Why Lower Numbers Mean Maximum Comfort When It Comes To Energy Efficient Windows

With winter just around the corner, many of us are starting to think less about our air conditioning units, and more about our heating systems. However, when taken together, heating and cooling homes makes up roughly 45% of a homeowner’s annual energy bills. This is why one of the most important things to find out and consider when choosing windows, doors, or skylights, is how energy efficient they are. No matter the season, you need windows which are efficient and perform well. But, how do we know which factors make energy efficient windows? Who decides? And, most importantly, does energy efficiency necessarily correlate with maximum comfort?

Thankfully, there is a straightforward framework for architects and homeowners who wish to make informed decisions about windows and doors. The National Fenestration Rating Council provides four key metrics, known as the NFRC rating, which provides context for prospective buyers. What makes the NFRC rating so useful to homeowners is the fact that the ratings are completely independent of the manufacturer or seller. In short, the NFRC ratings provide an objective score of the window or door’s efficiency. To make it easier to understand the numbers, we recommend this very simple tip: in general, the lower the number, the more efficient the window. More efficiency means more comfort and consistency.

Why Lower Numbers Matter

example chart illustrating nfrc rating.

As mentioned above, there are four key metrics which measure the efficiency of windows. These are: Air Leakage, U-Factor, Solar Heat Gain Coefficient, and Visible Transmittance. According to the NFRC, high performing fenestration products should:

  • Provide comfortable levels of heat and light.
  • Maintain a consistent temperature.
  • Protect against UV radiation.

Advances in fenestration technology has meant that energy efficiency has greatly improved over the course of the past century, and will almost certainly continue to do so. For now, however, the NFRC rating is the American standard. It provides homeowners with a good sense of the comfort windows with a specific rating will provide. Each homeowner’s needs will vary, based on a variety of factors. 

One of the biggest determining factors is whether your home is located within a warm climate. Homes in warmer climates need their windows to react differently to heat gain/loss than those in colder climates. For example, homes in colder climates can benefit from more sunlight. Similarly, homes which are southward facing will get more sun than those which are north facing (more on this below). 

Eliminating Air Leakage

One of the main concerns is eliminating Air Leakage. The Air Leakage rating specifically looks at how much air is able to enter or escape your home when the window is closed. From the design stage to installation, air leakage is taken very seriously. It is one of the leading causes for dissatisfaction with windows. This is particularly true in regions which experience cold weather. Ensuring that your windows are adequately protected against air leakage is an important step in ensuring your windows are energy efficient. Without it, your home’s temperature is unlikely to ever be comfortable or consistent, resulting in higher energy bills.

Although there is much emphasis placed on the U-Factor variable, Air Leakage (alternately known as air infiltration) is an equally important metric. In order to determine this, we calculate the “cubic feet per minute of air, per square meter of window area.” Basically, this helps us to understand how many feet of cubic air is able to pass through a window per minute, divided by the total area of the window. This sounds complicated, but all you really need to remember is to choose the lowest number you can. In order to achieve an efficiency rating, the highest score a window can have is 0.3. However, a high-quality Tilt and Turn window with compression seals and multi-locking points can achieve as low as 0.01. 

Also, if not installed correctly, the IGU can cause further air leakage from the window unit. Furthermore, it can lead to water leakages. This can lead to even bigger problems such as damp, rot, and, occasionally, structural damage within the home. It is very important that a licensed and certified installer team install all of your windows and doors. Here at Open AWD, we use an external licensed and certified team for our projects. Remember: a low air leakage rating also means lower energy bills.

Here at Open Architectural Windows and Doors, all of our products undergo rigorous testing. This is not only during the manufacturing process, but also carried out onsite by a licensed and certified team. This will ensure the ratings hold up within their environment.

Understanding U-Factors

U-Factor (also known as U-Value) is another important aspect when it comes to choosing energy efficient windows and doors. To put it very simply, it measures how effectively a window can retain heat inside a home and reduce heat loss. The glass, frame, dividers, and spacers all play a part in unwanted heat loss. To put this in context, older, single-pane windows had a very high U-Factor, usually of about 1. Again, the lower the number, the more heat it retains. So, for example, a window with a U-Factor of 0.15 will retain heat better than one with a value of 0.4. A low U-Factor is essential for people living in colder climates, or in locations with harsh winters.

Many of today’s window manufacturers will strive to achieve a Passive House standard with their windows. Passive House is a building standard that promotes thermally and energy efficient homes. Created in Germany at the end of the twentieth century, the Passive House concept has since become the global standard in sustainable design and energy efficiency. Through a combination of extensive insulation, thermally broken, airtight windows and doors, and an energy recovery ventilation (ERV) system, your home’s heating and cooling needs can be reduced by up to 90%. The result produces a home which is extremely cost effective and whose temperature is highly regulated. Here at Open Architectural Windows and Doors, all of our windows are Passive House standard. You can learn more about it here.


Harnessing Solar Heat Gain

Solar Heat Gain Coefficient (SHGC) is solar radiation which enters a home via its windows, doors, or its skylights. This is either transmitted directly and/or absorbed, before being released as heat inside a home. Much more so than Air Leakage or U-Factor, with Solar Heat Gain Coefficient (SHGC), a low or high number rating will really depend on the regional climate as well as the orientation of the property. The lower the number, the less heat it lets in. Conversely, windows with a high SHGC level will allow more heat into the home. 

Commercial buildings with a lot of large windows, or curtain walls systems, will often seek to minimize unwanted solar heat gain. Standard homes, however, may want to harness some of the solar heat. South-facing windows will be a great addition to a property in the winter months, but may introduce too much unwanted heat gain in the summer months. To avoid this, we would recommend installing overhangs or exterior shading systems. This will minimize glare and unwanted heat gain. We would recommend a low SHGC for east-west facing properties.

 Visibility Transmittance

Visibility Transmittance is the one factor in which lower does not mean not better. We use a scale to measure this and it helps determine how clear the glass is. The scale runs from 0 to 1. Standard double and triple-glazed windows will rank between 0.3 and 0.85. You want a higher number with this, because higher the number, the greater the visibility.

This factor is very important because it–along with the size of the window–will determine the amount of natural light that can enter a room. As a result, it will also have an impact on the amount or artificial light needed–another factor which influences energy efficiency.

Call Us Today

As you can see, purchasing windows is an important and often time-consuming decision. So, it makes sense to spend some time researching the ideal windows for your home in order to best serve you over the following decades. Although the window you need will be mostly determined by your climate, you will need to pay close attention to the variables we discussed here so that you get the most comfortable and consistent windows for your home. If you have a project in mind and want to learn more about what may be the best type of energy efficient windows for your home, contact us today for a free consultation. Call us at 718-403-0300, or email us at info@openawd.com and someone from our team would be happy to discuss this with you.