Most modern homes have now a spot exhaust ventilation system, that is, an exhaust fan in each bathroom and kitchen. That’s important, but in modern air-tight homes you may need more than just spot exhaust ventilation...
Spot ventilation is very important do exhaust internal sources of moist warm air: baths, cooking and laundry. To be effective it should be vented directly to outdoors, and never into the attic or any other room.
If shopping for spot exhaust fans prefer an Energy Star or a similar qualified exhaust fan. To compare models, pay attention to quietness (measured in Sones; 0.5 Sone is common in new energy-efficient models) and capacity (measured in CFMs, that is, Cubic Feet per Minute).
For bathroom exhaust fans you will need about 1 CFM (Cubic Feet per Minute) for every 1 sq. ft. of floor area (a 70 CFM exhaust fan for for a 7 x 10 bathroom...). Consider a range hood with an exhaust rate of 100 CFM per 10,000 BTU.
Quality and Prices
Spot exhaust fans have a long lifespan, and can be quiet and inexpensive to buy and run.
The best well-known brands in spot ventilation systems - Broan, Panasonic, Tamarack Technologies and Fantech - are offering through-wall ductless systems.
For prices and customer reviews, take a look at Amazon.com: Bath and kitchen exhaust fans.
Other simple ventilation systems
Fresh air is crucial for our health; after all, we spend most of our lives in our homes... And that makes mechanical ventilation critical, in many climates, in air-tight homes. Homes with reasonable or high levels of air-tightness need more than just spot ventilation.
If your needs of fresh air are moderate, consider a simple background ventilation system, based on a powerful enough exhaust fan (often installed in a central bathroom) or a simple supply ventilation system. Simple systems, with a relatively low capacity, can work in conjunction with spot ventilation and be scheduled to run according to your needs, or run full-time at an adjustable rate.
If you have a modern highly air-tight home (and if you live in a harsh climate) you may need a mechanical whole-house ventilation system (an HRV or an ERV system).
Simple Exhaust & Ventilation Systems
Balanced HRV and ERV Ventilation Systems
Mechanical Ventilation Guide
Natural vs. Mechanical Ventilation
Should you Have a Mechanical Ventilation System?
Pros and Cons of Supply and Exhaust Ventilation