- Q. With a cavity wall assembly consisting of load-bearing 8-inch CMUs (concrete masonry units), 2 inches of rigid foam, a 2-inch air space, and 4-inch concrete bricks covered with cementitious stucco and an acrylic color coat, how important is dampproofing? It seems to me that any liquid water that penetrates the stucco would be stopped by the layer of rigid foam. I am also concerned that the acrylic color coat on the exterior may act as a vapor barrier on the wrong side of the wall, trapping moisture within the wall.
A.William Rose, a research architect with the Building Research Council at the University of Illinois at Urbana-Champaign and the author of Water in Buildings: An Architect's Guide to Moisture and Mold, responds: If there's a cavity in the wall, it should always be designed to manage rainwater. That's because a CMU veneer — such as brick — allows some rainwater to enter the wall, travel along mortar droppings, and create local wetting of the block.
But saying the block wall should be "dampproofed" only begins to address the serious task of detailing the cavity water-management layer; coordinating it with insulation, ties, flashing, openings, and protrusions; and possibly having it serve as the air-barrier layer.
Moreover, the sequence of the dampproofing application needs to be scheduled so that any water collected in the block during construction has a sufficient chance to dry out.
A good source of information on this subject can be found in the Canada Mortgage and Housing Corp.'s Best Practices Guide: Brick Veneer Concrete Masonry Unit Backing ($89; 613/748-2003, www.cmhc-schl.gc.ca/en/).
Where dampproofing really pays off is in protecting a cavity wall against solar vapor drive, which occurs when the sun hits the masonry exterior and steams up the rain-wetted cavity. The rigid foam insulation may alone provide enough protection against this high vapor drive, depending on the type and the continuity of placement, but dampproofing behind the insulation doesn't hurt either. Whether it's necessary depends on the strength of rain wetting, the permeability of the insulation, the quality of the mortar joints, and a few other variables. I use modeling software called WUFI — which takes exterior wetting and solar drive into account — to help me make these decisions (www.ornl.gov/sci/btc/apps/moisture/index.html).
As for the acrylic coating, any finish applied to the exterior of a building will be wetted by rain from the outside and, to some extent, by moisture in the substrate. But the layer of rigid foam practically guarantees that indoor humidity will have a negligible effect. As long as the coating is an exterior-grade product intended for use with concrete block, it should be fine.
Old rules like the one concerning "wrong-side vapor barriers" are giving way to a wider palette of building-envelope designs, thanks to our ability to model their performance.