Glazing System U-Factor and Related Performance Requirements

Project managers in the glass and glazing business rarely receive training on the thermal performance of windows, energy code requirements, and thermal requirements in the design specifications. These can be particularly tough topics to learn, especially for new project managers who are faced with the challenge of learning many different things about the business on the fly.

This article is a quick primer on U-Factors and some related thermal performance concepts. It will give you the basic information you need to get through the day, including the overall system U-factor and other factors that play a role in the thermal performance of a building. By the end of the article you should be able to understand the specification requirements for a project and whether the systems you plan to use meets those requirements.

What is a U-Factor?

We can build a solid knowledge base by starting with some basic terminology:

• U-Factor is a measure of thermal transmittance through conduction, convection, and radiation. It’s the inverse of the R-value commonly used for insulation. The lower the U-Factor, the better for thermal performance.
• NFRC is the National Fenestration Rating Council, which determines the guidelines for energy ratings such as U-Factors.

The NFRC has developed testing standards such as NFRC 100, 200, 300, etc. for U-factors and other thermal performance measures. We’ll discuss NFRC guidelines in more depth, along with their uses and limitations, in future articles.

When it comes to the overall system U-Factor, the NFRC uses a component modeling approach (CMA) that factors together the U-Factor at the center of the glass (COG), the edge of the glass (spacer), and the frame itself. The flowchart below shows how these three factors are combined into a system U-Factor.

 

 

 

 

 

 

 

 

Every component of your system has a different U-Factor. And, taken together, these components combine into an overall system U-Factor. Check out the chart below from Tubelite, for example. It shows how different components affect the overall U-Factor of the system when combined. Remember, the lower the U-Factor, the more energy efficient the system.

 

 

 

 

 

What are U-Factors used for?

HVAC contractors use the overall U-Factor of glazing systems to design building systems. However, system U-Factor is only one piece of the puzzle. There are many other factors involved in the thermal performance of buildings. Some of these factors are briefly discussed below and will be explored in more detail in future articles.

• SHGC or Solar Heat Gain Coefficient: The solar heat gain of a window is the amount of solar radiation that passes through it into the building. The SHGC can be affected by sunshades, window coatings, tints, etc.
• VT or Visible Transmittance: The visible light transmittance of glass is a measure of how much visible light comes through the window.
• Air infiltration: Air entering the building through joints or seams can have a negative impact on the thermal performance of the building and HVAC systems.

Understanding these common thermal performance factors can help you avoid problems with submittals. For example, now that you know what a U-Factor is, you can make sure your system design always matches the specified U-Factor.

However, it’s important to note that sometimes the system design, including the specified glass, only meets some of the specified requirements. In this case, you can use different components that are not specifically called for to meet the U-Factor specification. These could include thermal pressure plates in a curtain wall or warm edge spacers in glass units. Consult the product U-Factor charts to see how they will affect the overall system U-Factor.

It’s important to always take the time to review the overall design specifications and the specs for all products. Product manufacturers should have all the information you need.

Questions?

Drop us a line if you have any questions about U-Factors or the thermal performance of glazing systems. We’ll be happy to help.

Is a 3D printer a worthwhile piece of equipment for your shop and/or office? We wanted to start exploring the possibilities, and ventured into the 3D printing world.

We researched high and low end pieces of equipment, and settled on a reasonably priced option to test the waters. Monoprice.com has many different options, and the Maker Ultimate 3D became our choice due to its price point and larger build size.

If you are looking to play around with the 3D printer we recommend going to https://www.thingiverse.com/ and you can download any number of fun and interesting prints very easily. These files do not require you to have 3D CAD software, and offer an opportunity to test the machine and its functionality.

Because we are primarily a drafting office we have AutoCAD 3D and Fusion available to us. You will need to take into consideration that if you don’t have 3D CAD capabilities in your office you will need to develop them, or pay an outside service to create the 3D files required for printing parts.

We discussed various uses for the machine and the best use we found for a glazing firm is printing prototype and/or customer components for review/testing or submitting to an architect. To see how easily we could create a prototype of a sunshade bracket assembly I talked a draftsman in our office to 3D model the various components needed. This took about 1.5 hours of time on his part to get the files ready.

Once inputted into the machine we decided to use different colors of filament to show the various components when assembled. It wouldn’t have taken too much more time to modify these components to fit whatever needs you may have for your customer assembly.

The printing for these files took more than a day, but once started you can simply walk away and let the machine work.

Overall the machine was easy to use, and within a couple of days we had an actual physical model for us to use and hold. While you couldn’t put much if any load on the pieces for testing it gives a great idea of what the finished product will look like.

Various options exist for how much the pieces are filled which will dictate how strong the finished product is and how long it takes to print.

For the low cost of entry we would definitely recommend this technology to anyone that has to create custom components and has a need to see them in the real world before committing money and resources to creating the actual product.