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Window Wall Structural Considerations

If you’re designing a home with large window walls, making sure your structural design and your window wall design adequately support the loading conditions is paramount.

Large vertical spans like floor to ceiling windows usually require stronger and/or deeper window frames. Large door openings require structural headers over them to support the weight of your window walls and structure above them. In addition, your structure and window design must at least meet minimum building codes, especially since certain geographic areas require stronger systems due to hurricanes, earthquakes or other specific loading conditions.

[Parish Center Andorf by [tp3] Architecture. Photo by Mark Sengstbrat.]

Most window wall manufacturers have standard details showing the depth of their window frames. This should give you a good idea of their approximate size when you’re shopping around. Just remember that the actual depth of the frame may vary depending on your particular project conditions. Once you start working with your window wall company, they should be able to tell you for sure how deep the framing members need to be if you have large spans.

One way to avoid deep window frames is add structural posts, beams, or braces to your design where additional support is needed. Many times these structural members can be concealed within walls, window frames, or even covered up by closure pieces to blend in with your window frame. This is helpful in keeping a consistent look.

In any case, it’s important to coordinate your window wall system capabilities and loading requirements with your structural engineer in conjunction with your window wall company. Your engineer will work with you to confirm if additional framing is required or if there are any other limitations that you might not be aware of. 

Thermal Performance of Window Walls

There’s many benefits to energy-efficient window walls.

• Energy and cost savings – Energy efficient glazing can account for a 10-25% cost savings in annual heating and cooling costs for new homes. In addition, if you can lower your HVAC demands, the size of your HVAC system can be smaller, thus reducing appliance costs and replacement costs. Lower HVAC costs. Lower utility bills. Lower replacement costs.
• Improved comfort – High-performance glazing makes homes feel more comfortable. Cold glass can create uncomfortable drafts as air next to the window is cooled and condensation forms. In addition, strong direct sunlight creates overheating and discomfort to people and on interior surfaces.
• Increased light and views – Energy efficient glazing means that instead of using shading devices, curtains, and blinds, you can take advantage of the advanced performing properties of the glass and have more unobstructed views and light.
• Reduced fading – Carpet, furniture, artwork, and wood are highly susceptible to fading. Using low-E glass, tints, and films will help reduce fading due to unwanted UV light and save you from needing to replace your damaged stuff.

How Is a Window Wall’s Thermal Performance Measured?

To understand a window’s thermal performance, look at it’s U-value and SHGC value.

U-value measures how much heat is lost in a material, the lower the number the better. This is a value that measures the glass performance (not including the frame) and has a value between 0.32-0.40. High-performance glazing has a value around 0.27.

Compare this to a solid exterior wall which has a U-value around 0.05 (with R-19 insulation).Without getting too technical, you can see that window walls are less efficient than solid walls.

But you’re probably wondering why people choose to go with window walls when they aren’t that energy efficient. Well, the answer is because some people value the other benefits of window walls more than their thermal performance. They like the amount of natural light and the big views. Once they’ve made the decision to go with window walls, they then look at higher performing windows and options to improve the thermal performance of them over the standard options.

The other value to look at is SHGC (solar heat gain coefficient) which measures solar radiation transmitted through a window unit. It’s a number between 0 to 1, the lower the value the less heat enters through the glass. Typical window systems have values between 0.25 and 0.40.  Obviously, in warmer climates especially, you want to choose a system with a lower value.

What Are the Energy requirements For MY Window Walls?

Energy efficiency requirements for windows vary throughout the country depending on geographic location and climate. Building codes specify U-value and SHGC requirements in eight different climate zones that show baseline requirements that all window systems must meet.

To check the residential window requirements in your area, find your location on this map and determine your climate zone. Then use the chart to determine the performance requirements that your windows must meet. (Just remember that some local jurisdictions have stricter requirements than these so it’s a good idea to check with your specific city/county to confirm minimum requirements.)

[International Code Council Zone Map & Residential Window Requirements courtesy Energy Efficient Collaborative.]

