Origins of WRBsThe use of a “back up” material to protect substrates (the wall structure) from water damage has been used since at least the 1980s. Usually, the WRBs’ materials were variations of the EIFS attachment adhesive or basecoat adhesive, applied as a separate layer on top of the substrate. Sometimes EIFS reinforcing mesh was added as well. This type of early WRB-like back up system was not the full EIFS with drainage system we know today.
The use of WRBs became widespread in the 1990s when problems surfaced on EIFS-clad buildings, particularly residences in the southeast. Substrates and framing were damaged by water intrusion and the lack of a drainage capability in traditional barrier type EIFS was blamed. The building code people and EIFS producers got together and developed requirements for WRBs for use when EIFS is applied over moisture sensitive walls such as walls using wood or gypsum sheathing, and wood studs. Moisture insensitive substrates, such as masonry, concrete and brick, did not require new regulations. Thus, for practical purposes, this new construction requirement applied mostly to wood frame homes and some light commercial wood frame structures. The code now requires WRBs and a drainage media of some type on these buildings.
When are WRBs Required or Specified?Although they are generally only required by code on supporting walls using water sensitive materials, some EIFS contractors chose to use them anyway on commercial structures too. This EIFS with drainage takes additional materials and steps to install. Thus, such types of EIFS cost more. EIFS contractors I’ve chatted with indicate there is a 15 to 20 percent increase in the total cost.
Types of WRBsWRBs come in two basic forms. The first is a paper, sheet or film material, such as building paper or house wrap. This type of WRB is attached to the substrate with mechanical anchors such as staples. The sheet, film or paper is overlapped shingle style.
The second is a trowel-applied or spray-applied coating. This is applied to the substrate as a liquid and bonds to the substrate. The coating is applied continuously and goes over the sheathing board joints.
WRBs as Part of an EIFS or as Separate Materials?Most EIFS producers make liquid WRB materials. None make the sheet, paper or film type. A number of other building product manufacturers make liquid-applied WBRs. This type of WRB can be used with EIFS, as well as with other wall claddings such as stucco, panels and siding.
EIFS producers would prefer that their insulation attachment adhesive and their WRB materials be used in conjunction with their EIFS. This results in making an EIFS cladding system that is assembled with compatible materials for proven performance. Given that the EIFS contractor can obtain these non-EIFS WRBs separately from the EIFS components, it’s hard to know how much of the EIFS market uses WRBs.
Drainage MediaThe term “drainage media” means the way by which the drainage cavity is created between the outside of the WRB and the backside of the EPS foam insulation. This media usually takes the form of vertical slots in the back of the foam, thick vertical stripes of attachment adhesive or some type of spacer such as plastic mesh or lath.
It’s also possible to have a WRB that is itself the drainage media. For example, paper-like materials that are corrugated can be placed over the substrate with the corrugations running vertically. The paper provides the channels for the water to drain out. These types of WRBs require the use of mechanical anchors to attach the EIFS foam insulation.
Holes Through WRBsOne might ask, “What good is the WRB if it has holes in it?”-when the EIFS is mechanically attached using screws and washers. Full-scale tests of such assemblies show that the many small fastener screw holes have little tendency to leak. Thus, in reality, the holes don’t matter.
Bonding to WRBsThe nonstructural nature of WRBs made of paper, films or sheets, makes bonding the EIFS foam insulation to them impossible. With liquid-applied WRBs, the EIFS foam insulation can be adhesively bonded to the WRB.
Cracking in WRBsPaper, sheet and film WRBs don’t tend to crack per se but can be split if provisions are not made to allow for movement. For instance, this can occur at expansion joints and floor lines in wood frame buildings.
This issue is more pronounced for liquid-applied WRBs. This is because the WRB is continuously adhered to the substrate and must be able to bridge the sheathing joints. Wood based sheathing can shrink and expand due to changes in moisture content as well as open and close if the wall racks out of square due to wind or earthquakes. To ensure this does not happen, a full-scale mockup of a stud-and-sheathing wall is made and is bolted down to solid materials such as a floor slab. Force is then applied to the frame in the plane of the wall, forcing the assembly to go out of square. The WRB is then examined to see if it is strong and flexible enough to resist such forces.