Draft November 19, 2012
For further information see:

IRC 2012

IBC 2012

IEBC 2012

ASHRAE Handbook – Fundamentals 2009

ASHRAE 90.1-2010 Energy Standard for Buildings Except Low-rise Residential

ASHRAE 160-2009 Criteria for Moisture-Control Design Analysis in Buildings
1 Scope
This document is limited to moisture control in exterior walls, and the focus will be on wood-framed walls.
Although moisture from roofs, foundations or other sources may impact exterior walls, the impact from these sources is not taken into account. The requirements of the IRC and the IBC/IECC will be evaluated.
This document will address the following sources of moisture as they might impact exterior walls:
1. Exterior sources of liquid moisture that may penetrate the cladding (e.g., wind-driven rain).
2. Moisture vapor diffusion through materials into exterior walls from inside and/or outside.
3. Air leakage transport of moisture into and/or through exterior walls from inside and/or outside.
In addition, one must be concerned about durability. One must either specify building material products that are tolerant of exposure to moisture, or one must pay greater attention to wetting/drying cycles and how the drying process works.
Mold can begin within two weeks under the proper moisture and temperature conditions.
Moisture movement and effects are also dependent on the climate zone where the wall exists. This needs to be taken into account as well.
2 Decision Flow Chart
As a designer is determining the exterior wall assemblies for the structure in question, in addition to structural issues, he/she will also need to address exterior finish, interior finish and insulation requirements. The following flow chart is based on Annex A of ASHRAE 160:
1. Define building assembly
2. Assign material properties
3. Determine initial material conditions (at time of construction)
4. Select outdoor climate conditions
5. Select exposure conditions
6. Determine indoor conditions
7. Analyze performance: a. If acceptable, record results b. If unacceptable generally, change construction design or HVAC design & repeat analysis c. If unacceptable due to initial material conditions, add an initial drying procedure & repeat analysis
3 Definitions (from IRC 2012):
AIR BARRIER. Material(s) assembled and joined together to provide a barrier to air leakage through the building envelope. An air barrier may be a single material or a combination of materials.
VAPOR RETARDER CLASS. A measure of the ability of a material or assembly to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method with Procedure A of ASTM
E 96 as follows:

Class I: 0.1 perm or less
Class II: 0.1 < perm ≤ 1.0 perm
Class III: 1.0 < perm ≤ 10 perm
WATER-RESISTIVE BARRIER. A material behind an exterior wall covering that is intended to resist liquid water that has penetrated behind the exterior covering from further intruding into the exterior wall assembly.
4 Discussion
The perfect wall has one or more of each of the following: a rain control layer, an air control layer, a vapor control layer, and a thermal control layer. Even with code compliant wall assemblies and materials, not all wall assemblies perform as expected, especially regarding moisture control.
Although there is every expectation that an exterior wall assembly that is designed according to the code will perform as expected in dealing with thermal and moisture concerns, some of the variables involved in real life conditions may make code compliant design inadequate. A factor as simple as wall orientation
(north facing vs. south facing) can significantly impact performance. The relative humidity in the interior conditioned space, which is not specifically addressed in the codes, will have a significant impact on the performance of the wall assembly.
An exterior wall assembly is expected to be able to resist moisture intrusion from outside and inside and at the same time be able to dry to either the inside (cold climate) or outside (warm climate) or both/either
(intermediate climate). It is never advisable to construct a wall with high vapor resistance (low perm) materials on both sides of the wall assembly (i.e., a “double vapor retarder”) because of the potential of water to become trapped within such a wall assembly.
Carefully constructed and maintained walls are challenging.
The best practice recommendation is to ensure that the wall assembly has sufficient ability to “breathe,” allowing moisture in the cavity to escape, or follow the above building code recommendation to ensure sufficient continuous insulation thermal resistance is provided to keep the wall warm (i.e., above dew point temperature under design conditions). liquid moisture - Control of liquid moisture intrusion from the outside is the easiest to understand and is typically addressed by the exterior cladding, roof overhangs, water resistive barrier (WRB) and flashing.
If adequate attention to detailing is provided, little liquid moisture should enter the wall assembly from the outside. vapor diffusion - Control of vapor diffusion has to be addressed from both the outside and the inside of the exterior wall assembly. This is typically addressed by the moisture vapor retarder permeability requirements. However, the permeability of other products making up the wall assembly will also impact the flow of vapor by diffusion. air leakage - Control of air leakage transporting moisture has to be addressed from both the outside and the inside of the exterior wall assembly. This is typically addressed by air barrier requirements and testing (e.g., blower door testing). However, the evaluation of the wall as an assembly to limit air leakage does not address the location of the layer(s) in the assembly which constitute the air barrier and how moisture trapped on the wrong side of the air barrier might impact wall durability and performance.
5 Requirements (IRC/IBC-IECC):
Description
Wall loads
Wall structure
Climate Zone Map
IRC Section IBC/IECC Section
Chapter 3 (gravity + wind and/or seismic Chapter 16
Section 602.10
Figure N1102.10
Material appropriate chapter
IECC Figure C301.1
Vapor Retarder Section 702.7 & Table 702.7.1
Water-resistive Barrier Section 703.2
Air Barrier Section N1102.4 & Table N1102.4.1
Section 1405.3 & Table 1505.3.1
Section 1404.2
IECC Section 402.4.1
Each one of these sections has an impact on the thermal and moisture performance of the wall assembly.
The following are a few examples of how these separate code sections can interact:

