When More Ventilation Isn’t Always Better: The Emerging Case for Outside Air Reduction
- Ava Montini
- Sep 12, 2025
- 6 min read
At the height of the COVID-19 pandemic, building operators were given one clear directive: get as much fresh outside air into the building as possible.
The reasoning was simple and sound: diluting indoor air with outside air reduced the concentration of airborne viruses and gave occupants a greater sense of safety. Schools cranked open dampers, office towers increased their minimum ventilation rates, and healthcare facilities invested heavily in boosting air exchanges.
That strategy worked in an emergency, but it also came at a cost. Energy bills spiked as HVAC systems struggled to heat and cool the constant flow of unconditioned outside air. Humidity control became more difficult. Comfort complaints rose. And in some regions, the “fresh air” being drawn inside was anything but fresh. Things like wildfire smoke, traffic emissions, and industrial pollutants all found their way indoors.
Fast forward to today, and the conversation has shifted. ASHRAE and other standard-setting bodies have recognized that the blanket approach of maximum ventilation isn’t sustainable as a long-term practice.
As we’ve moved past the emergency phase, a more nuanced picture is emerging. Outside air confers benefits (especially in terms of health), but it also imposes costs: energy, comfort, mechanical wear, sometimes even polluted air if your outdoor environment isn’t clean. ASHRAE, energy codes, and HVAC practice are now pushing toward finding balance. One big part of that shift is outside air reduction (or controlling outside air to what’s necessary, rather than “as much as possible”).
Why Reduce Outside Air? What Are the Trade-Offs
To see why reducing outside air is resurfacing, it's helpful to walk through what the costs are and what the benefits might be of dialling things back.
The Costs of Too Much Outside Air
Energy Use
Heating and cooling costs skyrocket when you have to condition large volumes of outdoor air, especially in extreme climates. In summer, bringing in hot, humid air means your cooling system works harder; in winter, cold air needs heating.
Beyond simply heating/cooling, there’s also fan energy. More outside air often means more airflow through dampers, larger pressure differentials, etc.
Visual Comfort / Thermal Discomfort
Cold drafts in winter; humid, sweaty feelings in summer if moist outdoor air isn’t adequately dehumidified.
Inconsistent thermal zones due to mixing outside air with return or recirculated air.
Mechanical Wear & Maintenance
Outside air includes particulates, pollutants, and moisture. Therefore filters, coils, ducts, and dampers need more maintenance.
When outside air brings in pollutants or high humidity, it can cause corrosion, mold, or damage to finish materials.
Indoor Air Quality Considerations
Ironically, bringing in outside air isn’t always “cleaner”; if outdoor air is polluted (e.g. wildfire smoke, high PM2.5, industrial pollution), ventilation could degrade indoor air quality.
The Benefits of Reducing Outside Air (When Done Right)
Energy Savings
Reduced heating/cooling loads → lower utility bills.
In some ASHRAE Standard 90.1 addenda / code changes, reducing outdoor air intake is explicitly a path toward improved energy efficiency. For example, changes made in standard 90.1-2019 (and later) allow reduced outside air intake in central systems and reduced minimum flows in VAV (variable air volume) boxes. Energy Codes
Buildings with moderated outside air approaches (versus maximum outside air strategy) can often hit much better energy performance, especially in climates with extreme temperatures.
Comfort and Building Stability
More stable indoor temperatures, less risk of humidity spikes or condensation issues.
Better ability to maintain indoor comfort metrics, which improves occupant satisfaction.
Cost Predictability & Maintenance Savings
Less strain on HVAC equipment.
Lower maintenance cost due to fewer introduced contaminants, less filter load, etc.
Health / IAQ Still Possible
By using strategies such as proper filtration (appropriately rated filters), UVGI, good air distribution, and periodic flushing, you can maintain healthy indoor air even with more controlled outside air.
ASHRAE guidance, post-COVID, suggests that ventilation + filtration + other engineering controls together are the path—not merely “open all dampers.” ASHRAE
How ASHRAE & Codes Are Shifting
The push to balance ventilation, energy and comfort is finding formal expression in updated standards and codes. Some key threads:
ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) has been the go-to for minimum ventilation. But recent addenda adjust how outside air rates are calculated, especially in Variable Air Volume (VAV) systems, enabling more dynamic or performance-based approaches. Energy Codes
ASHRAE Standard 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) is increasingly recognizing that “more outside air” is not always the optimal path for energy efficiency. The 2019 to 2022 versions include addenda that allow for reduced outdoor air intake in some scenarios and model outside air intake more precisely. Energy Codes
Post-COVID Guidance from the Epidemic Task Force and other committees acknowledges that increased ventilation is helpful for infectious disease mitigation—but also warns about the cost, feasibility, and trade-offs. ASHRAE’s filtration & disinfection guidance, for instance, emphasizes that filters should be sealed well, systems should be maintained, and energy impacts considered. ASHRAE
There is growing interest in “ventilation efficiency” (i.e. how well the outdoor air being brought in actually participates in diluting contaminant levels) vs simply “bringing in more air.” That opens doors for smarter design: placement of supply/exhaust, air distribution patterns, possibly recirculation with clean filtration, and technology like UVGI in ducts. arXiv
What Building Owners / Managers Should Do
If you’re in charge of managing indoor air quality, HVAC systems, or the budget, here are some practical steps, questions, and strategies to move toward smart outside air reduction without compromising health or compliance.
