Moisture Control and "Air Barriers" within the WCBC

Almost everyone is well aware of the importance of having a weather resistant barrier on the exterior envelope of buildings. Too much damage to the building will occur if we do not keep the wet out.

In Wisconsin's climate, cold cavities in the exterior envelope of a building are also subject to moisture build-up by condensation if the moisture in the conditioned interior air is not controlled. That condensed moisture can cause the deterioration of building elements just as surely as when the roofing or siding leaks.

Effective March 2008, the Safety & Buildings Division at the Department of Commerce dropped our previous language on "air barriers/retarders" that was particular to Wisconsin and adopted the 2006 edition of the International Building Code (IBC). IBC section 1403.2 reads as follows: Protection against condensation in the exterior wall assembly shall be provided in accordance with the International Energy Conservation Code . Guidance on this topic may also be found in the energy conservation provisions of the Wisconsin Commercial Building Code (WCBC) at section 63.05 0 2 (1) , as follows: ". . . the building design may not create conditions of accelerated deterioration from moisture condensation."

Damage to the building occurs when moisture is allowed into cold cavities of the building where condensation takes place. In heated buildings, the moisture that creates condensation cannot be controlled unless a building element, whether called an air barrier or a vapor retarder, is located on the warm interior side of the assembly and is continuously sealed to limit the free migration of moisture-laden air out of the heated space and into the cavities of the structure.

That moisture is controlled first by creating a continuous moisture barrier that is sealed to keep the warm moist air from freely moving into the cold space of the envelope.

Once sealed to prevent that free movement of air, the only way for the moisture to get into the cavity is if the vapor can move through the barrier via permeation. In essence, the most efficient way to control the migration of the moisture is by having a single moisture barrier in place.

To function, that moisture barrier must be located on the warm-in-winter side of the assembly. Just as we seal the exterior surface of a wall to control air infiltration from the exterior, the barrier on the warm-in-winter side must also be caulked, gasketed or otherwise sealed to provide a similar continuous barrier to control air exfiltration from the interior.

The code's performance requirement calls for a continuous element that will keep the warm moist air inside of the tempered space and away from any unheated cavities that exist. That includes the cavities found in multi-wythe masonry construction using concrete masonry units (CMU). When properly designed and constructed, the inside warm air that is carrying the moisture should not be able to move past the warm plane of the thermal envelope and into the unheated space where the problems associated with moisture condensation show up.

Standards for the control of moisture are specified in in sections 402.4, 402.5, 502.4 and 502.5 of the International Energy Conservation Code and section Comm 63.0502. Some of the requirements are written using performance language as opposed to prescriptive language. Whether you choose to use the performance or prescriptive standards in the code, compliance is required.

-- J.B. Smith, P.E.