A well ventilated underfloor is very to stop mould and rot, but it is very common for them to be present even with proper vents and the air quality is always horrible. But the air movement in the cavity under a house has never been quite such an issue until now, with the introduction of the heatpump.
I love my heatpump, but am aware that in an older villa in winter, when I close the doors and windows and turn it on, the new hot air in the room rises quickly, causing a vacuum down low. This is particularly an issue in two story homes where there is a lot of quick hot air movement from downstairs to upstairs. this forces the underfloor air into the home, and if you have ever stuck your head under your house, you can see that this is less than ideal.