Design – Ventilation

DESIGN

Ventilation

Ventilation is the method by which fresh air enters the home and stale air leaves it. In the past uncontrolled ventilation, known as natural ventilation in the building regulations, was the most common method. It is cost effective as it consists of permanent wall vents or installing windows prefitted with vents. Natural ventilation is complemented by extractor fans in bathrooms and kitchens to extract moist stale air more quickly.  

This method allows for plenty of fresh (cold) air to enter the home but also for plenty of heat to escape, which has led to the practice of draught-excluding, or boarding up vents, which in turn leads to moisture problems and the danger of carbon monoxide build up.

Natural ventilation is still relied upon in most existing homes and it is still possible to adopt the method under the building regulations for new builds, although in ROI since the introduction of the 2019 building regulations natural ventilation has become extremely hard to design into a new house: this is because the new regulations make it mandatory to make the house airtight, and at the same time require that you install mechanical ventilation if your airtightness result is very good. It was already difficult to install natural ventilation with the 2011 framework of the regulations.  

Don’t forget that a combustible appliance in your dwelling requires a permanent air supply from outside. A stove which has its air supply directly fed into it through a duct is therefore preferable to the alternative of a permanent vent in the wall, ceiling or floor.

Nowadays because most new homes are built to be airtight, some form of mechanical ventilation that controls the volume of air allowed in and out, has become mainstream. By definition an airtight home only allows a minimum amount of air to enter or leave the home so a dedicated system to bring fresh air in, for the house and its occupants to breathe, is literally vital.  

Windows are only part of a mechanised ventilation strategy in that they provide purge ventilation; they cannot be relied upon for background ventilation as a certain level of fresh air needs to be steadily maintained for a good level of indoor air quality.  

A fully mechanised system will rely on a centralised unit and ducting to each room of the house. There are many different types of mechanised systems but Mechanical Ventilation with Heat Recovery (MVHR) has become a go-to solution for self-builders for its efficiency, as it recycles heat from within the home through a heat exchanger. It is a fully mechanised system and may offer some form of cooling in summer, but this depends on the model. MVHR cannot act as an air conditioning device because it is a system designed to optimise air circulation. 

Other options that aren’t fully mechanised include demand control ventilation (DCV) which also has a centralised fan but relies on humidity sensor vents in the rooms to extract stale air. Meanwhile positive input ventilation is an age-old system that relies on the passive stack effect but may have trouble working in some areas with high winds. There are variants of all of these. 

Both natural ventilation and fully mechanised ventilation systems, and hybrid solutions in between, must be designed by a professional. The ventilation system must be designed for your specific home to get the ventilation system to work as intended. For example creating a voids in walls and ceilings will allow for the larger sized ducts which reduce the noise of the system and improves its overall efficiency due to lower resistance. 

In ROI the building regulations stipulate you must have the system both designed and commissioned (tested) by an independent energy assessor. In NI for fixed building ventilation systems which require commissioning, the regulations do not specify who does it but it must be done in accordance with the procedures given in the Domestic Ventilation Compliance Guide as specified in the Building Regulations, so it is advisable to at the very least get the company you are buying the kit from to take on these roles to maximise efficiencies.

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