When writing a guide spec for a rainscreen system, I had to include structural silicone adhesives. My client told me to specify the type of adhesive by reference to the appropriate ASTM standards. I would agree with this approach for many types of adhesives, but not for high performance structural silicone. Here's why:

Two of the leading producers of structural silicone are Momentive (formerly GE) and Dow Corning. Their product literature states:

Momentive: Do not use, "In structural glazing applications unless Momentive Performance Materials (MPM) has reviewed shop drawings for applicability and has performed adhesion and compatibility tests on project substrates, spacer materials and all accompanying accessories. Review and testing is done on a project-by-project basis. No blanket approval is given by MPM for structural glazing applications."

Dow - "Sealant should not be used In structural applications without prior review and approval by your local Dow Corning Sales Application Engineer"

It would be inconvenient for my client's rainscreen installers to have to go through a review with their silicone supplier for every project, and I suspect most of them will overlook the requirement. Few architects take the time to review submittals so closely that they will note the silicone manufacturers' limitation.

The result: The required review is not performed, and the silicone manufacture has a free pass if there is a failure on a project. All they have to tell the judge that the customer did not do the required review.

SOLUTION

I told my client to get letters of approval from leading silicone producers and make the letters part of their installation instructions. In return, the specific silicone products can be named in the rainscreen guidespec.