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Discussion Starter · #1 ·
Some shingle underlayment act as a vapor barrier. Ice and watershield is a vapor barrier and some synthetic underlayments are vapor barriers also.

I wonder if a vapor barrier can cause more humidity to build up in an attic.

In northern regions the roof could be covered with snow and outside temperatures can be below freezing for 2-3 months straight. It’s easy for moisture to condensate on the roof sheathing when the roof sheathing gets cold.

I know some of the humidity works its way out through the sheathing, the felt paper and eventually through the shingles. Now if the underlayment is a vapor barrier then the ventilation system would have to create enough air flow to remove 100% of the humidity in the winter time.

Here’s the dilemma; can you be sure that your ventilation system will create the amount of airflow that you need in the winter time and does an underlayment that acts as a vapor barrier increase the amount of ventilation that you need?
 

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If your ventilation system is designed and operating properly with continuous air flowage, you should not have to concern yourself about the permeance of the wood deck sheathing with a vapor barrier material installed on the top of it.

Ed
 

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Discussion Starter · #3 ·
If your ventilation system is designed and operating properly with continuous air flowage, you should not have to concern yourself about the permeance of the wood deck sheathing with a vapor barrier material installed on the top of it.

Ed
Are you speaking from experience, where you have been up in an attic that was covered with a vapor barrier in the winter and you saw no condensation?

Or are you speculating based a theory?
 

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Discussion Starter · #4 · (Edited)
Reservations about synthetic underlayment

My supplier just informed of a case he heard about that involves a mold filled attic.

The attic is properly ventilated with ridge and soffit vents and Synthetic underlayment was used. They found moisture under the Synthetic underlayment.
:thumbdown:
I don’t have all the details yet on what type of ridgevent and shingle were used.

I suspect that covering the roof with a vapor barrier will contribute to mold growth and I predict that we will eventually find out that it is a mistake.

Deck Armor is the only one I know of that is designed to let humidity pass through.
 

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The information i am about to submit has nothing to do with roofing but is related to the house wraps that are commonly used during residential construction.

Several years ago I sat in on a mold remediation and abatement class where the instructor was a life long resident of louisville / Bullit County Kentucky. The instructor stated Bullit County Kentucky had the most stringent rules and regulations for mold permisibility within the state of Kentucky. This instructor also worked for one of the leading mold remediation and abatement contractors withing the state of Kentucky.

He said there can be no denying that with the advent and use of the new house wrap materials there had been a several hundered fold percentage increase of house hold mold occurences in the state of Kentucky.
He stated that in the older more loosely built structures of the late 1800's and early to mid 1900's mold is not commonly found to be reported. Even in structures that were prone to water damage it was not often prevalent.
In the instances it is most often reported it is residential structures that had house wraps, silicone sealed outer wall joints and thermal pane windows.
 
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