800-334-0227 jimglaze@thomco1.com

Constructing Thermally Efficient Rainscreen Support Assemblies (2-21-2024)

Understanding thermally broken options, rather than generically referring to “thermally broken”, can result in a 10% thermal performance gain for your client.

We spend a lot of time talking about air/moisture/vapor barriers, critical aspects of their detailing and interaction with other building components, how they are applied, stopping air leakage, the transfer of water vapor and keeping bulk water out of the assembly.  We often get into great detailed discussions about this critical element of rainscreen design and installation.  For good reason, of course.

We spend much less time on insulation type and selection, considering R-Value of the components or U-value of the assembly with a given insulation product, rigid vs. semi-rigid and fire characteristics, most importantly NFPA-285.  Ultimately, we do settle on a specific insulation type to meet project parameters and requirements.

We spend a lot of time considering the cladding to be applied in the rainscreen assembly, including material type, cladding dimensions, colors, fire characteristics etc.

We spend very little time on the cladding support assembly, simply stating to use whatever the cladding manufacturer suggests or provide a “thermally broken” assembly.  That’s not good.

“Thermally broken” can mean many things. Some are not so good. Some very good.

Let’s look at an example using FRP clips, which are widely believed to be more thermally efficient than steel-based clips.  The difference may surprise you.

EXAMPLE: a wall assembly of Interior Gypsum with Steel Studs 16” OC; Exterior Gypsum, and R-16.8 nominal exterior insulation. That assembly using FRP clips with 16” x 26” spacing has an effective R-value of 15.7, a 26 percent loss.  Thermally isolated Knight Wall Systems MFI steel brackets placed 16” x 24”, result in an effective R-value of 17.2, or a loss of insulating ability of 17 percent. To put that another way, you gain about 10% in thermal performance using an efficiently isolated steel bracket.  Assembly U-.063 value for the FRP clip. Assembly U-.058 for the MFI thermally isolated bracket.

The same example can be expanded to other brackets and thermal isolation in general.  Before selecting a cladding support assembly, research thermally modeled results and understand what you are getting.  Since these assemblies are in a sense permanent (barring recladding of the structure at some point in time), maximizing thermal performance is a goal that can only be reached by understanding “thermally broken”.

Knight Wall Systems. Pioneering cladding support solutions.

Fortunately there is now a recognized specification section that assists the design community in defining the support assembly that is right for their project and enables a spotlight to shine on this marginalized, but critical design element.