October, 2007

Does Permeability Matter in a Housewrap?

The more quickly a wall dries, the better.

By Steve Easley

I often get questions regarding the permeability of building wraps. Permeability is the ability of any material to allow water vapor to pass through it. The 2006 International Residential Code requires a weather-resistive barrier or WRB, behind all cladding systems.

 

This code change is long overdue. Builders in recent years have been plagued with water intrusion and mold-related problems. These problems stem primarily from untrained field forces not installing weather-resistive barriers and flashing properly.  In addition, there is widespread lack of attention to moisture management detailing in general.

Because of this, builders are spending a great deal in unnecessary callback costs.

 

The concept of weather-resistive barriers is certainly not a new one. For years builders used building paper over walls with diagonal 1x4 or 1x6 skip sheathing. This system had 1-in. continuous gaps that required secondary protection. It was and still is a common practice to install paper directly over the studs for stucco cladding in some parts of the country where sheathing is not required for storm or seismic reasons. 

 

Why the Change?

 

Builders started moving away from weather-resistive barriers more than 50 years ago when the code was changed to allow for 4x8 panels to double as a weather-resistive barrier. This was a mistake, because the wood 4x8 panels absorb water and by nature, they are hydroscopic. Even walls constructed with 4x8 panels that are not wood-based still have more than 2,000 linear feet of seams in a typical home. These seams are common water entry points.

 

This practice is complicated by the fact that some of today's exterior cladding systems allow more water into the wall system. Vinyl siding, for example, is manufactured with weep holes to allow for water to escape.

 

Brick and stone installation often leaves excessive amounts of mortar behind the brick, acting as a reservoir to store water. Moisture always flows from wetter to dryer areas.

 

Porous synthetic stone is applied with no air space or capillary break, substantially reducing the wall’s ability to drain and dry. All cladding systems leak—it’s not a matter of “if,” but of when.

 

Building wraps and properly integrated flashing are an excellent solution to most water and air infiltration related problems. Most water, mold and moisture-related problems are caused by:

 

1. Water leaking through the claddings

2. Air laden with moisture piggy-backing air currents, thereby wetting building materials and in some

cases, actually condensing on cold surfaces.

There is no doubt that as more states adopt the new codes, builders will be faced with a lot of choices when selecting a building wrap.

 

Why Permeability Matters

 

Some people might be inclined to conclude that because building paper has been around for so many years, a perm rating of 5 or 6 must be optimal for building wraps. In my opinion, this isn’t true.

 

An excellent real world example is the number of failures associated with glue applied EIFS (Exterior Insulation and Finish System) over OSB. Expanded polystyrene, the major component of EIFS, has a perm rating in the 5 to 7 range.

 

When wall components get wet, you want them to dry out as fast as possible. The only way wet building materials can dry is by evaporation. A high perm wrap allows sheathing to dry much faster. The chart below shows the drying times of three pieces of OSB, saturated to approximately 25% moisture content. The three samples were:

 

1. Placed in a screen pouch (unwrapped)2. Placed in 58-perm pouch 3. Placed in a 5-perm pouch 

At the end of three days, the OSB sheathing covered with a 5 to 7 perm product was at about 23% moisture content. The OSB covered with a 58-perm product was very near the unwrapped OSB’s moisture content of 14 to 15%. After 10 days, the OSB in the 5-perm pouch was still at 20% moisture content. My conclusion is that the higher perm wrap works much better to allow the sheathing to dry faster.

 

Conclusions

 

1. All cladding systems leak.

2. When building materials get wet, you want them to drain and dry as fast as possible to reduce the

potential for mold growth.3. A building wrap is an economical solution to reducing water and air infiltration.

4. Buildings get wet very quickly as a result of leaks due to penetrations and so on. They take a long

time to dry out. They can only dry by evaporation and diffusion, which is a very slow process. The less

permeable the wrap is, the longer wet building components take to dry out.5. Wrap and flash all penetrations.

6. Create a structure that is watertight before the cladding is installed.