Permeability is a HUGE topic in our mixed humid climate for anyone involved in the construction industry. The perm rating of a product is the measure of the diffusion of water vapor through a material. This vapor drive through a building material can make or break the durability, efficiency, and indoor air quality of a project.
In our climate, the key is to have a wall and roof assembly that is air tight and vapor permeable. This is an almost impossible task so there are other things that need to be done for the house to be as efficient and durable as possible – but I will not go into that depth with this post.
Vapor permeability (air tight and vapor permeable) is sometimes referred to as the breath-ability of a system or ability to let water vapor pass. This should not be confused with stopping bulk moisture, which is a must for a wall and roof assembly. Bulk water is moisture in liquid form and water vapor is a gas.
So why is vapor permeability so important to the health, efficiency and durability of a building system? Because walls and roofs do get wet. Condensation will occur in some systems. Plumbing leaks do happen on occasion. Most important, we have two seasons here in Virginia and therefore have two different vapor drive directions. So walls and roofs need to be able to dry in both directions. When a wall cannot dry out, it becomes susceptible to moisture damage (rot, mold, insects).
Anything with a perm rating of less than 1 is a semi-vapor barrier. These products should be used sparingly for the thermal envelope in our climate. Anything less than 10 has low permeability and care needs to be taken to dry the assemblies out and not trap vapor inside.
Many products we use traditionally need extra thought due to their permeability:
Open Cell Foam = 15 Perms
Closed Cell Foam = 1 Perms
Concrete = 3.2 Perms
Gypsum wall board = 50 Perms
Plywood Sheathing = 10 Perms
OSB Sheathing = 2 Perms
Oil Paint = 1.6 Perms
15# Felt = 8 Perms
Kraft paper batts = 1 Perms
House wrap = range 5 to 50 Perms
Having an architect that understands building science is a must in our mixed humid climate. A well planned wall and roof assembly will be healthy, durable, and energy-efficient.