Austin Riesenberger joined us recently on our weekly free CEU webinar series to talk about Ventilation Without Renovation: Solving High CO2 and Poor Air Exchange in Existing Homes.

Review a summary of attendee feedback below, watch the recording, and/or read an article on the topic below.

Webinar attendees learned extensively about indoor air quality and the importance of proper ventilation in homes. Many were surprised to discover how high CO2 and VOC (volatile organic compound) levels can become in bedrooms, especially at night, and how indoor air can be significantly worse than outdoor air. The session highlighted that code requirements for ventilation can vary by jurisdiction and emphasized the value of actively monitoring indoor air quality using available sensors. Attendees also learned that typical store-bought air purifiers may not be effective against VOCs and that balanced ventilation, such as that provided by ERVs (Energy Recovery Ventilators), is crucial for healthy indoor environments.

Ventilation remains a critical but often overlooked aspect of residential building health and sustainability. As energy codes tighten and buildings become more airtight, achieving optimal indoor air quality in both new and existing homes becomes increasingly challenging. This article explores the evolution of residential ventilation, with a focus on affordable, scalable solutions for existing buildings—particularly the emergence of window-based energy recovery ventilators (ERVs) and smart ventilation technologies. Drawing exclusively from expert discussion and industry experience, the article provides a comprehensive, educational overview suitable for housing professionals and an interested general audience. Key considerations include technical standards, device comparisons, installation challenges, energy and health trade-offs, and the future direction of residential air quality management. A dedicated Q&A section addresses common concerns regarding maintenance, monitoring, filtration, and practical deployment.

Ventilation in the Modern Home: Challenges, Solutions, and the Future of Indoor Air Quality

The importance of ventilation in residential buildings has grown substantially as codes, green certification programs, and occupant health priorities have evolved. Today, ventilation is not only a code requirement in many jurisdictions but is also central to programs such as LEED, Green Star, and others that incentivize better air quality and energy performance. However, the reality for much of the housing stock—especially existing homes—is that effective, efficient ventilation remains a significant challenge. This article examines the underlying science and technology of residential ventilation, the barriers homeowners and housing agencies face, and the innovations that are reshaping what is possible for both professionals and the public.

The Ventilation Pyramid: From Baseline to Best Practice

A useful conceptual model for understanding residential ventilation is the “Ventilation Pyramid,” which mirrors the approach used in home energy efficiency. At its base are local code requirements, which increasingly include minimum ventilation standards. These are often derived from ASHRAE standards, specifically the 62.2 series, which has evolved over the years to define minimum rates for fresh air exchange in residential settings. Depending on region, these requirements may be triggered by state energy rebates or weatherization programs, and are now extending into renovations, not just new constructions.

Above code minimums are the requirements of green building programs, such as LEED for Homes and similar certifications. These typically mandate devices that either continuously or intermittently bring in fresh air or exhaust stale air—most often via bath fans, furnace intakes, or similar systems. While these approaches fulfill foundational standards, they are not always optimized for occupant health, nor do they always consider energy efficiency holistically.

The next tier in the pyramid is “balanced ventilation,” which involves systems that bring in and exhaust air in equal measures. Balanced systems are widely regarded as best for health and comfort, as they offer consistent, controlled air exchange. Verification and testing are critical at this level to ensure that systems are indeed delivering the intended rates of air change. However, balanced ventilation—especially when verified—can be energy-intensive, as the process of exchanging indoor and outdoor air often results in substantial heat loss.

At the pyramid’s apex are energy recovery ventilators (ERVs) and smart ventilation systems. ERVs reclaim thermal energy from outgoing air, using it to temper incoming air, thereby reducing the energy penalty of ventilation. Smart ventilation systems go a step further, using sensors to activate ventilation only when indoor pollutant levels (such as CO2, VOCs, or particulates) exceed healthy thresholds. This approach aligns ventilation rates with real-time need, maximizing both air quality and energy efficiency.

Why Ventilation Matters: Beyond Compliance

While meeting code and certification requirements is essential, the true value of effective ventilation lies in its impact on human health and cognitive function. Elevated indoor CO2 levels have been directly linked to poor sleep, headaches, brain fog, and reduced productivity. In tightly sealed homes, CO2 and other indoor pollutants can quickly accumulate to levels well above outdoor baselines, particularly in bedrooms and other spaces with limited natural air exchange.

Volatile organic compounds (VOCs), radon, particulate matter, and moisture are also common indoor contaminants. While air purifiers can address some particulates and odors, they do not remove CO2 or radon and are not substitutes for actual air exchange. In older homes, natural infiltration once provided a degree of ventilation, but modern weatherization practices have reduced this effect, making mechanical ventilation more critical than ever.

Current Solutions: Windows, Fans, and ERVs

The simplest form of ventilation remains opening windows, which is effective but highly dependent on outdoor conditions. High humidity, extreme temperatures, pollution, allergens, and noise are all factors that can make window ventilation impractical for much of the year. Bath fans and furnace intakes provide a mechanical solution, but these are often “supply only” or “exhaust only,” meaning they can create pressure imbalances and are not always energy efficient.

