Being a damnyankee, I know that I would normally use a plastic vapor barrier on the inside of insulated walls to prevent humidity and moisture from entering the structure. I haven't seen this in houses that are built down here in Texas -- it seems that if there's any barrier applied to the batting, it's tarpaper on the inside (up against the drywall).

What are the best/proper kinds of high R-value insulation that can be retrofitted into an existing structure, and what kind of precautions, moisture barriers, and other details need to be considered that my northern DIY renovation experience might not take into account?

In Texas vapor barriers act to keep out wind more than vapor. There is a formula that can be used to determine at what point in the wall cavity the vapor drops out. I do not remember the formula. Usually you see a "Tyvex" type of material on the exterior side of the wall stud assembly.
http://www.thebestconference.org/best1/pdfs/044.pdf

http://74.6.239.67/search/cache?ei=UTF-8&p=vapor+barrier+calculation&sado=1&fr=yfp-t-890-s&u=www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design&w=vapor+vapour+barrier+barriers+calculation+calculator+calculators&d=NRVSwO_EUTnK&icp=1&.intl=us&sig=wNAb5SQKhsK97jyl63gsrQ--

[This message has been edited by Mon Dow 2000 (edited 3/15/2010 3:14p).]

ok, so vapor barriers down here are an exterior thing? This is what I'm used to on the inside of the structure:



[This message has been edited by superspeck (edited 3/15/2010 3:22p).]

The really general rule of thumb is vapor barriers go on the outside within 100 miles of the gulf coast. Rules of thumb were made to be broken. I've seen several national maps where BCS was on, and either side of the outside/inside map.

The equation MonDow mentions is the key to determining where moisture will condense in the wall under different conditions. In yankeeland, its all about keeping the moisture and vapor pressure gradient from pushing moisture out of the warm interior in winter, hitting the cold air on the other side of the insulation, condensing and rotting your siding, etc.

Turn that around for high humidity days during a gulf coast summer. The vapor pressure gradient is reversed and warm, moist air moves into the wall. When it hits the cool air on the other side of (or in) the insulation, it condenses and rots things there.

Theoretically, you can't prevent it all...but you can look at typical conditions throughout the year to see where you need to be most of the time.

Rule of thumb, vapor barrier on the warm side.

In B/CS we have more hot days than cold days.