PIV vs MVHR vs extract vs dehumidifier
Every condensation product has a job it does well and a limit it hits. Here is the honest comparison, and a process for picking the right combination.
Quick answer
For most existing homes with a loft, positive input ventilation (PIV) at £400–£800 installed is the most cost-effective fix. MVHR (£3,000–£8,000) suits new build or major refurbishment. Continuous extract fans handle wet rooms; trickle vents supply replacement air; insulation and heating address cold surfaces. Dehumidifiers only manage the symptom.
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The measures, compared honestly
| Measure | Best for | Typical cost | Main limitation |
|---|---|---|---|
| Positive input ventilation (PIV) | Houses and bungalows with an accessible loft and a whole-home condensation and humidity problem. The most cost-effective single intervention for most condensation mould. | £400–£800 installed | Needs a loft or roof void to draw from — flats without one need a loft-less PIV variant or a different approach. Pushes air in one direction only, so it relies on the property having enough leakage or trickle venting for air to escape. Not a substitute for extract at the source in kitchens and bathrooms. |
| Mechanical ventilation with heat recovery (MVHR) | New build and substantial refurbishment, where the ductwork can be installed and the building fabric is airtight enough for heat recovery to be worthwhile. | £3,000–£8,000 installed | Retrofitting ductwork into an occupied period property is disruptive and often impractical. Only performs well in a reasonably airtight building — in a leaky property much of the benefit is lost. Filters and the unit need regular maintenance. |
| Continuous mechanical extract (dMEV / cMEV) | Flats and houses where whole-house supply ventilation isn't practical but the wet rooms can take a modern continuous-running fan. A strong baseline upgrade from old intermittent fans. | See cost guide | Only ventilates the rooms it serves — bedrooms and living rooms with condensation on external walls still need air movement. Relies on replacement air being able to enter the property (trickle vents, undercut doors). |
| Intermittent extract fans | Meeting the minimum ventilation requirement in a wet room that currently has no mechanical extract at all. The cheapest first step. | See cost guide | Does nothing between uses, so moisture produced overnight or from drying laundry is not addressed. Frequently found undersized, ducted poorly, or wired so tenants can switch them off. Often not enough on its own for an established mould problem. |
| Trickle vents | Providing the 'way in' for replacement air that mechanical extract and PIV both depend on, and restoring background ventilation lost when old draughty windows were replaced with sealed units. | See cost guide | Background ventilation only — not a fix for an active condensation problem by themselves. Retrofitting to existing sealed units can mean new sashes or new windows. Since 15 June 2022, Approved Document F requires that replacement windows do not make existing ventilation worse, which in practice means trickle vents on most replacements. |
| Insulation and cold-spot treatment | Properties where the underlying issue is cold surfaces and thermal bridging rather than moisture generation — the fix that removes the cause rather than managing the symptom. | See cost guide | Solid-wall insulation is a significant cost and, done badly, can move the condensation risk into the wall (interstitial condensation) — it needs designing, not just installing. Insulation without adequate ventilation can raise indoor humidity. |
| Heating improvements | Homes that are under-heated because heating them is expensive or the system is inadequate — a common driver of winter condensation mould. | See cost guide | Heating alone raises the air's capacity to hold moisture but doesn't remove the moisture — without ventilation, warmer air simply carries more water vapour to the next cold surface. Best paired with ventilation and insulation measures. |
| Dehumidifiers | Short-term control while a permanent fix is designed and installed, drying out a property after a leak, or a single problem room where structural work is scheduled. | See cost guide | Treats the symptom, not the cause. Needs emptying or plumbed drainage, uses electricity continuously, and only conditions the room it sits in. A property that needs a permanent dehumidifier to stay mould-free has an unresolved ventilation, heating or insulation defect. |
Full cost breakdown, including the measures priced individually: how much does condensation control cost? →
What each one actually does
Positive input ventilation (PIV)
A loft-mounted unit that draws filtered air from the roof space and gently pushes it into the home through a single central ceiling diffuser, slightly pressurising the property so stale humid air is displaced out through gaps and vents.
Best for: Houses and bungalows with an accessible loft and a whole-home condensation and humidity problem. The most cost-effective single intervention for most condensation mould.
Limitations: Needs a loft or roof void to draw from — flats without one need a loft-less PIV variant or a different approach. Pushes air in one direction only, so it relies on the property having enough leakage or trickle venting for air to escape. Not a substitute for extract at the source in kitchens and bathrooms.
Mechanical ventilation with heat recovery (MVHR)
A whole-house ducted system that continuously extracts stale air from wet rooms and supplies filtered fresh air to living rooms and bedrooms, passing the two airstreams through a heat exchanger so most of the warmth in the outgoing air is recovered.
Best for: New build and substantial refurbishment, where the ductwork can be installed and the building fabric is airtight enough for heat recovery to be worthwhile.
Limitations: Retrofitting ductwork into an occupied period property is disruptive and often impractical. Only performs well in a reasonably airtight building — in a leaky property much of the benefit is lost. Filters and the unit need regular maintenance.
Continuous mechanical extract (dMEV / cMEV)
Low-wattage extract fans in kitchens and bathrooms that run continuously at a low trickle rate and boost automatically on humidity or occupancy, removing moisture at the point it is produced rather than after it has spread through the home.
Best for: Flats and houses where whole-house supply ventilation isn't practical but the wet rooms can take a modern continuous-running fan. A strong baseline upgrade from old intermittent fans.
