Informed Mechanical Design Through Tested Air Leakage Rates

Energy efficiency initiatives, such as the 2030 Challenge, hold the integrated design approach as a key for success. Integrated design aims for a collaborative approach between sub-disciplines of building design. However, one relationship not commonly addressed is between building enclosure designers and mechanical engineers. As building insulation and air tightness measures get more sophisticated with corresponding improved performance, there is a need for mechanical engineers to make more accurate assumptions in design in order to reach the efficiency goals for projects. The focus of this paper will be the relation between assumed and actual air leakage rates.

Blower-door testing used to measure air leakage rates of larger new construction buildings is beginning to be required in jurisdictions including Washington State. As a result of the requirements for continuous air barrier design and testing of completed buildings, a body of data on tested leakage rates will soon be available and can be referenced to predict air leakage rates during the design process.

Currently, mechanical engineers make assumptions for air infiltration based on modeling guidelines, sometimes assuming leakage rates 4.5 times or greater more than the prescribed rates under codes. Assumed infiltration amounts typically represent about 35% of building enclosure heat load. This overestimates heating energy requirements and can lead to over sizing of systems when continuous air barriers are installed. By using tested leakage rate data as a basis of assumed leakages, mechanical engineers could create accurate sizing of heating systems which could lead to upfront cost savings, more efficient systems, and operational cost savings.

This paper will examine the potential interaction between post-construction air leakage testing and pre-construction mechanical design parameters. A comparison of various common guidelines for mechanical engineer’s infiltration rates will be compared to leakage rates prescribed by codes, protocols, and expected leakages for buildings where attention is paid to continuous air barrier design and execution. Several case studies will be discussed to illustrate current practices. Opportunities for improvement in current practices will be explored.