Tight Homes: Air Sealing, Ventilation, Moisture Control, and Proper HVAC Sizing

Krissa Miller
6 hours ago
5 min read

Our homes are constructed with air leakage rates below 3 ACH50, which means mechanical ventilation becomes an important part of maintaining healthy indoor air quality. Building a tight home is not the problem—the key is pairing a tight building envelope with a properly designed ventilation and HVAC system.

Exhaust-Only Ventilation and Uncontrolled Moisture

Builders often choose exhaust-only ventilation because it is a simple and relatively inexpensive approach. However, an exhaust-only system depressurizes the home by exhausting indoor air without providing a dedicated source of replacement air.

That replacement air still has to come from somewhere.

When the home is depressurized, outdoor air can be pulled through uncontrolled leakage points such as rim joists, attic and ceiling penetrations, electrical outlets, wall assemblies, plumbing penetrations, and other gaps in the building envelope.

During a hot and humid Kansas City summer, this becomes more than an air-quality concern—it becomes a moisture-control issue. The outdoor air being pulled into the home can contain a significant amount of water vapor. Every cubic foot of humid outdoor air that enters through uncontrolled leakage adds moisture that the home's HVAC system must remove.

This can increase indoor relative humidity and make the home feel uncomfortable even when the thermostat indicates that the indoor temperature is acceptable. Higher indoor humidity can also contribute to condensation on cold surfaces, musty odors, microbial growth, deterioration of building materials, and other long-term moisture concerns.

The tighter the home becomes, the more important it is that we control where ventilation air comes from and how moisture is managed. We do not want a mechanical ventilation system intentionally depressurizing the home and then relying on random cracks and openings in the building envelope for replacement air.

Instead of providing clean, filtered outdoor air from a known location, exhaust-only ventilation can draw hot, humid, unfiltered air through the building assembly. That air may also carry outdoor pollutants, dust, and particles from insulation and other building materials into the indoor environment.

Depressurization can also create concerns with naturally drafted combustion appliances. Under certain conditions, negative pressure can interfere with proper drafting and increase the potential for combustion gases, including carbon monoxide, to enter the home.

A tight home with properly designed mechanical ventilation provides a much better approach. It allows us to control where outdoor air enters the home, filter that air, manage pressure, and better control the moisture entering the building.

Concerns with Supply-Only Ventilation

Some builders have been advised to use an air cycler or similar supply-only ventilation strategy that introduces outdoor air directly into the return side of the HVAC system.

This approach requires careful design, particularly in a hot and humid climate.

Consider what happens on a summer day when it is approximately 100°F outside with high outdoor humidity. That hot, moisture-laden air is introduced directly into the HVAC return and mixed with the cooler return air from the home.

The system now has to handle both the additional heat and the additional moisture contained in that outdoor air.

There is also the potential for condensation when humid outdoor air comes into contact with ductwork or other surfaces that are below the dew-point temperature of that air. Condensation on or around ductwork can lead to wet insulation, microbial growth, deterioration of building materials, and other moisture-related problems.

The concern becomes greater if ventilation air continues to be introduced when the cooling system is not operating. In a tight home with a relatively small cooling load, the air conditioner may not have sufficient runtime to remove the additional moisture being brought inside.

During winter, the opposite temperature extreme occurs. Very cold outdoor air may be introduced into a return system carrying much warmer indoor air. We have heard reports from builders experiencing significant indoor humidity and comfort issues in homes using this type of ventilation strategy.

Simply introducing outdoor air into an HVAC return does not automatically provide good ventilation. The amount of outdoor air, operating schedule, filtration, HVAC runtime, duct configuration, climate conditions, pressure effects, and moisture load all need to be considered.

Balanced Ventilation with an ERV

An Energy Recovery Ventilator (ERV) is a balanced mechanical ventilation system designed to bring fresh outdoor air into the home while exhausting approximately the same amount of stale indoor air.

The incoming outdoor air passes through filtration before being introduced into the home. At the same time, exhaust air is removed from areas where moisture and contaminants are commonly generated.

An ERV provides another important benefit in our climate: moisture management.

During hot and humid summer conditions, the ERV transfers a portion of the moisture from the incoming outdoor air to the outgoing exhaust air before that outdoor air enters the home. While an ERV is not a dehumidifier and does not eliminate the HVAC system's latent cooling load, it can substantially reduce the moisture burden associated with bringing required ventilation air into the home compared with introducing untreated outdoor air.

Because the system is balanced, an ERV also minimizes the pressure imbalance associated with exhaust-only or supply-only ventilation. This reduces the amount of hot, humid outdoor air being pulled through uncontrolled openings in the building envelope.

The result is a home that is both tight and intentionally ventilated—rather than relying on random building leakage to provide replacement air.

Our experience with KCMO homes supports this approach. Homes equipped with ERVs that have also successfully completed the air-sealing requirements of the energy code have received positive feedback from homeowners. Builders and HVAC contractors working on these homes have also reported fewer comfort and indoor-air-quality complaints.

Proper HVAC Sizing and Humidity Control

These homes also had Manual J load calculations completed so that the heating and cooling equipment could be properly sized for the actual building loads.

Proper HVAC sizing is another critical part of controlling indoor humidity.

An air conditioner does two jobs: it reduces the indoor air temperature, but it also removes moisture from the air. To effectively dehumidify the home, the cooling system needs adequate runtime for moisture to condense on the cooling coil and drain from the system.

An oversized air conditioner can cool a tight, well-insulated home very quickly. The thermostat reaches its setpoint, the equipment shuts off, and the cooling cycle may end before enough moisture has been removed from the air.

This creates a situation where the home may be 72°F but still feel damp or uncomfortable because the relative humidity remains too high. Repeated short cycling can also increase equipment wear and reduce overall system efficiency.

This problem becomes even more significant when the ventilation system is continuously introducing untreated humid outdoor air. We can end up simultaneously adding moisture to the home while reducing the HVAC system's opportunity to remove it.

Undersized equipment creates the opposite problem. The system may operate continuously during design conditions, struggle to maintain the desired indoor temperature, and experience excessive runtime.

The goal is not simply to install a larger HVAC system. The goal is to install equipment that is properly selected for the home's calculated sensible and latent loads, while accounting for the home's ventilation strategy.

The Complete System

The solution is not to make homes leakier. The solution is to design the home as a complete system.

Proper air sealing, balanced mechanical ventilation, moisture management, and correctly sized HVAC equipment all work together.

Air sealing gives us control over the building envelope and reduces the amount of hot, humid outdoor air entering through uncontrolled leakage.

Balanced ventilation allows us to introduce filtered outdoor air from a known location while exhausting stale indoor air without unnecessarily pressurizing or depressurizing the home.

An ERV can reduce the moisture load associated with required ventilation during Kansas City's humid summer conditions.

Proper Manual J calculations and HVAC sizing ensure that the cooling system has the capacity and operating characteristics necessary to manage both temperature and humidity.

A tight home is not an unhealthy home. An improperly ventilated and improperly conditioned home can be.

When we control air, moisture, ventilation, and HVAC sizing as one complete system, we can build tight homes that are more comfortable, energy efficient, durable, and capable of providing better indoor air quality for their occupants.