How does the tightness of my building envelope, or infiltration rate, affect building performance?
The "tightness" of a building envelope refers to its ability to control air leakage into and out of the building. A tight building is one in which air leakage, or infiltration, rates are low. Low infiltration rates support better regulated temperatures (i.e. fewer uncomfortable drafts and less heat loss) and, as a result, often support smaller heating and cooling systems. From a results standpoint, this translates into lowered energy use and lowered expense to condition the building, and potentially lowered capital costs (with regard to heating and cooling equipment).
Note that a significant challenge building projects face is implementing the infiltration rate design values. One reason that energy models are poor predictors of energy use is that simulations assume perfect construction. Early performance assessments that suggest infiltration rates are significant performance drivers for your project should alert the team to the importance of sound detailing, sound specification and diligent CA oversight.
Consider reviewing the typical values guide here to help inform your early design studies.