April 27, 2007

Research

Quantifying the effects of window choice on human comfort levels

PETER KENTER

correspondent

An office worker sits at a desk beside a window designed to exacting specifications. It contains an inert gas that insulates it from the outside elements and uses a special coating to reduce the effects of ultraviolet radiation on building occupants.

The only problem: when the sun shines on the guy behind the desk his face gets too darned hot while his feet are freezing.

The problem with choosing suitable windows is that it often satisfies design and energy criteria without considering human comfort. Research conducted by the Center for the Built Environment at the University of California, Berkeley (UCB) aims to quantify the levels of comfort occupants might experience.

"Our group has a long history of studying

Charlie Huizenga research specialist

“We are seeing interest in window comfort ratings from mostly design professionals, though there are a few manufacturers that have taken great interest,” says Charlie Huizenga, research specialist in the Department of Architecture at UCB.

“The recent interest in green building design has put more focus on indoor environmental quality and on high performance facades that can improve both the energy performance of buildings and the comfort of the occupants.”

Huizenga notes that human comfort has lagged behind most technical research on window performance.

“The historical approach has been to deliver heating and cooling energy at the window surface to offset the impact of the window on comfort,” he says.

“But hot and cold complaints are the most common of all occupant complaints in commercial buildings. Some US $70 billion is spent annually on heating and cooling in the U.S., yet we have pretty crude approaches to making people comfortable. Our group has a long history of studying thermal comfort to create comfortable buildings that use less energy.”

Huizenga was one of the authors of a report, Window Performance for Human Thermal Comfort, prepared on behalf of the National Fenestration Rating Council, for further consideration as the basis for a window comfort rating standard.

The report looks at comfort predictions in a variety of settings, including asymmetrical environments, with cool or warm ceilings, walls, or floors.

Prior studies used the predicted mean vote (PMV) model to predict personal comfort, but it was limited to predicting comfort in uniform conditions, where air temperature and surrounding surface temperatures were roughly the same.

To conduct more precise research, UCB developed a detailed physiological model that distinguishes 16 different parts of the body.

CENTER FOR THE BUILT ENVIRONMENT AT THE UNIVERSITY OF CALIFORNIA, BERKELEY

The UCB Comfort Model predicts thermal comfort levels in 16 areas of the human body.

“The UCB Comfort Model can predict when your hand will become cold as your body starts to vaso-constrict blood vessels due to sitting next to a cold window,” says Huizenga.

“The second unique feature is that, based on the physiological prediction of skin temperatures over the body, it can predict how a person will feel. Discomfort is often caused by one or more local body regions becoming cool or hot and our model can predict this.”

In winter, Huizenga says, the dominant factor for human comfort is the outdoor air temperature, while in summer, the dominant factor is solar radiation.

The Window Performance report recommends two sets of window ratings be adopted — one each for summer and winter —to reflect that disparity.

The report was expected to be submitted to ASHRAE for peer-review this month.

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