Gareth Ryan (MCIAT) is a Member of the Chartered Institute of Architectural Technologists. Contact him at 023 8821807 or 087 7444568.
Ventilation of a dwelling is an important consideration when building a new or renovating an existing house, as part of our existence is reliant of breathing fresh air. This fresh air originates from outside with staler more odour-filled and moisture-laden air being created within a building. The main principles around ventilation of a building is to exchange the outside air with the inside air but at the same time minimise heat energy loss when carrying this out. There are two main different methods of completing this, with either natural ventilation or mechanical ventilation being the overall choices. Commonly a combination of both methods is required to be used in either choice but it depends on which is the primary method chosen, with the other acting as the secondary method. Both methods have the fall-back requirement of having what is called purge ventilation, which are openable windows and doors sized appropriately to the floor area of the house.
Natural ventilation, when used in a dwelling, sets out the requirement for the primary method of ventilation to be by natural means where reliance is placed on having enough background ventilators, which are openings in walls or windows, to provide general ventilation for a dwelling. The other characteristic of this type of ventilation is called the principle of cross ventilation where openings are placed at either side of a house so that air can flow naturally in and out across the building due to natural atmospheric pressures. Purge ventilation in the form of openable windows and doors are then used for dweller driven comfort demands to get rid of odours and to freshen up the air in spaces. Moisture laden air can also be controlled by these purge ventilation techniques but provision is also required for mechanical extract fans with minimum intermittent extract rates for spaces such as kitchen, utility and bathrooms. In general, this method of ventilation works well but one has less control over the heat energy loss of the building. The overall ventilation rates for the dwelling is dependant on the air tightness test results, which indicates the amount of air filtration in and out of the buildings fabric. In practice, to get natural ventilation to work exactly correctly, one needs to install the purge ventilation methods to at least the minimum required size and seal the house airtight as much as possible, then do an airtightness test and then come along to determine the amount of additional background ventilators required in the building to achieve the required ventilation rate for the dwelling. The design professional you employ should be able to advise you best how to achieve the appropriate solution for your dwelling.
Mechanical ventilation with heat recovery systems in my opinion allow more control over the ventilation rates in a dwelling and I consider the requirements of the building regulations easier to achieve using this method. The control of the ventilation rate is much simpler as one mechanically sets the rate at which the house exchanges internal and external air depending on it volume. The requirement for mechanical extract fans in rooms with moisture in the air is mitigated by the design of the system having extract points located in these rooms and a boost function use to increase the ventilation rates when required. The purge ventilation methods are still required for increased ventilation rates to get rid of excess odours or to remove high concentrates of pollutants or water vapour that may develop from time to time but the opening of windows or doors just for general fresh air is not required, which ultimately gives one more control over the convectional heat losses for the building through the air. The MVHR systems also recovers the heat that is in the air by transferring the heat from the air that is being extracted into the air that is being taken into the building.
Next month’s column will depart from commentary on designing new or renovating buildings to explore the features of a building of historical importance in Ireland as an interest piece.