Originally Posted by Lumen
Surely mechanical extract is fundamentally better for mould and condensation?
Think about the airflow.
Assuming the building fabric is leaky (otherwise you'd get HRV) then if you pressurize a room with PIV some of the air is going to leave via the hole in the wall, and some of it via leaks in the building fabric. Those leaks will take moist, warm air and condense it in the building fabric.
Alternatively, if you're depressurising a room with MEV, then that same leaky fabric is going to be pulling fresh air in, which won't cause condensation because it carries less moisture and is being warmed up as it enters.
I know this is all theoretical but I just can't see how PIV makes sense over MEV.
It makes sense because it s a cheap and quick fix. Hence companies selling it to cure your damp.
I like your explanation and you might be right, but in my case the fabric is brick/concrete so the volume of damp that's going to build up, will have little effect... i hope.
The "damp" in a house that condenses on the cold surfaces is initially relative humidity. My understanding is that its key to reduce RH as much as possible, to prevent it condensing and turning to moisture/damp. Air and its RH will exit via the easiest path, especially with the sort of pressure being put in by the PIV... its next to nothing. So PIV must be coupled with trickle venting and open windows or background vents drilled through walls. Without these, PIV will not work well..IMO.
The RH outside my house right now is ~65%. Before we had the PIV, inside would match and exceed this, 70%++.
We have trickle vents permanently open and the heating controlled via smart thermostat. The house has so far stayed warm and thankfully no sign of the damp we saw last year. Granted, we both now WFH so maybe the heating on more regular has helped too. We are getting the house wrapped next month so that will seal up more of the gaps in our fabric. I'll be watching the RH and increasing the trickle vent size if it starts to go up as a result.
PIV is worth a shot, if you can do it yourself its cheap as chips.