>The two most important things about vapour barrier are:
>1) Vapour barrier should be continuous around walls and
>ceiling, on the inside (warm side in winter) of the
>insulation. It goes on the inside because the interior
>remains at a relatively constant temperature, so the vapour
>barrier doesn't cause and trap condensation, as it would if
>it were outside the insulation in varying temperatures.
As far as the continuous nature of the vapor barrier,
I like to think of the room in terms of a fishtank and the warm air like the water, with that in mind it's easy to imagine any pinhole in a fishtank will let water escape, just like any rips, tears, gaps in the vapor barrier will let moist air get into that space. Usually there's enough attic ventilation and so on for the small amount that does get past the vapor barrier to escape to the outside.
What gets me is folks who put on aluminum (or vinyl) siding and the siding is basically one big vapor barrier but on the WRONG side of the wall!
I have read of bad install jobs that didn't allow for that and add more vents, and the moisture in the house condensed on the inside of the metal siding and drenched the studs and wood to the point where they were rotted to the point of needing to be replaced.
Good points - thanks! I'll be sure to use unfaced as much as possible. I don't know if I'll have a problem keeping the insulation between the rafters until I install the plastic and the strapping, both of which will hold it in place.
I know the strapping is typically installed before the insulation, but that gets in the way of putting in the fiberglass batts. It's easier for the framers, while they're there, to do everything at once. I thought I'd just do the strapping after to make it easier on myself, since I am both the framer and the insulation installer. Maybe, though, I should just follow convention and put up the strapping, followed by unfaced batts in the walls and between the rafters, and then apply the plastic over the strapping, to be covered by the drywall. Sounds like a plan!
P.S. PasterPaul: How thoughtful you are! :) A most kind and generous offer!!! ;)
One thing I have seen: when you need to use paper-back insulation because of the need to be able to staple it in place (whether ceiling or wall), go ahead and install the insulation and then slice the paper every foot or so. Then put your sheet of poly vapor barrier over that.
With the poor seal you get just stapling the edges of two adjoining batts together, combined with the slashing of the kraft paper, eliminates the kraft paper as a vapor barrier.
If you want to use the poly vapor barrier, and can do without the kraft paper on the insulation, by all means use the unfaced insulation. It will save you a few bucks.