I’m getting ready to put up fiberglass insulation. I understand that it’s important to staple the edges of the kraft paper to the faces of my studs to act as a vapor barrier.
So what happens when I cut the insulation to fit into spaces that aren’t standard width? And how about the ends of the insulation, which don’t have tabs for stapling? If it’s so important to seal the edges, so water doesn’t easily penetrate, what should I do about these?
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Normally we use unfaced fiberglass in the walls and then apply a vapor barrier. With the unfaced batts, you can see how complete the fit is. Some use the kraft faced on the first layer on ceilings. Typically the paper is fastened to the sides of the joists, not the face. If the insulation has to be cut, enough size is added so that the insulation can be tucked back. This doesn't happen too often since the joists are evenly spaced. The insulation contractors in my area will provide the materials and do the installation for about what I pay for the materials alone. I haven't touched a roll of fiberglass in years. I don't miss it at all!
Beat it to fit / Paint it to match
I envy you working with evenly space joists. It seems like there aren't two joists or trusses in my house that have the same spacing!
If you have unevenly spaced joists (like in lots of old houses) maybe a blown in insullation would be a better choice. I did that to the floor between the finished attic and the second floor because it was full of wires, along with lots of overlapping joists. I wanted to be able to seal off the third floor when it wasn't in use.Anyway, just a thought.
FG is a waste of time and money.
Blow cells in it, stay warm.
Joe H
I thought someone here on BT and maybe on the Building Sciences web site said kraft paper is not a vapor barrier.
I was just on a job and when I suggested we staple the insulation to the sides of the joists (ceiling) I got looks from the people I was working with like I had suggested we fill the cavities with prunes or something. Then the stapler (not the hammer type) wouldn't drive the staples flush, so I was hammering them. I was told not to bother; then the crew putting up the drywall had trouble getting it tight to the joists. Most of these people were volunteers working on Katrina relief and were Dow retirees or still working as chemists and chem engs at Dow. I do remodeling for a living. But everyone else thought they knew more about construction than I did. After a week of that I was definitely ready to come home. One guy kept criticizing (jokingly) my attire--I had paint on my pants. I finally mentioned it to someone else and said I had thought we were down there to work, not to be fashion models.
Sorry to get off the subject with a rant!
I've gotten more different answers on vapor barriers than on anything else since I started my remodel. My understanding of the information on the Building Sciences web site is that polyethylene would be a real mistake for me, and that kraft paper is the best choice for me for a vapor retarder (OK, not a vapor barrier). I live in the Chicago area, where spring lasts a day and a half and the rest of the year it's either too hot or too cold.
Yeah, vapor retarder is probably the best name for kraft paper. What you say makes sense (i.e., no poly). Someone mentioned blown in cellulose; that would probably be good.
To answer your specific question, here's what I learned to do, back when I used to do some insulation. Staple the flanges to the face of the studs. For odd size cavities, cut the fiberglass (but not the paper facing) to fit snuggly between the framing members. Then cut the paper face an inch or so wider to provide a stapling flange. No poly. This is not as air-tight as covering the walls with plastic sheathing, but it does allow moisture within the walls to dry to the inside.
Kraft paper is vapor-retarder, not a vapor barrier. My understanding is that if you have a vapor barrier elsewhere in the wall (for instance as sheathing) you do not want another vapor barrier (e.g., poly) on the inside. Recommended wall configurations (for vapor barriers, vapor retarders, and insulation) for various general climates can be found at http://www.buildingscience.com. You should take a look at the information there, and consider it along with any feedback you get on this forum.