More moisture is typically moved by air movement than by diffusion. Thus the 2012 IECC made the air barrier requirements of the structure more stringent. The IRC requires documented testing (N1102.4.1.2) to have an air leakage rate of 5 air changes per hour in zones 1 and 2 and an air leakage rate of 3 or less in zones 3-8. The IECC for commercial buildings allows both testing and deemed to comply methods. Air leakage typically occurs through gaps, cracks, holes, windows/doors, fireplaces, recessed lighting fixtures, venting fans, and HVAC system components. With the implementation of the 2012 air leakage requirements, one of the major factors in moisture movement has been limited. However, since the requirements relate to the entire structure, the movement of air into or out of an individual assembly can still impact that assembly ’s performance.
For example: if a given wall assembly relies on the gypsum wall board as the primary defense against air leakage from the inside, attention to detailing can result in the resistance of most air movement from the inside to the outside. However, if the same level of detailing is not provided on the exterior, moisture can move from the exterior of the assembly to the backside of the interior gypsum wallboard.
The reverse is also true.
If an equally high level of detailing is provided on both the interior and exterior surfaces of the wall assembly, moisture movement by air intrusion is extremely limited.
However, the low levels of air exchange in the structure can lead to high relative humidity i n a structure’s interior space and even to air pollution. The code does not require any monitoring or control of interior air quality.
Diffusion through the wall’s materials of construction is then the remaining method of moisture movement.
This movement has typically not been too significant in residential structures. However, since the movement of moisture by air leakage has been minimized, this now can become a more significant factor.
Material performance is impacted by both pressure and temperature differentials as well as water absorption properties of each material.
6 Guidelines
6.1 Colder Climates (Zones Marine 4, 5, 6, 7 & 8):
1. Keep sheathing warm – if using foam insulated sheathing by itself or over or with other sheathings, use
IRC Table R702.7.1 or IBC Table 1403.3.1 to calculate foam sheathing thickness to reduce impact of moisture in cavity from freezing or condensing on inside of sheathing.
2. Do not use Class I or II Vapor Retarder on inside face of exterior walls in colder climates. Use Class III
Vapor Retarder – see IRC Table R702.7.1 or IBC Table 1403.1 to determine when this is acceptable. This allows wall to dry to the inside when a lower permeable product is on the exterior.
3. Using cavity insulation that reduces air movement through cavity will reduce passage of moisture carried by air.
4. Keep sheathing dry from the outside – use ventilated siding or rainscreen gap and pay attention to flashing details at WRB layer (see IRC R703.2 or IBC 1404.2). In the case of foam insulated sheathing, the sheathing may qualify as the WRB, if the joints are taped.
5. Carefully detail interior Class III Vapor Retarder joints and penetrations to reduce moisture transmitted by air flow and to help meet assembly air barrier requirements.
6.2 Warmer Climates (Zones 1, 2, 3, & 4):
1. Use foam insulated sheathing by itself or over or with other sheathings to keep wall cool. Keeps humid outdoor air from entering the building cavity, which is critical during the cooling season.
2. Don’t use Class I or II Vapor Retarder on inside face of exterior walls (IRC R702.7 or IBC 1405.3). This allows wall to dry to the inside.
3. Using cavity insulation that reduces air movement through cavity will reduce passage of moisture carried by air.
4. Keep sheathing dry from the outside – use ventilated siding or rainscreen gap and pay attention to flashing details at WRB layer (see IRC R703.2 or IBC 1404.2). In the case of foam insulated sheathing, it may qualify as the WRB if the joints are taped.

5. Carefully detail interior covering and exterior sheathing joints and penetrations to reduce moisture transmitted by air flow to help meet assembly air barrier requirements.
Figure 1 - IRC Figure N1101.10
Insert image of Type III VR usage (IRC Table R702.7.1)/(IBC Table 1403.3.1)

7 Best Practice Recommendations for Moisture Control
1. Control of indoor humidity (25-35% RH is recommended target unless walls are designed for higher humidity level for occupant health or special occupancy and use needs).
2. Control moisture at the source (e.g., adequately sized bathroom exhaust fans piped to the exterior and use of cook-top hoods directed to the exterior).
3. Avoid use of direct-vented gas appliances, such as a fireplace or heater, which release combustion by-products (e.g., moisture and carbon monoxide) to the interior of a building.
4. Conduct blower-door tests to identify and seal air leakage pathways that permit moisture to circumvent moisture control strategies.
5. Use a heat recovery ventilator where ventilation must be provided to meet or enhance indoor air quality.
6. Use foundation water-proofing (as opposed to code-minimum damp-proofing), ground vapor retarders and enhanced drainage methods (ground moisture is a major source of interior building moisture vapor that can impact roof and wall system performance).
7. Ensure wall materials are dry before closing in the wall (recommended moisture content of wood should be 12% or less at time of wall close-in).
8. Store construction materials in a manner that keeps them dry.

9. Apply a water-proof roofing underlayment (membrane) at roof eaves to prevent ice dams from trapping roof water and causing to run into the roof assembly and also into wall assemblies through concealed interior spaces (generally applies to cold climates where roof design snow load exceeds 20psf).
Disclaimer
This guide addresses only moisture control code requirements related to use of continuous insulation in wall assemblies. Use of this information is not a substitute for verification of building code compliance and design of specific wall assemblies and materials in actual conditions of use. In addition, other matters of code compliance must also be addressed, including structural, fire safety, sound transmission and other matters common to all wall constructions. In addition, compatibility of all materials should be verified and addressed in construction details that clearly convey the intended use of materials and installation requirements for a complete and code-compliant installation.