Insulating floor before subfloor goes in.
![](https://www.greenbuildingadvisor.com/app/uploads/assets/avatars/5712-avatar-1548442991-100x100.png)
I need to insulate the floor of Dear Old Mom’s house in zone 4c to r-38 to earn enough green “credits.” Conditioned crawl is not an option because of our hot radon potential. 22″ crawl space makes working down there very difficult.
Would this work:
Cut 3″ poly iso (R-19.6) to fit between the webs of the 9 ½” TJIs (19.2 o.c.) and rest them on the bottom flanges before the subfloor is installed (or stage the cut pieces in the crawl before the subfloor goes in). Later, cut access holes and blow cellulose to fill the remainder of joist bay.
Polyiso = R19.6, remaining joist bay, 5 1/8″ (9 ½” TJI – 1 3/8″ flange – 3″ poly iso) blown to R3.6/inch = 18.4.
(The attached picture is from https://www.apawood.org/i-joist-fire-assemblies. I don’t need gypsum for a fire block, but they are showing using the flange to support a small load, similar to my insulation idea.)
Is there a problem having relatively cold bottom flanges facing the crawl space ground cover?
(3″ Polyiso @ $0.65/sf.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
The short answer is "no". In zone 4C with a vented crawl space and a good ground vapor retarder/radon-barrier the flanges will be fine. As a rule zone 4C vented crawlspaces have a MUCH lower moisture adsorption/condensation risk than say, zone 4A.
Would it be easier to just screw the entire 4x8 polyiso sheet to the bottom of the flanges and tape the seams vs cutting all the pieces to fit?
Oops--missed the 22" space you have to work in!
Thanks, Dana.
Kevin, I would have were it not for the 22" crawl depth.
Mark,
The only issue concerns rain. If the floor assembly gets wet from rain before the roofing is installed, make sure that there are no puddles (no standing water) on the polyiso -- you want those joist bays to be completely dry before you install insulation.
Mark,
If you have any other work to do in the crawlspace, and the floor is concrete, invest in a creeper.
https://www.amazon.ca/Torin-TRP6240-Rolling-Creeper-Garage/dp/B002E1AYTU/ref=sr_1_4?s=automotive&ie=UTF8&qid=1500665209&sr=1-4