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Tackling Damp and Mould

 5 reasons why dMEV is preferable to PIV

As we hit another damp and mould season, Zehnder is urging landlords to invest in preventative solutions to deter damp and save tenants from months of potential misery.

Condensation is the most common cause of damp in rented properties and poor ventilation is a precursor to this. Research has uncovered that 5.8 million tenants experience damp and condensation issues in their rented homes in the UK.

This year, the number of damp and mouldy homes looks set to be at its worst yet, with many people spending more time indoors as a result of the pandemic and occupancy levels rising as people continue to work from home. More cooking, bathing and drying clothes indoors will increase humidity levels and condensation and unfortunately that means a greater likelihood of encountering problems with damp. And as research shows a strong association between damp housing conditions and a wide range of respiratory illnesses, given the current COVID climate, more needs to be done.

As properties become increasingly airtight, intermittent extract fans are no longer fit for purpose. The good news for landlords - there are now cost effective and easily installable solutions, which can protect properties from damp issues, offering significant long-term cost savings. For retrofitting, landlords must essentially choose between two solutions, PIV (Positive Input Ventilation) and dMEV (decentralised Mechanical Extract Ventilation).

What is dMEV?

dMEV - decentralised Mechanical Extract Ventilation - is a whole house ventilation methodology which consists of extract fans and background ventilators which are fitted in bathrooms and kitchens, running continuously at low trickle speeds to draw moisture-laden air out of the home.

What is PIV?

PIV - Positive Input Ventilation - is a system that supplies fresh filtered air from a well-ventilated loft space or via a wall-mounted unit and delivers it into the house via a ceiling-mounted diffuser. The system dilutes moisture in the air, which is then dispersed through trickle vents and natural ventilation leakage.

In well-insulated, airtight properties, dMEV offers distinct advantages.

Here are five reasons why dMEV comes out on top:

  1. PIV can shut off at 27 degrees

    Some PIV systems can shut off at 27 degrees at which point they offer no ventilation at all.

    Temperature makes no difference to dMEV fans, which run continuously throughout the year.


  2. dMEV offers more effective airflow than PIV

    PIV is much less effective because it relies more heavily on natural ventilation leakage. PIV moves air to other rooms through doorways, and then out through openings in the walls, floor or ceiling of each room. While trickle vents solve this problem to a degree, it is impossible to accurately predict where air will actually go, which rooms will be ventilated and if airflow is adequate. In the winter months it is also much more common for occupants to block up trickle vents, further impacting airflow and making PIV more unsuitable.

    Where trickle vents need to be installed in all habitable rooms and in combination with dMEV, airflow is established throughout the dwelling more effectively than with PIV.


  3. dMEV offers better ventilation control

    dMEV offers localised control whereas PIV airflow is supplied centrally to the home and relies on building permeability for adequate circulation.


  4. dMEV offers humidity control at source

    With a dMEV ventilation solution, continuously running extract fans are installed in decentralised locations, meaning they are placed directly in the rooms that are subject to the most moisture, such as bathrooms and kitchens. This approach helps prevent the moisture from spreading into other rooms and offers humidity control at source (not the case with PIV). In a field study carried out by BRE, the world’s leading building science centre, it was discovered that PIV ventilation systems in homes were not consistently effective in reducing relative humidity.


  5. dMEV recommended as a solution in the Future Homes consultation document

    Although PIV is classified as a ventilation strategy under Approved Document F of the Building Regulations 2010, it has since been excluded from the Future Homes consultation - a new document setting out plans for the Future Homes Standard. The document proposes options to increase the energy efficiency requirements for new homes by the end of 2020, but also addresses the need for changes to Part F in terms of ventilation. PIV isn’t considered a viable solution for modern airtight homes because its capability relies on adequate natural ventilation leakage. dMEV, on the other hand, has been included as it is a viable low-energy ventilation solution.


Futureproof your properties with Zehnder’s dMEV.

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