Both PIV ventilation and dehumidifiers reduce indoor humidity. They work in completely different ways. The right answer depends on your problem.
Quick TL;DR: PIV wins for whole-house condensation problems (mould, streaming windows, musty air across rooms). Dehumidifiers win for one-room temporary fixes (drying out a flooded basement, a single problem bedroom, or post-leak drying). For most UK homes with persistent condensation, PIV is much cheaper long-term and addresses the cause rather than the symptom.
How they work — the basics
A dehumidifier is a self-contained appliance you plug in. It cools air past its dew point, condenses the moisture into water inside the unit, drains it (or you empty the tank), and blows the dried air back out. Works on the room it's in. Has to run for hours a day.
A PIV (Positive Input Ventilation) unit is fitted in your loft. A quiet low-energy fan pushes filtered fresh air down into your home through a single ceiling diffuser at the top of the stairs. The slight positive pressure forces stale humid air out through every gap, vent and trickle vent. Whole-house, continuous, 24/7. Full PIV explainer.
One removes moisture in one room. The other replaces moisture-heavy air with dry air across the whole house.
The cost comparison — 5 years of running
Most homeowners look at upfront cost and stop. The real comparison is total cost over the typical lifespan.
Over 15 years a PIV is roughly £3,500–£7,500 cheaper than running a dehumidifier — and that's per problem room, since most homes need 1-2 dehumidifiers minimum.
The energy use difference is staggering
A typical 12-litre dehumidifier draws around 200–350 watts continuously while running. To dry one bedroom adequately you need it running 6-8 hours a day in winter. That's 1.2–2.8 kWh per day per unit.
A modern PIV unit draws 5–15 watts continuously, 24 hours. That's 0.12–0.36 kWh per day for the whole house.
At UK 2026 electricity prices (~30p per kWh), that's:
- Dehumidifier (one room): £130–£300 a year
- Two dehumidifiers (multi-room home): £260–£600 a year
- PIV (whole house): £15–£25 a year
The effectiveness difference
Dehumidifier removes moisture from the air it processes. Whichever room it's in is drier. Other rooms? Moist. Move the unit to another room? The first one re-humidifies. Your kitchen, bathroom, hallway, kids' bedrooms — none of them get touched unless you have a dehumidifier in each.
PIV replaces humid air with dry air for the entire home, simultaneously, continuously. One unit, every room benefits. Bathroom steam clears in minutes. Kitchen condensation drops. Wardrobes stop smelling musty. The whole house breathes drier.
The hassle factor
Dehumidifier: Empty the tank daily (sometimes twice daily in autumn/winter). Move it between rooms. Keep it running. Listen to the noise (40-55 dB, like a quiet conversation). Buy a new one every 5 years. Hot exhaust air it produces makes the room slightly warmer in summer (annoying) and is wasted heat in winter.
PIV: Change the filter once a year (5-minute job). That's it. The unit is silent (you typically can't hear it from below the loft hatch). Lasts 15+ years.
When a dehumidifier IS the right answer
PIV isn't always the right tool. Dehumidifiers genuinely win for:
- Drying out after a flood — high-capacity dehumidifiers reduce moisture fast (faster than ventilation alone). Time-bounded use.
- One single problem room in an otherwise dry house — e.g. one north-facing spare room with mould, no whole-house issue.
- Properties with no loft access AND where the hallway-mounted PIV alternative isn't viable (rare).
- Renting — you can't fit a PIV in a rental (capital improvement), but you can plug in a dehumidifier.
What about a desiccant dehumidifier?
Desiccant units (Meaco DD8L, EcoAir DD1) work better than refrigerant dehumidifiers in cold UK rooms (below 12°C) and use slightly less power. But they still cost £200–£500 a year to run vs £15-£25 for PIV. Same conclusion.
Why your friend's dehumidifier "fixed" their condensation
Plug-in dehumidifiers do reduce visible condensation — for the room they're in. So when someone tells you their dehumidifier "fixed it", they typically mean: their bedroom windows stop streaming. Their kitchen ceiling mould slows down (because they moved it there). The mould comes back when they go on holiday and the unit's off for a week.
PIV doesn't have that limitation. It runs whether you're home or not, in every room, every day, year-round.
The honest verdict
For most UK homes with whole-house condensation (streaming windows + multi-room mould + persistent musty smells), PIV is the right answer. Cheaper to run, hands-off, whole-house, addresses the cause not the symptom.
For one-room or temporary problems, a dehumidifier is fine.
For renters, dehumidifier is the only option — but ask your landlord about PIV: a £900-£1,400 install permanently fixes their property's mould risk and is a far better investment for them than dealing with mould complaints under Awaab's Law.
Not sure which is right for your situation?
Take our free 60-second condensation check — based on your symptoms, we'll tell you whether PIV is likely the right fix or if it's something else (rising damp, penetrating damp, drainage failure all need different solutions).
Or just book a free 30-minute on-site survey. We diagnose first, recommend second, never push PIV if it's not the right answer.
Stop guessing. Get a free diagnosis.
60-second condensation check or 30-minute on-site survey — both free.