Examine the wall system’s temperature profile
Designers have been concerned about condensation in wall assemblies for decades. Since the mid-1970s, the greater amounts of insulation specified in the building envelope has increased the likelihood for condensation somewhere in the assembly. Many articles have been written over the years that describe the physics of this problem. And for the vast majority of this time there has been a laser-like focus on one method for controlling condensation: permeability.
The rule of thumb has been to place low permeability materials/retarders on the warm side of the wall and higher permeability materials on the cold side of the wall. In this fashion, the designer strove to make it difficult for water vapor to enter the wall (making it difficult for much water to condense in the wall) and easy for water vapor to leave the wall (drying out any water that managed to get into the assembly). And to answer this siren call, manufacturers began to introduce high permeability air barriers, water barriers, and sheathings along with “smart” vapor retarders for the warm side of the wall.
Unbeknownst to many designers, there is another, far more significant method for controlling condensation in wall systems: temperature. Or more precisely, the temperature profile of the wall system. Why should designers care about wall temperatures? Because temperature plays a huge role in the condensation process. Condensation does not occur unless the temperature is low enough. And since a wall is not equally water sensitive in each layer, we can manage the temperature of the wall at any given location by placing insulation in the appropriate locations. By doing this, we can force condensation to only occur (if it occurs at all) in the wall location that can tolerate the condensate.
Here’s how: Place as much of the wall insulation as is possible on the outbound side of the assembly (Figure 1). This is easy to do whether the base wall is metal stud, block or poured concrete. In cold weather conditions this will warm the entire wall on the interior. This will change the temperature profile of the wall
Changing the insulation location changes the temperature profile. The dotted line temperature profile occurs when insulation is in the stud cavity. The solid line temperature profile occurs when the insulation is moved to the exterior side of the wall. Changing the temperature profile in this manner has far-reaching consequences. Consider a standard rainscreen veneer wall assembly. Certainly the veneer is not sensitive to water, it is exposed to the elements on a constant basis. The support elements for the veneer are also not sensitive to water, they are in the drainage space behind the veneer and quite a bit of water reaches that space. And the insulation layer on which the supports rest has to be moisture resistant as well for the same reason as the supports. If condensation can be forced to only happen (if at all) somewhere in or around these components, which are essentially immune to water, then the wall design is completely robust in its resistance to condensation.
Alter the profile
Designing wall assemblies by adding or altering the permeabilities of the wall components gains only mild improvements in condensation resistance. To create a truly robust wall system with the greatest condensation resistance, designers need to look at altering the temperature profile of the wall assembly by moving insulation as far as possible to the exterior of the wall.
Does all this mean that we no longer think about or design with the permeability of materials in mind? No, not at all. It means that we relegate the water permeability part of our design efforts to the proper place in the design consideration hierarchy. That would be the second place, behind the wall temperature profile design effort.
Daniel Tempas is a building science expert at The Dow Chemical Co., Midland, Mich. To learn more, visit www.dow.com.