Insulation and Vapor Barrier

[rookie4411]

Would it be a better idea as far as reducing infiltration on 2x4 or 2x6 exterior wall to use 6 mil plastic over fiberglass insulation rather than the usual roll insulation with a vapor barrier?

[pushkins]

No, it is far better to use faced insulation, reasoning is that the paper (faced) has the ability to breathe. Plastic can trap moisture in the insulation.

[FordMan59]

Although more expensive than plastic another option would be perforated radiant barrier. Radiant barrier is available on eBay for as low as $107. for a 1000 square foot roll. This not only has small holes so it can breath but as long as there's an air space next to it it'll reflect a large amount of the heat that would otherwise enter your home in the summer and reflect the heat that's already inside in the winter back toward the living space instead of allowing it to escape through the walls. I put radiant barrier underneath the new metal roof on my house in 2011 and can tell it's helped with both cooling and heating. I also have it on the roof and inside walls of my garage. On a hot summer day it probably lowered the inside temperature of the garage by at least 20-25*. If I ever have to replace the siding on my house I'm going to put radiant barrier underneath the new siding.

[Drywall DIY Guy]

I have been told that up by us, use unfaced and plastic over the top so the plastic is on the heated side right under the drywall. Moisture forms when moisture vapor hits cold. The plastic stops the moisture and it stays within the heated wall as vapor (not water), and it can escape back to the heated space. If the moisture vapor gets through to the colder part of the insulation it then can condense. I put plastic on and tape all seams. I live in N.WI where summer AC is not an issue, so if you are farther south this might be different. Ask for advice at your local lumber mill (not big box).