Heat Tests - U & R Values

Understanding the terminology
R-value & U-factor are the two common measures to rate how well a specific material or system resists or conducts heat flow. 

U-value  rate how much heat is conducted through the combination of materials, which means how much heat the component allows to pass through it.

• Low U-value indicates good energy efficiency - U-0.35 would be better than U-0.43.
• U-value is based on scale of zero to 1.

R-value  used to rate how well a material resists the flow of heat through it. The higher an R-value is, the more heat loss it resists, and therefore the more energy efficient window is. For example, R-19 is better than R-11.

Converting from U-values to R-values
You may have noticed that U-values tend to be very small numbers. Because R-values rate heat loss resistance and U-values rate actual heat loss, they are the opposite - or reciprocal - of each other. Therefore, you can convert from one to the other using the following formulas:

• 1/R-value = U-value, or 1/U-value = R-value.

Solar Heat Gain Coefficient (SHGC)measures a window's ability to reduce heat gain by blocking the heat-producing rays. It's based on scale of zero to 1.

• A high coefficient will allow more sunlight and warmth in to the house.
• A low coefficient will block more sunlight to reduce summer heat gain.

Visible Transmittance (VT) is an optical property that indicates the amount of visible light transmitted. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.