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Part F Ventilation

General Information


Ventilation outlined

This appendix outlines the provisions of Part F -- Ventilation of the Building Regulations 2009 insofar as they relate to new dwellings. Subject to the transitional arrangements outlined, this will determine whether the provisions of Building Regulations 2009 or Building Regulations 2002 apply.

Ventilation is vital for occupant safety and comfort as it provides a controlled amount of fresh air into the dwelling which helps control condensation and humidity as well as diluting or removing any pollutants that can build up inside a dwelling.

Ventilation and airtightness

Modern construction methods and advancements in building materials now mean dwellings are achieving greater levels of airtightness than in the past. Advancements in construction materials mean the air leakage through building materials has been significantly reduced. The effect this has had on buildings is that consideration of ventilation has become of increased significance.

Considerations for ventilation need to be made at the design stage: certain factors such as the BER rating of a dwelling or the comparison between energy-saving and good air quality must be examined during the process of choosing a ventilation system.

Traditional vs. modern ventilation

Traditional ventilation systems in Ireland were dependent on the weather conditions. This resulted in cases of over and under ventilation causing the following effects:

  • Over ventilation can cause discomfort to inhabitants by increasing the interior air temperature.

  • Under ventilation can contribute to poor air quality within the dwelling and the build-up of condensation and mold.

  • Over ventilation can have the negative effect that more energy is consumed in the heating of the dwelling.

These points reinforce the importance of installing a ventilation system that is fit for purpose and does not over-impact on energy efficiency. The right level of air tightness to ventilation must be achieved also to prevent the buildup of pollutants in the dwelling. More detailed information on airtightness is contained in Right on the Site 47 -- Air Leakage in New Dwellings.

Pollutant types

Pollutants in dwellings can be produced by a number of common sources:

  • Allergens from the accumulation of dust.

  • Odors from people, waste, cooking, aerosols, etc.

  • Carbon dioxide from combustion appliances and people.

  • Carbon monoxide from combustion appliances.

  • []{#bookmark2 .anchor}Volatile organic compounds, formaldehyde (found in certain types of furniture), and aerosols.

Water vapour build-up

Water vapour is created through household activities such as cooking, clothes drying, showering, etc. If this water vapour is not ventilated properly from the building it can cause condensation. Condensation is not always visible on the interior of the dwelling; in some cases moisture-laden air can pass through permeable building materials, cooling down as it passes through external walls and turns back into water in the cavity.

If this condensation is left for long periods of time unseen, it can cause moisture damage to the fabric of the building. Improved building materials and techniques and improvements in building airtightness help combat this but the most effective way to control ventilation is to ensure that there is a ventilation rate of 0.5-1.5ACH (air changes per hour). Research indicated that humidity levels over 70% increase the frequency of condensation, which can encourage the growth of mould. Mould will appear as furry patches or spots on material surfaces.

Mould growth and prevention

Mould growth occurs when spores in the air react to the right living conditions of finding a suitable food source such as dust, grease, etc. and when the environment is damp.

The following steps can be taken to help reduce condensation and mould growth from occurring:

  • Ensure that moisture-filled air is removed from the dwelling as soon as possible.

  • Ensure that ventilation is adequate to remove the volume of air required.

  • Try and minimize the production of moisture as much as possible.

  • Use the dwelling's heating system to remove cold areas where condensation may occur.


Homeowners often wrongly believe that condensation is due to defects in the material of the dwelling. This condensation is not caused by any defect or malfunction; it naturally occurs. It is dealt with by either opening windows or using the car's heating system to remove the condensation.

Similarly, in a house, condensation will naturally occur due to everyday tasks undertaken and from differences in temperature.

Ventilation Systems

Types and purpose

Typical types of ventilation are as follows:

  • Passive ventilation

  • Natural ventilation

  • Mechanical ventilation

  • Purge ventilation

  • Single room heat recovery ventilation

  • Extract ventilation

The whole purpose of all these types of ventilation is to provide fresh air into the dwelling for safety and comfort whilst performing in an energy efficient manner.

This publication concentrates only on natural ventilation and MVHR systems, as requirements for both these systems are contained in Part F of the Building Regulations.

Natural vs. Mechanical systems

In Ireland, traditionally natural ventilation was the main type of ventilation used. It incorporated the use of vents in each room of the dwelling with the back-up of a mechanical ventilation system to remove moisture-laden air from bathrooms, kitchens etc. Mechanical ventilation removes moisture-laden air and pollutants from the dwelling at a far quicker rate than natural ventilation.

These mechanical vents can be operated both manually or automatically. Mechanical vents located in walls, roofs, cooker hoods, etc. are used as a support system to the natural ventilation in a room. Because mechanical ventilation is not always occurring, it is important that background vents are sufficiently sized for ventilation of the areas. The minimum provisions for background vents are outlined later in the section, Ventilation and the Building Regulations.

The extract ducting for mechanical ventilation should be directed to the outside through cavity walls; care must be taken to ensure that ventilation ducting is correctly installed and not just vented into the attic space.

The negative side of natural ventilation is that it can be affected by the prevailing weather conditions. This effect can cause the dwelling to be under or over ventilated depending on the weather conditions. Due to improvements in construction detailing and materials, there is a higher level of airtightness, thus increasing the priority to design a competent natural ventilation system to compensate for this.


System outline

MVHR (mechanical ventilation heat recovery) is a form of ventilation system that recovers heat from waste air that is being ventilated from the building. It uses this recovered heat to warm incoming air. MVHR systems are often used as an alternative to natural ventilation. These types of systems are commonly designed and installed as ducted whole house systems.

The system works off the basis of extracting warm moisture-laden air from kitchens, bathrooms etc. The air is drawn through a heat exchanger box unit, where the heat exchanger removes the heat energy from the air as it passes to the outside. This in turn provides heating to incoming air in the opposite side of the transfer box going to habitable rooms.

Levels and operation

These systems can operate via two levels of ventilation:

  • High speed, which boosts the extract flow for rooms of high moisture content production.

  • Low speed, which the most typical setting left running on these systems and provides low-speed continuous ventilation.

* Depending on the specifications of the MVHR system, it may be possible to eliminate some other forms of natural ventilation.

These systems also create a huge energy saving benefit. The recycling of heat energy aids the dwelling's energy efficiency. This effect is amplified by improved airtightness as this in turn increases the effectiveness of the MVHR systems.

Benefits and conditions

As well as the major financial benefits from energy efficiency, MVHR also has the following benefits:

  • The controlled ventilation eliminates the likelihood of draughts.

  • Air filters within the heat exchanger box can remove pollutants from the air passing through it.

  • As the warm moisture-laden air is being drawn in as a heat source, it does not remain in the building or pass through the building fabric, meaning the likelihood of condensation is reduced.

  • Because the MVHR is a mechanical system at low speeds providing a constant rate of ventilation, it also means that there is a constant to the air changes per hour.

Regular maintenance of MVHR systems should be carried out as per the manufacturer’s instruction. MVHR systems should be designed, installed and commissioned only by qualified personnel; they should be installed in locations where they are easily accessible for service.

All MVHR units should be fitted with a drain and water trap, and a filter to remove pollutants.

Servicing involves using the drain to remove any moisture present in the unit and the filter must also be replaced.

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