When More Ventilation Isn’t Always Better: The Emerging Case for Outside Air Reduction

Learn why outside air reduction is reshaping building operations. Balance indoor air quality, energy savings, and ASHRAE standards in the post-COVID era.

Ava Montini

Sep 12, 2025

Written by

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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.

Air Filter Media: Choosing the Best for Improved Indoor Air Quality

Jennifer Crowley
Jul 31, 2023
5 min read

Updated: Jul 9, 2024

Close up photo of various filter media types — When choosing air filter media, it’s important to consider factors such as compatibility with your HVAC system, specific air quality concerns, and maintenance requirements.

One way to improve indoor air quality is by using air filters, which trap airborne particles and pollutants. However, not all air filters are created equal. Air filter media, the material used in air filters to capture particles, can vary widely in terms of effectiveness, lifespan, and cost.

This blog will explore the different types of air filter media available, including pleated and non-pleated filters, fibreglass, paper, and carbon filters. We’ll also discuss the efficiency ratings used to measure air filter effectiveness and the benefits of using high-quality air filter media. When choosing air filter media, it’s important to consider factors such as compatibility with your HVAC system, specific air quality concerns, and maintenance requirements.

By understanding the different types of air filter media available and the factors to consider when choosing them, you can make an informed decision and choose the air filter media that best meets your indoor air quality needs.

Types of Air Filter Media

Air filter media is a material used in air filters to trap and remove harmful particles and allergens from the air. The effectiveness of an air filter depends mainly on the type and quality of the filter media used and the type of material being filtered. Some common types of air filter media are:

Pleated Air Filters

Pleated air filters are made from polyester or cotton and have a longer lifespan than non-pleated filters due to their increased surface area. They can effectively trap particles such as pollen, dust mites, and pet dander, making them suitable for residential and commercial settings with high traffic or high air quality needs. Higher MERV-rated filters can capture smaller particles, such as bacteria and smoke. However, they are typically more expensive than non-pleated filters.

Non-Pleated Air Filters

Non-pleated air filters are generally made from materials such as fibreglass or polyester and have a shorter lifespan than pleated air filters. However, they are effective at trapping large particles such as dust and debris, making them ideal for residential settings and protecting HVAC equipment from large particles. They are generally less expensive than pleated air filters but offer lower effectiveness and longevity.

Fibreglass Air Filters

Fibreglass air filters are made from spun glass fibres and are the most economical choice. They typically last for about 30 days before requiring replacement and are less effective at trapping small particles. They are most suitable for residential settings with moderate air quality needs.

Paper Air Filters

Paper air filters are made from pleated paper material and are effective at trapping larger particles, such as dust and pollen, but less effective at capturing smaller particles, such as bacteria and smoke. They typically last for about three months before requiring replacement and are suitable for residential settings with moderate air quality needs. They are generally less expensive than pleated air filters due to their lower MERV rating.

Carbon Air Filters

Carbon air filters are made with activated carbon and are most effective at removing odours and certain chemicals from the air. They typically last for about three months before requiring replacement and are suitable for commercial settings, such as hospitals and laboratories, where odour and chemical removal is a priority. However, they are generally more expensive than other air filter media due to their specialized use and effectiveness.

Efficiency Ratings for Air Filter Media

When choosing an air filter media, it’s important to consider its efficiency rating. The most common efficiency rating systems used are:

MERV (Minimum Efficiency Reporting Value)
CADR (Clean Air Delivery Rate)

MERV Rating

The efficiency rating refers to the ability of an air filter to trap and remove airborne particles of different sizes. The most common rating system used for air filters is the Minimum Efficiency Reporting Value (MERV), which rates filters on a scale of 1-20 based on their ability to remove particles from the air. The higher the MERV rating, the more effective the filter removes smaller particles.

It’s important to note that while higher MERV-rated filters are more effective at removing smaller particles, they may also restrict airflow more than lower-rated filters. This can cause issues with HVAC system performance and increased energy usage. For this reason, it’s important to consult with an HVAC professional to determine the most appropriate MERV rating for your indoor air quality needs and to ensure that your system can handle the increased airflow resistance that comes with higher MERV-rated filters.

CADR

The CADR (Clean Air Delivery Rate) is a rating system that measures the amount of clean air delivered by an air purifier or air filter. It is a standardized rating system recognized by the American Home Appliance Manufacturers (AHAM) and is used to compare the performance of air purifiers and air filters from different manufacturers. The higher the CADR rating, the more effective the filter will remove particles from the air.

The CADR rating is calculated based on three categories of particulate matter: smoke, pollen, and dust. Each category is assigned a numerical value, and the overall CADR rating is the sum of the three values.

Factors to Consider When Choosing the Best Air Filter Media

HVAC Compatibility

The first factor to consider when choosing air filter media is compatibility with your HVAC system. Different types of air filters may be designed for specific types of HVAC systems, so it’s important to choose a filter that is compatible with your system. Some filters may also require modifications to your HVAC system, so it’s essential to consult with a professional before making any changes.

Air Quality Concerns

Another factor to consider when choosing air filter media is any specific air quality concerns you may have. For example, if you live in an area with high levels of outdoor pollution, choose an air filter designed to capture smaller particles, such as those found in smoke and exhaust fumes. On the other hand, if you have pets or allergies, you may want to choose an air filter designed to capture pet dander and other allergens.

Maintenance Requirements

Finally, it’s important to consider the maintenance requirements of different types of air filter media. Some filters may need to be replaced more frequently than others, and some may require more intensive cleaning or maintenance. It’s important to choose a filter that you can realistically maintain, as neglecting filter maintenance can lead to reduced effectiveness and increased wear and tear on your HVAC system.

Benefits of High-Quality Air Filter Media

Using high-quality air filter media offers numerous benefits for indoor air quality and overall health. One of the primary benefits is that these filters are more effective at removing airborne particles, such as dust, allergens, and pollutants, from the air. This can significantly reduce indoor air pollution, which is especially important for individuals with allergies or respiratory conditions.

Improved indoor air quality can also have a positive impact on productivity. Studies have shown that indoor air quality can have a significant effect on cognitive function, with poor air quality leading to decreased productivity, reduced concentration, and increased absenteeism. By removing airborne particles and pollutants from the air, high-quality air filter media can help to create a healthier indoor environment that promotes better cognitive function and increased productivity. This is especially important in workspaces, where employees may spend a significant portion of their day indoors.

In addition to health benefits, high-quality air filter media can also be more cost-effective in the long run. While they may initially cost more, their increased efficiency and longer lifespan can result in lower maintenance costs and energy bills over time. Additionally, by improving indoor air quality and reducing the need for frequent cleaning and maintenance, these filters can save homeowners both time and money.

Choosing the right air filter media is crucial for improving indoor air quality. We hope this comprehensive guide has helped you understand the different types of air filter media available, their efficiency ratings, and what to consider when choosing one for your home. Remember, always choose high-quality air filter media for the best results.

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