Posted by: lcowie | November 16, 2010

Pressure vs. Thermal Boundary

One of the most important concepts for you to understand when you are studying for your BPI Building Analyst exam is air movement and the factors that enable movement. For one thing, air movement requires a pressure differential. This means that the pressure boundary and thermal boundary are not aligned.

The pressure boundary is also called the air barrier. This is the barrier that prevents infiltration of outdoor air into the conditioned space and exfiltration of indoor air to the outside. For BPI purposes, exfiltration is a word! Infiltration is the uncontrolled intake of air, and exfiltration is the uncontrolled movement of air that leaves the home. Both infiltration and exfiltration are bad. That means there is a gap in your home that is enabling this uncontrolled air to enter or leave your home. The pressure boundary blocks air movement by convection.

The thermal boundary is the part of the home that physically separates the conditioned spaces from the unconditioned spaces. You may think of the thermal boundary in terms of insulation. The thermal boundary blocks air movement by conduction.

As you may have noticed, both boundaries block air movement, but through different means. Just because the pressure boundary blocks air movement through convection doesn’t mean it will also stop air movement through conduction! Air will find a way to bypass the boundary that is in place, using a different mode of heat transfer (convection, conduction, or radiation).

That being said, the pressure boundary must be continuous and aligned with the thermal boundary. If both boundaries are continuous and aligned (no gaps), air will be less likely to move. The ideal house is 100% airtight, which means no infiltration and exfiltration.

Be sure you know the difference between these two boundaries because they will play a larger role in the BPI Envelope Professional training course!

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