What Makes a Home Livable?
Humans are highly adaptable to different climates, but much of that adaptability is technology enhanced. Without heating, warm clothes, and so on, we're fairly fragile.
LiveScience notes that "people can live indefinitely in environments that range between roughly 40 °F and 95 °F (4 °C and 35 °C), if the latter temperature occurs at no more than 50 percent relative humidity."
So basically we've got a 55 °F comfort window, and the closer we get to the outer "extremes" of that window, the more uncomfortable we are. Thus, a drafty home with excessive humidity (or, less commonly, dry air) can really mess with our happiness. Add in an unhealthy noise level, and you have a trifecta of invisible problems.
Fortunately, we now understand building science well enough to address each of these aspects of home comfort. A combination of good design, the right products and "systems" can solve them.
Modern homes tend to be tightly built and insulated. That means that any moisture created inside the home is likely to be trapped for a while, unless there's some mechanism to remove it. Dishwashers, showers, faucets, cooking and leaking ducts from clothes dryers are just some of the internal sources of moisture, along with human respiration (breathing) and perspiration. To remove moisture and other contaminants, building standards and codes now require a certain amount of mechanical ventilation₁ in new homes, typically between 40 and 70 cubic feet per minute.
Bathrooms are a major source of indoor humidity.
Unless you run your bath fans 24 hours a day (some fans have a lower speed setting for this purpose), they may not be adequate to remove enough moisture to get it down to a range that's comfortable—yet not likely to result in mold or mildew. That's especially true in high performance homes. According to a report by Building Science Corporation, that range should be around 60% RH in the summer, and somewhat lower in the winter.
To retard the migration of water vapor through the wall assembly, building codes also require "vapor retarders" in wall assemblies in some climates. The International Residential Code (IRC) defines vapor retarders according to their vapor permeance: Class I has ≤ 0.1 perm (essentially impermeable), Class II has from 0.1 to 1.0 perm (semi-impermeable) and Class III has from 1.0 to 10 perm (semi-permeable). Kraft paper, which usually faces insulation, is a Class II vapor retarder. Since the conditions in houses change with time, it would be interesting to have a vapor retarder that would change its vapor permeance based on the conditions: low permeance when the humidity is low and high permeability when humidity is higher. While innovation has produced sophisticated "smart" vapor retarders that adjust their resistance to moisture flow automatically, kraft paper also has this variable permeance.
In cold weather, moisture generated within the house can find its way to the attic and can damage insulation, rafters, roof deck and other materials. Roofing ventilation can remove the moisture with air entering the attic through soffit vents and leaving through a ridge vent along the top of the roof peak.