Building wraps are now required behind all cladding by the International Residential Code. If you are building over a wood frame you must use a secondary water resistive barrier (WRB). Recent research has shown that as much as 10 percent of the wind driven rain is getting past the cladding onto the back-up layer. Intruding water must be redirected toward the exterior and not allowed to accumulate within the wall assemblies. This is the function of the water resistive barrier.
When selecting a building wrap you need to consider:
- Install-ability – Is it tough enough to withstand exposure to the elements over a period of time? Building wraps need to hold up to winds, not rip over staples and stay in place once installed.
- Balanced set of physical properties – Select a material that is somewhat vapor open but not too vapor open. There is a relatively healthy balance for a variety of cladding assemblies based on what the appropriate permeance is at that layer. Much of the computer modeling that I have done over the years indicates a good general number for the permeance is between 10 – 20 U.S. perms for most U.S. climates.
- Always install in accordance with the manufacturer’s recommendations – This becomes your principle means of defense for moisture. Moisture intrusion must stop at this line.
Building wrap needs to be installed in a shingle-like manner, so that it sheds water to the outside. It should also be installed in coordination with flashings and other accessories to ensure that any moisture that does intrude the assembly gets stopped at this layer. It must then be redirected to the exterior and not become entrapped within the wall somewhere else.
Common sense is something that our fathers and grandfathers did far better than we do today. Today the tendency is to rely on the silver bullet of technology to help pull us out of the fire. The fact is, if you take these materials and install them with a great deal of common sense and construction knowledge, as older tradesmen did, then they work really well. In the best circumstances, the marriage of good technology and practical knowledge coincide and we construct durable and sustainable buildings. The correct application of a proper water resistive barrier is an example of such an installation. It’s not rocket science, it’s good building science.
Lucas Hamilton is Manager, Building Science Applications for CertainTeed Corporation