likely air pathways are the air channels between your bricks and the interior plaster.

Under the circumstances, sealing these air channels in the attic makes sense. You can so this by stuffing the tops of the channels with either fiberglass or mineral wool insulation. Push the fibrous insulation down a few inches, and seal the top with two-component spray foam.

Answered by Martin Holladay
Posted Jan 14, 2018 7:00 AM ET

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