Treating crawl spaces (with ductwork) when the adjacent basement ceiling is insulated.

I am currently speaking with a homeowner who would like to replace the existing FG batt insulation in the ceiling above his crawl space. The crawl is connected to the basement, and the basement ceiling is insulated with FG batts as well. The crawl has ducts insulated to R-4.2 throughout it. The crawl floor is a thin layer of concrete (about 1").

I spoke to him of two options:

1. Slap R-19 un-faced FG batts in the ceiling, and then slap 1/2" to 1" of foil-faced rigid foam to the underside of the floor joists.


2. (This is my preference). Lay down 6-mil poly across the floor of the crawl, and adhere the poly to the crawl walls (up a couple of feet). Then install 2" of foil-faced rigid foam to the crawl walls. 

I will also be insulating the rim\band joists throughout the basement.

Any advice?

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I agree with your preferred option.  If the connected space is climate controlled then stopping moisture and outside air from entering is the best option.  Only putting the FG batts in the ceiling of the crawl space would still leave the influx of air to deal with and it would decrease the efficiency and comfort of the home.

Thanks Tim. I have been pushing this option, and I think the homeowner is going to bite.

I would suggest a true vapor barrier product or ASTM  E-1745

From my short education on the product you want to make sure that the product is rated for the application. It is critical that the product is mechanically fastened to the wall and sealed. If there are any piers the product should be fastened and sealed to it as well.All seams sealed with tape approved for the product

I use 1x2  and ramset for fastening and duct mastic for sealing.

It is tedious, boring and often unappreciated in a challenging space.

I would always be concerned about the walls ability to dry. If the outside stem wall is buried I would think twice before insulating it

That is some really valuable input Glen. I definitely will continue to think this one through.




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