Guide to Conversion of Traditional Buildings

6.6 Mechanical ventilation and air conditioning

Mandatory Standard

Standard 6.6

Every building must be designed and constructed in such a way that:

a) the form and fabric of the building minimises the use of mechanical

ventilating or cooling systems for cooling purposes, and

b) ventilating and cooling systems installed are energy efficient and are capable of being controlled to achieve optimum energy efficiency.

Limitation:

This standard does not apply to buildings which do not use fuel or power

for ventilating or cooling the internal environment.

6.6.1 Application of standard to conversions

In the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

6.6.2 Commentary

This standard acknowledges that it ought in most cases to be possible to avoid mechanical ventilation and air conditioning for cooling purposes, and that this is desirable in order to avoid the associated carbon emissions and costs. Where such systems are necessary, they should be designed to minimise energy consumption as far as possible.

In most cases, this standard should be relatively straightforward to meet because traditional buildings tend to represent less of an overheating risk than contemporary buildings due to smaller window sizes, greater levels of thermal mass, higher ceiling heights and greater availability of natural ventilation options.

On the other hand, climate change is increasing risks of overheating in all buildings. Conversion schemes for historic buildings may be constrained by the fact that the fenestration arrangement is existing and upgrades to insulation and airtightness may increase the risk of overheating.

Furthermore, risks to the character of culturally significant buildings from insensitive insertion of mechanical systems can be high, especially where options are limited or where there are particularly demanding servicing requirements.

The guidance in this standard is therefore intended to promote designs that avoid the need for mechanical systems. As such, there is considerable overlap with Standard 3.28 which sets out ways to assess summer overheating risk and guidance within Standard 3.14 on ventilation in dwelling types that have limited capacity for cross-ventilation.

6.6.3 Issues to be considered

Typical risks in the conversion of historic/traditional buildings

1. Changing risk profile

Climate change is increasing the risk of overheating (and thus the need for cooling) in all buildings. Some contemporary use of traditional buildings, particularly where large amounts of equipment or machinery are present, can significantly increase the amount of heat generated internally. This means that traditional solutions to avoid overheating may no longer be adequate.

2. Overheating risk

While overheating can be a relatively mild problem in some cases, it can also become a serious health and comfort risk in more extreme cases and needs to be considered at the design stage. It is usually relatively simple to ‘design out’ the risk, using the assessment tools noted in the technical standards.

3. Location and installation of services equipment

Where it is not possible to design out the need for mechanical ventilation and /or air conditioning, careful design and planning will be needed to avoid or minimise impacts on the building.

Inadequate design or planning can mean installations are left to site operatives who may not have the necessary understanding of the special needs of historic buildings.

Note that unlike water pipework, movement of air can often involve large ductwork which can be difficult to hide, especially when insulated and / or installed within the fixed layouts of existing buildings.

Potential negative impacts of new building services may include:

• Covering or obscuring of historic features or details.
• Causing direct damage during installation or subsequent maintenance.
• Inducing staining through leaks or condensation where uninsulated ductwork carrying cold air passes through warm or humid areas.
• Strengthening of the building may be required where large or heavy new equipment is needed.
• Vibration and noise from installed equipment must be adequately controlled.
• Increased maintenance requirements of mechanical services can be caused not just by the services, but by associated safety, access and maintenance infrastructure (e.g. access hatches, walkways, ladders and safety barriers).

The environmental and ongoing financial costs of air conditioning can be offset by photovoltaic panels which tend to produce most electricity when the need for air conditioning is highest. However, these have a visual impact and will need to be considered holistically within the overall context and design.

4. Significant alterations to humidity conditions

Mechanical air handling systems which significantly alter the ambient humidity levels in a building for a particular purpose or localised requirement may have unintended consequences on the structural and moisture-related aspects of the wider building which should be carefully considered.

5. Expert advice

Where mechanical ventilation and / or air conditioning is required within a traditional building, it is strongly recommended to use a Services Engineer experienced in working with traditional and protected buildings.

6. Service life of mechanical systems

The service life of mechanical systems can be relatively short, meaning that access for maintenance, repair and replacement must be considered.

