The £600 fix for the damp problem you've been mopping up for years.

A PIV unit sits in the loft. A low-speed fan pulls air from the loft space, passes it through a filter, and pushes it into the central hallway at a gentle but constant rate (typically 50–60 litres per second). That positive pressure displaces the damp, stale air inside the home, pushing it out through every leaky window, door and trickle vent.

It works in leaky houses precisely because the house is leaky. MVHR needs airtightness to function; PIV needs leakiness. Most pre-1990 UK housing stock leaks more than enough for PIV to do its job effectively.

The 'air from the loft' bit confuses people. Loft air in a typical UK home is drier than internal air for most of the year; the loft is ventilated by the eaves and breathes with the outside. The PIV is replacing moisture-laden kitchen-and-bathroom air with drier loft air, which the heating then warms.

PIV fixes condensation mould (the black spots around window seals and in corners), it fixes that 'stuffy' smell when you walk back into the house after a weekend away, and it dramatically reduces dust mite populations because they need humidity above 50% to thrive.

PIV doesn't fix rising damp (that's a wall problem), penetrating damp (that's a roof or render problem), or condensation that's actively pouring off cold-bridged surfaces (that's an insulation problem). If a damp survey identifies any of those causes, PIV is the wrong tool; fix the cause first.

PIV also doesn't filter to MVHR standard. It typically uses a G3 or G4 filter; adequate for pollen and gross dust, not adequate if your priority is removing PM2.5 from urban traffic.

Two flavours exist. Cold PIV is cheaper (£300–£500 for the unit) and pushes loft-temperature air into the hallway. In winter this can feel chilly at the inlet diffuser, though the temperature differential is usually smaller than people expect.

Heated PIV (£500–£900 for the unit) includes a small electric pre-heater that takes the chill off the supply air. Sounds attractive until you do the maths; the heater runs whenever the loft is below ~14°C, drawing 400–800W, and over a UK winter adds £30–£60 to your electricity bill. For most homes, cold PIV plus a well-positioned diffuser is the better answer.

No loft means no PIV; the unit needs somewhere to draw air from. A flat, maisonette or warm-roof loft conversion rules it out. In those cases you're choosing between MVHR (if airtight enough), constant-extract fans in wet rooms, or just better behavioural ventilation.

A properly insulated, recently retrofitted home with good airtightness also doesn't need PIV; it needs MVHR. PIV in an airtight house just pressurises the space and forces moisture into the fabric, where it can condense in wall voids. Wrong tool for that job.

Severe pre-existing mould or rot needs remediation first. PIV will stop new mould forming once you've killed and cleaned the existing colonies.