Positive Input Ventilation (PIV) is marketed as a cheap ‘Silver Bullet’ solution to damp, mould and condensation problems in poorly ventilated houses. Below are some key things to consider before installing one in your home. For a more general overview on why ventilation is important in your home see my post on ventilation here

It may compromise the fire strategy for the building.

Most houses in the UK are designed to resist fire sufficiently to allow the people inside them to escape if they catch fire.  

To achieve this the structural integrity of the building is designed so that things like floors, walls and supporting beams will have enough resistance to fire that they don’t fail or collapse during the early stages of a building fire.

This strategy also relies on the occupants being alerted to the presence of a fire early enough to get out of the building within a certain period of time.

This part of the strategy is normally achieved through smoke alarms and heat detectors.  These may be battery operated or mains wired and interconnected.  These detectors are normally positioned in the hall and landing of the property, although some newer properties also have heat detectors in the kitchen this is not the case in most houses built before around the turn of the century.  

This system relies on smoke making its way from the source of the fire to the smoke detectors in the circulation spaces.

Mechanical Ventilation systems are designed to extract air from the bathroom and kitchen with air supplied – via supply ventilation, trickle vents etc.  Air moves in through the supply vents and migrates via the circulation spaces to the wet rooms where its extracted passing the fire detectors on the way.

Positive Input Ventilation typically pressurises these spaces making it harder for the smoke to get to the detector, potentially increasing the time the fire can burn before the occupants are alerted to it.

This risk can be mitigated by increasing the number of detectors in the property and linking it to the PIV unit, but this is far from standard installation practice, and it is not always practical or desirable to fit interconnected smoke and heat detectors in every room.

Houses are designed to have their air sucked out not pushed in.

Ventilation is covered under the Building Regulations part F.  All of the systems described within this document rely on some form of extraction provided by either passive or mechanical means.

Natural Ventilation  

This is the most common form of ventilation for homes in the UK.  It relies on the building fabric being pretty leaky (around 1 air change per hour) with fans in the wet rooms to ventilate intermittently when cooking/washing etc.  Air is sucked out of the wet rooms and drawn in through the trickle vents in the windows, or air bricks through the walls, or through anywhere else in the building the air can come from to balance the pressure.  The problem with this is that if the vents are closed or there is insufficient wind, the house is draughty, cold, and expensive to heat.

Mechanical Extract Ventilation

Air is extracted from a centralised fan – normally in the loft, at a constant rate which can be boosted as required for cooking/washing etc

Whole House Balanced Mechanical Ventilation without heat recovery.

Air is extracted by a centralised fan – normally within the loft – at a constant rate which can be boosted as required for cooking/washing.  Air is also supplied to all habitable rooms at the same rate balancing the pressure in the building ensuring the air is supplied at the correct rate but at the same temperature as the outside air

Whole House Balanced Mechanical Ventilation with Heat Recovery

Air is extracted by a centralised fan – normally within the loft – at a constant rate which can be boosted as required for cooking/washing.  On its way out of the building the air passes through a heat exchanger cooling it down.  Air is also supplied to all habitable rooms at the same rate balancing the pressure in the building ensuring the air is supplied at the correct rate.  As it enters the building the air passes through the same heat exchanger warming the air up to almost the same temperature as the building ensuring thermal comfort.

All these systems are acceptable to building control in new and existing buildings as they work with the fabric of the building ensuring that the warm moist polluted air is taken out of the building in the correct way through the correct parts of the building. 

Positive Input Ventilation

PIV is not directly supported by the Building Regulations because it works against this by supplying air into the house – normally via the first-floor landing.  Where that air ends up is unclear – it is always uncontrolled as there is no guidance on sizing exhaust air outlets nor any recommendations on where to locate them.

Where the air goes will be the path of least resistance which can basically be anywhere, and as soon as that air hits its dew point it will release moisture, which can create huge problems where it can’t be seen.  

For example, one of the hardest areas to treat for air movement is where joists are built into solid walls.  If moisture forms here, it can cause the ends of the joists to rot and in extreme circumstances for the floor to collapse.  This applies to all suspended floors – whether at ground or upper floor levels.

Up to 50% of the air goes straight back into the loft anyway.

What we do know from a study published by the BRE (IP12/00)  is that up to 50% of the air supplied goes straight back into the loft through the existing loft hatch and other air leakage around light fittings etc.

This is not where it needs to go.

The evidence doesn’t support their use.

There are no peer reviewed papers to back up any of the claims in sales literature just woolly claims about scientific studies.  

The only paper on this we are aware of which looked at PIV in any detail is BRE IP12/00 which notes the following with regards to the field tests they undertook:

The fans were generally only providing 50% of the air they were supposed to as the rest was escaping through the loft hatch etc.

In 1/3 of cases the input ventilation was not consistently effective in reducing relative humidity

Input ventilation was found to be effective in reducing relative humidity levels by around 10% – although no baseline is provided for comparison.

People were more enthusiastic about the effectiveness of input ventilation than the measured results suggest.

Those with the highest humidity levels in their homes before the study were most impressed.

Where it was effective it was differently effective between different rooms in the same house.

Do you really want to breath the air from your loft?

Loft spaces often have odour, condensation, and mould problems of their own – do we really want to blow these into our living spaces.  Most units have G4 filters which will clog up over time and if they are not replaced regularly will start to form ideal environments to breed bacteria and mould which will then be blown into the building – they will need to be changed regularly – there is no such thing as a maintenance free filter. 

If you do have issues with damp, mould, condensation, and indoor air quality and would like to deal with it as part of a whole house retrofit strategy…