6.6.4 Recommendations to meet the standard

Clause 6.6.1 notes five issues to consider regarding minimising overheating and the need for mechanical services:

• The proportion of glazing, with reference also to the need for natural light in apartments (Standard 3.16) and artificial lighting (Standard 6.5).
• The orientation of glazed areas.
• Solar shading or other solar control measures where relevant to solar gain.
• Natural ventilation (including night cooling).
• Thermal mass.

Note that the properties of the glazing should also be considered. Where a dwelling has little or no cross ventilation or a high proportion of glazing facing south (or within 90 degrees either side of south) clause 6.6.1 notes that the risk of overheating is increased but note that all five of the above aspects may be fixed or constrained in an existing building.

Standard 3.28 is then referenced and this provides two options for assessing overheating risk as part of the design process which should be followed. The first is known as the ‘Simple method’ and focuses on two factors: limiting the extent of solar gain through glazing and establishing minimum levels of ventilation openings. The second option involves the use of Dynamic Thermal Analysis modelling, which should be in accordance with CIBSE TM 59 ‘Design methodology for the assessment of overheating risk in homes’ (2017) although note this is for domestic properties only.

Clause 3.28.4 in the Technical Handbook notes several practical constraints that may affect potential solutions. Noise and air pollution from the local environment may compromise otherwise reasonable solutions, while security concerns can prevent otherwise effective use of windows for ventilation, particularly on lower floors.

Standard 3.28 makes general recommendations against active cooling (using ‘active’ mechanical processes), but acknowledges that in conversions and existing buildings, the limitations imposed by the building’s form and fabric may preclude an effective passive solution that fully mitigates risk.

In introducing the subject of overheating, Standard 3.28 mentions two useful guides: ‘Overheating in New Homes’ (Good Homes Alliance, 2019) and ‘Avoiding Summer Overheating’ (Passivhaus Trust, 2021). Both refer largely to new homes and while many aspects, such as orientation and window sizes, will be fixed within existing buildings, the following may also offer passive approaches to reduce overheating risk:

• If windows can be altered, they may be upgraded to reduce solar gain (through changes to g-values) and to allow effective ventilation. This can be combined with improvements to safety control (fire escape and accidental fall prevention) and security provisions.
• In warm periods, wind levels are often low and so cross-ventilation can be less effective than stack ventilation. In this way, both low level and high-level openings are effective in venting warm air at high level and bringing in cooler air from low level and / or shaded areas. Note that where passive options may not be adequate, mixed mode options, where mechanical assistance is provided to boost stack or other forms of natural ventilation, may help. Refer to Standard 3.14 of the Technical Handbook for more information.
• Preventing solar gain by using external shading is far more effective than internal shading, so it is important to try and find a suitable solution to external shading wherever possible. Where fixed or adjustable shading is not permitted, alternative solutions might include temporary shading which can be removed completely. ‘Microclimate’ options, such as the planting of deciduous trees or plants to passively reduce summer solar gain, might also provide a solution. Note that these are not considered under the Technical Standards and advice should be sought on planting near to existing walls and foundations etc.
• Occupancy can be a significant variable in overheating, so educating occupants in effective measures to reduce overheating, e.g. via quick start guides (section 6.8) can be a simple and effective tool with no implications for the fabric of the building.
• Following the general point above, it is important to establish that heating controls are located and set up effectively (e.g. with thermostatic control and weather compensation), and that they are fully understood by all occupants.
• It is important that all hot water pipework is insulated for energy efficiency generally (section 6.4) but note that uninsulated pipes can also contribute to overheating in warm weather.
• Internal heat gains from electrical and other equipment can be a significant factor in overheating as well as being an issue for energy efficiency more broadly. In some cases, specification can extend to some internal equipment, but this is usually beyond the scope of the practitioner and may instead be addressed (to an extent) within the quick start guide.

Where mechanical ventilation and / or air conditioning are required, guidance on minimum levels of efficiency of the systems is given in sections 10 and 11 of the Domestic Building Services Compliance Guide for Scotland (2022).

Note that the corresponding Non-domestic Standards reference the Non-domestic Building Services Compliance Guide for Scotland, along with reference to CIBSE Technical Memorandum 37 (TM37) Design for Improved Solar Control. CIBSE Guide TM52 ‘The limits of thermal comfort: avoiding overheating’ (2013) may also be useful in understanding and managing overheating. Overheating is also addressed in section 3.28 of Environment: Overheating risk.