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Window Performance

By: Brian Albrecht
Posted: Nov 19th, 2021

“All windows are the same” is a refrain we’ve heard plenty of times before. As long as you get a good brand that will come out to service them if anything goes wrong, you’ll be fine. In reality, windows are one of the more complex and important pieces of your home when it comes to overall comfort as well as energy efficiency. While windows can be a major line item cost when building a new home or remodeling, cutting the window budget is something to seriously think twice about. 

In the following sections, we will discuss the benefits that a good window can offer. While the science behind the thermodynamics of the entire system and chemistry involved in various coatings can be complex, we will try to keep things simple (so try not to get too upset when we simplify the concepts of radiant and convection heat transfer).

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Home Comfort

What drives the level of comfort that you feel in your home? Temperature (despite the eternal struggle over the thermostat settings)? Lighting? The finishes and furnishings around you? All of these play a role in how we perceive a space and how comfortable we feel in them. A gorgeous room can turn into a place you don’t want to be if it is always too cold or too warm, and the perfect temperature room can become dull and bland without access to adequate lighting or a comfortable place to sit.

Figure 1: A room such as this could suffer from being too hot or too  cold during the summer and winter if poor performing glass and  windows were used.


Windows let in daylight. Duh. But how much daylight they let in can change from window to window. There are few windows on the market these days that don’t have a coating on them. You may see coatings touted as Low-E, or some proprietary version of Low E, and all the benefits they can offer. While the coatings are both necessary and beneficial, the consequence of them is a lower amount of daylight getting through to light up the inside of your house. When looking at a window, they will almost always indicate the Visual Light Transmittance (VT), or how much daylight actually makes it through the window. A single pane of clear, uncoated glass typically allows 90% of light through, losing 10% to some of the light being reflected back to the outside. Coated, double pane windows frequently have a VT of somewhere between 40% and 80% depending on the coating. Losing 20% of the light that is coming into your home can make a big difference and it is important to look at the specifications the window offers to weigh the pros and cons that sacrificing some light may provide. While it may seem like picking the window with the highest VT is the best option, there is more at play here than simply the amount of light getting through the glass, which we will discuss more when it comes to energy efficiency.

Figure 2: The difference in visual light transmittance can make a big difference in how bright a space feels, but you may be  surprised to learn how much light your current windows block. 


While windows obviously play a large role in the light component of what makes a space comfortable, they can also be a major driver of comfort when it comes to temperature. Picture this, it’s a cold winter day and you are sitting in your car shortly after scraping the ice from the windows. The heater is on full tilt trying to warm up the space. Have you ever noticed that the side of you closest to the window, regardless of how warm the inside of the car gets, always feels a slight chill? That chill is the window glass sapping you of heat. The same can be said of windows in your house. Old drafty homes are frequently drafty because of poor quality windows that sap the inside of your house of warmth at a far greater rate than modern windows. Those old drafty windows are what are called single-pane windows, meaning they only have a single piece of glass between the inside and outside. In modern windows there are almost always two panes of glass, and sometimes three or four, between the inside and outside, improving the window’s ability to shield you from the heat or cold.

Another component that plays a role in how drafty a window can be is what is called “air  infiltration,” or the amount of air that is able to get through the window assembly. Continuing with the earlier example of your car on a cold day, the heater is already fighting a losing battle since cars only have single pane windows, but how much longer would it take for the car to heat up if you cracked a window just a tiny bit? Depending on how much you cracked the window, it might not take that much longer for the car to heat up, or, it may never heat up if the crack is big enough and it’s cold enough outside.

Figure 3: Sitting close to the wall of windows in this space could be chilly or drafty if  windows with poor performance or a high air infiltration rate were used.

Think of air infiltration like cracking your car window, except in this case, you don’t have the ability to close it. All windows leak air; the question is simply how much? A cheaper window likely leaks more than a higher quality one and thus can impact the overall comfort of your home as it becomes a little more drafty and your furnace and A/C have to work harder to keep things at the right temperature, which also impacts the energy efficiency of your home as a whole. 

Energy Efficiency

In today’s age, energy efficiency is a common topic of conversation with clients. No one wants to see high monthly bills coming in due to an inefficient home. What is often overlooked by clients is the importance of windows in the home efficiency puzzle. Windows are the single weakest point in the home when it comes to energy efficiency. Glass simply isn’t a good insulator, and as such, more glass means a less efficient home, so anything we can do to improve the energy efficiency of the windows is a huge step up when it comes to the overall quality of the home.

