Wall Sheathing – It’s what’s inside the wall that counts
By Paul E. Beers
With the popularity of lightweight wall construction using studs and sheathing, water intrusion due to poor design or construction errors is inevitable. One of the most critical decisions with wall system design is what type of wall sheathing to use. I have investigated hundreds of water intrusion failures with stucco, EIFS or brick veneer cladding and the degree of damage can vary widely depending upon the wall sheathing used.
This article will consider three wall sheathing types – wood (plywood or OSB), paper faced gypsum sheathing and fiberglass faced gypsum sheathing. Wood sheathing is primarily found in residential construction and gypsum sheathing is used predominately in commercial structures.
Oriented strand board (OSB) sheathing and exterior grade paper faced gypsum sheathing are typically treated to resist some wetting during construction. Fiberglass faced sheathing by design also resists some wetting during construction. All three sheathing types are recommended to be protected with a moisture barrier or water resistant air barrier as part of wall system design. The theory is that any water that infiltrates exterior cladding, joints, wall penetration or windows and doors is shed away from the sheathing and drained back to the outs ide. In effect, the intent is that the sheathing never gets wet.
The reality is that the moisture barrier must be perfectly designed and constructed to prevent any water from contacting the sheathing. Another reality is that this often does not happen. Any design concept that relies on perfection, particularly in construction, is likely to result in eventual failure.
When OSB sheathing, plywood and paper faced gypsum sheathing are subjected to water leakage inside a wall cavity, they become saturated and deteriorate. This causes them to lose their ability to support the cladding and can also lead to mold and other environmental problems. Most failures of these systems during severe wind storms reveal degradation of the sheathing due to water intrusion. Figures 1 – 4 below illustrate water damaged sheathing at a small commercial building type with similar wall construction design except for the sheathing:
Figure 1. – Water damaged OSB Sheathing
Figure 2. – OSG Sheathing rotted away
Figure 3. – Water Damaged Plywood Sheathing
Figure 4. – Water Damaged Gypsum Sheathing
In contrast, fiberglass faced gypsum sheathing is better able to withstand prolonged water intrusion within a wall cavity. Fiberglass faced gypsum sheathing manufacturers recommend that all board joints and fastener penetrations be sealed prior to the application of cladding. However, even if this is not done, the performance of this sheathing material when subjected to prolonged water leakage is far superior to the others. Figure 5 below is from the same group of projects as Figures 1 – 4 above but used fiberglass faced gypsum sheathing. Note how much less damage there is to the sheathing.
Figure 5 – Fiberglass Faced Gypsum Sheathing
I have investigated many projects with water intrusion into the wall cavity that used fiberglass faced gypsum sheathing and the damage is much less that with other types of sheathing. Fiberglass faced gypsum sheathing is more expensive that paper faced sheathing, although it is not enough to discourage its use. Based upon our field experience, GCI always recommends fiberglass faced gypsum sheathing for use as an exterior sheathing. It still needs an air and water moisture barrier over it inside the wall cavity, but if leakage does occur, the damage and subsequent repairs are greatly reduced.
Paul Beers is the Managing Member of Glazing Consultants International, LLC (GCI), a building envelope consulting firm in business since 1988. He has over 25 years experience in the window and glazing trade and with building envelopes. He is a leading expert with glazing systems and hurricane damage and protection and was instrumental in the development and implementation of missile impact tests after Hurricane Andrew hit Dade County, FL. His expertise includes windows, doors, glass and wall claddings with an emphasis on water leakage and damage. He has served as an expert witness in federal and circuit courts for windows, doors, glass and wall systems and water leakage. Paul can be reached at email@example.com.