View Full Version : Vapor Barrier Question

John Yogus
03-28-2010, 12:31 PM
Not sure if this is the correct forum for this, but we'll give it a shot. I want to put blown insulation in my attic. There is a floor in my attic, so I will have to lift boards to get to it. I will be blowing in over 3 bedrooms and a bathroom. Do I need to worry about putting vapor barrier down before blowing insulation? If so, this would go down first and then insulation over top, correct? Also, I was told that insulating my vertical walls in my attic would help. Any truth to this? I was always under the impression that the attic floor would be best. If this would help, in my climate (PA), which side should I consider my warm side to put the barrier on (between wall and insulation OR staple barrier to studs on top of insulation)?


Paul Ryan
03-28-2010, 1:05 PM

I am in MN and just blew in about 60 bags last autum to add to my insulation. You don't have to put a barrier down unless it is code in PA. It doesn't hurt it would stop any moisture if you had any. But if you have adequate venting you shouldn't have moisture issues. Insulating the verticle walls again would help, but in the case of an unheated attic it will do very little. Heat rises so you don't need nearly the same insulation on the walls as you do celing/floor. As far as I know insulating the walls of the attic would just keep some of the heat in the attic a little longer before it escapes the roof.

Terry Achey
03-28-2010, 8:14 PM

Definatley do not want to have your vapor barrier on top of the insulation. You can trap moisture from teh house in your insulation. The vapor barrier (attic or side walls) always goes next to the living space. Unless you heat your attic, insulating the attic walls will serve little purpose since most attics have louvers or ridge vents.

While not required, vapor barriers will help keep humidity in the home in the cold, dry winter temps. Humidity is important to keep room temps feeling comfortable. You homes humidity will easily and quickly pass right through drywall and plaster in the winter w/o a vapor barrier.

In you case, taking up folor boards, it will be difficult to place an tight and effective plastic barrier in the attic floor. Most important part is that your adding the insulation....the more the better.


Rob Fisher
03-28-2010, 9:39 PM

...The vapor barrier (attic or side walls) always goes next to the living space...

Terry is right. In PA climates the vapor barrier always goes on the living space side. This changes in different climates so I always like to remember it that the vapor barrier always goes on the hot side of the wall or ceiling. In PA we heat more than cool so it goes on the inside of the insulation. In Florida they cool most of the time so the vapor barrier should go on the outside of the insulation.

You also mentioned that the new insulation and vapor barrier will be going over a bathroom. If it does not have a bath fan in it now, you probably want to seriously consider putting one in as you will now be trapping much more the the hot humid air into the bathroom.