Central ERV systems are the gold standard in new construction, providing balanced, energy-recovering ventilation throughout the home. However, retrofitting these systems into existing buildings is typically expensive and invasive, requiring ductwork, wall penetrations, and skilled labor. For renters, housing agencies, or owners of smaller homes and apartments, these solutions are often out of reach.

Emerging Technologies: Window ERVs and Smart Ventilation

Recent advances have led to the development of window-based ERVs—compact, ductless devices that can be installed much like a window air conditioner. These units combine the core benefits of ERVs (energy recovery, balanced exchange) with the accessibility and affordability needed for existing residences. They are especially valuable for applications where traditional ducted ERVs are impractical, such as in rental units or older buildings without easy access for renovation.

Window ERVs use heat exchanger cores to transfer thermal energy between outgoing and incoming air streams, minimizing heat loss or gain depending on the season. Some models employ a “regenerator” type core, which can offer higher efficiency in a smaller package. These devices may operate in different modes, including continuous, dual (simultaneous supply and exhaust), and smart modes that adjust operation based on real-time air quality measurements.

Smart ventilation, increasingly integrated into modern devices, uses sensors to monitor CO2, temperature, humidity, and sometimes particulates. These systems can throttle ventilation up or down as needed, preventing both under- and over-ventilation. Automated controls, combined with user override options, allow for both energy savings and occupant empowerment—a critical requirement under many green building standards.

Technical Considerations: Installation, Filtration, and Controls

Installation of window ERVs typically involves a foam block and bracket system to ensure a tight, insulated fit within a variety of window types—most commonly double-hung or sliding windows. While some customization is possible, compatibility with casement windows remains a limitation for many devices on the market. Security concerns are addressed through the use of window locks and careful installation to prevent tampering or forced entry, especially on ground floors.

Filtration is another key aspect of modern ventilation devices. MERV 13 filters with embedded activated carbon are increasingly standard, providing protection against particulate pollution, smoke, and odors. Maintenance requirements are minimal, usually limited to filter replacement every six months and occasional cleaning of pre-filters or cores. Some devices include washable components, though care must be taken not to damage sensitive coatings or electronics.

Controls on smart ERVs often include manual settings, auto modes, and the ability to integrate with mobile apps for data tracking and remote management. Some systems can connect to local weather and air quality APIs, providing additional context for operation. Importantly, user control is mandated by many building standards, ensuring occupants can override automated operation as needed.

Performance and Validation: Measuring Success

Verification of ventilation system performance is a critical step in both compliance and real-world efficacy. In the field, airflow can be measured using anemometers and hoods to confirm that devices are delivering the design-specified cubic feet per minute (CFM). However, real-world factors such as stack effect, wind, and building pressure differences can impact performance, making ongoing monitoring valuable.

Increasingly, experts argue for a shift from one-time airflow testing to continuous air quality monitoring—using CO2 levels as a direct proxy for effective ventilation. This approach recognizes that prescribed CFM rates may under- or overestimate actual needs due to occupancy patterns, building characteristics, and environmental conditions. Alerts and notifications can be integrated into smart systems to inform users if air quality falls outside acceptable ranges.

Cost and Accessibility: Making Ventilation Work for All

One of the most significant barriers to improved residential ventilation is cost—both for equipment and installation. Central ERVs and ducted systems can easily run into thousands of dollars, with installation often costing more than the device itself. Wall-mounted ERVs, while less expensive, still require specialized tools and expertise. In contrast, window ERVs offer a dramatically lower installed cost. Most can be installed by a homeowner or with minimal professional assistance, with contractors typically charging $200–$500 per unit for installation if needed. This accessibility opens the door for widespread adoption in both single-family and multifamily settings, including rentals and affordable housing.

The Role of User Experience and Future Directions

As residential ventilation technology matures, user experience is coming to the forefront. Occupants must be able to control, monitor, and maintain their systems easily. Devices that are difficult to access, require specialized knowledge, or limit user intervention risk non-compliance and poor long-term outcomes. Modern ERVs address this by providing accessible controls, modular components, and clear maintenance instructions.

Looking ahead, further integration of predictive controls and AI-driven optimization is likely. However, user override and manual control will remain essential, both for compliance and occupant trust. Additionally, the field is moving toward “permanent commissioning,” where continuous monitoring and alerting ensure that ventilation remains effective throughout a building’s lifecycle.

Question and Answer Discussion

Q: Can window ERVs be installed in homes with casement windows or other non-standard window types? A: Most current window ERV models are designed for double-hung or sliding windows. Compatibility with casement windows is limited, though future models and adapters may expand options. For now, custom installation is possible but not generally recommended unless undertaken by a skilled professional.

Q: What is the best location in a home for installing a window ERV? A: The ideal placement is in the room where the occupant spends the most time and where air quality is poorest—typically bedrooms or home offices. Bathrooms and kitchens, while important for moisture and odor control, are not prime candidates for these devices.

Q: Are window ERVs compatible with horizontal sliding windows? A: Yes, most modern window ERVs are fully compatible with horizontal sliding windows as well as vertically sliding double-hung and single-hung windows.

