Two vapor barriers or one?
We had planned on using Benjamin Obdyke UV Flatwrap between the strapping for our exterior rockwool and our open jointed cladding. However, I have been worried about the deterioration of the Flatwrap over time. It is warrantied for 10 years but we don’t want to have to remove the cladding in 15 years to replace it because it is in tatters. Also I watched John Straube’s video on using rockwool under open jointed cladding and he warns against using two vapor barriers. So my question is this Flatwrap a good idea? We will already have taped and sealed Zip under the rockwool. For aesthetic reasons, I can use black insect screen to provide a darker shadow gap and bug protection. I should add that the home will be in Virginia and get quite a bit of sun. Cheers, Lisa
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Flatwrap isn't a vapor barrier.
Yes you are right! It is a "water resistive barrier" which implies that some moisture could become trapped. I still could use some help with my problem. Lisa
Lisa,
I cannot speak to the longevity of Benjamin Obdyke product, but it is not a vapor barrier. I think you may be confusing weather barriers and vapor barriers.
Most weather barriers (house wrap products like the BO UV Flatwrap) are used primarily to keep bulk water (rain and melting snow) from getting into the walls. Sometime they are also used as the primary air barrier. Most builders who use ZIP sheathing with taped seams for example, are providing the weather barrier and primary air barrier with that product ("primary" means that there is still a lot of air sealing to do for an air-tight house). These products have a perm rating, but most are not typically vapor barriers.
Poly sheeting installed inside the walls is historically the most common vapor barriers required in colder climates, but now that builders are installing more and more rigid foam on the exterior of houses, and sometimes that foam is also a vapor barrier (depending on perm rating and thickness), it's important to make sure that we don't put a vapor barrier on both sides of the wall and trap moisture inside.
In your case, neither the weather barrier or the insulation are a vapor barrier, but we'd need to know about the rest of the wall assembly to be sure that it makes good sense from a building science and durability perspective.
Here are some articles that may be helpful:
https://www.greenbuildingadvisor.com/article/understanding-air-barriers-vapor-barriers-and-drainage-planes
https://www.finehomebuilding.com/2014/09/10/whats-the-difference-control-layers
https://www.greenbuildingadvisor.com/?s=vapro+barrier+vapor+retarder
Lisa,
Jon is right -- most water-resistive barriers (WRBs) and quite permeable to water vapor (deliberately). They don't significantly restrict drying.
I think you are confusing vapor barriers, air barriers, and WRBs. For more information, see "All About Water-Resistive Barriers."
So using the Flatwrap would not trap moisture--thank you for that! My second question is--is it needed behind the open joint rainscreen assembly? Could I use insect screen and trust that the two inches of rockwool, the strapping and the wood cladding will protect the Zip or do I need an additional "water resistive barrier"? Lisa
ZIP's factory applied WRB is enough.
In VA you don't need or want a true VAPOR barrier anywhere in the assembly. Build AIR tight, but not vapor tight (to water vapor).
Thank you Dana!!!