Building Science 101 for HVAC Contractors w/ Bill Spohn and Joe Medosch
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Some critical design points to consider for comfort, health, durability, and energy efficiency can be remembered by ABCDE: airflow, barriers, control, dampness, and exchange. Airflow refers to the slowing or stopping of airflow into or out of a building. Barriers prevent the flow of air, vapor, or heat. Control over our equipment requires us to make sure the equipment has accurate sensing capabilities. “Damp” refers to keeping a building dry. Exchange requires us to exchange stale, wet air for fresh air that dilutes pollutants. To ensure that the building and HVAC design touches on all five areas, we need to use data to back up our decisions and consider upgrades as permanent improvements.
The building blocks of building science can be summed up with the acronym “HAM,” which refers to heat, air, and moisture. We are trying to control all three of those to give customers the best in terms of health, comfort, durability, and energy efficiency. To control those, we need to look at the building enclosure (envelope), pressures, and what’s contained within a building. Everything in a house is connected, and we can rely on measurement and detection to help us identify problem areas (such as poor drainage) that affect the whole building.
Controlling the average home is a challenge due to all of the holes in the structure, including windows, doors, and openings for piping and wiring. The air that comes in sticks to everything inside, and low-quality air can significantly impact occupants’ health and comfort if they spend 50-75% of their time inside their homes. Fresh air isn’t always “fresh” either, especially if neighbors are burning leaves, if you live in a smoggy area, or if the pollen count is high. The bottom line is that air leakage is a contaminant pathway (ESPECIALLY in vented attics), and we need to control it. Mechanical ventilation (like range hoods, dryer vents, and bath fans) can actually exacerbate air leakage.
Indoor air can be even worse. Carbon dioxide, skin cells, pet dander, and cooking and cleaning chemicals are also pretty bad for indoor air quality. Filtration can help a lot, though.
Building science fundamentals overlap with physics fundamentals; for example, one of the core principles is that heat moves from an area of higher temperature to lower temperature. If air can get in during the winter, then heat can escape your house during the winter. Air pressure is also a major player, especially when you think about the stack effect, the neutral pressure plane, and wind.
In theory, insulation isn’t an air barrier, but it’s a bit more complex in reality. Insulation may or may not act as an air barrier; it must be up against the air barrier for it to be effective against air leakage. For the insulation to be effective, air sealing must be done BEFORE adding insulation.
Air leakage can also accompany vapor diffusion through permeable materials. Moisture can also find its way into a home, especially as it moves from an area with a higher concentration of moisture to one of lower moisture inside the home. Moisture that gets through to the roof deck can rot it out over several years.
Duct leakage is another significant driver of infiltration, especially because it affects the pressures in the house. Supply leakage causes air to be drawn into the home through the gaps and cracks in a home. Return leakage causes conditioned air to escape the home through gaps and cracks in the home.
We can get an idea of the air leakage in a building by doing a blower door test. A blower door test requires you to replace a door with a blower door, and it usually depressurizes the house; you can watch the pressure with a precision manometer, and the blower door test gives you the data to determine how many air changes happen in a building per hour at 50 Pascals (if you also know the building volume). Knowing the ACH50 allows you to categorize a house as a passive house (0.6 ACH50 or fewer). You can combine a blower door test with infrared imaging to locate major leakage points and seal them with guidance, but you have to be mindful of solar gains. Pressure pans also allow you to record pressure data.
In any case, a properly sealed house is more comfortable, healthier, and more energy-efficient than a home with uncontrolled air leakage. An airtight house can benefit from well-thought balanced ventilation (NOT a mere bath fan).