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Balancing Comfort and Control: The Critical Role of Building Pressurization
When we talk about building performance, we often focus on insulation, HVAC system efficiency, and ventilation rates. But one subtle factor — building pressurization — can make a dramatic difference in comfort, energy use, and indoor air quality.
Put simply, building pressurization refers to the balance between the air coming into a building and the air being exhausted. A properly pressurized building (typically slightly positive) keeps unwanted outdoor air, odors, and contaminants from seeping in through cracks, doors, and windows.
Why Pressure Matters
If a building is negatively pressurized, it means air is being pulled in from outside — often unfiltered, cold, or humid. This leads to:
- Drafts near doors and windows
- Cold spots and discomfort
- Excess dust, pollen, or odors
- Higher heating and cooling loads
- Potential moisture intrusion, including roof leaks in some cases
Negative pressurization doesn’t just affect comfort. It can compromise the building envelope. According to industry sources, excessive negative pressure can actually draw in rainwater and vapor through gaps in the structure — including the roof. This increases the risk of moisture damage, mold, and building degradation.
“Maintaining proper pressurization is essential to prevent unwanted air and vapor movement through the building envelope. Even small pressure differences can cause large airflow volumes, leading to infiltration and moisture issues.”
— FSB Engineers’ Guide to Accurate, Precise Building Pressurization - Source
A related article by Consulting-Specifying Engineer identifies three primary drivers of building infiltration:
- Stack Effect – Temperature differences drive vertical airflow through stairwells and shafts
- Wind Pressure – External wind pressures cause uncontrolled infiltration at building surfaces
- Mechanical Pressurization – An imbalance in supply and exhaust airflow can unintentionally depressurize the building
The article also stresses the value of compartmentalization (like sealing vertical shafts and stairwells) and air balance verification to maintain appropriate pressure levels throughout the building envelope. These are all factors that can be investigated by taking regular pressure readings across different times of day and in different building zones. This approach helps identify whether issues are persistent or conditional, and whether they align with known changes in occupancy or weather.
Additionally, ASHRAE Addendum f to Guideline 36-2018 addresses how return air tracking strategies affect building pressure. It specifies a differential airflow setpoint (S-R-DIFF) between supply and return fans, adjusted based on occupancy. This ensures pressure control is dynamically responsive to building load. The guideline also recommends methods for decoupling fans during unoccupied modes to avoid unintended excess outdoor air intake — an area where monitoring and recording pressure over time can validate proper behavior and expose faults.
“The airflow differential between supply air and return air fans [should be] required to maintain building pressure at desired pressure (e.g., 12 Pa [0.05 in. of water]) using a handheld sensor if a permanent sensor is not provided.”
ASHRAE Addendum f to Guideline 36-2018
A Real-World Example: University Building in Vancouver
Years ago, I was asked to troubleshoot a four-story university building in Vancouver, BC. Nearly every perimeter office had a plug-in heater beside the window. Many air diffusers were taped over or stuffed with cloth to block what occupants felt were cold, unpleasant drafts.
We conducted a pressure investigation and found that the building was strongly negatively pressurized. The cause? Insufficient outside air and possibly excessive exhaust. Once we adjusted the outside air control settings and increased fan volume, we were able to shift the building into a slightly positive pressure — and the comfort complaints disappeared. That small change in pressure made a big impact.
How Much Pressure Is Enough?
Believe it or not, even a small pressure differential — on the order of 0.02 to 0.05 inches of water column (5 to 12.5 Pa) — is enough to influence airflow through a building envelope. Maintaining this gentle pressure differential helps ensure that air is moving out of the building, rather than being drawn in through uncontrolled paths.
ASHRAE Standard 62.1 emphasizes the importance of maintaining appropriate pressure relationships between spaces as a way to promote acceptable indoor air quality and prevent the spread of contaminants — a principle that applies equally to whole-building pressurization.
Measuring and Logging Pressure Differentials
Building pressurization is not a fixed condition — it’s highly dynamic. A single, static measurement provides only a snapshot in time and may not reveal systemic issues. Depending on the time of day, weather conditions, internal occupancy, or HVAC operation modes, a building may switch between positive and negative pressure.
To properly evaluate a building’s performance, it’s important to monitor pressure trends over time and ensure the control systems are responsive. Logging differential pressure continuously allows facilities teams to:
- Detect inconsistent pressurization patterns
- Correlate pressure shifts with system schedules or weather changes
- Confirm that controls maintain desired pressurization across varied conditions
Pressure differences this small are impossible to feel and difficult to measure without the right tools. That’s why low-range differential pressure loggers are essential for anyone investigating HVAC performance or building pressurization.
ACR Systems offers a range of tools to support this type of investigation. Two key options are the SmartReader Plus 4 LPD and the ACR Air™ Low Pressure Differential Logger, the latter of which is also available as part of the ACR Ventilation Assessment Kit.
These instruments are designed to:
- Record pressure changes over extended periods
- Detect subtle pressure fluctuations as small as 0.001 in. WC (0.25 Pa)
- Provide long-term profiles of pressurization trends across multiple areas in a building
- Verify pressurization stability under various occupancy and HVAC conditions
- Identify ventilation imbalances between zones
Small Pressure, Big Impact
In today’s world of energy optimization and IAQ concerns, building pressurization is an often-overlooked metric that can reveal a lot with just a little data. Whether you're commissioning a new system, tuning an old one, or just want to verify performance, consider starting with pressure.
👉 Learn more about our ACR Air™ Low Pressure Differential Logger and Ventilation Assessment Kit
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