Heavenly Thermal Comfort

Median Radiant Temperature Exposure

The relatively large windows, e.g. in the living room, while aesthetically pleasing, can be perceived uncomfortable in terms of thermal comfort. The human body is specifically sensitive to radiant sensation, much more than to air temperature.

For example, if a person sits on a chair in some proximity of the window, he/she is significantly exposed to lower surface temperatures from windows. While the surface temperatures of our high-performance exterior walls will be close to the actual room set-point temperature, the surface temperatures of R-3.5 windows will be noticeable lower under cold outside conditions.

Our analysis showed that the interior temperature on a day with -5°C (23°F) exterior temperature drops the interior surface temperature to 11°C (52°F) for a R-3.5 window, while it only drops to around 15°C (~60°F) for a triple-glaze R-7.5 window. This may seem not seem significant – however, a quick estimate utilizing the Median Radiant Temperature (MRT) method provides some more detailed insight.

For the MRT method the surrounding temperatures are scanned and recorded according to their angles of exposure. The Figure above depicts this scenario, where the exposure to the window surface comes to around 90°, as compared to the wall, which can be approximated to be roughly at room temperature. The radiant exposure to the floor or ceiling is neglected for now, as our body surfaces seeing these areas are comparatively small as well.

Averaging the radiant temperatures according to the exposed angles we can write: 

T(sens) = 52°F x 0.25 + 70° x 0.75 = 65.5°F

In an attempt to offset this sensation with raising the setpoint temperature in the room, a person would have to overcompensate by 1.4, as radiant sensation is perceived by 40% more significant than actual air temperatures. If no other measures are taken, this would mean that the setpoint would need to be adjusted to  

T(set) = 70° + (70°F - 65.5°F) x 1.4 = 76.3°F

We therefore have chosen to install triple-pane windows to increase next their improved energy efficiency, also the thermal comfort for occupants. For these triple glazing windows we would calculate: 

T(sens) = 60°F x 0.25 + 70° x 0.75 = 67.5°F

To compensate with the setpoint temperature for a target sensation temperature of 70° we would need to adjust the thermostat to 

T(set) = 70° + (70°F - 67.5°F) x 1.4 = 73.5°F

While an occupant may chose to not adjust the thermostat in every case, the high performance windows selected for our project will mitigate that tendency, and thus may actually save energy cost in the longer term.