For energy upgrades to existing homes, homeowners should follow logical priorities, including indoor-air quality (IAQ). If a home has one-half a natural air change per hour  (0.5 ACH50) of air leakage, an expensive central ventilation system makes no sense. With a leaky home, the air leakage dominates air exchange with the outdoors and not the ventilation system. Therefore, the first priorities for a leaky home are air sealing to reduce unintentional air leakage and removing pollution sources. Also, no ventilation system can effectively remove large quantities of pollutants carried in on infiltrating outdoor air.

Remove Pollution Sources

back-drafting, fire safety, flame roll out

This water heater shows signs of flame rollout. Remove safety and pollutant hazards before retrofitting a ventilation system to an existing dwelling.

Don’t expect a central ventilation alone system to provide healthy air and minimize ventilation’s energy costs. First air seal the building and remove sources of pollution.
Existing homes have three advantages over new homes for implementing ventilation systems.

  1. You can see signs of air-quality problems such as mold, peeling paint, and dust from building materials.
  2. Building materials have had time to dry out and release their gases, which reduces the pollutants that a new ventilation system must remove.
  3. The homeowner may identify problems that aren’t visible, such as periodic back-drafting of combustion appliances.

Reduce Air-Leakage and Install Insulation

Install insulation in any empty building cavities bordering the exterior. Provide a continuous air barrier next to the insulation wherever possible. Homes need effective air barriers to minimize heating and cooling costs. The air barrier also minimizes unwanted flow of polluted outdoor air into the dwelling.
Upgrade combustion appliances if possible. Open-combustion appliances are more likely to back-draft after workers tighten the dwelling. Also these older appliances are less than 78% efficient. Consider installing 90% efficient appliances with sealed combustion chambers and fan-assisted draft. These efficient appliances benefit your household with reduced heating costs, improved safety and better IAQ.

Choose a Ventilation Option

spillage, back-drafting' corrosion

Shows signs of combustion byproducts escaping the chimney.

A homeowner should choose ventilation equipment depending upon how much they’ve improved their home’s airtightness. A blower-door test can inform you how successful that air-sealing efforts were at air-tightening home.
To aid the natural air leakage, the leaky home may require only bathroom and kitchen exhaust fans. However, an airtight home may require a central heat recovery ventilator (HRV) to provide healthy indoor air and to minimize ventilation energy.

Further reading

Residential Energy Chapter 11