Is it necessary to tape joints in under-slab insulation?

STEPHEN SHEEHY | Posted in Energy Efficiency and Durability on August 11, 2014 09:08am

I’m putting recycled 4″ XPS under my slab. Zone 6. Do the joints need to be sealed with anything? I’ll put poly on top, under the concrete.

Answer

GBA Prime

Join the leading community of building science experts

Expert Member

Dana Dorsett | Aug 12, 2014 11:10am | #1
Nope- the taping would have negligible value at best. The amount of thermal bypass air movement under the slab is miniscule, and the poly layer makes it air & water-vapor tight.
STEPHEN SHEEHY | Aug 12, 2014 11:19am | #2
Thanks, Dana-That's what I hoped was the case. Your explanation makes sense.
dankolbert | Aug 12, 2014 08:42pm | #3
You doing one layer of 4" or 2 layers of 2"? Good to have staggered seams.
STEPHEN SHEEHY | Aug 13, 2014 09:21am | #4
Dan-we're using 4".
easyrider470 | Aug 18, 2014 12:54pm | #5
I used 3" XPS under my basement floor and didn't tape the seams, we did however ensure we taped the seams of the poly vapor barrier extremely well.