In a recent BSC Building Science Insights post, Joe Lstiburek revisits failed crawlspaces, noting, "crawlspaces continue to be a mess ... We are continuing to see buckled wood floors over crawlspaces and moldy vinyl floor coverings." Continuing in his pointed style, he writes, "You can’t solve the problem building vented crawlspaces with 'fluffy' stuff smushed into floor joists."

In this piece, Dr. Joe walks us step-by-step through what it takes to build an effective sealed crawlspace (in), and how we need to detail an effective ventilated crawlspace (out). Sealing a crawlspace is pretty straightforward; the tricky part is making sure it is also conditioned. With a ventilated crawlspace, the tricky part is at the floor line, or crawlspace perimeter, which must be sealed and insulated - no easy trick in most floors crammed with utilities.

On bringing a crawlspace "in": "So what does a conditioned crawlspace look like? It is insulated on the interior with rigid insulation. It has a continuous vapor impermeable ground cover….do not forget the columns and posts. It is air sealed isolating it from the exterior air and the ground below. Now for the conditioning part…folks tend to forget this part. You have to provide air change between the crawlspace and the house or you have to provide dehumidification. The building code calls for 50 cfm per 1,000 ft2(approximately 25 l/s per 100 m2) of supply air or dehumidification."

On building a crawlspace that is "out": "Being the nervous conservative old guy I have become I worry about critters getting into the rigid insulation and pretty much always recommend some type of protection board over the underside of the rigid insulation. Fiber-cement boards work well in this regard."

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