Step | What to Do | Key Questions & Considerations |
1. Audit your current system | Measure how much outside air is being brought in currently. Identify how often dampers are fully open, what settings for minimum outside air are. Document past energy bills, thermal comfort complaints. | Do you really need to run at 100 % outdoor air all the time? What’s the outside-air fraction during non-peak periods? How often are you using demand-controlled ventilation? |
2. Model / simulate | Use energy modelling (or vendor/engineering consultants) to simulate what energy & comfort impact you’d see from reducing outside air to code minimum vs current levels vs maximum “pandemic level.” Include local climate, outdoor pollutant levels. | What’s your climate? How extreme are winters / summers? What are outdoor pollution or humidity challenges? Can your HVAC system handle variable loads well? |
3. Filter & clean | If you reduce outside air, you’re inherently relying more on “recirculation / indoor air cleaning” to maintain IAQ. Ensure your filters are appropriate efficiency, well sealed, replaced regularly. Consider supplementary measures (UV, air cleaners, HEPA, etc.). | What is the MERV rating you’re using? Can your fan/coil handle higher efficiencies without losing capacity? How about maintenance cycles? |
4. Design flexibility & control | Make systems adjustable—both in terms of outdoor air intake (dampers, controls) and monitoring (CO₂, PM2.5, VOCs). This allows ramping up when needed, and reducing when risk is low or when conditions are unfavorable. | Do you have sensors to detect indoor air quality? Do your controls allow override or programmed changes? Are occupants/management aware and aligned with policy? |
5. Engage stakeholders | Staff, occupants, board members often worry that reducing outside air means compromising health. Transparency helps: show them energy/comfort data, IAQ readings, trade-offs. Sometimes policies (e.g. open windows during good outdoor air, closed when it’s bad) help. | What are occupant expectations? Do you have health policies in place? Who signs off on trade-offs (e.g. budget vs comfort)? |
6. Monitor & adjust | After changes, monitor indoor environment (temperature, humidity, CO₂, pollutant levels), energy, comfort complaints. Be ready to adjust. Outside air isn’t a static setting; it’s dynamic. | How often will you review? What thresholds trigger change? For example: high CO₂ or PM2.5, or outdoor air pollution alerts, might warrant reducing outside air. |
What This Means for Policy, Standards, & the Future
Energy codes & carbon targets
As jurisdictions push toward net zero or carbon reduction, the HVAC energy penalty of over-ventilating becomes a liability. Efficient outdoor air management helps reduce energy use, which helps reduce emissions. ASHRAE 90.1’s newer addenda are already projecting energy savings from smarter outside air settings. Energy Codes
Health & resilience
Pandemics have taught us that buildings need flexibility—not fixed, extreme settings. Systems that can adapt: e.g., crank up ventilation when risk is high, pull back otherwise—are more resilient. Outdoor air reduction is part of enabling that flexibility.
Indoor air quality (IAQ) & occupant wellness
People increasingly expect buildings (schools, offices, public spaces) to deliver both clean air and comfort without extreme energy waste. Outside air reduction done thoughtfully helps spread the benefits: lower energy bills, better comfort, less waste.
Cost pressures
Energy costs are volatile. Running massive outside air loads just to “play it safe” all the time may no longer be financially justified, especially in regions with high energy costs or challenging climates.
Getting Outside Air Right, Not Just More
After so many years where the message was “more outside air, more safety,” we’re entering a more mature phase—one where how outside air is managed, rather than just how much, becomes the critical question.
Reducing outside air (when it can be done safely) doesn’t mean lowering standards or compromising on health. It means using all the tools: ventilation, filtration, controls and monitoring, to deliver indoor air quality that is healthy, comfortable, sustainable and cost-effective.
If you’re managing buildings, this is the moment to rethink your default settings. Push for audits, invest in systems and sensors, communicate clearly with occupants. Because the buildings that get this right will be healthier, more resilient, and much more efficient in the long run.
