Although most people are oblivious to the issue, in more than half of the homes I inspect ventilation is a challenge. “Ventilation” means “air-exchanges” with the outside air.
From our society's energy efficient mind-set of the last 25 years, most people think that bringing outside air into the house envelope is a bad idea because it is not “conditioned” — (i.e., warmed or cooled). This logic is horribly flawed.
Ventilation is REQUIRED for healthy living.
Ventilation is required in the kitchen, laundry, and bath to exhaust warm moist air to the exterior. It is required of some appliances like the dryer and stove to exhaust lint or smoke and grease to the exterior. It is required in the under floor crawl space and attic spaces to evaporate moisture - ESPECIALLY in winter months when so many people attempt to close off those vents.
Think of your house as a small part of the larger atmosphere. During the winter your house is warm and filled with moist air — basically, a mini high-pressure area. The outside air is dryer and cold — basically, a low-pressure area.
The natural movement of air is for high-pressure to move to low-pressure. When it does, it carries with it the humidity (moisture) from inside your house. This moisture condenses on the cold building components.
Ventilation brings cool dry(er) outside air into attic and crawl spaces to evaporate the moisture that condenses on metal building parts like nails and joist hangers. Without adequate ventilation, that moisture begins to rust the metal, dampen the wood, and then mold begins to grow - just like it would if a dead tree fell over onto the forest floor, (read “dead tree” as “floor joist,” “rafter,” or other “framing member”).
Natural ventilation means just that. The crawl spaces of your home should have enough screened vents so that when you are inside the crawl space on a sunny day and turn off your flashlight, you can see to get out1. If not, then adding additional ventilation is recommended.
In the underfloor crawl, vents should be located on all four sides of the house. For the attic, about half of the vents should be at the eave, and the other half at the ridge or located in the gable end or through a roof vent.
Mechanical ventilation means that there is an electric fan that forces humid or contaminated air to the outside from inside your home - (thus reducing the amount of moisture or humidity inside the house). When the mechanical fan is on, outside (replacement) air is being sucked into your home from some place — or at least it should be. Building too tight a home means that when mechanical fans are turned on they don't work very efficiently - (along with a half-dozen other things I don't have space to mention).
If you have one of those “tightly-built” homes, contact a heating contractor to install a mechanical “air exchanger” so that the house can draw-in adequate replacement air.
In summary, ask your home inspector to detail the ventilation of the house, (both natural and mechanical), so you can decide if you want to make it your home.
1 Source = The Rules of Thumb™ for Home Inspecting from the UNIFORM HOME INSPECTOR'S CODE BOOK™