The House Always Wins! A Class On Building Science Basics – Short #294

In this short episode from the Bry-X stage of the 7th Annual HVACR Training Symposium, Tessa Murry gives a class on building science basics: The House Always Wins! Tessa is a building scientist who works with TEC.

Tessa’s class is about how the house puts HVAC contractors in difficult situations. When people have indoor air quality or comfort concerns, the HVAC often takes the blame, even if the issue is with the house. There are usually several little home improvement decisions that create unintended consequences and add up. In many cases, when there aren’t exhaust fans in kitchens and bathrooms, moisture stays inside and can cause indoor condensation. Air-sealing, adding or removing insulation, replacing windows, replacing furnaces with a different efficiency model, and even moving people in can all affect comfort and air quality.

Comfort issues and complaints, such as hot and cold spots, often point to issues with the house, not necessarily just the HVAC system. Pressure boundaries and thermal boundaries in the building envelope need to be aligned, continuous, and consistent for HVAC systems to do their job well, but many houses don’t have that. Those boundaries need to be clear between attached spaces like attics or garages. Those spaces create problems with energy efficiency and comfort, and humidity is a problem in some climates. Garages also have fumes we want to keep out of the house. 

Heat moves from hot to cold, and air moves when there is a pressure differential. Mechanical equipment and wind can drive pressure differentials. If there is a pressure difference and a hole, there will be air movement. In the winter, cold air sinks and displaces warm air, which rises and creates positive pressure at the top of the house. That air will push through gaps around can lights, vents, and more. When that humid air gets into a cold attic, the moisture will condense on the roof decking surface and cause an ice dam to form. In the summer, hot, humid air comes into the structure. Regardless of the house’s issues with air movement, it’s on HVAC contractors to make the decisions that put the occupant’s health and safety first, including calling the contractors with the knowledge to diagnose the house’s problem.

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