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Zip Sheathing on Vented Crawlspace Walls

JFord24 | Posted in General Questions on

Hello All,

New to GBA and have been browsing the site in search of answers to the many questions I have for my residential full remodel in Southern Los Angeles. Currently have the building torn down to the studs interior and exterior and am at the stage where I will be putting up sheathing soon. The goal is to get the interior very well air sealed and make the home as efficient as possible in a budget friendly way.

Project Details:
900 sqft
Sloped plot which makes the “crawlspace” (basically an unfinished basement) about 6ft tall on one end and 4ft on the other.
Original design was a vented attic and crawl space
Converting to all electric
Attic space will be left as vented
7/16″ Zip Sheathing
Rockwool 1.5″ Comfortboard 80 Exterior Insulation

Got a great deal on enough 7/16″ zip sheathing and rockwool comfortboard 80 for the exterior walls but my main question currently is how to address the crawl space. It seems like it would be a waste to put the zip system on the crawl space exterior walls, only to cut a bunch of holes in it for vents. Likewise, exterior insulation in the same area seems pointless if the walls have vents in them. But I also don’t see a need to have the crawl space be sealed and conditioned due to the climate we have in Los Angeles. Maybe I’m wrong?

Can I simply put the zip over the entire house and just leave the crawl space unvented without truly encapsulating it like a basement? Due to the large size of the crawl, I intend to put a heat pump water heater and the HVAC interior air handler + ducting all in the space. I figure the heat pump water heater would help to dehumidify the crawl, especially if I vented the exhaust to the exterior.

I’d love input as to what the best practice for this situation should be and reasons why I should or shouldn’t vent the crawl space. What’s the best bang for the buck? Thanks in advance for any advice or resources to turn to.

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  1. matthew25 | | #1

    > "The goal is to get the interior very well air sealed and make the home as efficient as possible in a budget friendly way."

    If this is really your goal and you are putting mechanicals in the crawlspace you should encapsulate the crawl space. If you plan on venting the crawlspace, what are the details for the underside of your floor? Do you have a WRB and exterior insulation planned for that? You should, just like the other exterior surfaces of your house.

    1. JFord24 | | #2

      The underside of the floor is actually already insulated with fiberglass batts from the previous owner. They don't look to be in all that great of condition though. I hadn't considered a WRB for the underside of the floor but I was intending to air seal using Prosoco joint and seam filler at the subfloor level if I kept the crawl as a vented space.

      Considering I'm using zip sheathing, it's looking like the most logical thing for me to do is to seal the crawl space and get the air barrier on the exterior walls and crawl space floor. It makes sense to me but I've been told by others that sealing the crawl isn't the most crucial thing to do. But then I've read reports that crawl space encapsulation increases efficiency and air quality of the home.

      1. Expert Member
        BILL WICHERS | | #3

        The biggest part of the crawlspace encapsulation you'd need, assuming you are already insulating the EXTERIOR of the crawlspace walls, is the vapor barrier membrane over the floor. Put some 10+ mil (code is 6 mil, but that's really not thick enough) polyethyelene down, run it at least one foot up the walls (I prefer to run it all the way up the masonry walls), and seal it to the block (polyurenthane sealant is great for this, and will hold the poly in place as you work, negating the need for additional fasteners). That poly will help with moisture levels and mold issues, which is how it can help with indoor air quality. There really aren't any downsides to doing this work, aside from the cost (which isn't really all that bad) and that it's not exactly a fun type of project.

        You should pull out the old fiberglass insulation, especially considering it's in bad shape already.


        1. JFord24 | | #4

          Thanks Bill. That all makes perfect sense to me and is something I can definitely handle. The climate here is very mild but it can definitely get a little humid. My concern is if a dehumidifier will be necessary or if I can get away with just having a HPWH do that work for me. I guess I can see how it is after a while and proceed as necessary.

          1. matthew25 | | #5

            Yeah wait and see, worse case you can put a simple plug in dehumidifier model from the big box store. Might want to think about a drain location while you’re doing this work now though.

  2. Expert Member
    DCcontrarian | | #6

    If you have mechanicals in the space you want to condition it
    You don't want to vent your heat pump water heater to the outside. It's producing nice cool dry air which dehumidifies the crawl space and helps cool your house. If you vent to the exterior the air you vent is going to be replaced by outdoor air which will be warmer and more humid, not what you want in your house.

    Where does the idea of venting heat pumps to the outside even come from? It seems like a fairly common misconception.

    1. JFord24 | | #7

      You're right. I was thinking of it incorrectly. Makes sense to not vent it to the exterior as it is essentially an air conditioner.

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