This question has always been a tough one and coincidentally it has been a hot topic lately. We’ve received multiple calls in the last few weeks from varying climate zones – each homeowner wondering what the best practice is.
Before sharing, please note we are insulation manufacturers not building scientists, so by all means, if you are facing this issue please consult an expert. Caveats aside we do speak to smart people in the built environment on a regular basis and are therefore comfortable sharing the following commentary – in no particular order of importance.
Vapor barriers are often misplaced in buildings. However, a dirt floor in a crawl space is a great place to put a vapor barrier. In addressing the exterior wall which may be (partially) below grade we assume proper drainage and exterior sealing is in place. As such, any potential moisture would come from the interior. More than likely, the framing is flush with the exterior wall. A slight air gap would be great but it is often not in the equation. We of course would advocate for wool insulation as there is no better fiber to manage inevitable moisture and avoid mold growth, but that decision is yours to make. To abate the moisture which will come from the interior we recommend an airtight vapor variable membrane. Intello+ is an example. A useful note from the field is ensuring that the insulation in the cavity touches the membrane. In order for the vapor variability to work and the escape path to be created it is important for the insulation to be in contact with the membrane.
The ceiling of the crawl space is another area to address. I had this issue and after much consternation, the following path was selected. Again, a vapor barrier on the dirt. Wool insulation in the cavity and an airtight membrane over the top. As an extra step, and to help with any varmints that are pervasive in our area, we added Gutex wood fiberboard. The tong and groove style offers protection from our furry friends while providing an exterior continuous insulation that adds significantly to the overall R-value of the cavity.
Our interior experience improved significantly. The floor is no longer cold and the temperature in the room is suddenly similar to the rest of the house.
There are of course varying options for how to insulate a crawl space and for sure many of them have idiosyncratic issues. However, after much debate and experience, we have found that these steps are a sound and useful way to address the matter if not a best practice.
Hopefully, this helps with your crawl space project. Please don’t hesitate to comment here or reach out directly with additional points to consider.
Talk soon friends…….