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Musings of an Energy Nerd

Well, They Got It Half Right

What's Best For a Crawl Space — Fiberglass Batts or Rigid Foam?

At the International Builders’ Show in Las Vegas, an Environments for Living show home vaunted its “ventless, conditioned crawl strategy.” While the strategy makes sense, many visitors to the home would probably prefer to see rigid foam on the crawl space walls instead of fiberglass batts between the crawl space joists.
Image Credit: Martin Holladay

At the International Builder’s Show, several demonstration homes have been set up in the parking lot outside the Las Vegas convention center.

The Environments for Living show home has a display promoting the advantages of ventless conditioned crawl spaces; so far, so good. But instead of following best-practice advice and insulating the crawl space walls with rigid foam, the Environments for Living home designers chose to install fiberglass batts between the floor joists — a feature proudly displayed behind a Plexiglas viewing panel.

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  1. Joe Lstiburek | | #1

    Lighten up
    The Masco show home crawlspace is not as dumb as you imply. AEC
    promotes a strategy where the crawlspace is sealed (continuous ground cover, perimeter walls airtight but not insulated) and pressurized continuously with air from the house. The crawlspace insulation is now located within the floor joists. This solves the termite infestation and termite inspection problem (no inspection gap to worry about at the perimeter). For this to work the subfloor needs to be sealed airtight (which AEC recommends). In this configuration the thermal barrier is at the floor as is the primary air barrier. The crawlspace is moisture controlled via the exhaust air from the house which is always drier than the outside air during the summer and not humid enough to cause condensation in the winter. This is a pretty elegant solution. Personally, I am not ecstatic about it because it needs a continuously operating fan and you still do not want to put HVAC ducts in the space because if they leak air this air does not find its way back to the house. But all in all, not bad.
    So, lighten up.

  2. GBA Editor
    Martin Holladay | | #2

    Thanks for the details
    Thanks for taking the time to provide more details on the Environments for Living approach to sealed crawl spaces. Your posting gives me an opportunity to note that Masco / Environments for Living has consistently promoted above-code housing based on firm building science principles, for which they should be applauded.

    It is certainly true that in termite-infested regions of the US, rigid foam insulation may not be the best choice for foundation insulation. I was unaware that this design required a continuously operating fan; like you, I feel that the need for such a fan is not ideal.

    With the details you note — a mechanism to keep the crawl space pressurized with conditioned indoor air and an airtight subfloor — I don't doubt that these crawl spaces can perform well. However, I think the jury is still out on the question of whether fiberglass batts are an appropriate insulation for crawl space ceilings. I have inspected many crawl spaces with fiberglass batts overhead, and this insulation method does not age well. I wouldn't do it in my own house. Done with attention to details, and with a secure air barrier or protection board to encapsulate the fiberglass, it may be an acceptable approach. I'd be interested in hearing other readers' opinions on the topic.

  3. Expert Member
    CARL SEVILLE | | #3

    Fiberglass floor insulation
    I'm with you on the batts in the floor joists. It is difficult if not impossible to get a complete airseal on the underside of the joists (the subfloor is manageable), so the insulation is never fully encapsulated, and over time, it almost always starts to fall and gets damaged during maintenance. I would be happy if there was a complete air seal at the bottom, but by the time you invest in that, you might as well insulate the crawlspace.

  4. Sam Duncan | | #4

    non venteded crawl space/sealing/insulating
    I am building my own house. I believe that I want to build an unvented, conditioned crawlspace. I'm thinking of installing a downdraft hvac unit that heats/cools the crawlspace, no ducts just cut in openings in the floor where needed. no insulation under the floor to permit the floor itself to be heated/cooled and spray foam closed cell insulation on the perimeter stem walls and floor joist bands making the crawl space virtually airtight. I know this must be controversial since it is hard to find info on this and I see much conflicting info. Am interested in learning more. All help is appreciated.

  5. GBA Editor
    Martin Holladay | | #5

    The information you are looking for
    Here's the information you are looking for:

    There's also a lot of information on this Web site:

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