Building Air Leakage and Air

I just love this subject. Do you know whether your building is sucking or blowing? Did you know that even a new building, built to the usual building methods, will leak air? In a new project, the key is to make allowance for that, and design, build and validate accordingly. On an existing building, it is important to determine how much and where your building blows or sucks, so you can take action, if need be. Windows, doors, and construction joints all have a normal tendency to leak air or allow infiltration. Varying pressures within the building and across each envelope, causes air to move through these openings.

“Blower door” tests are a key tool for determining how much leakage occurs and in which direction, in (negative pressure) or out (positive pressure).

The major issues surrounding air leakage are indoor air quality, compromised comfort due to drafts or uneven temperatures, and energy usage. A good HVAC designer will account for all of these, and assure that undesirable effects, such as sucking cold or hot-humid in through the building joints is prevented. Likewise, a good architect/builder will account for the behavior of the envelope assembly, air leakage being but one behavior, in his/her design. (You can’t have a 100% tight envelope in practice, it rather more expensive to achieve 0% infiltration, so we plan for it, and plan to blow rather than suck. This rule does NOT apply to “clean rooms”, of course, where 0% infiltration is mandated.)

Blower door testing is a key component of new building commissioning or re-commissioning an existing building. If you know where all the intakes, exhausts, and door openings are, and their size and type, and then the remainder (theoretically) is the “tightness of the building’s construction”.

So building HVAC systems are commonly designed to create a slight positive pressure (blowing) at all times, to CONTROL infiltration. Design should always include some common sense and experience. ANSI/ASHRAE 62-2004, Ventilation for Acceptable Indoor Air Quality (IAQ), is the most commonly adopted code the prescribes indoor air quality and ventilation rates. The input of the ventilation code rules, combined with a input of air leakage rates, will guide the decisions on course of action.

Common reasons for buildings that suck and are uncomfortable include

• obsolete HVAC controls,
• building usage changes over time,
• Increasing complexity of tenant build-out without corresponding update to HVAC
• Increasing demand and complexity of schedules not adaptable to existing building
• Increasing technological demand ( ie growing data communication rooms) not well integrated with the buildings older existing controls.

Solutions to these problems include

• Periodic recommissioning of the HVAC systems and controls
• Upgrading the HVAC controls
• Sometimes existing building systems are just waiting to be replaced with simpler systems.

Many of us experienced energy modelers have been reliant on spreadsheets and/or dedicated heating and cooling calculation programs from the beginnings of our careers. But these methods are labor intensive and lately, we look more and more to BIM to help accelerate its use, or sometimes we are being required.

TECHNICAL POINT: How to model the air leakage in a building information model has been problematic up until very recently , I think. The recent releases last spring of new Revit add-ons (see the BIMOLOGY link to the right to read about ECOTECT ) give us some hope. I think that they are finally getting the details into the model. I could be wrong. But ideally it would be nice to specify and analyze air leakage across geometric boundaries on the fly.

In the end, its better to have a building envelope that blows a little, than a building that sucks at all.

ASHRAE 62-2001 synopsis on Internet(out-of-date in the exact details, so be careful)