7) Raise the core temperature of the building ............. i.e. Heating
This comes up a lot on here. I don't wish to be unkind, Cockatiel, but we do have a rather strange mindset in this country when it comes to using heating.
We have possibly one of the most miserable, damp, and unpleasant climates in the Western world in this country. Yet, we still probably spend more on shoes than we do on heating.
I've fixed this problem in plenty of properties in the past with a combination of gentle (trickle) ventilation; serious extraction; and just a little heat.
If there is Off-peak in the house, then even the smallest storage heater will do it. If not, a modern small (maybe 750w) panel heater with the thermostat set quite low.
If there is central heating, then just a modest amount of heat during the evening will help keep the core temperature up.
About the only thing I disagree with, is using de-humidifiers. I only ever use them after floods ;o)
They do work, but are simply addressing the symptoms rather than the problem. It's rather like curing a roof leak by putting a bucket under it.
Seriously... with someone who has asthma, I wouldn't even think about being mean with the heating... especially at this time of year.