Passive Window Design for Optimal Performance

Passive Window Design

You thought windows were really simple and easy, right? Well, think again, my friends! When you pair high-efficiency windows with high altitude and differing sun exposures, you soon discover there are many possible ways you can not only optimize your window performance, but also set yourself up for failure!

Thin Air

Altitude is no joke, and you can literally feel it when you come from lower lands or sea level to visit a state like Colorado. The locals will all tell you to drink lots of water and if you try to go for a light jog, you may feel like you actually just attempted a marathon instead. The altitude has a real and major effect on the body. The same is true with windows. 

Why Does Altitude Matter?

High-performance windows today are equipped with triple and even quad glazing to form insulated glazing units (IGUs). IGUs have an airtight seal around the perimeter and trap the gas between the panes. Typically, gasses like argon and krypton are used. 

IGU’s perform best when they are assembled at the altitude where the final installation is. When IGUs are transported from a factory at sea level to a place with high elevation like we have in Colorado, it’s a recipe for disaster. When the altitude delta exceeds 1,600 feet, it becomes problematic because the barometric pressure varies enough to create complications. For example, when going up to 5,000 feet in elevation, the atmospheric pressure is much lower than at sea level. Think about what happens to a bag of potato chips. Most mainstream companies don’t gas fill and insert a capillary tube so the glass doesn’t break or even blow up in transit. Alpen HPP uses breather tubes and balloons to regulate barometric pressure of the gas used in their windows. There are other ways to manage barometric pressure to include pressure release valves as well as pre-pressurizing in the factory to the average barometric pressure for the installation location.

Hello, Mr. Sun!

If you’ve ever been in Colorado on a sunny day in the winter when it’s around 50 degrees, you may have experienced some hot flashes in your car where you scurry to get your air conditioning going. Wait a second! In the winter?? Yes, my friends, the sun is that intense here at higher altitudes and especially when it’s streaming through the glass of our cars. 

Now, let’s apply that to our homes. Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a skylight, door, or window, whether directly and/or absorbed. That radiation manifests as heat within the home. 

A lower SHGC window reduces solar heat and provides greater “shading” essentially. Glazing products with low SHGC ratings are more effective at reducing cooling loads during the summer by blocking some of the heat gain from sun radiation. Even up in the mountains, low SHGC glass is often used on the west elevation of a home to reduce summer overheating.  A glazing product with a high SHGC allows more solar heat gain which is great in colder months. Your home’s climate, orientation, and shading will drive the optimal SHGC for each window, often depending on the elevation or which side of the building the windows and doors are installed.

Please do not mistake SHGC with UV; these are different things. That said, one of the benefits of Alpen’s High Performance glass is it protects against up to 99.5% of UV rays. UV rays are damaging to virtually everything, including our skin. Related to windows, UV rays can damage nearly every finish in our homes and UV protection keeps interior finishes, wood, and furniture from getting damaged and discolored.  

Where Higher SHGC is Helpful:

In colder, heating-dominated northern climates, both SHGC and a window’s U-factor should be taken into account for energy efficiency. In heating climates where heating is a bigger concern than air conditioning, a higher SHGC in the range of 0.30 to 0.60 can be helpful especially on our south windows. During the winter months, the sun is lower in the south sky and the solar heat that is gained from the sun helps warm our homes.

Where Lower SHGC is Ideal:

In the Southern and Central states, where the climate can get extremely warm and air conditioning is used extensively throughout the majority of the year, a lower SHGC is ideal and desired. This equates to less heat gained from our windows. 

With most of our projects, we tune the windows by the elevation (North, East, South, West).  There have been rare occasions that windows even on the same elevation might have different SHGCs.  A north-facing window is going to behave differently than an east, south, or west-facing window simply based on the position of the sun throughout the day and the seasons. If you “tune” each window to be a specific value, make sure that each window is tagged properly and that the contractor pays attention to where each window needs to be installed, which is critical if you have a lot of the same sized windows. 

Of course, specifying this level of detail on each window is as much a science as it is an art. Note that SHGC levels generally run parallel with the visible transmittance of natural light.  While not noticeable by most people, there are slight hue differences with different SHGC glass. We know that every home’s windows are not just built to function well, but also to enhance a view and afford us natural light. In the mountains, we have some pretty spectacular views! Often, it’s a real-world balance to get the best performance while considering natural light in the home. [NOTE: it’s recommended to consult with an energy modeler in the process.]

So, those simple windows don’t seem so simple anymore, eh? There’s actually a whole lot more going on to make sure they will work optimally for your home. When you put this level of attention into your window design, you really can achieve some amazing results in performance, and as fans of sustainable, efficient homes, we love that! Love the idea of high-performing windows in your home too? Not sure where to start? Let us help you break down some of the basics and how they might apply to your home’s strategy.

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