R-Value is a measure of thermal resistance used in the building industry. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value. R-value is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu.
Full Frame R-Values
The thermal image below shows the cross section of a standard dual-pane low-e aluminum framed window and the cross section of Alpen Windows’s frame with two panes of glass and one layer of suspended film. The colors depicted show the amount of thermal energy (Btu/h-ft2) passing through the frame. Purple means virtually no thermal energy is transferring through. The more green, red, orange and yellow means the more thermal energy that is passing through with red representing the most energy being transferred. The white areas in the image at left indicate the highest level of heat transfer - this is like a highway for heat to leave the home.
The thermal image clearly shows that in well made multi-paned, low-e windows, the main source of thermal energy transfer is through the framing system. Knowing the full frame R-value of a window is critical in ensuring that you are getting a high quality, energy efficient window.
Modeled via THERM 6.1 Simulation software, Lawrence Berkeley National Labs
Five Factors That Affect Full Frame R-value of a Window
- The type of glazing material (e.g., glass, suspended film, treated glass)
- The number of air chambers created by multiple layers of suspended film or glass panes
- What type of gas, if any, is used to fill the air space(s)
- The thermal resistance of the frame and spacer materials
- The "tightness" of the window – how much air leaks through
Many windows available today can achieve a relatively high center-of-glass insulation performance number. However, many are poorly manufactured and are made of a material that is not an effective insulator so building owners will experience draftiness from increased condensation and air leakage.