HVAC School - For Techs, By Techs
The House Always Wins! A Class On Building Science Basics - Short #294
· Summarized
Key takeaways
- The house should be viewed as a system where every modification impacts the overall performance and safety.
- Proper alignment and continuity of thermal and pressure boundaries are essential for energy efficiency, comfort, and moisture management.
- Many indoor air quality and comfort issues originate from building envelope flaws rather than HVAC equipment faults.
- Understanding how heat and air move through a house helps diagnose problems related to moisture, comfort, and durability.
- HVAC contractors should recognize their limits, identify signs of house-related issues, and collaborate with specialists when necessary.
Summary
Building Science Fundamentals and House as a System
Tessa Murray discusses the importance of understanding building science when assessing HVAC performance in residential homes. She emphasizes the house as a system, where small decisions over time can bring a house close to failure or 'the cliff'—a point of critical failure involving safety, mold, or durability issues. She highlights how many indoor comfort or air quality problems are rooted in the house's design, installation, or modifications rather than solely the HVAC system. Understanding the house's system helps in reducing liabilities and making HVAC jobs more effective.
Pressure and Thermal Boundaries in the Building Envelope
The episode stresses the importance of properly aligned and continuous thermal and pressure boundaries in the building envelope. Problems such as comfort issues, moisture, and mold are often caused by gaps, leaks, or improper insulation that disrupt these boundaries. Examples include attic bypasses, ductwork outside the thermal boundary, and unsealed penetrations like can lights or plumbing. Correct boundary management is crucial for energy efficiency, comfort, and indoor air quality.
Moisture Management and Condensation Risks
Tessa explains how moisture issues arise from air leakage and temperature differentials. She shows how improper sealing, insulation, or thermal boundary issues can lead to condensation, ice dams in winter, or mold growth. Both winter and summer conditions present unique challenges—cold dry outdoor air can dry out the house in winter, while hot humid air in summer can cause moisture buildup if vapor barriers are misplaced. Proper detailing and sealing of boundary penetrations, as well as understanding climate-specific needs, are essential for moisture control.
Building Envelope Challenges and Common Failures
Various building envelope problems are discussed, including houses with no insulation, spaces with non-continuous insulation, and ductwork located outside conditioned space. The episode illustrates how these issues cause comfort problems, energy inefficiency, and moisture concerns. Examples include houses with clay tile walls, vaulted ceilings with improper insulation, and unsealed attic penetrations that lead to moisture and ice dam issues. Proper design, construction, and retrofitting practices are necessary to achieve effective thermal and pressure boundaries.
Safety Considerations and Multi-Disciplinary Approach
Tessa emphasizes that HVAC contractors should be aware of potential safety issues like asbestos in vermiculite or orphaned water heaters that can backdraft. Recognizing when an issue is beyond HVAC and requires specialists is crucial. The episode advocates for a team approach, where understanding and respecting building science helps reduce liabilities, improve indoor conditions, and ensure occupant safety.
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Show notes
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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