Building Performance – Lesson 5

Building Performance – Lesson 5

5. Air behaves like a fluid.

Air and moisture move the same as any fluid by pressure. When we design pipes to carry water we are very careful to make those pipes water tight. Water pipes are pressure tested with air to detect any potential leaks. If pipework reticulation is found to be air tight, it will certainly be water tight.

As mentioned in the previous example, air will leak through holes that are too small for water to pass through. The problem is that we don’t manage air like we do water, and we are pushing air through ducts that even water would leak out of. We should be using the same principles of fluid movement for air that we do for water.

Most often, central heating /cooling ductwork is installed in the attic space or under the floor. Poorly installed supply ductwork leaks air into these spaces. Poorly installed return ductwork draws air from these spaces.

In winter, leaky heating ducts in the attic space can expel warm, moisture laden air which will rise and condense on the underside of the roof or on structural members. In turn this can cause structural decay, mould growth, and unhealthy air. This unhealthy air can then be directly injected back into the house via the leaky ductwork.

By simply sealing the ductwork, we will improve occupant comfort, improve heating/cooling system performance, and reduce the potential for building failures. It’s just as important to have leak free ducts, as it is to have leak free plumbing.


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