Replies

1. GBA Editor
   Martin Holladay | Feb 18, 2015 05:34am | #1

   Bob,
   The best thing to do would be to remove the interior polyethylene so that the wall assembly can dry to the interior during the summer.

   However, you have a potential problem. Removing the polyethylene may result in the insulation spilling onto the floor (depending on how densely it was installed), and you don't want that.

   Slashing the poly won't do much; it really has to be removed entirely if you want to get effective drying. I would experiment with removal to see if removal of the poly is even possible.

2. 
   bobhol | Feb 18, 2015 06:17am | #2

   Martin,the drywall team used mesh to blow the insulation in then applied the poly.I should be ok to remove it .The building inspector is not asking for the vapour barrier.Should I leave it installed on high moisture areas? I was thinking of the exterior bathroom walls...and also is the use of moisture resistant drywall still applicable in the bathrooms. Thanks

3. GBA Editor
   Martin Holladay | Feb 18, 2015 06:23am | #3

   Bob,
   Even in your bathroom, it's still good for your walls to be able to dry to the interior during the summer, so I'd vote to remove the poly even there. (However, if the walls in question are covered with a tub/shower surround or ceramic tile, you can't expect much drying in any case.)

   The best material to use would be a "smart" retarder like MemBrain, but it probably isn't worth buying a whole roll of Membrain just for your bathroom.

4. 
   davidmeiland | Feb 18, 2015 09:58am | #4

   Your insulators/drywallers get bonus points for installing netting before the poly. Even so, there is a good chance the insulation would stay put if it's properly dense-packed.

5. Expert Member
   MALCOLM TAYLOR | Feb 18, 2015 11:05am | #5

   Bob, It's worth remembering that the poly was installed with the intention of stopping air movement into the wall, and appropriately sealed at sills, openings and penetrations. An alternate strategy needs to be developed using gaskets and different sealers to achieve the same result if you remove the poly and rely on the drywall. How are you dealing with the ceiling?

6. 
   bobhol | Feb 18, 2015 01:07pm | #6

   Malcolm,the ceiling has poly holding the r70 insulation...it has been strapped for the drywall to hang...all the electrical boxes have plastic covers around them taped to the mesh ...the windows and doors are foamed in place to prevent drafts....what else is necessary?

7. Expert Member
   MALCOLM TAYLOR | Feb 18, 2015 03:35pm | #7

   Bob, right now the sills of all the exterior walls probably have a bead of acoustic sealant in which the poly is bedded. If you remove the poly you need to make sure that the sill is still sealed but also that either sealant or a gasket is placed under the drywall. Similarly the sealing round electrical and plumbing penetrations, window and door trim etc.
   Why I asked about the ceiling is that if you are relying on the poly, you have a strip over all interior walls sealed to the sheets over the rooms. If you remove the poly a lot of air can leak into the roof system through interior walls. You need a strategy there too.
   I get short shrift as a dinosaur here for my reluctance to condemn poly. I think it's often a bad idea to have it on the walls, but from an air-sealing point of view, eliminating poly isn't as simple as just using the drywall instead. There are a whole range of techniques necessary which don't get much play when people discuss airtight drywall as an alternative.

8. 
   bobhol | Feb 18, 2015 03:55pm | #8

   Malcolm,that is food for thought but if the drywall is totally sealed will it not trap any moisture and lead to mold ...or am I missing something here .My sill plate does have a gasket under it and the foam sheeting on the outside wall goes past the sill plate by about 18 inches to help with air infiltration at that joint ...It would still be easy to caulk from the inside as well though ..I will check the edge of the ceiling poly for leakage ...

9. Expert Member
   MALCOLM TAYLOR | Feb 18, 2015 09:27pm | #9

   "if the drywall is totally sealed will it not trap any moisture and lead to mold "

   If you mean "sealed" as in leaving the poly then yes. The benefit to getting rid of the poly is that moisture in the wall can escape through the drywall to the inside by diffusion. You have got rid of your vapour barrier.

   My concern is that you have also got rid of an effective air barrier and you need to make sure moisture can't enter the wall by piggybacking on air leaks. Your new air barrier is either the well sealed exterior sheathing and foam, or air tight drywall. If you rely on the drywall, it needs to be sealed wherever it is not continuous. That is at the sill plate of exterior walls, window and door frames and all penetrations.

   Your ceiling doesn't have either sheathing or foam to act as an air barrier. It needs to be sealed at any penetrations but also at the top plates of interior walls where air can make its way up to the roof space between the framing and drywall. This is usually done with either gaskets or sealant applied to the interior top plates prior to being drywalled.

10. 
    bobhol | Feb 18, 2015 09:41pm | #10

    Thanks for the detailed explanation. Much appreciated u