VAPOR BARRIER LOCATION

[craig pedersen]

I bought a house built in the 90s with vapor barrier on inside of wall behind drywall, no exterior sheathing, only 'bearhide' and vinyl siding. I want to replace the vinyl siding with fiber cement siding but am concerned about moisture getting trapped in wall, if I install a second vapor barrier under new fiber-cement siding. Installing a second vapor barrier seems like a bad idea as that would trap summer moisture in wall cavity/insulation.

[Dom D'Agostino]

Start with reading the guidelines by James Hardie, one manufacturer of fiber cement siding, for a primer on what's required.

http://www.jameshardie.com/pdf/install/hardieplank-hz10.pdf

[craig pedersen]

Thanks, I am an architect, familiar with installation guidelines. Hardie suggests a VB behind siding. I believe two vapor barries, interior and exterior are a problem

[Raymond Wand]

You require an air barrier such as Tyvec behind the siding not a vapour barrier.

[Jim Luttrall]

You are correct to worry about two vapor barriers. The wall has to be able to dry to one side or the other. It depends on the climate as to the best location for a vapor barrier. There are however vapor "retarders" that are somewhat vapor permeable. You definitely need a WRB (water repellant barrier) to stop liquid water behind the siding but some products like old fashioned building paper installed shingle style will fulfill both the WRB and permeable barrier.
For a deeper understanding of the physics behind vapor barriers, thermal barriers, WRB and the like; visit the Building Science Corp. website.

Building Science Information (http://www.buildingscience.com/search?SearchableText=vapor+barrier)