Are your energy-efficient windows really efficient?


Many window brands can be generally classified as “energy-efficient” if the profile is made with a highly thermal-resistant material, such as uPVC (vinyl), which acts as an insulating barrier between the interior and exterior of a home or building. Add insulating glass (double or triple glazing) filled with argon gas, and you can achieve very low U-values. Typically, a low U-value will lead to improved comfort and reduced energy costs.


Mission accomplished; you now have an extremely energy-efficient window. Right? Not so fast. Let’s take a look at how this window is designed specifically to prevent heat loss.


The ultimate goal for an energy-efficient home or building is to control heat loss, which happens through conduction, radiation and convection. A truly efficient window is able to prevent heat loss across all three conditions:



Now let’s get technical. (Or you can jump to the layman’s summary toward the end of the post!)


Take a look at the NAFS2011 minimum requirements for different levels of window air leakage:




*Source: NAFS 2011 (North American Fenestration Standard / Specification for windows, doors, and skylights)



For Performance Class R, LC or CW windows, when there is a positive pressure (the pressure difference between indoors and outdoors) of 75 pa, the air leakage of windows should be less than 1.5 L/s∙m2 = 5.4 m3/m2·h (Unit Conversion: 1 L/s∙m2 = 3.6 m3/m2·h).


For Class AW windows, when there is a positive pressure of 300 pa, the air leakage of windows should be less than 0.5 L/s∙m2 = 1.8 m3/m2·h.


According to the thermodynamic formula, heat loss, Q, can be calculated:


Q = C ∙ ρ ∙ ∆T ∙ V


C – Specific heat capacity of air (kj/kg·K), Typically: 1.005;

ρ – Air density (kg/m3), Typically: 1.2041;

∆T – Indoor and outdoor temperature difference, 1K;

V – Leakage (m3/m2·h)

Unit Conversion: 1 kj/h∙m²∙K = 1 W/m²∙K = 0.17611018191798222 Btu/h∙ft²∙°F


Qr/lc/cw = 1.005 x 1.2041 x 1 x 5.4

= 6.53 kj/h∙m²∙K = 6.53 (W/m2·K)

≈ 1.15 Btu/h∙ft²∙°F


For Performance Class R, LC or CW windows, unit heat loss Qr/lc/cw is 6.53 W/m2·K ≈ 1.15 Btu/h∙ft²∙°F


Qaw  = 1.005 x 1.2041 x 1 x 1.8

= 2.18 W/m2·K = 2.18 (W/m2·K)

≈ 0.38 Btu/h∙ft²∙°F


For Performance Class AW windows, unit heat loss Qaw is 2.18 W/m2·K ≈ 0.38 Btu/h∙ft²∙°F


Can you sum that up in layman’s terms, you ask?  


Basically, all these calculations mean that for most of the popularly chosen windows on the market today the U-value is not significant, the real U-value should include both conductive heat loss and heat loss due to air leakage, which as a result is:


Real U-value = 0.16 ~ 0.51 Btu/h∙ft²∙°F + 1.15 Btu/h∙ft²∙°F = 1.31 ~ 1.49 Btu/h∙ft²∙°F


Do you think this makes for a super energy-efficient window? If you said no, you are correct. This result is from a test pressure of only 75 pa. For cases with higher pressures, the heat loss and resulting energy bills to make up for it would be huge.


To sum it up: Truly energy-efficient windows feature profiles made with highly thermal-resistant materials, Low-E coated insulating glass, an air-tight window design and a U-value that is significantly lower than those advertised by popular brands.


Now that you know the differentiating elements of a high-performance window design, challenge the “energy-efficient” claim many window brands tout; ask specifically how the window is designed to prevent heat loss.


By Forest Song

Technical Designer, REHAU Window Solutions









Suggested Window Examples


REHAU Tilt-Turn System 4500 — Easily achieves AW tightness level with air leakage value less than 0.02 L/s∙m2 = 0.072 m3/m2·h; and heat loss due to air leakage at only 0.015 Btu/h∙ft²∙°F.


REHAU Tilt-Turn System GENEO — This window system meets the stringent air-tightness requirements of Passive House.


REHAU Casement System ÄSPEKT