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Community and Q&A

Fireblocking and Potential Moisture Problems

SPCummings | Posted in General Questions on

I’m in the process of renovating part of my basement. It’s a capecod house where the brick runs all the way to the top of the first story. 1″ furring strips are attached to the inside of the exterior walls for the first story followed by a plasterboard and then a layer of drywall (each approx. 1/2″). When insulating the knee wall on the second story, I noticed that this 1/2″ to 1″ gap is generally accessible all the way to the basement. When I was insulating the kneewall, I started to seal this gap for fire safety, but began to think that the reason the wood was in generally great condition for being 60 years old is partly due to the presence of these gaps. The basement has embedded rim joists complicating everything.

I would like to improve the safety of the house and am inclined to fireblock in the basement and the knee wall. However, I also wonder if this would be bad regarding moisture as I would be getting ventilation with the cool humid air being pulled up into the attic and eaves.

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  1. SPCummings | | #1

    I'd like to update -

    I am going to fireblock between the brick and furring strips at the top of the rim joist (about a 1" gap between the outside wall).

    However, I think I will not air-seal the rim joist as originally intended because of the embedded rim joists to prevent wood rot there. if I put any roxul or fiberglass, I'll not put it in the first several inches so it doesn't contact the concrete. I'm in the process of re-pointing much of the outside of the house to help water ingress during rains.

    In your thoughts, should this help allow for improving fire safety and the thermal envelope of the house while still keeping my chances of wood rot relatively low (Zone 4A).

  2. GBA Editor
    Kiley Jacques | | #2

    A related question was asked this way: Will fireblocking over balloon framing stop airflow and promote rot?

    Peter Yost responded with:

    “The key principle here is a continuous air control layer. Air moving the way you design for to ventilate and dry an assembly is a good thing. Air moving by happenstance--air leakage--often wets as much if not more than it dries assemblies. Fire stops can compartmentalize cavities and reduce convect currents but regardless, continuous air control layers are key in any climate, even mild ones.”

  3. SPCummings | | #3

    Thank you Kiley. I'm glad you were able to visualize my question!

    Because I'm reading too much about wood rot due to poor air sealing I'm dreaming only of the floor falling through due to wood rot rather than sugar plums and a more comfortable home.

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