We see it all the time in the old city cores; new additions attached onto the old existing solid masonry or double brick. New construction techniques often rely on flexible air barriers like Tyvek or Typar and it begs the question “What do we tie the exterior air barrier into when we transition from new to old?”
New addition on an old double brick house: After installing the CN Tower Antenna the crew will have to decide how to tie the air barrier from the new to the old existing structure.
New Construction
Typically – in new construction – the flexible air barrier will be strapped from the outside to create a drainage plane, ditto if they used insulated sheathing. This drainage plane will ensure that water isn’t held in the gap by capillary forces and can drain out of the wall assembly to allow the wall to dry. This is good.
New Construction detail: The flexible membrane embedded in a mortar joint will help drain the drainage gap. Note above that even in new construction, the mortar squeeze-out is significant and may block free passage in drainage plane.
Vintage Double Brick
Typically, the vintage load-bearing double brick with regular tie courses produced an air gap between the two wythes and this inner gap is usually partially filled with excess mortar squeezed out when the bricks were pressed together. A close examination of any masonry wall on any of the unexposed faces shows how inconsistent the mortar joints were. Voids very common in the rough work.
Typical example of a double brick wall showing the air/mortar gap between wythes of brick. Note on the inside the white parging in vertical strips ~16″o.c. Often, a parge coat was applied on the inside face between the wood strapping that held up the wood lath on which plaster was applied.
Beam Pockets: Though this is an exterior face of a 1900 building, and the retrofit done in the 1950 (note “Corduroy” brick as bearing plates) the same applies to the inside where the joists inlet into the brick -ends fire cut – were only covered by one layer of brick on the outside. Notice how much mortar is missing in the joints, the same holds today on most hidden courses of brick.
Many confuse this gap as the modern equivalent to the drainage plane behind modern brick veneer. Unlike the modern non-load bearing brick veneer walls that have a drainage plane, the vintage walls have no intended drainage plane and have no through flashing to contain or drain water out of the wall assembly.
Getting it from both ends: The first course of brick on a foundation wall are subject to accelerated moisture damage either from wicking water from the foundation wall below or from rain water intrusion from the wall above. The brick faces are popping off (spalling) in the winter months
Because vintage solid masonry homes have no effective way of draining water out of the wall assemblies, it is critical that driving rain doesn’t get into the assembly in the first place and the best way to ensure that is by repointing mortar where necessary and by ensuring that all penetration (vents, windows, wires, doors) be sealed and flashed well to avoid gross water ingress. The other is to ensure that the foundation wall is waterproofed on the exterior (yes, the exterior) to prevent water from traveling up the wall by capillary action.
Brick is not an effective exterior air barrier
Brick is not an effective exterior air barrier either in new or vintage construction. BlueGreen Group has been doing research on these old buildings and the air leakage is significant and can range from 9 to 14 air changes per hour at 50 Pascals of depressurisation in the original state. Brick is too porous to be an effective air barrier and is difficult to seal against. So where do we tie the new into the vintage? Ideally, the house wrap would be bonded continuously into the old air barrier – the lath and plaster – and at least offer a place for the future vintage retrofit to tie into the new if and when that renovation takes place.
As the new gets woven onto the old, the additions on the front, back and top all require a continuous seal to the vintage structure.
Fiber vs Foam in Vintage Homes
Because brick is subject to such high air leakage, it will hamper the performance of fibrous insulations like cellulose and Roxul. To improve their performance, these fibrous insulations would ideally have an air barrier layer on the cold side to prevent wind from washing heat out. The alternative is to use 2LBS spray foam as it provides an all inclusive solution (air, vapour and inuslative layer all in one coat) and may be quicker to install that laying in an barrier against the brick.
An interesting take on installing an air barrier on the cold side was explored recently in an article in Fine Home Building called “Buttoned Up for a New Century” where author Jeremy R.M Shannon applies a liquid air barrier to the inside face of the brick. In this case they used an elastomeric Sto Guard product called Gold Coat. This allowed them to get extremely air tight performance – better than spray foam – and allowed them to use cellulose insulation which was more environmentally friendly than spray foam.
A product like this might be a great place to tie the exterior air barrier to from additions as long as the drainage planes are diverted outside the brick.