1. Attic knee-walls. Installing an air barrier on the attic side of the wall helps tremendously. Rigid foam is commonly used, but other materials will work if insulation requirements are already met. Spray foam or sealant should be used to seal all edges and joints. The barrier needs to extend into the joist spaces below the knee walls. These joist spaces often allow massive air leakage.
2. Adiabatic walls, aka common or party walls. These walls can also leak a lot of air, both from the outside and from the next unit. Infiltration tests do not differentiate the source of air. Unwanted odors, microorganisms and other airborne particles should be kept out, regardless of the source. It is best to devise a plan beforehand for air-sealing these common walls. This plan should take into account the specific construction details.
3. Ceilings. Seal all penetrations for lights, wires etc. through the ceilings, and also seal the top plates. See Common sources of air leakage, Part 1 for more detail.
4. Bottom plates on slab. Make sure the slab is free of dust and moisture before sealing the bottom plate to slab. Also be sure not to skimp on the sealant. Please see Slabs: in Common sources of air leakage, Part 3 for more detail.
There is much crossover between air leakage in townhomes and single family dwellings. Common walls are the unique element and we'll plan to address these in more detail in a future post. Thanks!