Tags: #architecture #buildingdesign #buildingscience #EIFS #materialsscience #resilience

A failed sealant joint is often thought of as a moisture problem, not a thermal problem. Actually, it’s both. 

For example, here I’ve evaluated thermal performance of typical expansion joints through EIFS. These joints penetrate the lamina and 3” EPS insulation – but not the backup wall. When designed and installed perfectly, R-values are reduced by only 3 to 4%. In failed or defective joints, we see a reduction of 8-14%. These numbers do not include convection, which can double this loss.

It seems the most important variables are backer rod size and placement. In other words, even if the sealant fails, the joint may retain much of its original thermal performance – assuming the backer rod is properly sized and positioned. Closed cell rods are designed to accommodate 25% – 75% compression. The low end of this range is inadequate and will not offer the redundancy and performance that is often assumed.

We tend to think of backer rod as a means for properly configured sealant, not as a thermal stopgap. But they deserve greater attention as does the placement of joint insulation. These considerations are particularly critical for thru-wall joints, which reduce performance even further – well in excess of 25%.