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rusty71

04:51PM | 11/30/06
Member Since: 11/29/06
2 lifetime posts
Bvhvac
We are at the insulation stage of our new home. Fairly large home with cathedral ceilings. We have chosen the blown insulation for the majority of our home and the question arose regarding the use of a vapor barrier for the blown insulation. I know the ole plastic trick has been famous for years but is the same true for blown. I am hearing so many sides both pro and con and I am officially confused. Any assistance is appreciated.

I was all over the web and talked with local vendors and contractors and just could not find any consistency to the answer. Does any one really know or is this just a guess?

Appreciate any help and have a great holiday season everyone.

James Straley Chambersburg, Pa (I live in Southern Pa so I also found that location is important as well when deciding on Vapor barriers....aahhhhhhhhhh)

Billhart

07:18PM | 11/30/06
Member Since: 04/25/05
1918 lifetime posts
It looks like you are in a mixed-humid climate.

http://www.buildingscience.com/designsthatwork/hygro-thermal.htm

Go through this and match up with what you are doing.

http://www.buildingscience.com/designsthatwork/default.htm

http://www.buildingscience.com/designsthatwork/mixedhumid/default.htm

You have to control the movement of moisture, but that might not be a poly barrier.

But be warned with a cathedral ceiling can lights or other ceiling penatrations can allow moisture into and through the insulation to the underside of the roof where it can condenses.


homebild

07:05AM | 12/01/06
Member Since: 01/28/03
694 lifetime posts
The answer depends upon what type of blown insulation you chose to use.

Blown fiberglass would normally require a vapor retarder on the warm-in-winter side of the thermal envelope, but polyethylene plastic is a poor choice for mixed climate use. Such can lead to moisture issues inside framing cavities.

Blown cellulose requires no vapor retarder at all in most cases because of the manufacturer's installation sepcifications.

Even with blown fiberglass or other blown insulations, however, you may not want a vapor retarder at all depending on whether you have a continuous rigid foam on the exterior of the structure and your house is considered "unusually tight construction."

In such a case, the vapor retarder should not be used on the warm-in-winter side of the thermal envelope since it could actually trap moisture inside of framing cavities. No vapor retarder should be used in such a case allowing for deying to the inside of the structure (as in the Building Sciences Links cited in the previous post)

Pennsylvania Uniform Construction Code requires you to conform to one of several approved Energy Compliance paths for your structure, inncluding the 2003 International Energy Conservation Code, The US Department of Energy's RESCheck program, PA's Alternate Residential Energy Provisions, or others...

As a reminder, Pennsylvanai State Code requires you to first indicate to your local code office when you apply for permits which compliance path for energy conservation you will follow, be issued permits on that basis, and have a mandatory state energy inspection once installation is completed based upon your compliance path.

rusty71

05:23PM | 12/01/06
Member Since: 11/29/06
2 lifetime posts
Appreciate the great information. I will sit down and discuss further with my builder and insulation sub.

Appreciate it!!

James
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