Wednesday, December 26, 2007

Building Science and Integrated Design

The cover story in November's HPAC magazine made a critical point about integrated design: In order to optimize building performance, HVAC designers need to take a more proactive role in the design of the building their systems are serving.

The author describes the traditional 'reactive' role of the HVAC designer, and advocates for a more active role in the design of what was traditionally left to the Architect. He provides a compelling example in the illustration of the effect of architectural detail on overall building component thermal performance.

A recent study focused on architects' building-envelope details and their impact on HVAC designers' heating- and cooling-load calculations. Four identically insulated and glazed building models were set up with R-19 wall insulation, but with variations in window frames, wall detailing, and wall construction. The models had the same window areas and center-of-glass thermal performance. The overall wall thermal resistance varied from a low of R-4.8 to R-12.6. (This included adding the R values of the glazing and frames to the wall thermal resistance to get an "overall" R value).

Thermal bridging caused by accepting cheaper/faster construction products and details can degrade wall and roof thermal transmission by more than 25 percent below insulation values. For example, R-20 wall insulation, coupled with poor thermal-bridging details, may provide an actual R-13 to R-14 thermal-transmission barrier.

But to be effective, the HVAC designer must, of course, understand the building science behind the way these components perform. The author suggest the use of free software modeling the effect of glazing details at
or more basic education at and

ASHRAE members also should not overlook the excellent series "Building
Sciences" by Joseph W. Lstiburek, Ph.D., P. Eng. found in the ASHRAE Journal (membership required for access). Dr. Lstiburek brings humor and insight to a subject over which mastery is demanded by today's energy performance goals.

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