18 June 2020

It is increasingly recognised that ventilation is an important aspect of maintaining a healthy environment in homes, offices and all other types of buildings and commercial properties.

The main purpose of ventilation in any building is to remove polluted indoor air and replace it with ‘fresh’ air from outside. There are a number of ways you can extract stale air and introduce fresh air into a building.

In this blog we’ll discuss why dMEV is preferable to PIV as a form of residential ventilation. Let’s start by taking a closer look at some of the common issues around air flow in housing, before exploring what dMEV ventilation is and why it is advantageous to PIV.

What is ventilation and why do homes need it?

Ventilation is the process of removing stale indoor air and providing clean outdoor air to replace it. Appropriate ventilation helps to remove air pollutants, reduce humidity, and minimise the potential for condensation and the subsequent development of mould.

Without adequate airflow an insulated and airtight house seals in pollutants, such as carbon monoxide, and moisture can condense leading to problems with damp. This can damage the fabric of a property and lead to health problems.

All dwellings should have provision for ventilation either by natural means, mechanical means or a combination of both. Mechanical ventilation is increasingly recognised as an additional necessity, particularly in new homes which are constructed to be energy-efficient and are as a result more airtight.

Even with the addition of a mechanical ventilation solution, proper indoor air control still requires an element of human input (opening windows). In urban areas where people are less inclined to open windows because of poor air quality and pollution, a lack of ventilation is accelerating problems of damp and mould. Fuel poverty also contributes to poor ventilation with residents preserving heat by keeping windows closed.

London has a widescale problem with cold, damp and mouldy homes. Low-quality social housing, overcrowding and fuel poverty are being further compounded by poorly executed retrofits that aim to save energy but fail to address the need for adequate ventilation.

As people are spending more time in controlled environments than ever before - research by insight consultancy Opinium found that Brits spend 90 per cent of their time inside - ventilation in indoor environments has become increasingly important.

Part F of building regulations provides guidance on building ventilation, including building air quality and preventing condensation in a domestic or non-domestic structure.

Common ventilation problems in residential properties

In 2017, Passive House Plus reported an emerging ventilation crisis. Despite increasing standards in insulation, it was reported that housing developers faced few requirements to provide better ventilation and improve indoor air quality.

“We may have made our homes more airtight, beefed up the insulation and installed much better windows, but the quality of air in these homes might be – if anything – getting worse.”

We have become increasingly intolerant of draughts and housing standards are getting better. Modern homes are now airtight, but the importance of ventilation is all too often overlooked.

Greenspec explain why ventilation is an equally important issue in older buildings. “In modern homes it’s recommended that 40 per cent of the total volume of air within the building is replaced every hour, in other words around 0.4 air changes per hour (ac/h). In a solid-walled traditional building up to 0.8 ac/h may be required because of the extra moisture present in the structure.”

Poor ventilation has serious consequences, both to the fabric of the building and to the health of the occupants. Excessive humidity in homes poses health risks. Mould and mites, known allergens and a common aggravating factor in asthma, thrive in high humidity environments. Overly low humidity is also known to help the spread of infection.

What is dMEV?

dMEV stands for decentralised Mechanical Extract Ventilation. It is a whole house ventilation methodology which consists of continuously running extract fans and background ventilators.

It works on the same basis as a continuously running central system (MEV), but is a local, in-room approach. Fans are located in every wet room and kitchen, running continuously at low trickle speeds to draw moisture-laden air out of the home. They can be boosted via a light switch or manual control switch.

What is PIV?

PIV stands for Positive Input Ventilations. A PIV system creates fresh and healthy living environments by supplying fresh filtered air into a property at a continuous equal rate throughout. A PIV system usually works from a loft-mounted unit with a central ceiling diffuser. It works by drawing air continuously from a well-ventilated loft space and delivers this air into the house via the ceiling-mounted diffuser. The system is designed to dilute moisture in the air and disperse through trickle vents and natural ventilation leakage.

Why is dMEV preferable to PIV?

PIV is increasingly being installed in many residential properties, especially social housing, as a supposed cure for damp and mould.

However, PIV systems are not nearly as effective as a dMEV ventilation solution. Here’s why.

PIV actually works poorly on airtight buildings and can have a detrimental effect on the fabric of a property.

Firstly, some PIV systems can shut off at 27 degrees, offering no ventilation at all. Secondly, a PIV system works by pressurising a property with clean air and pushing contaminated air out, which relies on natural paths or trickle vents. In modern airtight homes, this kind of ventilation isn’t appropriate. It’s also not uncommon for people to inadvertently block up vents in older homes, which renders PIV even more unsuitable.

PIV is classified under Approved Document F of the Building Regulations 2010 (As Amended) as an alternative ventilation strategy, but has been removed as a recommendation from a new consultation document, which sets out plans for the Future Homes Standard in 2020.

In a field study carried out by BRE, the world’s leading building science centre, it was found that PIV ventilation systems in homes were not effective in reducing relative humidity. While effectiveness improved in the most humid houses, the systems did little in drier houses and in the cases where PIV was successful, there were still inconsistencies between rooms in the same house.

Mark Brinkley, author of the House Builder Bible, says of PIV solutions, “Of all the varieties of ventilation systems you can fix into a building, this type has to be the worst.”

dMEV offers distinct advantages over PIV. Firstly, dMEV offers localised control (with PIV air flow is supplied centrally to the home and relies on building permeability for adequate circulation).

Secondly, dMEV offers humidity control at source with continuously running extractor fans located in bathrooms/wetrooms. It is worth noting that with a PIV system, any wetrooms without openable windows will still require extractor fans to be fitted. The dMEV system is also easily retrofitted into existing bathroom and kitchen fan locations.

Poor ventilation harms not only the building’s health but that of the people living in it. The key to achieving necessary air exchange, while maintaining energy efficiency in a building, is controlled whole house ventilation. dMEV systems comply with existing regulations and are recommended as a solution in the forthcoming Future Homes consultation document (PIV isn’t). Ultimately, dMEV offers a quiet and energy-efficient solution to ambient home ventilation.