The Professional Services Perspective: Thermal Performance

The building industry is shifting in its demands for thermal performance and thermal analysis on wall systems. More projects require thermal analysis, factors such as dew point and edge-of-glass U-values have become more important, and collaboration across the project team is on the rise.

In this blog, I’m addressing thermal analysis, energy efficiency and the warming of glazed curtain wall systems. My context is purely from a high level, and from my and our team’s overall experiences on projects from the pre-sale and pre-construction stage, through final performance evaluation and final installation. Here are some of those experiences, observations, stories and opinions on the issue of thermal performance and related topics.

1. Demand for thermal analysis.

Thermal performance related to U-values of wall systems has been in specifications for a long time, but for years was less-frequently substantiated in many areas of the country (yes, even cold areas). This is no longer the case. We have seen that most projects, particularly custom curtain wall projects, are requiring thermal analysis, and validation of U-values.

2. Inclusion of dew points.

Although it’s not always specified, thermal analysis should include dew point temperatures to inform design, and to mitigate, eliminate or better manage condensation. Don’t miss the importance of this. Specifications that request U-values per NFRC100 may not address the need to calculate and verify that surface temperatures on interior surfaces or surfaces behind the seal line are to be above the dew point temperature (unless there’s a way to manage condensation in non-visible areas.) This is important for the main body of the system, and very important for non-typical frames, perimeter conditions and transitions within or between systems. Much moisture can accumulate because of dew point issues, and this can be destructive to systems and interior environments.

3. Connected design.

Specifications are more clearly defined and tied into the mechanical engineer’s analysis and requirements for total building envelope performance. This has a lot to do with commissioning of buildings and actually validating all the values for the exterior enclosure. It’s good to see more “connected design” and less “throw it over the wall” compartmentalization.

4. Collaboration challenges.

As a result of all this, I still see levels of disconnectedness, differences of opinion, questionable application of standards, and a bit of “disruption.” I see this particularly between some on the A/E side of the project team versus those in the industry side working as delegated designers. The mixing of provisions from ASHRAE and THERM is one of the problems we’ve encountered, as have been issues regarding opaque areas at stone or panel spandrels, and the correct way to assess or analyze these.

5.  Use of edge values. 

Frame edge and glass edge have a big impact on reduction of U-value, and it is not uncommon for us to hear, “That can’t be right. The center of glass U-value is so much lower; how can the total U-value be so much higher?” Aluminum mullions and aluminum spacers in glass units are conductive. They have a higher U-value than the center of glass. Consequently, thermal improvements such as thermal separation and warm edge spacers have a significant impact in reducing total glazed wall U-value.

6. Non-conductive or less-conductive attachments.

FRP, polyamide, co-extrusions, and other non-conductive or less conductive attachment devices continue to grow in popularity and for use on rain screen panel, stone, UHPC, and other opaque cladding systems. There is a reduction in thermal performance when metal girts or components penetrate insulation seams and are screwed to the substrate behind. Do not forget to account for these penetrations through insulation areas in rain screen cladding if they are being utilized.

That is my story for now, for today. What is most exciting about blogging is creating conversation, trying to communicate experiences, to generate conversation and to somehow be of service to our industry. Please communicate back to me and let’s advance our work in glass and glazing.

John Wheaton is the founder & co-owner of Wheaton & Sprague Engineering, Inc., also known as Wheaton Sprague Building Envelope. The firm provides full service design, engineering and consulting services for the curtain wall/building envelope/building enclosure industry, and works at “Creating Structure” for clients. He can be reached at and on Twitter, @JohnLWheaton1.

The opinions expressed here are those of the individual author and do not necessarily reflect those of the National Glass Association, Glass Magazine editors, or other glassblog contributors.


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