Air Barrier Testing: Bigger is Better

building image In the world of whole building air barrier testing, bigger is in fact better.  Larger buildings tend to perform much better, with significantly less air leakage measured per square foot of building envelope, when tested.  Why is this … more money for construction materials, more experienced personnel, a better quality control program?  While these are all possible and reasonable answers, the answer is actually much simpler.  Bigger buildings test better due to a combination of their overall size and geometry.  Let’s analyze why this is.

Naturally, any simple analysis must be underscored by a key assumption.  In this case, we assume that air leakage primarily occurs at critical interfaces (floor-to-wall, wall-to-roof, etc.) within the building envelope.  While uncontrolled air leakage could occur within the field of a roof, wall or floor, this “field leakage” is generally negligible compared to the air leakage at interfaces, and can be ignored.  While this may seem like a rather bold assumption, in the 150-plus whole building air barrier tests we have performed over the past several years, the most common and significant areas of air leakage for a building of any size generally occurs at the building interfaces.

Another key point to keep in mind is that larger buildings typically exhibit greater air leakage values while still meeting established allowable leakage rates.  Allowable leakage rates are usually a function of the building size itself  – the United States Army Corps of Engineers (USACE) air barrier testing protocol, for example, states that the allowable leakage rate should not exceed 0.25 CFM/ft2 of building envelope.  For example, a building with a 100,000 ft2 building envelope can have 25,000 CFM of air leakage and satisfy the USACE requirement, while a building with a 10,000 ft2 building envelope can only have 2,500 CFM of air leakage and satisfy the requirement.

As you may have already guessed, our assumption and key point described above work against each other.  A bigger building has more interface length and is therefore potentially leakier, but the same bigger building also has a larger building envelope and thus is allowed to “leak more”.  While we will not get bogged down in the mathematical relationship between building envelope area and interface length, we easily recognize that building envelope area increases more rapidly than interface length as building size increases.  And since envelope increases more rapidly, the “leak more” effect outpaces the potential leakage, which results in better results for bigger buildings.

         

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