Do I need a PIV unit to stop condensation and mould?

What this usually means

Positive input ventilation works on a simple principle: a unit, usually mounted in the loft, draws in air, filters it, and supplies it gently into the home — typically through a central hallway diffuser — at a slight positive pressure. That continuous trickle of drier air dilutes the moisture-laden internal air and encourages it to leave through the building's natural leakage points, lowering the average humidity throughout the dwelling. Because the whole house is gently flushed with fresh air, a PIV can be genuinely effective against condensation and mould where the underlying problem is simply that the home is too humid overall — moisture from cooking, washing, drying and breathing building up faster than the existing ventilation can remove it.

But condensation and mould are not always a whole-house humidity problem, and this is where PIV can disappoint. If the mould is concentrated on a particular cold surface — a thermal bridge in a reveal, a cold corner behind furniture, an uninsulated wall — the issue is that surface falling below the dew point, and gently lowering the average humidity may not raise that surface above it. If the moisture has a specific source, such as a leak, penetrating damp or rising damp, no amount of ventilation will fix it. And if a bathroom or kitchen produces large bursts of moisture, that is best removed at source by effective extraction, not diluted across the house after it has already spread. In these cases a PIV treats a symptom partially while the real cause continues.

There is also the question of whether the home suits a PIV. The unit relies on slightly pressurising the dwelling, which works best in a reasonably airtight home where the introduced air actually displaces the stale air rather than escaping immediately; in a very leaky house the effect is diluted. The air source matters too — drawing from a cold loft can introduce cool air that occupants find uncomfortable, and the loft air must itself be suitable. For many homes, the more appropriate answer is good extraction at source, or a heat-recovery system that ventilates while retaining warmth, rather than PIV. Deciding whether you need a PIV therefore means first establishing why the home is humid, where the mould actually forms, and which ventilation strategy fits the building — which is exactly what an assessment determines.

Common causes

Signs and symptoms

Condensation and mould occur where the relative humidity at a surface stays high enough for long enough — which happens when the air's moisture content is high, the surface is cold, or both. Ventilation strategies act on the moisture content of the air; they do not, in themselves, warm surfaces. PIV reduces the average vapour content of the indoor air by continuously introducing drier air and displacing humid air through the leakage paths, which lowers the room-air dew point. Where the surfaces are only marginally cold and the problem is generally elevated humidity, lifting the surface relative humidity off the threshold can be enough to stop mould. Where a surface is well below the dew point because it is a genuine thermal bridge, modestly lowering the air's moisture may not raise the surface humidity sufficiently, so the mould persists despite the ventilation.

The distinction between distributed and localised moisture is therefore central. A whole-house dilution approach like PIV is well matched to distributed moisture generation — the cumulative output of occupancy spread through the dwelling — because it lowers the background humidity everywhere. It is poorly matched to point sources: a plumbing leak, penetrating or rising damp introduce liquid water that ventilation cannot remove, and concentrated vapour bursts from cooking and bathing are most efficiently captured at source by extract ventilation before they disperse and condense elsewhere. Diluting that moisture across the house after it has spread is far less effective than removing it where it is produced, which is why source extraction frequently outperforms whole-house dilution for these cases.

Finally, PIV's mechanism assumes the dwelling can be held at a slight positive pressure, which depends on the envelope being reasonably airtight; in a leaky building the introduced air escapes too readily for effective displacement, and the supply air's temperature and quality — often drawn from the loft — affect comfort and acceptability. A heat-recovery ventilation system, by contrast, supplies and extracts in balance while recovering warmth, suiting airtight homes that need controlled fresh air without heat loss. Choosing between source extraction, PIV and heat-recovery ventilation is thus a building-physics decision driven by where the moisture arises, how cold the affected surfaces are, and how airtight the home is — which an assessment of humidity, surface temperatures and the moisture sources resolves, rather than assuming a PIV is the answer.

How to decide whether a PIV unit is right for you

Diagnose why the home is humid and where the mould forms first, then choose between source extraction, PIV or heat-recovery ventilation to match the cause and the building.

1. 01

   Identify the moisture pattern

   Establish whether the humidity is general across the home or localised to a cold surface or source.

2. 02

   Rule out a moisture source

   Check for a leak, penetrating or rising damp that no ventilation can fix.

3. 03

   Check source extraction

   Ensure kitchen and bathroom extraction removes moisture bursts at source before they spread.

4. 04

   Assess suitability for PIV

   Confirm the home is airtight enough and has a suitable air source for positive input ventilation.

5. 05

   Warm cold surfaces where needed

   Where mould is on a thermal bridge, address the cold surface as well as the humidity.

6. 06

   Choose the right strategy

   Select source extraction, PIV or heat-recovery ventilation to suit the cause and the building.

How to prevent it coming back

• Match the ventilation strategy to where the moisture arises.
• Remove moisture bursts at source rather than diluting them later.
• Address cold surfaces as well as humidity where mould is localised.
• Fix any leak or rising damp before relying on ventilation.