The second crucial difference between insulations leads us into a Gordian knot of building science problems. The closest thing I've found to a solution is a book at this link:
http://www.buildingsciencepress.com/Builders-Guide-to-HotHumid-Climates-P2.aspx
Consider this image: as you, your movie-star husband, and your two over-achieving kids enjoy an Independence Day barbecue on your patio, your ice-filled julep makes a puddle on the table. The glass isn't leaking, humid air is condensing on its cold surface. There's a good chance you've seen this happen on your home's windows as well. This phenomenon doesn't just make rings on coffee tables, it can cause serious damage where you don't see it, in your walls, under your floors, and in the attic.
Condensation occurs when water vapor in the air comes in contact with a surface at or below the dew point. The dew point temperature varies according to relative humidity. Water vapor enters your home in a variety of ways, for example:
-from outside air that is more humid than the air inside the house;
-showering;
-cooking;
-clothes dryers;
-human breathing.
Building materials have different degrees of resistance to penetration by water vapor. Glass and sheet vinyl have very low permeability, whereas wood and drywall have fairly high permeability. What happens in a worst-case scenario? On a muggy summer day, the inside of your house is air-conditioned real cool and pleasant. Hot air leaks inside, say through the hole where your cable comes in. It gets in your wall, carrying that sticky humidity, moves through the insulation, gets cooler as it permeates the drywall, then runs into that handsome, semi-impermeable wallpaper in the kitchen. It condenses into liquid water, over time growing mold, making the drywall soft and crumbly, attracting termites to the framing lumber, and saturating the insulation. The reverse of this happens in winter, when warm, more humid air in your house moves through your walls towards cold, potentially condensing surfaces outside.
Fiberglass and cellulose are very permeable, and they are rendered useless as insulations once they've been wet. Closed-cell foam is impermeable, while open-cell foam has low permeability. More on this next time, when I'll get to what you can do about it while renovating your house, especially in regards to insulation.
For the moment, some things you can do to keep water vapor from causing damage:
-Reduce it and control it.
-Modern central air systems do a lot of the work for you.
-Using your range hood fan while cooking helps immensely.
-Clean your dryer's lint catcher, and shorten its vent duct if possible.
-Ensure that your doors, windows, and weatherstripping are in good working order.
-And please, use your bath fan to remove the steam from showers and baths! It is not there for smells, it is there to remove excess water vapor.
