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Climate Zone 3 subfloor encapsulation

FiveDs | Posted in General Questions on

I am renovating a 70 yr. old house in San Diego that I have owned for 5 years. It is Climate Zone 3 here.  The original floors were 4×6 post and beam with 2×10 planks (no joists) and no insulation. The 2×10’s were spongy and kind of  had a white’ish tint to them (maybe from years of salty air)?. So I gutted the entire subfloor which was ventilated at the rim boards and at the 2 accesses. The new floor system is 2 x 6 joists with 3/4” tongue and groove OSB. Initially, I was going to not insulate the floor and reinstall the vents at the rim boards. My main concern is the salty air causing a problem with the OSB subfloor and possibly leeching up into the laminate flooring. I am considering  1 1/2” closed cell spray foam on the bottom of the OSB and floor joists or encapsulating the crawlspace with by spray foaming the stem wall and installing a 12 mil vapor barrier product and install a LOMANCO unit. Encapsulating the crawlspace is about 70% cheaper. A spray foam contractor told me that encapsulating the crawlspace in this climate zone would be a big mistake and would definitely cause a mold issue in the subfloor area. I have been researching and looking into this for several weeks And I am leaning toward encapsulating the subfloor. For some reason I don’t know if I can believe him so I am looking for some input. I have a bout 18 to 24 inches of area to crawl so space is not a factor for the installation of spray foam. So the question is do I leave the OSB and floor joist as they are and then laid the crawlspace as it was? Or should I consider encapsulating the crawlspace or spray forming the underside of the floor deck? Thanks for your input.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Five.

    In a general sense and when detailed correctly, unvented crawlspaces tend to be healthier and more energy efficient. That said, there are some regional considerations. For example, unvented crawls are not a great idea in areas prone to flooding. And some research has shown that it hot and dry climates, vented crawlspaces are just fine and insulated floors may perform better than insulated crawls. Check out this article before you make your decision: Building an Unvented Crawlspace.

    1. FiveDs | | #2

      Thanks for the reply. My main concern is how the elements will affect the OSB subfloor decking material utilized. Even if it was plywood I would still have the same concern with moisture issues with the material.. The article that you suggested that I read stated that installing batts insulation within the joists and venting the crawlspace is a recipe for mold. I was surprised to read that and that may not apply in San Diego. Do you have any experience or information regarding how well the OSB subfloor decking will be affected by the elements if left uninsulated or without The application of a The application of a spray foam? I Have thought about spraying some type of sealer or a primer to the bottom side of the OSB To battle the elements and not insulating the floor joists And then encapsulating the crawlspace. Thanks

  2. charlie_sullivan | | #3

    If you are aiming for green building (this being green building advisor), you should think about some of the issue with spray foam before you proceed with either concept. If you specify an HFO blowing agent and work with a highly skilled installer, you can avoid many of the issues, but it's still an expensive approach that I don't think is necessary in your situation.

    Your climate has pretty low humidity, so you could vent the crawl space, but you can keep it drier if you encapsulate. And you can get good insulation for cheaper.

    If you closed the crawl vents and had a dirt floor with no vapor retarder, and you had no conditioned air supplied there, you could develop moisture and mold problems. But if you do it right, it should be easy to avoid any problems in your climate.

    I don't see a need to put any treatment on the wood, but you could put a borate treatment on it if you want.

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