Tommy Mac Discussions › Forums › Fix-it Forum: Home Improvement & Do It Yourself Repair Forum › Vapor barrier, no or yes?
- This topic has 2 replies, 1 voice, and was last updated 17 years, 5 months ago by Unregistered-The Insulator.
June 28, 2005 at 7:21 am #52758tjlmbklrParticipant
I recently insulated my attic. The angle portion ((12-12 PITCH) from floor to ceilings was sprayed with corbond. I was responsible for insulating the ceiling and exterior walls. I use kaft faced R-13, in some areas I had to cut and fit, and was not able to get it stapled so there is not a seal in all areas. Should I use vapor barrier on the walls over the Kraft faced, or is this not recommended. By the way the ceiling was done withR30 non-faced with a 6-mil vapor barrier.
June 28, 2005 at 8:35 am #263257homebildParticipant
By Building Code, As long as the attic space is ventilated, no vapor barrier is required at all.
6 mil plastic sheeting should never be used as a vapor barrier in walls and ceilings and it should be immediately removed.
6 mil plastic is designed for use under concrete floors only and is not the correct product to use in walls in ceilings.
6 mil plastic will only serve to allow for condensation to form and pool and can create some very severe water damage and mold growth.
The R-13 in your walls may be too little depending upon your location.
Your rafter (ceiling) insulation should also include rafter vents which extend from the attic spaces behind the knee walls up to the ridge peak and vent for each rafter space.
July 2, 2005 at 9:35 am #263400Unregistered-The InsulatorGuest
I am an insulation contractor in Northern WI. In fact I’m based within an Hour of the infamous “Tri-State Homes” factory (Former factory). Though I work primarily in new construction, I specialize as a problem solver for heatloss & moisture issues. Thought I might clear up some misconceptions about VBs. I will preface this by mentioning that in a relative way, Vapor barrier is a minor issue. The primary concern is eliminating air leakage and no, that is not what a VB by definition does, though under the proper conditions, the VB can also be an air leakage barrier.
1/ Does The use of spray foam mean that you do not need a vapor barrier? Well, the foam in itself is a vapor barrier and thereby fulfills the code requirements, over the insulation. functionally, it does nothing as far a VB over construction material. moisture can still move throuigh the wood. Still, the foam solves 95% of the issue. If you want to solve the rest, put on the poly. Will doing so cause condenstion problems? Absolutely NOT! Unless you put it on the outside, ala Tri-State Homes.
Depending in the interior finish material, it may be advisable to install the poly. If it’s drywall, since finished drywall is an air-tight barrier, it’s not needed, but it won’t hurt. If it’s T&G panelling, it would be a good idea, since the panelling is not an air-tight barrier. Though the foam is at time of installation an air-tight barrier, I’m still concerned over the long-term effect of expansion & contraction on this seal.
2/ Over Fiberglass, there needs to ba a VB of some sort. Kraft facing qualifies, but is not in any way, shape, or form an air barrier. So again, it depends on the finish material. Drywall, nothing more needed. T&G Panelling, put on the poly. Does putting poly over kraft constitute a double VB? No. It’s a continuation of the same VB Layer, unless again you put it on the exterior.
3/ By WI code since 1998, it is required that all penetration through thermal walls & ceilings be sealed. This includes electrical boxes & plumbing pipes. These need to be made air-tight. This functionally will have a greater impact on performance thae VP. If you take a 4 X 8 sheet of drywall air-tight without a vapor barrier, approximately 1/3 cup of water will move through it during the heating season. If you take the same sheet, with a VB but a 1″ hole through it, approximately 32 cups of water will pass through it. Air- tightness is job 1 for a quality insulation job. Came a house be made too tight. NO! This is a myth that just won’t jo away. However, and adequate air management unit (HRV) should be considered mandetory. Remember indoor air quality in not an insulation issue.
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