Article #1384 - The Importance of Vapor Retarders
[2000 IECC, 2003 IECC, 2006 IECC]
Vapor travels from high pressure to low pressure. Moisture-laden air is literally "pushed" through penetrations in the building envelope. The moisture could damage the insulation and even the framing members, but a continuous vapor barrier helps prevent this from happening.
Vapor Retarder
A component that retards water vapor diffusion but does not totally prevent its transmission. Vapor retarder material is usually a thin sheet or coating. However, a construction of several materials, some perhaps of substantial thickness, could also constitute a vapor retarder system.
Code Citation*
IECC 2000 and 2003, Section 502.1.1 Moisture Control
The design shall not create conditions of accelerated deterioration from moisture condensation. Frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder having a maximum permeance rating of 1.0 perm when tested in accordance with Procedure A of ASTM E 96. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.
There are exceptions to this requirement, such as where moisture or its freezing will not damage the materials, hot and humid climates, or where other approved means to avoid condensation in unventilated framed wall, floor, roof and ceiling cavities are provided.
IECC 2006, Section 402.5 Moisture Control (Mandatory)
The building design shall not create conditions of accelerated deterioration from moisture condensation. Above-grade frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.
Exceptions include where moisture or its freezing will not damage the materials; frame walls, floors and ceilings in jurisdictions in Zones 1 through 4 (crawl space floor vapor retarders are not exempted); and where other approved means to avoid condensation are provided.
*Copyright, 2000, International Code Council
, Inc. Falls Church, Virginia. Reproduced with permission. All rights reserved. 2000 International Energy Conservation Code; Copyright, 2003, International Code Council, Inc. Falls Church, Virginia. Reproduced with permission. All rights reserved. 2003 International Energy Conservation Code; Copyright, 2006, International Code Council, Inc. Falls Church, Virginia. All rights reserved. 2006 International Energy Conservation Code.