Before You Buy: Understanding Thermal Properties in Windows and Doors

When selecting new doors or windows for your home or property, understanding thermal properties can be the difference between a quality purchase and a poor one.

The right window can transform a room from dull to beautiful—but if its thermal properties aren't suitable for the climate, that beauty will come at a steep price. 

Let's look closely at the most common terms used when manufacturers list the thermal properties of windows and doors. Also, we'll show you how you can leverage that knowledge to make informed decisions. 


Common Terms on Window Information Listings

When browsing catalogs or reading stickers on window models, here are common terms you'll see and what they mean in plain language:


Thermal Conductivity

This term refers to the general ability of a material to conduct heat from one point to another. A low-conductivity door or window will thus insulate your home more effectively.



The r-value is the measurement of a window's insulating level. A higher R-value rating means the product works better in keeping heat in or out. 

You'll see this more often in doors than windows, as it was initially invented as a way to assess the stopping power of insulation. Windows are more commonly marketed and sold based on their U-factor. 



This term rate measures how a window or door conducts heat flow not directly related to sunlight. This is the opposite of the R-value, in that a lower U-factor indicates a window or door will let less heat flow through it. 

Windows are generally considered efficient when their U-factor is below 0.40. 

It's important to note that U-factor measures the entire product's ability to control heat flow. This is in contrast to R-value, which only assesses a specific part of a window or door (such as the glass). 

This means a high R-value product may still leak heat in several ways, rendering that R-value is meaningless when shopping for windows. 


Solar Heat Gain Coefficient

SHGC describes what fraction of solar radiation passes through a window or door, either by passing through uninterrupted or by being absorbed and then leaking through. 

A lower number means the window or door does a better job of keeping solar energy outside of your home. 

If you live in a high sunlight area where UV rays constantly bombard your home, make sure the SHGC is low!


Air Leakage

Air leakage ratings measure how much air should be able to leak around the window or door, assuming proper installation and a standard pressure difference between the inside and outside. 


Visible Transmittance

Visible transmittance, or "VT," describes what fraction of the visible light spectrum can pass through the glass of your door or window. A higher fraction means more light will pass through. Again, keep this number in mind if you live in a region with lots of sunlight. Miami and Los Angeles residents, this notification is for you!


Light-to-Solar Gain

Light-to-solar gain helps you assess the relationship between a window's visible transmittance (how much light it lets in) and its solar heat gain coefficient (how much heat from the sun it lets in). 

A window with a high LSG ratio allows in more light relative to the amount of heat it lets in; if you want brightly lit rooms but don't want the heat that comes with that sunlight, a high LSG is ideal. 


Condensation Resistance 

Another rating on windows, with implications for climate control, is condensation resistance. This term simply describes the ability of a window or glass door to prevent condensation from forming on its exterior or interior due to factors of moisture and temperature. 

As controlling humidity is critical toward efficient climate control, this can be very important for maintaining thermal efficiency in humid climates. 


Putting This Information into Practice

If you're simply looking for windows and doors that are appropriately efficient for your home, then the basic NRFC labeling on most products should be enough. 

If you want to dig deeper, then the Department of Energy has a list of state fact sheets you can refer to.

Of course, if you want to go beyond simple efficiency ratings, use this guide to find the perfect window or door match for your home. 

For example, if you want low condensation, minimal air leakage for air quality reasons, or want light without heat-- you'll need to dig deeper into the ratings or get help from an experienced professional. 

Still feeling a bit lost or overwhelmed by this information? Don't worry, you're not alone. If you would like to learn more about choosing the right windows and doors to match the thermal needs of your property, Alco's customer service team is ready to help. 

Knowledgeable, professional, and attentive to your needs, the Alco Windows and Doors service team can guide you to the exact products necessary to protect your home, business, or property—whatever your exact needs may be. 


Contact Alco Windows and Doors for your next impact product installation!


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