Gary from Hamilton, Ontario writes :
I know that brick veneer homes have vent holes in the lower rows to allow air to equalize the pressure and help keep water out. We have just bought a 50 year old solid brick (two layers of brick) house. I notice there are no vent holes and it doesn't appear that there ever were some but they were plugged. My question is: did they not realize the pressurisation theory 50 years ago or do solid brick homes not need these vent holes? Secondly, should I knock out the mortar between some of the bricks in the lower rows for this pressurisation or is it unnecessary?
As you seem to read my stuff thoroughly, you are aware that you are talking about a "rain screen". (You can see a whole animation explaining what is a rain screen by looking up "Rain Screen" in the keywords.) The vent holes (and drainage holes at the same time) require that a whole system be in place: a wind break behind the brick that will stop the wind from penetrating into the wall (the building paper or house wrap); building paper on the wall and brick flashing on the bottom that will direct water back out the drain holes.
Without these two elements, opening holes in the wall could be dangerous as they could become entrance ways for water to get into the house, rather than drain holes. Hence, when an old house was not designed with all of these elements (especially the brick flashing from the inner wall into the mortar joint below the weeping holes) and was built simply counting on the wall resisting all winter elements, your best bet is not to try and make it a modern "rain screen", but simply keep the mortar in good repair to help it to resist as best it can. A brick wall with proper rain screen elements will stay dryer and dry out better than a solid masonry wall, but with proper maintenance, the old system has proven itself quite reliable for years.