Where Vapor Retarders Are Not Needed.

[MountainDon]

In researching some other stuff I can across some interesting information about vapor retarders, specifically where a vapor retarder is not needed.

These are hot and humid locations as in our South. Some viewers may recall seeing previous postings on the dangers of mold growth when vapor retarders or vapor barriers are used on the inside of buildings in a hot & humid climate. Here is a list of locations as deemed hot & humid by the DOE. If you live there I'm sure you already suspected  .   Not only are they not needed, they are a bad idea.

4.2 Simplified Vapor Retarder Exemption
Section 502.1.4 of the 1992, 1993, and 1995 MEC, Section 502.1.2 of the 1998 IECC, and Section 502.1.1 of the 2000 and 2003 IECC require that vapor retarders be installed on the warm-in-winter side of the thermal insulation in walls, ceilings, and floors. The following locations in hot and humid climates are exempted from this requirement:
• locations where 67°F or higher wet-bulb temperatures occur for 3000 or more hours during the warmest six consecutive months of the year, or
• locations where 73°F or higher wet-bulb temperatures occur for 1500 or more hours during the warmest six consecutive months of the year.
Most builders and code officials will not have access to temperature data of this type and will therefore be unable to determine whether a building qualifies for the exemption.
To simplify this exemption, we evaluated Test Reference Year (TRY) and Weather Year for Energy Calculation (WYEC) data for over 200 locations. Based on these data, locations exempted from the vapor retarder requirement on the warm-in-winter side of the wall were presented by state and climate zone. (The climate zones, presented in the maps that accompany the Prescriptive Packages, fall along county boundaries [DOE 1995b].)
The TRY and WYEC data provided annual totals of all hours above the cutoff wet-bulb temperatures and all the hours were assumed to occur in the warmest six consecutive months of the year. All cities in Florida, Hawaii, Louisiana, and Mississippi had more than the required number of hot and humid hours, therefore qualifying for the exemption. Six states had some locations that qualified for the exemption and some locations that did not qualify. Table 2 shows the number of hours at or above the cutoff wet-bulb temperatures for cities in these six states with the HDD for each city. All other states had no locations that qualified for the exemption. Based on the results shown in Table 4.2, we selected climate zones in the six southern states that qualify for the exemption.



Source reference:  http://www.energycodes.gov/rescheck/documents/rescheck_tsd4.4.pdf