Back to basics: achieving thermal comfort next to windows
We love to sit next to windows because of how it makes us feel. Access to daylight and views of nature have been proven to support human health, healing, learning, productivity and well-being.
However, sitting next to low-performing windows often can be uncomfortable. Several mechanisms impact this discomfort: Radiative heat transfer, direct solar radiation impingement, convective currents (downdrafts) and air leakage (AL).
Air Leakage
Cold or hot air can enter the building around or through fenestration. This causes cold drafts for anyone sitting near a window and increases building energy use through higher loads on heating, ventilation and air-conditioning systems. AL is estimated to be responsible for as much as 33% of heating and cooling loads. Ensuring that the window-wall interface is well insulated and sealed is important.
Direct Solar Radiation
Design strategies that minimize direct sun, such as reducing glass on east and west elevations, using exterior shading, employing solar control low-E glass coatings, internal shades or dynamic glazing, can help reduce this discomfort and energy use.
Radiative Heat Transfer
When occupants sit next to a surface that is colder or hotter than their skin (92-98 degrees Fahrenheit), heat will flow radiatively from the hotter to colder surface. If window room-side surfaces are significantly lower than skin temperature, which they often are in winter if not well insulated, occupants will feel cold because their heat is transferring to the window surfaces. Likewise, they will feel hot in the summer when poorly insulated room-side window surfaces often exceed 100 degrees Fahrenheit.
Downdrafts
Convective currents occur next to window surfaces that are colder than room air, causing an uncomfortable draft. Remember, hot air rises, and cold air falls. Cooled by its surfaces, the air next to the window falls, displacing warmer room air and creating vertical convection loops.
Reducing radiative heat transfer and down-drafts requires fenestration with low thermal transmittance (U-factor), thermally broken frames, warm-edge spacer and high-performance insulating glass. The lower the U-factor, the closer to the window occupants can comfortably sit.
Lower U-factor fenestration maximizes rentable space and reduces the need for perimeter heating, providing a significant up-front cost benefit for choosing higher thermal performance and long-term benefits from energy savings and occupant well-being.