When conducting a blower door test, an incense stick is a good tool to actively demonstrate where the air leaks are. (A smoke pen works as well but doesn’t smell as nice).
I just tested another 20 year old house today. 15 air changes at 50 paschals. Again, exhaust fans, and downlights are a massive leakage problem. These items are well worth considering seriously if you are building a new house.
As mentioned in previous posts, warm air rises, (stack effect). When warm air inside the house rises it will disappear into the roof space via ceiling penetrations such as these downlights and exhaust fans. Replacement air enters the house via other leakage points such as windows, external doors and service penetrations.
Ceiling penetrations are a chimney for warm air. The stack effect requires air flow at both high and low level. If you seal the ceiling penetrations you will effectively prevent the stack effect from transporting the air; this will in turn keep most of the warm air inside the house. Seal the other gaps too but for goodness sake have a decent think about what you’re doing when you punch holes in the ceiling.
Check out some of the smoke trails in the attached videos. The downlight leaked so badly that it almost extinguished the incense stick.