Replies

1. 
   RussMill | Apr 21, 2019 09:28pm | #1

   Well, im not sure exactly what you have there but, i like to use acoustical sealant on the concrete then copper flashing then acoustical sealant again, then foam. But, im normally over a slab with a good vapor barrier. We dont have enough details.

1. 
   smokey059 | Apr 21, 2019 10:01pm | #2

   So u lay down some acoustical sealant to the concrete then if the polyiso is 2" thick u cut a piece of copper flashing 2" wide and lay that onto the sealant and another bead of sealant on top of the flashing and then just lay the edge of the polyiso right on top of the flashing. This will stop wicking and the acoustical sealant will create a vapor tight edge.
   The slab has a vapor barrier and foam under it so there isn't any moisture issues. I just had read that polyiso shouldn't be placed directly on concrete and that a strip of xps along the edge would prevent any wicking. Just couldn't find anything on how to seal It. Thanks

2. 
   RussMill | Apr 22, 2019 01:26am | #3

   You can put a few inch strip of xps wont hurt. Mske your flashing a little wider wont hurt.

3. GBA Editor
   Martin Holladay | Apr 22, 2019 06:46am | #4

   Smokey,
   I'm not sure what you mean by a "vapor seal." Both polyiso and XPS are vapor retarders -- either type of rigid foam will limit inward vapor diffusion.

4. 
   smokey059 | Apr 22, 2019 08:34am | #5

   Martin my understanding is that taped foil faced polyiso creates a vapor barrier on the wall and I simply want to carry the vapor barrier layer all the way down to the concrete floor. I have read your articles on vapor barrier and vapor retarders and how to install foam on a wall but couldn't find any details on how to make the transition to the concrete from the foam. You guys have so much great info and help here but sometimes its hard to find the little details in the broader picture for someone like me who is doing his own building. I already have a question for the next step of putting the furling strips over the foam and how long a srew I will need to go thru the 2" foam into the studs. I found your article on fasteners for foam on the exterior of building but I'm not certain the same would apply to installing strapping over foam on the interior. Thanks for all the help

5. GBA Editor
   Martin Holladay | Apr 22, 2019 09:50am | #6

   Smokey,
   Here's a key point to remember about vapor diffusion and vapor barriers: If a vapor barrier covers 98% of an area, it is 98% as effective as a vapor barrier that covers 100% of the area. There is no need to worry about small holes or cracks. (In that way, vapor barriers differ from air barriers. With an air barrier, continuity matters -- and every seam needs to be carefully sealed. The same concerns don't apply to vapor barriers.)

   For more information on this issue, see "All About Vapor Diffusion."

6. Expert Member
   BILL WICHERS | Apr 22, 2019 11:12am | #7

   You’re not going to get tape to stay stuck to bare concrete. If you need to tape to concrete, you have to paint the concrete first with a heavy paint (like blockfill, or an epoxy paint) to leave you with a smooth plastic-like surface for the paint to adhere to.

   That said, I wouldn’t try to tape to concrete. You’re best with an overlapped joint and some sealant in between. You didn’t mention your climate zone, but I’d be a bit concerned with using acoustical sealant in an area subject to freeze/thaw cycles since that might affect the sealant. I’ve never tried using acoustical sealant outside before. I’d probably use a few heavy beads of a good silicone caulk myself.

   Bill

7. 
   Peter Yost | Apr 22, 2019 01:24pm | #8

   Hi Smokey -

   I commend you for installing a cap break between bottom plates and concrete flatwork, even on interior walls. And pretty much any thickness of XPS (which is closed cell) can function as a cap break. Some folks put down as much as 1-inch or more thickness XPS in case a leak means standing water on that slab.

   But I am not sure why you want to "vapor seal" the bottom of an interior wall--what is your rationale for this?

   Peter

8. 
   Jon_R | Apr 22, 2019 01:55pm | #9

   Anything that doesn't prevent air movement will allow a lot of vapor to pass. But avoid confusion and call it an air barrier.

9. 
   smokey059 | Apr 22, 2019 10:29pm | #10

   Thanks for all the input. Martins explanation about vapor barriers and vapor diffusion cleared up some confusion I had. I realize now that the air barrier matters much more than the vapor barrier or vapor retarder. The polyiso is foil faced so in this case it is a vapor barrier on an interior wall. I know that in most cases a vapor barrier is not needed but I'm right on the edge between zone 6a and 7 and I have 2x6 walls with roxul in them and the polyiso. Don't need Ac just heating here especially if building is insulated well. I have a very good separate air barrier so I'm not relying on the polyiso for that. The tip about the isopropyl helped a lot and the flashing tip thanks.

10. 
    Peter Yost | Apr 24, 2019 09:45am | #11

    The difference between vapor and air control is one of the most often misunderstood building science basics.

    In addition to Martin's blog, please note that Joe Lstiburek did a podcast on this topic for GBA back in 2010; another good resource on the topic.

    https://www.greenbuildingadvisor.com/article/air-barrier-or-vapor-barrier-building-science-podcast

    Peter