Replies

1. Expert Member
   Dana Dorsett | Dec 01, 2015 01:18pm | #1

   For 10 mil polyethylene almost any tape would be suitable (or even no tape at all).

   Under an insulated slab it's better to put the polyethlene on top of the insulation. Just overlap the seams by a foot. you can staple it to the insulation. If the polyethyene is under the foam there is potential for it to capture pockets of liquid water, which takes forever to dry through a few inches of foam, which can ruin susceptible floor finishes. When it's between the foam and the concrete there aren't any pockets where moisture can pool.

   I'm not quite sure why "radiant" is a separate line item- the radiant heating tubing is withing the slab.

2. GBA Editor
   Martin Holladay | Dec 01, 2015 01:19pm | #2

   Josh,
   There are three issues here.

   1. Your sub-slab vapor barrier belongs directly under the concrete, not under your horizontal insulation. Here is a link to an article that explains why: Polyethylene Under Concrete Slabs.

   2. Polyethylene seams can be sealed with Tremco acoustical sealant, duct mastic, or any of the following tapes: Polyken Shadowlastic, Siga Wigluv, Siga Sicrall, Pro Clima Tescon No. 1, Venture 1585 HT/W, or Dow Weathermate.

   3. The seams of the vapor barrier installed under a slab do not have to be sealed. The poly does not serve as an air barrier. The concrete slab is your air barrier. To address vapor diffusion, all you have to do is overlap the plastic; there is no need to seal the seams.

3. 
   learner15 | Dec 01, 2015 01:30pm | #3

   thanks to the two of you. radiant was not meant to be separate, as it will certainly be within the slab. i would feel better taping the seams (even if its just for peace of mind), as it is a relatively simple (and perhaps zen) element in this overall show.

   i will also have a vapor barrier on the wall. any thoughts on continuing the floor sheeting up the wall?

   thanks again.

4. GBA Editor
   Martin Holladay | Dec 01, 2015 01:32pm | #4

   Josh,
   Here is a link to an article about basement walls: How to Insulate a Basement Wall.

5. 
   learner15 | Dec 01, 2015 01:39pm | #5

   martin-i have read this and followed the links contained within. my lower level is 3 feet in ground and about 4 above. stone and mortar foundation. i plan to use airkrete for wall insulation. will poly up to a 3' height really cause a problem for the insulation. so much is cost/risk v benefit. the reason i want the poly is to direct water vapor/liquid down to the french drain. if you still recommend not using poly, what would be an appropriate solution to prevent vapor/liquid from hitting my drywall?
   thanks much.

6. GBA Editor
   Martin Holladay | Dec 01, 2015 01:47pm | #6

   Josh,
   Here at GBA, we have never gotten a positive report from any reader who has tried Air Krete. However, we have gotten reports that Air Krete insulation is fragile, crumbly, and disappointing.

   If you install polyethylene as you propose, the polyethylene probably won't cause any problems. Good luck.

7. 
   learner15 | Dec 01, 2015 01:56pm | #7

   well, i'm just running in circles here! if i remember correctly, your article suggested closed-cell spray for a setting like mine. However, the health concerns are bothering me. https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/does-spray-foam-insulation-gas-poisonous-fumes

8. Expert Member
   MALCOLM TAYLOR | Dec 01, 2015 09:28pm | #8

   Josh,
   The best reason to tape your sub-slab poly is to keep the concrete from making its way between the laps during the pour. Up here in Canada we use red Tuck Tape.