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Crawlspace Air for Rainscreen Venting

jons_headache | Posted in General Questions on

I have an old 1800s log home in Maryland. The way they built it was a small layer of stacked stone foundation, 6” max off of the grade. Bed rock is at the surface so I can’t excavate down much below the sill log. To remove the log would be a very expensive task. I have all my materials to apply new siding(cedar) with a layer of exterior rockwool and a 1.5” rainscreen behind the siding. We really want to add a covered concrete patio in front of the log, but I worry about that creating moisture problems and rotting out the sill log. My question is could I design the vented rainscreen, maybe using slotted pvc pipe in a way that it could pull air out of my crawl space, across the surface of the sill log then up and out thru the soffits? I might be crazy but it seems like it could act as a chimney that could also help with air circulation in my crawl space. Or would there be a better design for that sill log but still including a concrete slab in front of it? Thanks.

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  1. Expert Member


    There are a few difficulties with the plan.

    - Crawlspaces are best sealed and insulated, but when they are vented it is primarily to remove moisture. You want the moist air to dissipate outside, not add to the moisture you are trying to remove from the wall via the rain-screen.
    - Vented crawlspaces should be designed with inlets and outlets so that pressure imbalances don't draw air into or out of the house above.
    - The 1 1/2" continuous cavity from crawlspace to roof would be a concealed space, and represents a path for fire to spread between the two. Our code would not allow it.
    - These Heath-Robinson inventions are hit or miss. Without precedent it's hard to know if they will be effective or not.

    Sorry to be so discouraging, but if it were me I'd try and think of a more conventional way to ensure the log could stay dry.

    1. jons_headache | | #2

      Darn, I thought I was on to something. But that’s a good point. Guess it’s back to the drawing board.

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