The metrics used to evaluate the energy efficiency of a material or assembly of materials (the entire wall) are what is called the U-value or R-value, both of which relate to one another. It is typical to see an R-value for the walls in your house and a U-value for glass and window units. R-value can be converted to U-value using the equation U value = 1/R-value making the two easily comparable. The current energy code (2019 Ohio Energy Code) requires residential walls to meet a value of R-20 or U=.05. By comparison, most windows range in performance from R-2 to R-5. What makes this difference in efficiency even more stark is that the thermal performance of materials is not a linear relationship. You get a far greater benefit in thermal performance moving from R-1 (U=1.0) to R-2  (U=0.5) than you do moving from R-10 (U=0.1) to R-11 (U=0.09) as is evident in the U-value comparisons; meaning if you have the opportunity to add a little more insulation in your walls versus going with a higher efficiency window, you’re probably going to get a better bang for your buck with the improved windows (See Figure 3).

Figure 4: The relationship between R-value and U-value is not linear, and as such, better windows, which have a lower R-value offer much greater improved performance with incremental improvements than other materials and assemblies that have a higher R-values.

Unfortunately, the R-value and U-value relate negatively to the visual light transmittance (VT) that was discussed earlier. The better the window performance, the more it blocks the light coming into the house. If possible, it can be helpful to find out what the visual light transmittance of your current windows is as a benchmark. It may sound like a lot to block 50% of the light coming into the house, but you might be surprised by what your current windows block and will then have a baseline of comparison when looking at new options. Keep in mind, the better performance you get from your windows, the more you help your HVAC system and therefore reduce your energy bills, and as such, balancing the window thermal performance (U-value and R-value) and the light transmittance (VT) can be an important decision. 

Beyond the thermal performance of the glass is the other metric that frequently relates to the visual light transmittance (VT) that was mentioned earlier. The Solar Heat Gain Coefficient (SHGC) is the measure of how much heat the glass allows to pass through from the exterior to the interior. Why should we care about that? In the winter, we would love to have that extra heat from the sun so the furnace doesn’t have to work as hard to get the house up to temperature (a high SHGC), thus reducing your heating bills. However, by contrast, we don’t want that heat in the summer when the A/C is trying to cool the house down (a low SHGC). In the south, this is easy since there aren’t many days they need to heat the house, and in the far north they have the opposite issue where they rarely need A/C. Unfortunately for us in Ohio, we are in a mixed zone where we have a fair number of days that need A/C and a fair number that require heating. So, what’s the solution? If you are worried about energy efficiency, then the answer is simply that it is more energy intensive to heat the house than it is to cool it, so lean towards a window that harnesses more heat to help out the furnace. It is also beneficial to lean heavily towards harnessing the heat because, while Dayton is a mixed climate, we require about 9x the amount of heating as we do cooling. If that sounds higher than expected, consider a 100 degree day (fairly uncommon in Dayton) and a 20 degree day (somewhat common in Dayton). If your home is set at 70 degrees, your HVAC system needs to make up 30 degrees of difference on that scorching 100 degree day, while it needs to make up 50 degrees of difference on the 20 degree day, meaning it will need to work substantially harder to maintain that interior temperature on the 20 degree day. So, anything we can do to help the furnace out on those 0 degree days (just as common as the 100 degree days in Dayton) is a bonus; thus a higher SHGC is substantially more help in the Dayton area than a lower one.  

As mentioned, the SHGC is also tied closely to the Visual Light Transmittance. The more heat you block, the more light you block. So, if we want to harness some of that heat gain, we get the benefit of getting slightly more light transmittance than our southern neighbors who prefer to block the heat (finally, some good news!).


Windows are complicated, and choosing the right one is made even more complicated by the a mixed climate that we have in Ohio. Window performance is dependent on a variety of factors, all of which can make a big difference in the comfort and efficiency of your home, but if you know what metrics to look for, you can greatly improve the quality of your home. In a nutshell, for the Dayton market, we look for a window that isn’t leaky (low air infiltration rate), has a great U-value (the lower the better), but a high SHGC (as high as we can get while maintaining a solid U-value) and high VT (the more light the better). While windows can be expensive, it is important to remember that improved window performance is one of the few things in your home that gradually pays for itself. The better your windows perform, the less you pay in heating and cooling, slowly paying for the increased upfront cost. And while we won’t claim that the improved performance will pay for itself monetarily in 5 or 10 years, or even in the life of the window, it is important to remember that the improved performance also benefits you from a comfort standpoint day in and day out as you live in the home. 

If you are looking at a remodeling project that includes a few windows, or maybe you have a room in your house that consistently overheats in the summer and is too cold in the winter, consider replacing the windows with a higher performance option which could help the furnace and A/C keep up with properly conditioning the space. 

In the end, the windows you choose are likely the ones you will be living with for quite some time. Changing out windows is no small expense, and as such, is not done on a frequent basis. So, getting it right the first time can help prevent buyers remorse down the line when a poor performing window has impacted your home’s comfort and efficiency, and by extension, your bank account through more expensive utility bills.

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