Ventilation: what to look for

Whether you’re building or improving your home it’s clear that insulation should be on top of your must-have list. Perhaps less obvious is the need for ventilation, which every house requires, but most of all those that are airtight and well insulated.

In not-so-well insulated homes, purpose-built vents in the windows and walls provide unregulated ventilation. The lack of control over how much air is allowed into and out of the house makes these vents unsuitable for low energy homes. In a new build or in an existing home that’s undergone an energy upgrade, it’s therefore more than likely you’ll need to rely on a mechanical system.

With the number of ventilation systems on the market specifically designed for domestic use, it can be a minefield. So here’s an overview of what systems are available, what is suitable for where and what to bear in mind when specifying your unit.

Types of ventilation systems

First off, know that there is no ‘best system’: each ventilation system serves a purpose and it’s very much a case of ‘horses for courses’. As with all building projects, your ventilation strategy needs to be ‘fit for purpose’, which means that it must comply with the Building Regulation requirements for your build and, just as importantly, it must meet your needs.

Houses built before the recent wave of energy efficient measures (2011 in ROI and 2012 in NI) traditionally relied on a combination of natural ventilation (‘trickle vents’, also known as ‘background ventilators’, in the living rooms/bedrooms) and standard mechanical extract ventilation (extractor fans in the bathrooms). The trickle vents in the windows are incorporated in the frame in the form of sliding louvers; in the wall they consist of a hole with a grille on each end – usually plastic on the inside which can be opened and closed, and galvanised steel on the outside. This system is commonly found in older houses or those with air leakage superior to 5m³/h/m² whereby air leakage or permeability is measured at 50 Pa; Pascal is the unit used to quantify pressure.

In many cases issues arose because the mechanical extract fans were fitted with long runs of ducting. This practice can lead to a significant drop in performance which in turn causes persistent humidity problems in wet rooms as the fans are meant to be fitted at the extraction point. You can now get fans that are designed to be used with long duct runs but they are considerably more expensive.

The use of flexible ducting was also prevalent in the past; these are no longer commonly used as installation must be precise to avoid condensation within the pipe and eventual leaks. That said, if fitted correctly natural ventilation and standard extractor fans can, from a ventilation point of view, provide adequate air changes.

The main drawback of this combination from an energy point of view is that the amount of fresh air allowed into and warm air flowing out of the home is often excessive as it is unregulated. This has led to the common practice of blocking up the vents to avoid draughts but doing so has disastrous consequences for indoor air quality, i.e. more pollutants and more humidity build up in the home.

This is why positive input ventilation, which allows you to control the amount of fresh air that is brought in, is an attractive option if upgrading an existing ventilation system – or had none at all! Another popular option for retrofits is demand controlled ventilation as it not only regulates the amount of incoming air based on demand (sensors detecting humidity/pollutants in the rooms regulate how much air is allowed in), but also extracts stale air from each room in the house. The ducting required to be installed is only on the extraction side, which makes it less cumbersome to retrofit than a ducted system on both ends. In fact demand controlled ventilation is becoming an increasingly popular alternative on new builds as well.

Heat recovery ventilation

The most popular choice for the new build market, but also the most costly, is Mechanical Ventilation with Heat Recovery (MVHR). Most one-off houses are extremely energy efficient with high levels of insulation and airtightness below 5m³/h/m² (as opposed to the minimum of 7 in ROI and 8 in NI).

It is in these conditions that heat recovery ventilation can, if designed and installed correctly, be the most efficient option. Even in less well insulated and airtight homes, when calculating your energy rating you will find that heat recovery ventilation contributes to a higher grade (the other systems don’t rank as high).

There are two options within MVHR systems, tee’d (commonly known as ‘branch’) or manifold. Your M&E consultant will be able to advise you on which system to choose in your particular circumstances.