Download PDF version (66.6k) Log In or Register to view the full article as a PDF document.
Q.Since 1983, we have been using unvented crawlspaces under many of our energy-efficient housing projects. Our county has recently hired a new building inspector who is unfamiliar with this building practice and can find no provisions for it in the BOCA code. He would like us to provide evidence that unvented crawlspaces are sound building practice in our climate.
We build in a 9,500-degree-day climate. Our crawlspace construction includes 2 inches (R=10) of rigid foam insulation on the outside of the foundation from footing to subfloor, a continuous 6-mil poly ground cover sealed at the joints and the outside wall, and R-19 batts in the floor joists. We have inspected most of these systems over the years and have found no evidence of wood rot or crawlspace moisture problems.
What is the latest building science information on unvented crawlspaces, and how is it integrated into building code?

A.Paul Fisette responds: As your own experience suggests, unvented crawlspaces pose no real problems — as long as there isn’t excessive moisture. In your case, you have provided a continuous sealed vapor barrier across the crawlspace floor and, I would guess, proper foundation drainage on the outside.

Nevertheless, you still have two code hoops to jump through: the Model Energy Code and the BOCA code. Unfortunately, in this case, the two codes don’t necessarily jibe.

The Model Energy Code tells you how much insulation you need and where and how to use it to define the thermal envelope that protects the "conditioned space" in a home. (To qualify as conditioned, the temperature within the space must be maintained at 50°F or higher.) The 1995 MEC (602.2.5) allows for an unvented crawlspace as long as the perimeter insulation meets a specified value. However, your design appears to define the thermal envelope twice. You should insulate either the floor of the living space or the walls of the crawlspace, but not both. If you remove the R-19 batts from the crawlspace ceiling and upgrade the exterior foam as needed, you could convert your "unconditioned" crawlspace into "conditioned" space, and treat it like any other conditioned space in the house where venting is not required.

But then there’s the BOCA code. The bottom line is, unvented crawlspaces are not allowed under BOCA. Section 1210.2 treats crawlspaces as "special spaces," and requires that they be ventilated by either natural or mechanical means.

Still, given your track record, you might be able to convince your inspector to treat your conditioned space the same as a basement, which would typically not require any special ventilation measures. If the inspector disagrees, you still have a few options. You can get an engineer to review your plan, deem it acceptable, and stamp it to certify that it will perform as intended. This seems like the most practical approach. You could also install a mechanical exhaust system in the crawlspace that is governed by a humidistat. This may satisfy the code, but using an exhaust fan in the crawlspace could draw moisture and radon into the crawlspace. And lastly, don’t forget that you have the right of appeal. You can petition your state building officials to review the code as it relates to your specific case. Refer to the code commentary (your code official might have this), which discusses the line of reasoning that went into the development of this particular code provision. If you can prove that your building system works and does not violate the intent of the code, you have a shot at gaining an approval.

Paul Fisette is director of the Building Materials and Wood Technology program at the University of Massachusetts in Amherst.