Modern homes are built tight. Improved insulation, better windows, and careful air sealing have made houses dramatically more energy efficient than they were a few decades ago. But this efficiency comes with a tradeoff: these homes no longer breathe on their own. Without intentional ventilation, indoor air becomes stale, pollutants accumulate, and air quality suffers.
The Problem with Tight Houses
Older homes were leaky enough that fresh air continuously seeped in through cracks, gaps, and poorly sealed windows. This natural infiltration was inefficient from an energy standpoint, but it did provide a constant supply of outdoor air. Modern construction eliminates most of this infiltration, which is excellent for energy bills but problematic for air quality.
Without adequate ventilation, indoor pollutants have nowhere to go. Carbon dioxide from breathing accumulates. Volatile organic compounds from furniture, cleaning products, and building materials build up. Moisture from cooking, showering, and simply being human has no escape route. The result is air that becomes progressively less healthy to breathe.
What Mechanical Ventilation Does
Mechanical ventilation systems bring fresh outdoor air into your home while exhausting stale indoor air. Unlike opening a window, these systems operate continuously and in a controlled manner, ensuring consistent air exchange regardless of weather conditions or whether anyone remembers to open the windows.
The most basic approach is exhaust-only ventilation, where bathroom and kitchen fans remove air and fresh air infiltrates through small openings in the building envelope. More sophisticated systems use balanced ventilation, bringing in fresh air and exhausting stale air in equal measure through dedicated equipment.
Heat Recovery Ventilators
A Heat Recovery Ventilator, or HRV, brings in fresh outdoor air while capturing heat from the outgoing stale air. In winter, the warm air leaving your home passes through a heat exchanger where it warms the cold incoming air. This means you get fresh air without losing all the heat you have paid to generate. In summer, the process can work in reverse, with the cooler outgoing air helping to pre-cool hot incoming air.
HRVs transfer heat only, not moisture. This makes them particularly effective in cold climates where indoor air tends to be dry in winter. By not transferring moisture from the outgoing air, HRVs can actually help reduce indoor humidity when needed.
Energy Recovery Ventilators
An Energy Recovery Ventilator, or ERV, works similarly to an HRV but also transfers moisture between the air streams. In winter, when indoor air is humid and outdoor air is dry, the ERV retains some of that indoor moisture instead of exhausting it all outside. In summer, when outdoor air is humid, the ERV can reduce the moisture content of incoming air.
ERVs are generally better suited to climates with hot, humid summers or very cold winters where maintaining indoor humidity is desirable. They reduce the load on both your heating system in winter and your air conditioning system in summer by managing both temperature and moisture.
Choosing Between ERV and HRV
The choice between an ERV and HRV depends primarily on your climate and your home's specific humidity characteristics. In most climates, an ERV is the better choice because it helps maintain balanced humidity year-round. ERVs work well in humid climates by reducing incoming moisture in summer, and they help retain needed humidity during dry winters.
HRVs may be preferable in homes that generate excessive moisture, such as those with many occupants in a small space, or in very cold climates where condensation is a concern. Some manufacturers offer units with interchangeable cores, allowing you to switch between ERV and HRV operation seasonally.
Signs You Need Better Ventilation
Stuffy or stale-smelling air is the most obvious indicator. If your home feels fresh only when windows are open, ventilation is inadequate. Persistent odors from cooking or other activities that linger for hours suggest insufficient air exchange. Condensation on windows during cold weather can indicate both humidity problems and inadequate ventilation.
If occupants experience headaches, fatigue, or difficulty concentrating that improves when they leave the house, poor indoor air quality from inadequate ventilation may be contributing. These symptoms can result from elevated carbon dioxide levels or accumulated volatile organic compounds.
Implementing Fresh Air Ventilation
For new construction or major renovations, whole-house ventilation should be part of the HVAC system design from the start. For existing homes, adding an ERV or HRV is a retrofit project that requires professional installation but can dramatically improve air quality.
At minimum, ensure that bathroom and kitchen exhaust fans are properly sized and actually vent to the outdoors, not into an attic or crawlspace. These spot ventilation systems remove moisture and pollutants at their source and are required by most building codes.
Your home may look solid and sealed, but that does not mean the air inside should be stagnant. Fresh air is not a luxury. It is fundamental to healthy indoor environments.
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