Outdoor-Air Ventilation

Outdoor air ventilation diagram

Outdoor-air ventilation is a key component of better Indoor Air Quality IAQ. It's a simple concept: replace indoor-air with outdoor-air. These measures combined with an airtight, building envelope are smart choices for optimal health. 

Building envelope thumbnail What is a Building Envelope?        Blower door test thumbnail   The Blower Door Test. 

A small, but growing body of evidence suggests we should be providing near, or above the minimum ventilation rates required by international codes. Most local codes don't enforce minimum ventilation standards. As an important matter of health, all new construction should include outdoor-air ventilation.

Passive or Active Ventilation?

Window ventilation illustration

Active ventilation also known as mechanical ventilation uses a fan. The best form of passive ventilation is an operable window. Windows work great for nice weather but offer little control when we need to close-up and condition the indoor air. Air infiltration and some earthtubes are passive but they introduce considerable risks of contamination. For those seeking to meet minimum building codes aligned with healthy indoor-air research, mechanical ventilation is required.

Building researchers addressing this subject categorize active, outdoor-air ventilation systems into three types. Supply-Only, Exhaust-Only and Balanced. They work like they sound, with the balanced systems using two fans, and usually a heat/energy recovery core. We strongly prefer Balanced systems.


Exhaust only ventilation diagram Supply only ventilation diagram Balanced ventilation diagram

Exhaust-only systems depressurize.
Often risky

Supply-only systems pressurize a home.
Better, with some risk (WNC climate)

Balanced systems use two fans with energy recovery.

Exhaust only and supply only effects on the building envelope illustration

Outdoor-Air Ventilation System Types:

1. Exhaust-only systems are the most problematic. The source of outdoor-air is unknown. It may be coming from nasty places like the garage, ground, attic or through building cavities containing mold. Exhaust-only systems make it more difficult for other exhaust vents to do their job; bath vents, dryer vents and the range hood will have a harder time exhausting those pollution sources. Indoor combustion appliances like woodstoves, gas furnaces, gas water heaters or gas dryers in a de-pressurized house, risk back-drafting flue gases. Exhaust-only systems have no capabilities of cleanable filtration or energy recovery.

2. Supply-only systems are less risky in our climate. Outdoor-air is supplied from a known location. Where the indoor-air escapes from the house is usually unknown. This can be a problem when indoor-air is humid and surfaces inside hidden building cavities are cold from outdoor weather. The escaping, humid-air will condense on these cold surfaces leading to mildew, mold and rot. Supply-only outdoor air can be filtered, but cannot utilize energy recovery.

3. Balanced systems are the safest and most energy efficient outdoor-air ventilation systems. They are usually referred to as Heat or Energy Recovery Ventilators HRV/ERVs. They supply and exhaust from known locations reducing risk of pollution and potential of exfiltrating humid-air condensing in hidden building cavities. They usually include filters and are capable of capturing 75-95% of the energy used to condition the indoor-air.

HRV core illustration ERV cutaway view HRV system illustration
  Heat exhanger core of HRV/ERV is low tech            Efficient models use small amounts of electricity  Stale indoor-air replaced for fresh outdoor-air


HRV/ERVs and Ductwork  Illustration of balanced ventilation with best practice duct design.                    

HRV/ERVs use varying amounts of ductwork. Ducts offer the best control for exhausting and supplying specific inside areas. Good duct design supplies outdoor-air to bedrooms and living areas while exhausting indoor-air from bathrooms and kitchens.

The bigger and more compartmentalized a home, the more ductwork makes sense. Ducts deservedly have poor reputations, but proper design and maintenance offer a lot of control for improving indoor-air quality, especially for multi-level homes of 3 bed 2 bath sizes and up.


Balanced, Ductless Outdoor-Air VentilationLunos ventilation image

The Lunos is a balanced, HRV concept without ductwork, well suited to small and open floorplans. This category of ductless outdoor-air balanced ventilation device is poised for explosive growth. They are better suited for existing homes. They work in pairs, reversing direction for certain periods of time. This can work great for small, open floor plans but may have a hard time getting the necessary outdoor-air rates distributed through a multi-level home with numerous bedrooms and partitions.



Picture of HRV outdoor-air intake with red arrow for homeowner manual.Good systems usually include some screens or filters which need to be inspected, cleaned and possibly replaced. Supply-only and balanced systems have outdoor-air intakes that need attention similar to dryer vents. Most ducted, HRV/ERV units feature filtration that protects the heat exchanger and supply-air ducts from dust or pollen. Maintenance of these filters is similar to forced-air heating and cooling. Builders that include outdoor-air ventilation systems should make the maintenance needs known in homeowner's manuals and on-site education.

A picture from one of our custom home's maintenance manuals showing the outdoor-air intake location.


People are rightfully intimidated by new technology, but there are few things with new construction, that offer such a positive impact on indoor-air quality and personal health. The technology is simple, little more than a fan. Systems have small electricity requirements which is met easily with renewables.

Personal and public health benefit from outdoor-air exchange. International level codes require meeting ASHRAE 62.2 minimum ventilation rates. We urge everyone to include minimum levels of airtightness and outdoor-air ventilation in renovations and home building projects.

ASHRAE 62.2 ventilation thumbnail Indoor Air Quality Understanding ASHRAE 62.2                  Breathing walls thumbnail Breathing Walls? 


Springtime Builders are Asheville NC custom home builders specializing in building science craftsmanship.

Posted in Indoor Air Quality

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