Replies

1. 
   ProBozo | Nov 24, 2003 04:38am | #1

   Stapling up the plastic after insul is common around here (NC).  What prob does you DW guy have with it? 

2. 
   JDRHI | Nov 24, 2003 05:32am | #2

   Just be sure to use unfaced insulation batts.

   J. D. Reynolds

   Home Improvements

   "DO IT RIGHT, DO IT ONCE"

3. 
   dIrishInMe | Nov 25, 2003 06:16am | #3

   As Pro said, plastic vapor barriers are (were?) very common here in NC.  Drywall guys probably see it more often than not, and don't really use that much glue anyway.  I'm gonna get flamed for saying this but I think plastic VB is declining in popularity though because of the possibility of resultant condensation, and then mold.  If you want to do the plastic vb thing, go ahead, but more importantly, think about sealing up holes in your drywall.  The amount of hot/cold air and moisture that is carried on leaking air currents is just as important if not more so than cold/heat/moisture that moves through wallboard/insulation, etc.  So, plan on sealing around electrical box wires in the walls, wire run holes in the top and bottom plates, around pipes, ducts and especially under bathtubs and showers, etc before drywall.  After drywall, but before ceiling insulation, go up in the attic and seal up any place that you see light coming through from below - around light fixture boxes, pipes, ceiling exhaust  fans, etc.  On the outside of the house, before siding, seal carefully around electrical penetrations - light fixtures meter box, and outlets. 

   A few other air sealing procedures are install backer rod insulation around windows and doors.  Even if you make your plastic VB tight around these openings, the drywall guys will cut the plastic with their roto-zip as they cut out the drywall around these openings.  Check the insulation job carefully for voids.  Install backer rod insulation around windows and doors.  Check the insulation job carefully for voids.

   Then, once you get everything all tight, look into a HVAC system with some kind of fresh air induction... ;^) Oh - and when you are building a tight house, it's very important that you have proper ventilation for bathrooms and the kitchen - moisture "point load" areas.

   Matt
4. 
   csnow | Nov 25, 2003 08:57pm | #4

   If you do all of what Matt said, the VB would likely do more harm than good.

   In the summer, the VB would be on the wrong side.

   Best case, it would do nothing at all.

   Search the web for "airtight drywall" or "airtight drywall approach".

1. 
   dIrishInMe | Nov 26, 2003 07:54am | #5

   OK, good.  It sounds like we agree.  Discourage the use of plastic vapor barrier and seal against air leakage.

   Matt

1. 
   DaveRicheson | Nov 26, 2003 02:15pm | #6

   This is an on going discussion about vapor retarding membrains. The local climate is more inportant than anyone has yet mentioned in the use and/or proper placement of a vp. I agree with both of you that air sealing is of paramont importance, but in a northern climate you will get moist air moving into the building envelope from inside the home. Condensation on cold exterior sheathing and wet wall cavity will become an issue. Controling and planning for moisture is another topic for discussion, but leaving a vp out in some climates is courting trouble.

   Dave

1. 
   csnow | Nov 26, 2003 05:44pm | #7

   "in a northern climate you will get moist air moving into the building envelope from inside the home. Condensation on cold exterior sheathing and wet wall cavity will become an issue."

   Obviously one of the topics of greatest contention here and just about everywhere:

   It is my position that if there is no moist air moving into the wall cavity from inside the structure in the first place, the VB becomes irrelevant at best.  When the seasons change, and moisture starts migrating into the cavity from the outside, the VB does more harm than good.

   In practice, the VB is 'randomly' breached a thousand times anyways, and does not make for a reliable continous air barrier in a heating climate.

1. 
   marv | Nov 26, 2003 06:28pm | #8

   It is my position that if there is no moist air moving into the wall cavity from inside the structure in the first place, the VB becomes irrelevant at best.

   Moisture does not have to travel by air to get into a wall.  It can "wick" throught just like a paper towel picks up water.  The rate at which moisture travels through a material is the perm rate.

   Some examples - Plaster on wood lath  2.4 Perm

                               Gypsum wall board 50. Perm

                               Exterior plywood  .7 Perm

                               Air      120 perm

                               6mil Polyetylene  .06 Perm

                               Aluminum Foil       0 PermYou get out of life what you put into it......minus taxes.

   Marv

2. 
   csnow | Nov 26, 2003 06:52pm | #9

   Understood, however I suppose the issue is really whether this vapor load is significant (or not) in the real world.  Depends a lot upon the climate in question, and the humidity differential on either side of the wall.  The published perm rate will only be realized under specific conditions.