Crash Course: 5 Minutes, 5 Replacement Window Performance Values

So, you’ve made the decision to invest in replacement windows for your home.  Congratulations!  So what comes next?  Where can you start making sense of the many brands available?  How will you know which is best for your home?

Research!  To sort through the dozens of window manufacturers and product lines, you need to understand how window performance is rated and compared.  Only then can you compare the different windows to determine which best meets your needs.  

But how?  Read our 5 minute crash course on the 5 replacement window performance values so you can compare different windows in a standardized way.  By understanding how one window performs better than another, you will gain confidence as you narrow down your list of preferred products.

1.  U-Factor and R-value

U-factor measures how well a window prevents heat from escaping.  The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly.  U-Factor ratings generally fall between 0.20 and 1.20. The insulating value is indicated by the R-value, which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

2.  Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat from sunlight. The SHGC is the fraction of incident solar radiation admitted through a window and directly transmitted, absorbed or subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.  However, lower isn’t always better based on climate.  In Phoenix, a lower SHGC will keep out more of the desert sun’s brutal rays.  Conversely, if you spend more money over a year heating your home than cooling it (like here in the Metro DC area), then you want some of the passive solar sun to warm your home in the winter.  

3.  Visible Transmittance

Visible Transmittance (VT) measures how much light comes through a window. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.  Coatings like low-E can have an significant impact on VT, and this varies among brands based on their manufacturing processes.

4.  Air Leakage

Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

5.  Condensation Resistance

Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.

At Windows on Washington, we don’t believe that one window fits every home. Our experts have done all the research to provide you with an array of products that not only look great, but perform above Energy Star guidelines to maximize your return on investment.  Don’t be fooled by salesmen that push one product for every project.  Instead, make an informed decision based on careful research with help from our experts.

Windows on Washington has helped thousands of homeowners in the Metro DC area make their home more comfortable, beautiful and energy efficient with best-in-class replacement windows.  To see how Windows on Washington can make your home improvement dreams a reality, give us a call today.  We are happy to schedule a free in-home consultation to provide the answers you need to make the right decision for you home.