Is a PIV unit causing damp in your home? When and why it can happen

A Positive Input Ventilation (PIV) unit causing damp in your home can be confusing. After all, the purpose of PIV is to reduce moisture, improve air quality and prevent mould and condensation. So why does damp sometimes still appear?

In many cases, the issue comes down to installation, airflow balance, or how suitable the home is for this type of system. Below, we explain the most common reasons a PIV system may contribute to damp and what you can do to resolve it.

Why can a PIV unit cause damp?

When correctly installed and used in a suitable property, a PIV system should help reduce damp rather than create it. Problems usually arise when air movement through the home is restricted or poorly managed.

PIV unit types work by gently introducing filtered fresh air, typically from the loft, and pushing stale, moisture-laden air towards the outside of the building. Ideally, this moist air escapes through background ventilation such as trickle vents, wall vents or natural leakage points.

If there is nowhere for the air to exit, moisture can become trapped around colder surfaces such as external walls and windows. This can lead to condensation forming, which may eventually result in damp patches or mould growth.

Another issue occurs when moist air is forced through gaps, cracks or penetrations in walls instead of being vented outdoors. In homes with insulated walls, this moisture can become trapped within the building fabric, increasing the risk of interstitial condensation and hidden damp problems.

Ways to stop a PIV unit causing damp

If a PIV system is contributing to damp, it’s important to act quickly.

The first step is ensuring that air can circulate freely throughout the home and escape to the outside. Without adequate exit routes, moisture will remain indoors.

In older homes, air often escapes naturally through gaps and cracks in the structure. While not energy efficient, this reduces the risk of moisture becoming trapped. Newer, well-insulated homes are much more airtight, which means planned ventilation becomes essential.

To address the problem, you may need to:

• Seal unnecessary gaps and cracks in walls and ceilings
• Install trickle vents in windows
• Add dedicated wall vents where needed

At the same time, it’s important to prevent moist air from circulating into unwanted spaces such as lofts or cupboards. Loft ventilation, service penetrations and cupboard doors can help ensure moist air exits the home rather than condensing elsewhere.

If damp problems persist, a professional assessment can determine whether your home is suitable for a PIV system or whether an alternative, such as Mechanical Ventilation with Heat Recovery (MVHR), would be more effective - particularly in modern, airtight properties.

Try these PIV units and accessories to ventilate your home

Will a PIV unit stop loft condensation?

A PIV unit alone will not solve loft condensation if the loft itself is poorly ventilated.

Because most PIV systems draw air from the loft, that space must have adequate ventilation. Without it, moist air can build up and condense on cold roof timbers and surfaces.

Installing eaves vents or roof ventilation can improve airflow in the loft. Combined with the continuous movement of air created by a PIV system, this can significantly reduce condensation problems.

Do PIV units work if windows and doors are closed?

PIV units do not rely on windows or doors being open, but air must still be able to move freely through the home.

To allow this:

• Internal doors should have small gaps at the bottom
• Windows should be fitted with trickle vents

If air cannot travel from the centre of the house to the outside, moisture may become trapped indoors, reducing the effectiveness of the system and increasing the risk of damp.

FAQs

Is a PIV unit better than a dehumidifier?

Both systems aim to reduce moisture, but they work differently. PIV units improve whole-house ventilation and air quality, running continuously with minimal maintenance. Even the best dehumidifiers are only more suitable for tackling high moisture levels in specific rooms and require manual emptying or drainage.

Does PIV make your house cold?

Because a PIV system introduces fresh air, there may be a slight cooling effect. In well-insulated homes, this is usually minimal. If cold air becomes an issue in winter, a PIV unit with an integrated heater can gently warm the air before it circulates.

Can you turn off a PIV unit?

While it is possible to turn off a PIV unit, doing so reduces its effectiveness. These systems are designed to run continuously. More advanced models use sensors to adjust airflow automatically, maintaining efficiency throughout the year without manual intervention.

Positive Input Ventilation is just one part of an effective home ventilation strategy. In areas such as kitchens and bathrooms, local extraction - such as a properly sized extractor fan - may still be required to manage moisture at its source.