A very helpful resource for learning more about energy efficient windows is the Energy Efficient Collaborative. They have a lot of useful information about windows and go into more detail than we can cover in this Window Wall Series.

Once you know the minimum performance requirements for your area, you can verify with your window wall manufacturer that those values can be met with your chosen window product. If not, you’ll have to see if there are options available to improve the performance rating such as those described below.  

Finding Ways To Optimize Your Window Wall’s Energy Performance

Select a Higher Performing Glass

As we mentioned in our article on window wall glazing types, there are various glazing options that can offer improved thermal performance. If you want to increase the energy performance of your windows, the first and easiest option is to switch out the glass type to a better performing glass or increase it to a triple-pane system.

Select a Thermally Broken System

When it comes to window wall design and thermal performance, you want to avoid transferring heat through the window wall. Obviously, the more inefficient a window system, the more you need to either heat or cool your space to remain comfortable… which results in higher HVAC costs, something none of us want.

Fiberglass, vinyl, and wood are good insulators, while aluminum and steel are good conductors of heat. Accidentally burning yourself by touching a pan on the stove is a good reminder of how heat can be transferred easily through metal.

Thankfully, because of the high popularity of metal windows, new improvements in window frame development have led to the introduction of thermally broken window frames.

In traditional, non-thermally broken metal frames the extrusions are exposed to both the interior and the exterior temperatures making them easy targets to transfer heat back and forth. That’s a big thumbs down. 

With thermally broken systems, the window frame is split in two: interior and exterior pieces, with an additional PVC insulator piece joining the metal pieces. This minimizes the ability of the frame to transfer heat through the system, thereby improving the energy performance of the window frame as well as potential condensation.

You can see in the thermally broken profile here: the all the black PVC pieces break the flow of heat and separate the interior and exterior pieces of metal.

Similarly, think of an insulated glass unit. The air or gas filled space acts as an insulator and stops thermal energy from passing through the glass. It’s a similar principle.

With newer, more strict energy codes thermally broken systems are becoming all but required in modern window wall systems in most parts of the country. So if you plan to use an aluminum or steel window walls system, you will probably want to use a thermally broken system.

Acoustical Performance of Window Walls

Are you in an urban area with a lot of street traffic or are you in the peaceful country where sounds are welcoming? Do you have specific needs for quiet spaces?

You and your specific project requirements will determine how important it is to focus on acoustical requirements.

Similar to energy performance, acoustical performance properties of window walls are lower than an insulated solid wall. Most exterior walls have an STC (sound transmission coefficient) rating between 34 and 38. The higher the number the better. In contrast, a standard window has an STC around 27. And since sound travels in the path of least resistance, it will use travel through the windows before the walls.

So if sound control is important to you, consider ways to improve the acoustical performance of your window wall system so that its STC rating is closer to that of your walls.

Here are some options to improve the acoustical rating of your windows:

• Increase the total glass thickness.
• Increase the air space thickness in insulated glass units.
• Evaluate different gas fills in the insulated glass units.
• Consider using different glass thicknesses for individual glass lites or a laminated interlayer for one or both of the lites.

If you increase the STC of your solid exterior walls, then by all means, try to increase the STC of your windows to match. But remember: The entire wall’s STC is only as effective as the weakest element. Also, conversely, if your window’s STC rating is higher than your walls’ rating, it won’t be of any benefit because, at that point, your walls would be your lower-performing component and thus, sound would just travel through the walls instead.

Acoustical performance for windows isn’t something you’ll find in the codes. It’s really at your discretion. And oftentimes the standard window wall systems are sufficient for most cases. It’s just those few cases if you live in a noisy location or you need it to be quiet for personal reasons that you explore ways to improve the STC rating.

While window walls look great architecturally, they do need to perform well and meet the high-performance standards of current codes and your own personal needs or else you won’t be happy with them no matter how nice they look. It’s obviously a tradeoff that you’ll accept if you feel strongly about wanting window walls. But as we shared above, there are ways of optimizing your window wall system to perform better. The key is to find a happy medium where you can be happy with the performance level and while still enjoying the light, views, and other benefits of window walls as they meet your other needs.