Limitations: Only ventilates the rooms it serves — bedrooms and living rooms with condensation on external walls still need air movement. Relies on replacement air being able to enter the property (trickle vents, undercut doors).
Intermittent extract fans
Standard bathroom and kitchen fans that run while the room is in use (and usually for an overrun period afterwards), triggered by the light switch, a pull cord, or a humidity sensor.
Best for: Meeting the minimum ventilation requirement in a wet room that currently has no mechanical extract at all. The cheapest first step.
Limitations: Does nothing between uses, so moisture produced overnight or from drying laundry is not addressed. Frequently found undersized, ducted poorly, or wired so tenants can switch them off. Often not enough on its own for an established mould problem.
Trickle vents
Small controllable slot vents in the head of a window or door frame that provide a continuous background supply of outdoor air without opening the window.
Best for: Providing the 'way in' for replacement air that mechanical extract and PIV both depend on, and restoring background ventilation lost when old draughty windows were replaced with sealed units.
Limitations: Background ventilation only — not a fix for an active condensation problem by themselves. Retrofitting to existing sealed units can mean new sashes or new windows. Since 15 June 2022, Approved Document F requires that replacement windows do not make existing ventilation worse, which in practice means trickle vents on most replacements.
Insulation and cold-spot treatment
Raising the temperature of the surfaces condensation forms on: loft top-up, cavity or internal/external solid-wall insulation, insulated plasterboard to a cold reveal or a specific cold patch, and thermal upgrades at junctions.
Best for: Properties where the underlying issue is cold surfaces and thermal bridging rather than moisture generation — the fix that removes the cause rather than managing the symptom.
Limitations: Solid-wall insulation is a significant cost and, done badly, can move the condensation risk into the wall (interstitial condensation) — it needs designing, not just installing. Insulation without adequate ventilation can raise indoor humidity.
Heating improvements
Making the home affordable to keep at a steady temperature: a more efficient boiler or heat pump, better controls (thermostats, TRVs, timers), and addressing fuel poverty so rooms are actually heated.
Best for: Homes that are under-heated because heating them is expensive or the system is inadequate — a common driver of winter condensation mould.
Limitations: Heating alone raises the air's capacity to hold moisture but doesn't remove the moisture — without ventilation, warmer air simply carries more water vapour to the next cold surface. Best paired with ventilation and insulation measures.
Dehumidifiers
A portable or fixed appliance that draws in room air, removes water from it, and returns it drier — either refrigerant (compressor) or desiccant type.
Best for: Short-term control while a permanent fix is designed and installed, drying out a property after a leak, or a single problem room where structural work is scheduled.
Limitations: Treats the symptom, not the cause. Needs emptying or plumbed drainage, uses electricity continuously, and only conditions the room it sits in. A property that needs a permanent dehumidifier to stay mould-free has an unresolved ventilation, heating or insulation defect.
How to choose: a five-step process
- 1
Diagnose the weak link
Is the problem moisture load, air exchange, cold surfaces, or a combination? A humidity reading, a check of the existing fans, and a look at insulation and window ventilation usually tell you.
- 2
Fix extraction at the source
Working, correctly-sized extract in the kitchen and bathroom, ducted to outside. Upgrade old intermittent fans to continuous (dMEV) where practical.
- 3
Restore background ventilation
A way in for replacement air — trickle vents, unblocked airbricks, undercut doors. Mechanical extract and PIV both depend on it.
- 4
Add whole-house ventilation if needed
PIV for a house with loft access; MVHR for new build or substantial refurbishment.
- 5
Warm the cold surfaces
Where cold walls and thermal bridges drive it, insulate and make the home affordable to heat steadily. This removes the cause.
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Condensation solutions — FAQs
- PIV or MVHR — which is better for condensation?
- For an existing house with an accessible loft and a whole-home humidity problem, PIV is usually the right answer: it is effective and costs £400–£800 installed. MVHR performs better and recovers heat, but it needs ductwork throughout and an airtight building, so it mainly suits new build or a substantial refurbishment. MVHR retrofitted into a leaky period property loses much of its benefit.
- Do extractor fans stop condensation?
- They help, and modern continuous-running (dMEV) fans in the kitchen and bathroom are a strong baseline. But extract only ventilates the room it serves — bedrooms and living rooms with condensation on external walls still need air movement, either from a whole-house system like PIV or from background ventilation such as trickle vents. Old intermittent fans that only run with the light are often not enough on their own.
- Is a dehumidifier a permanent solution?
- No. A dehumidifier manages humidity while it runs and is useful for drying out a property or holding a problem room while permanent work is planned. It does not fix the building defect. If a home needs a dehumidifier running permanently to stay mould-free, it has an unresolved ventilation, heating or insulation problem.
- Will more heating fix condensation?
- Only partly. Warmer air can hold more moisture and warmer surfaces stay above the dew point for longer, so heating helps — but heating without ventilation just moves warmer, wetter air to the next cold surface. The reliable fixes combine heating with ventilation and, where surfaces are cold, insulation.
- Do I need trickle vents?
- Usually yes, if you don't have them. Trickle vents provide the background supply of outdoor air that mechanical extract and PIV both rely on to work. Since 15 June 2022, Approved Document F requires that replacement windows do not reduce a home's existing ventilation, which in practice means fitting trickle vents on most window replacements.