What are the problems with Positive Input Ventilation (PIV)?
This article has been substantially revised.
The original version focused primarily on the technical limitations of Positive Input Ventilation (PIV). While those limitations remain valid, experience, evidence and lived accounts have shown that such a framing is no longer sufficient.
In recent years, the consequences of poor indoor air quality, damp and mould have become impossible to ignore. Cases involving serious illness, displacement and, in the most tragic circumstances, loss of life have demonstrated that ventilation is not merely a compliance exercise — it is a matter of health, duty of care and trust.
This update expands the scope of the discussion. It examines not only how PIV systems can fail technically, but how risk can be underestimated, how reassurance can replace investigation, and how human evidence is too often discounted until harm becomes undeniable.
The intent is not to condemn PIV indiscriminately, but to set out, clearly and responsibly, where the risks lie and why they demand deeper scrutiny.
When reassurance becomes a risk
Positive Input Ventilation (PIV) is often marketed as a simple, cost‑effective response to condensation, damp and mould. The premise is attractive: introduce fresh air at low velocity, dilute internal moisture, and gently displace stale air from the dwelling.
On paper, the logic is neat.
In real homes, it is far messier.
Homes are not controlled environments. They are layered systems shaped by age, alteration, maintenance history, occupancy patterns, health vulnerabilities and hidden defects. When ventilation systems interact with those variables, outcomes are not theoretical. They are lived.
This article asks a harder question than “does PIV work?”.
It asks:
What happens when a system designed to improve air quality becomes the pathway by which harm is distributed — and why are early warnings so often dismissed?
1. PIV depends on air drawn from spaces we rarely interrogate
Most PIV systems rely on air sourced from a roof void or loft. This design choice rests on a critical assumption: that the loft environment is benign.
In practice, many lofts are anything but.
Common conditions include:
- Cold surfaces prone to seasonal condensation
- Historic or ongoing roof leaks
- Long‑standing mould growth on timbers, felt and insulation
- Accumulated dust, fibres and debris from decades of occupation
- Disturbance from retrofit works, rewiring or insulation upgrades
These spaces are rarely inspected to the standard we would apply to occupied rooms. Yet a PIV unit can transform them into active supply plenums, drawing air continuously and delivering it directly into living spaces.
If contamination exists, the system does not neutralise it. It mobilises it.
2. Filtration is frequently assumed rather than proven
PIV systems are commonly described as filtered. That description conceals several practical realities:
- Filter grades vary widely and are often unspecified
- Installation tolerances can allow air to bypass filters entirely
- Filters degrade over time and require maintenance that is inconsistently communicated
- Occupants are rarely given the means to verify ongoing effectiveness
In contaminated environments, partial filtration is not a safety margin. It is a vulnerability.
Where mould spores or fine particulates are present, even small bypass routes can undermine the entire premise of protection. Once distributed throughout the dwelling, contaminants are no longer localised or containable.
3. Dilution is not the same as control
UK Building Regulations are explicit on a core principle of ventilation design: moisture and pollutants must be controlled at source.
Bathrooms, kitchens and utility spaces require direct extract ventilation capable of removing moisture and contaminants at the point of generation. PIV does not do this. It relies on dilution and displacement.
In some limited contexts, that may contribute positively. In many real‑world dwellings — particularly those with:
- High occupancy
- Intermittent heating
- Poor fabric performance
- Existing mould reservoirs
Dilution alone is insufficient and can mask underlying problems rather than resolve them.
This distinction matters. A system can move air continuously and still fail to protect health.
4. Pressurisation and the unintended consequences for building fabric
Positive pressurisation assumes a level of predictability that many UK homes do not possess.
It presumes that:
- Airtightness is reasonably uniform
- Moist air will escape harmlessly through the building envelope
- Pressure differentials will not drive moisture into cold cavities
In hybrid buildings — part original fabric, part retrofit — those assumptions often collapse.
Positive pressure can:
- Force warm, moist air into interstitial spaces
- Increase condensation within walls and floors
- Promote hidden mould growth beyond the reach of inspection
The result is not always immediate or visible. But invisibility does not equate to safety.
5. The most serious failure is not technical
Across damp and mould cases, a recurring pattern has emerged:
- Occupants report odours, discomfort or health changes
- Concerns are minimised or normalised
- Assurance is offered based on compliance or specification
- Investigation is delayed or avoided
Reassurance becomes a substitute for inquiry.
This is the most dangerous failure mode of all.
Ventilation systems are defended through documentation while the lived experience of occupants is discounted as subjective or coincidental. Yet bodies respond to exposure, not to certificates.
A family’s experience: why this matters
The following video documents a family’s experience after the installation of a PIV system drawing air from a contaminated attic space. Their account includes severe and escalating health impacts, including life‑threatening reactions in a child, repeated displacement from the home, and long‑term consequences.
This video is included deliberately.
Not as proof by anecdote, and not as a generalisation, but as human counter‑evidence — the kind that too often enters the conversation only after irreversible harm has occurred.
What is striking is not just the severity of the symptoms described, but the pattern:
- Early sensory warnings (odour, discomfort)
- Repeated reassurance that nothing dangerous was present
- Escalation to medical crisis before the system was seriously questioned
- Rapid improvement on leaving the property
- Relapse on re‑exposure
That sequence demands attention.
6. Compliance does not equal safety
Building Regulations are performance‑based. They assess airflow rates, commissioning and system descriptions. They do not measure health outcomes.
A system can be compliant and still be wrong for:
- The specific building
- The specific occupants
- The specific conditions present
When ventilation is treated purely as a regulatory hurdle, the gap between compliance and consequence widens.
Where this leaves PIV
Positive Input Ventilation is not inherently harmful. But neither is it neutral.
It is frequently:
- Oversimplified
- Installed without adequate assessment of the air source
- Relied upon in place of proper source extraction
- Defended through reassurance rather than investigation
Any system that actively distributes air must be treated as a potential health vector.
That demands a higher standard of scrutiny.
The question that must be asked earlier
The most important question is not:
“Does the system comply?”
It is:
“What if the occupants are right?”
Ventilation is about breath.
And breath is not negotiable.
If you would like to discuss ventilation strategies, risk‑led assessments or system selection grounded in health outcomes as well as compliance, please contact VENTI Group.