Q: How is security addressed when installing a window ERV? A: Security can be maintained using specialized window locks that prevent the window from opening further once the ERV is installed. These locks are available for different window types and can be matched to the installation for added safety.

Q: What are the filtration options, and how often should filters be replaced? A: Most window ERVs use a MERV 13 filter with activated carbon, replaced approximately every six months. Pre-filters may be washed or vacuumed as needed. Running the unit without a filter is not recommended, as it increases dust accumulation and noise.

Q: Can third-party air quality sensors be integrated with window ERVs? A: Currently, most devices do not support direct integration with third-party sensors. However, standalone sensors (such as AirThings View Plus or Air Gradient 1) can be used for independent monitoring. Future models may offer more integrated solutions.

Q: What are the sensible and latent heat recovery efficiencies of window ERVs? A: Sensible heat recovery is typically around 85% at standard test conditions (32°F, 0°C), peaking at up to 90% at optimal fan speeds. Latent recovery (moisture transfer) is slightly lower, generally between 80–83%. Higher fan speeds reduce efficiency.

Q: How do window ERVs handle wind-driven rain or condensation? A: Devices are designed with a slight tilt and drainage channels to direct water away from the interior. Field testing in various climates has not revealed significant issues with ingress, but ongoing monitoring continues as more units are deployed.

Q: What is the ongoing maintenance required? A: Maintenance is minimal: filters are replaced every six months, pre-filters may be washed, and the core may be wiped down if needed. Full washing or hosing of the core is not recommended as it may damage sensitive parts.

Q: What are typical installation costs for window ERVs? A: Professional installation ranges from $200–$500 per unit, though most homeowners can install the devices themselves. The process is generally easier than installing a window air conditioner.

Q: I have an older home but upgraded with casement windows that rotate outward. Can you make a unit for a bedroom that can be mounted to a wall using a customized sheet metal housing to fit the mechanism of your window device?
A: Unfortunately, the SWERV is not currently compatible with casement windows. It is only designed for sliding windows that are at least 23″ wide.

Q: My SWERV just arrived yesterday! Would you recommend I install it in my office or my bedroom?
A: The bedroom is a great option, but a home office is also a good choice—anywhere you spend the most time and want better air quality.

Q: I’m concerned about security if the ERV unit is installed in a first-floor window, as the window would no longer be lockable. Is there a way to secure the window after the ERV is installed?
A: Yes, retrofit window locks are available at hardware stores or online. Different types of locks work better with different window types, so make sure they are compatible. We also offer window locks on our site as an option at checkout.

Q: Which CO2 monitors do you recommend?
A: We recommend the Airthings View Plus for its high reliability, clean app user interface, and its ability to measure CO2, temperature, humidity, pressure, particulate matter, radon, and VOCs.

Q: What is the sensible heat recovery at standard test temperatures of 32°F (0°C) at different CFMs?
A: Sensible heat recovery is around 85–90%, depending on fan speed. Lower fan speeds have higher efficiency (about 90–94%), while higher fan speeds (e.g., 120 CFM) can go down to about 75%.

Q: Is there a rule of thumb for the number of window ERV units required based on apartment square footage? For example, is one unit sufficient for a 900 sq. ft. living space with 9′ ceilings? If only one, should it be placed in a central window?
A: You should run a 62.2 calculation to determine the exact requirement. If you have blower door test data, that helps, but generally, two units are recommended. Each unit can serve one room up to 1,000 sq. ft.

Q: Is there a risk of wind-driven rain ingress when a window has a window ERV, AC, or heat pump installed?
A: Window ERVs and heat pumps tend to seal much better than window ACs, as they are designed for year-round use, including winter. They provide better protection against sideways rain, and we haven’t had issues with rain intrusion, though it is possible.

Q: What is the price point for these units?
A: The price starts at $799. Two extra filters are $49, and sensors that measure temperature, humidity, and CO2 (both inside and outside) for autonomous operation and app dashboard are $99.

Q: Is the ERV core washable?
A: The cores do not need cleaning if you keep the filters installed, but you can blow or wipe them off if needed.

Q: What size options are available?
A: Currently, there is only one size available, but we plan to offer multiple sizes in the future.

Q: Can you have custom sizes made?
A: Not at this time. However, the foam blocks are cut to size for any sliding window at least 23″ wide, and airflow is adjustable from 10–120 CFM.

Key Takeaways

• Ventilation is essential for both health and energy efficiency in residential buildings, especially as airtightness increases.
• Existing homes face unique challenges in achieving effective ventilation due to cost, complexity, and building constraints.
• Window ERVs provide a practical, affordable solution for balanced, energy-recovering ventilation in a range of settings.
• Smart ventilation technologies enable demand-controlled operation, optimizing air quality and energy use.
• Installation and maintenance are accessible to most homeowners, with minimal ongoing costs and user-friendly design.
• Verification of performance is shifting from one-time tests to continuous monitoring of air quality metrics, especially CO2.
• User control remains critical, with manual override and accessible controls required by many building standards.
• The future of residential ventilation will integrate predictive controls, ongoing commissioning, and a broader range of compatible devices.