A PIV (positive input ventilation) unit draws fresh, filtered air in, usually from the loft, and gently pushes it into the house, which displaces moist air out through the natural gaps in the building. In the right property it can make a real difference to condensation, but it won't fix leaks, rising damp or penetrating damp.
PIV units are one of the most effective tools I use for condensation, and also one of the most oversold. Let’s get both sides on the table.
How a PIV unit works
A typical house unit sits in the loft, with a diffuser in the ceiling, usually on the landing. It draws in air from the loft space, which is fresher and often drier than the air in the house, filters it, and pushes it gently into the property. That puts the house under a slight positive pressure, so the stale, moist air is pushed out through the gaps around windows, doors and vents and replaced with fresher air. Many units have a small heater that tempers the incoming air in cold weather so it doesn’t feel draughty. There are also wall-mounted versions for flats and homes without a suitable loft.
When it helps
- Houses with general condensation and mould in several rooms, especially bedrooms.
- Homes that have been made more airtight with new windows, doors and insulation without the ventilation to match.
- Rental properties, where it gives a steady background ventilation that doesn’t depend on anybody remembering to open a window.
When it won’t
- Rising damp, penetrating damp, leaks and bridged DPCs. A PIV unit moves air, it doesn’t stop water getting into walls.
- Rooms with a big moisture source of their own. Bathrooms and kitchens still need their extractor fans.
- Properties where it can’t be installed or set up correctly, such as a poorly sealed loft hatch drawing the unit’s own air back in.
Things to check
- It needs fitting and wiring properly, with the electrical work done by someone competent.
- Settings matter. Too low and it does nothing; too high and you may feel it. It’s set to suit the size of the property.
- Filters need cleaning or changing as the manufacturer says.
Be wary of anybody who tells you a PIV unit is a guaranteed cure for all damp, or who makes their guarantee on other work conditional on you buying one. It’s a good bit of kit for condensation when the diagnosis supports it. Get the cause confirmed first.
When to call a specialist
If you're considering a PIV unit, a condensation survey will confirm whether it's the right fix for your home and how it should be set up. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Do PIV units work in flats?
Wall-mounted versions are available for flats and homes without a suitable loft.
Will a PIV unit make my house cold?
Many units have a heater that tempers incoming air, and correct setup keeps draughts to a minimum.
Does a PIV unit replace extractor fans?
No, bathrooms and kitchens still need extraction at the source of moisture.