That air space we talked about in Volume I should be at least and inch, and two inches is better
In some areas of Europe, they make it 6″. In Chicago, due to developers proclivity for increasing all available square footage in their projects, this space has been smashed down to an inch or less. A 1″ cavity is not large enough for the mason to clear out excess mortar, and that excess mortar can cause major problems.
The cavity in a cavity wall assembly has to be clear of cement to work properly. If there is cement in the cavity, drainage is blocked and the water can bridge from the exterior wythe to the interior wythe. When water bridges to the interior wythe, it soaks the block and migrates into insulation, wood framing, and drywall causing poor interior air quality, mold growth, and damage.
So, why do new buildings leak?
1) Flashing isn’t installed properly and the cavity can’t drain. Flashing has to be located above and below all windows and doors, at floor platforms, where the masonry bears on the foundation, and around and under coping and decorative masonry details.
2) The cavity is too small, jammed with cement, and it can’t drain.
3) Usually a combination of the previous two. It’s all about drainage. If you don’t have it, the building leaks.
The building code and all masonry industry recommendations indicate the cavity must be kept clear of mortar. There are tools and materials for managing mortar in the cavity that prevent moisture bridging across to interior wall. These materials work very well. The problem is, they are not used by masons working in Chicago. I have never seen any mortar or moisture management details in any masonry residential building in my entire career.
Weep holes are necessary for letting moisture that accumulates in the wall drain back to the exterior. If the bottom of the cavity is full of mortar, water can’t get to the weep holes and it can’t drain out. Additionally, masons in Chicago use rope wicks in the weep holes. The ropes do nothing to facilitate drainage, and in fact, just plug up the weep and restrict drainage. There are much better products (this and this) for providing weep capability, but for some reason, their use is nonexistent in Chicago.
So, why are all these necessary requirements for masonry walls to work satisfactorily ignored or dismissed?
I don’t know. I suspect it is largely due to the complete lack of continuing education in the trades coupled with the individualistic nature of the average tradesman. “If we don’t do it, you don’t need it” tends to be the operating motto of most tradespeople, to the detriment of the consumer.
So, if you are anticipating building a masonry house, you would do well to consider these important elements of how your building is supposed to be constructed. Ask you contractor if his mason uses mortar nets or water control membranes as part of the installation. Ask what sorts of weep hole materials they use. Problems in masonry construction can be largely or completely avoided by simply knowing the right questions to ask and then making sure your contractor integrates these critically important materials and methods into your new house.
Documents from our own local Masonry Advisory Council show various details of masonry wall construction. I will be putting up more materials and methods for how to correctly construct masonry walls, but for the time being, feel free to ask questions about any of these methods and how they relate to your house.