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Designing compliant ventilation systems for existing buildings

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How to design a good ventilation system that complies with the new regulation for an existing building

By Ruth MacEachern

Product Manager

Feb 27, 2023

Thus far, our blog series has focussed primarily on the installation of mechanical ventilation systems in new build properties. With a great deal having been said regarding the essential need for advance planning and design in order to achieve an efficient and effective installation, it stands to reason that renovating existing installations or installing new MVHR systems in existing residential properties is not without its challenges. In this article, we will look at the factors to be taken into account when installing a ventilation system in an existing building, highlighting the legislation and requirements that must be complied with, and offering tips to help you achieve compliance in an installation or refurbishment.

System upgrade or new installation?

Whether you are attempting to improve the thermal efficiency of an older building with a view to reducing heating costs, or are extending a property, considering whether and how to upgrade the building’s ventilation is essential on two fronts:

  • Firstly, cold bridges, such as areas in the home that cannot be insulated as effectively as others, can result in the formation of condensation, which in turn results in the development of mould. It has been shown that upgrading the glazing installation, and draft-proofing in a property can increase the risk of black mould three-fold. Updating the ventilation of a property alongside a renovation project is an essential factor in ensuring a healthy and comfortable living environment within the property.
  • Secondly, updates to government legislation regarding air flow rates and emissions values applicable when renovating existing properties mean that careful consideration in home ventilation is no longer just about making the home mould-free and reducing heating bills.

One key decision here is whether to upgrade an existing ventilation system, or to install an entirely new system. In a bungalow with a cold loft, for instance, renovation work may be minimal, with an MVHR unit installed in the loft, with ducting running beneath the soft insulation and only small openings required for vents in the various rooms. Two-storey homes may pose a greater challenge, although in some cases major renovation can be avoided by concealing riser pipes in built-in wardrobes, for example.

If adding an extension to a project, single-room MVHR units are an option, although you will have to plan where in the new room the unit will be installed when planning your construction work. If limited to a less invasive approach, you could of course replace existing air vents and trickle vents with more modern and efficient models, and upgrade extractor fans in kitchens and bathrooms.

Approved document F and the new homes strategy

The government has recently set ambitious targets aiming to achieve net zero carbon emissions by 2050. Under the “Future Homes and Building Standard”, the government has set new targets and implemented new requirements regarding the energy efficiency and green credentials of new build homes, however, renovations to existing requirements are also subject to new and more stringent requirements.

The section of the UK building regulations concerning ventilation and air flow in residential buildings, Approved Document F, contains an entire section (Section 3) addressing ventilation requirements when renovating existing properties, while also providing a commissioning checklist to help developers and homeowners ensure compliance. So what exactly are the requirements? They document a list of work in existing dwellings that involves work on ventilation systems:

  • adding a habitable room
  • adding a wet room
  • replacing part of the ventilation system

as well as work that will affect the ventilation system of the respective dwelling:

  • replacing a window or door
  • carrying out energy efficiency work

In the former case, the requirement is that the renovations must meet the same standards as outlined in the document for a new build, while in the latter case the work must either meet the standards in the relevant approved document, or should not be less satisfactory than before the work was carried out. In other words, if you want to reduce energy consumption when designing a residential ventilation, improve the living environment, and ensure compliance with the relevant legislation, the standard you must aim to achieve is the same as that applicable in a new build. More detailed information on the ventilation requirements when constructing a property that is compliant with the government's future homes and building strategy can be found in our article on the subject, available here.

How to ensure compliance

For each of the projects identified above, Approved Document F provides detailed information regarding the required air flow rates, and the various methods of ventilation. Specifications vary depending on the “background ventilator equivalent area’ of the added or renovated space, along with other factors, such as whether or not the room is connected to another habitable room. Table 3.1 in the document even goes so far as to categorise required work according to whether they require major or minor renovation. All that being said, the level of investment involved undoubtedly merits the consultation of an expert. Measuring air flow rates, the whole dwelling ventilation rates, and planning an appropriate ventilation strategy requires specialist equipment and expert knowledge. While the document claims to offer a “simplified method” by which to determine which additional ventilation provisions are required, it also suggests “seeking expert advice, which may include carrying out an air permeability test that follows the procedures given in Approved Document L, Volume 1: Dwellings”.

If the “simplified method” doesn’t seem quite so simple at all, the experts at EnviroVent would be happy to provide advice on the selection of appropriate products and the planning of the installation, assist you in ensuring compliance, and put you in touch with a qualified contractor. Furthermore, if these requirements represent an additional unexpected expense, on top of already climbing material costs, you should check out our article on how to save costs in building ventilation design, available here.