Since my column on vapor barriers first appeared, I’ve gotten more reader questions on this topic than on any other that I can remember. I’ve had the opportunity to talk with several experts in the field as well, and here are some questions and answers that might help clear up a few more issues about this important subject:
Q: I just purchased a house a few months ago and installed R-19 batt insulation in the attic (with paper facing up). I realized afterwards this was not ideal, but to save cost I left it as is. The house is shaded by lots of trees and is still very cold so I’d like to add a second layer of insulation to the attic, perhaps another unfaced R-19. I read in your comments that the insulation paper has to face down when installing batt insulation.
What should I do with the existing insulation? To allow the air to flow, can I just make incisions on the paper on the first batt, or can I remove the paper from the first batt? Or do I have to reverse the first batt such that the paper is facing the inside of the house?
Can I use unfaced R-19 as a second layer of insulation, or are there better choices? I am trying to save cost. –Diana N.
A: The vapor barrier on the insulation is designed to face toward the heated space, so yes, it should have been installed facing down toward the house. However, most experts agree that between the drywall and the paint on the ceiling, you’ve have a pretty good vapor barrier already, and whatever moisture makes it into the attic has sufficient avenues of air movement to escape to the outside through the attic vents (assuming your attic is properly vented). So that being said, you shouldn’t have any problems installing a second layer of unfaced R-19 over the existing layer. Install it perpendicular to the first layer for best coverage over the joists.
As far as better options are concerned, blown-in insulation will give you more uniform coverage over all the joists. To save money, you can rent the equipment from most home centers to do it yourself.
With either option, check with your local building department to be sure there aren’t any code restrictions against insulating over the existing vapor barrier.
Q: I am remodeling a bedroom and I’m not going to insulate my interior walls until I remodel the adjacent rooms (only insulating for sound deadener) so I don’t tear up new insulation on the next remodel. Do I need to put a plastic vapor barrier in the current bedroom before I put drywall up? –Bruce S.
A: The purpose of the vapor barrier is to deal with moisture that might get into your exterior walls and condense there in the cold weather, turning to liquid water and creating mold or structural damage. There’s no need to create a vapor barrier in walls between conditioned spaces, since the heat in those spaces keeps the moisture from condensing.
Q: I recently had a new "pole building" garage built. It has a metal roof and siding, and a loft/storage area above the garage. The whole building is wrapped with 2-inch insulation blanket, including between the roof and the two-by-eight rafters. I want to fill the area in the ceiling between the rafters with insulation and then sheet rock. My question is about whether a vapor barrier or faced insulation batts should be used or not. I would be putting new insulation up against the existing blanket, which has the smooth plastic side facing the inside. I want to eventually do the same thing on the walls. –Darrell N.
A: Here’s a bit of controversy for you. I’m of the opinion that when you install a second layer of insulation, you can create a situation where moisture can pass through the first layer and potentially be trapped against the vapor barrier on the second layer (the smooth plastic facing), where it could condense into a liquid and cause damage. How much of a risk that actually is depends in large part on the humidity and temperature conditions in your area, as well as how much moisture is being created by whatever you’ll be using the pole barn for.
However, not all the experts agree with me on this. As with the previous question, others feel that there is ample opportunity for the moisture to escape, and that it won’t be trapped.
I’m not comfortable telling you to proceed with additional insulation, especially over a continuous vapor barrier such as the one you describe. Since you just had the building constructed, you need to err on the side of caution and talk with both your local building department and the original contractor before you make any final decisions.
Q: I am a Realtor and I recently got the results of a house inspection where the inspector noted that the vapor barrier did not go all the way to the foundation. The property owner contacted the company that installed the vapor barrier and was advised that they always leave a couple of feet of the soil exposed, because they want the crawl space to breathe. They say if they covered the entire crawl space all the moisture would be trapped under the vapor barrier. Well, yes, I thought that was the purpose of the vapor barrier. Shouldn’t it extend all the way to the foundation? –Dave R.
A: You’re correct in assuming that the vapor barrier is there to stop moisture from the soil from coming up into the crawl space. If it’s only partly covering the soil, it’s only partly doing its job. The crawl space does indeed need to "breathe," but it does that through its ventilation openings, not by allowing excess moisture to come up through the ground.
If they’re going to install a vapor barrier, then the accepted standard is 6-mil black plastic sheeting installed directly on the ground, with joints lapped 12 inches at all seams and the plastic extending up the foundation walls 12 inches all the way around the crawl space. In addition to the vapor barrier, foundation vents are required at a rate of 1 square foot of vent area for every 1,500 square feet of under-floor space, and there has to be one vent opening within 3 feet of every corner of the building. If they’re not going to cover the ground completely, then the amount of ventilation needs to increase dramatically, to 1 square foot of vent area for every 150 square feet of floor area.
The people who installed the vapor barrier can’t have it both ways. I’m assuming they don’t want to come out and install a bunch of foundation vents, so your homeowner should have them come back and finish properly installing the remainder of the vapor barrier.
