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Vented crawlspace insulation in San Francisco Bay area

vishketan | Posted in Energy Efficiency and Durability on


We are first time home owners, and it is overwhelming. So please bear with me if I get the jargon or naming wrong.

We own a single family ranch in Saratoga, and as per we are in climate zone 4. Our climate is fairly mild (never goes below freezing in winter, and only a few weeks of warm weather during summer). Our home was built in the 1950s and had very poor insulation. I am doing a set of energy efficiency upgrades which include installing a new heat pump hybrid furnace, moving the ductwork to the attic, air sealing and insulating the attic etc. As part of this I would also like to insulate the crawlspace.

Currently the crawlspace is uninsulated and has a dirt floor, with 12″ to 18″ of clearance in most places. There is just about enough room to “crawl”. Only the water pipes and sewage pipes are in the crawlspace. The floors are mostly hardwood with vinyl in a small area in the kitchen and laundry. The bathrooms have tile floors. We have lived in the house less than one year, but so far we have not seen any moisture issues in the crawlspace.

I read quite a bit about crawlspace insulation and most of the articles seem to suggest converting it into a conditioned space. However, given our mild climate and because of what I read about radon issues, I prefer to leave it as a vented space.

I read the excellent article and Figure 7 there seems like the ideal way to go. However, I have quite a few questions:

– How is the 2″ polyiso rigid board secured to the floor joists? I am thinking of using Thermax and was wondering if simply using an adhesive like enerfoam suffices or do I also need to use some fasteners like Tapcon.

– Is it an overkill to add a layer of insulation between the thermax and the subfloor. Can I use denim insulation? It does not have Kraft paper on one side. Is that an issue? If fiberglass, i am assuming I need something which either has Kraft or foil backing. Which one is better? Which side should the foil backing face? Alternatively, what about roxul mineral wool?

– Can the Thermax boards touch the foundation wall or is there some concern about absorbing moisture?

– If I am planning to repipe the plumbing in the near future, should I wait for the plumbing to be done before taking up this project?

Thanks in advance for reading through the long post, and for sharing your knowledge.


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  1. GBA Editor
    Martin Holladay | | #1

    The best approach is to create a sealed crawl space with insulated walls (especially if you plan plumbing work). This article explains what you need to know: Building an Unvented Crawl Space.

    If access is difficult, it's possible to deepen the crawl space by removing soil to provide more room to work.

    It makes little sense to insulate under the joists if you plan plumbing work.

    Once you have finished your plumbing work, you can insulate under the joists if you want (as shown in the Building Science Corp. document you linked to). The illustration shows fiberglass batts between the joists and a continuous layer of foil-faced polyiso under the joists. I would use fiberglass, not denim, in this location. Unfaced fiberglass batts make more sense than kraft-faced batts. Roxul mineral wool can also be used here. If the polyiso touches the foundation walls, there is no reason to believe you will have problems.

    Foil-faced polyiso is fastened to joists with cap nails (nails with large plastic washers attached to their heads).

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