Bulk water management is one of the main functions of a wall assembly. When rain hits the side of a building, the wall needs to direct the water down and out, away from the structure. The wall also needs to be able to dry if it does get wet.
That’s all very simple in theory, but it can get complicated in practice—especially with brick walls. Brick is porous and holds water. Some of the water that hits a brick wall soaks in, and in the case of a single-wythe brick veneer, some of it even soaks through to the back of the brick. Water doesn’t hurt brick, but it could damage whatever lies behind the brick.
The masonry trade has traditionally detailed brick to handle soaking rains with a generous air space behind the brick and flashing. The air space not only allows water to flow down the back face of the brick, but—provided it has air inlets top and bottom—also allows air to flow in and help dry the brick. The flashings collect any water that leaks behind the brick and direct it out weep holes and onto the face of the wall.
Some building scientists, however, are increasingly taking another approach. Instead of focusing on the masonry, they focus on waterproofing the structural wall that sits behind the brick. If you create an effective water-control layer on the structural wall—which is possible with today’s self-adhesive and liquid-applied WRBs—you may not need through-wall flashing. And if you use a drainable filter fabric or drainable insulation material—materials that will allow the wall system to drain, even if some mortar falls down behind the brick—on the face of the backing wall, you may not need the air space either.
Building-science expert Joe Lstiburek lays out the justification for this view in a recent paper titled “Vitruvius Does Veneers.”Lstiburek points to the “Ten Books of Architecture,”written in about the year 15 A.D. by Roman architect Marcus Vitruvius Pollio: “... if a wall is in a state of dampness all over, construct a second thin wall a little way from it on the inside, at a distance suited to circumstances, and in the space between these two walls run a channel ... with vents to the open air. ... when the wall is brought up to the top, leave air holes there. For if the moisture has no means of getting out by vents at the bottom and at the top, it will not fail to spread all over the new wall.”
Vitruvius went on to specify brick ties: “In the course of time, the mortar has lost its strength … and so the monuments are tumbling down and going to pieces ... He who wishes to avoid such a disaster should leave a cavity behind the facings, and on the inside build walls two feet thick, made of red dimension stone or burnt brick or lava in courses, and then bind them to the fronts by means of iron clamps and lead.”
There you have it: 2,000-year-old instructions for ventilated cavity walls with a structural backing wall, brick veneer, and masonry wall ties. As for the air space between the walls, Lstiburek observes, Vitruvius left wiggle room with his phrase “a distance suited to circumstances.” According to Lstiburek, the strict 1-inch air-gap requirement in today’s building code doesn’t always suit today’s circumstances. On the contrary, he argues, a practical approach is to modify the air gap depending on the main wall characteristics and the climate.
Lstiburek also takes issue with the traditional practice of through-wall flashing at window heads and windowsills. If a flanged window taped to housewrap is sufficient for wood or vinyl siding, he argues, there’s no reason it can’t work as well behind brick veneer.
Masonry wall systems have evolved rapidly since the middle of the last century. Early on, Lstiburek observes, designers embraced a two-layer concept: pairs of single-wythe brick walls tied with 2-inch to 3-inch ties (based on the strength limits of the ties). Next, designers beefed up the inner masonry walls, evolving the classic concrete block masonry wall with brick cladding—again, with an air space of 2 to 3 inches.
Those traditional walls had no waterproofing over the masonry backing wall, notes Lstiburek. They needed a hefty air space so that masons could keep the gap free of mortar droppings—otherwise, mortar would keep the space from draining, and water trapped inside the wall could pool against the masonry, creating hydrostatic pressure that led to trouble.
But these days, Lstiburek says, masonry backing walls are routinely waterproofed, as are light-gauge steel or wood-framed walls behind brick veneer. Builders now also use drainage mats, too, which drain walls, relieve hydrostatic pressure, and create a capillary break, despite any mortar droppings. Even better, Lstiburek notes, we now have materials such as mineral wool that can insulate between the brick and the house wall while still allowing drainage. If you’re building a system with a waterproofed main wall and a drainage mat or drainable insulation layer, argues Lstiburek, you don’t need any additional air gap at all—at least not for drainage or drying.
Code challenge. In an email, Lstiburek told JLC, “A code change proposal is coming.” Lstiburek and his organization, Building Science Corporation, have been successful at spurring code revisions in the past—most notably in the case of interior-wall vapor barriers, where Lstiburek was instrumental in the effort to repeal requirements for poly under drywall in most climates. Lstiburek said his suggestion to omit the air space behind the brick—provided the wall has waterproofing and a drainage mat or drainable insulation, and the plans have been stamped by an architect or engineer—has been accepted many times on a case-by-case basis by code authorities.
But Lstiburek offers a couple of cautions. In climates with freeze/thaw conditions, he says, the air space helps dry the back of the brick veneer. This space should be kept as protection against frost damage. Lstiburek says experience teaches that a gap of 3/8 inch should be plenty for those purposes. Lstiburek also notes that the gap allows masons to meet visual construction tolerances. If the backing wall isn’t perfectly regular and flat, the gap allows masons to maintain a flat face on the side people can see. The larger the building and the masonry wall area, the more room masons are likely to need. So even if building science doesn’t justify requiring the gap, the gap will remain an option that many builders will continue to choose for practical reasons. —Ted Cushman
