Stroma Guidance Note

Overheating - TM59

Technical Bulletin

Overheating: Technical Bulletin

stroma.com/built-environment

BUILT ENVIRONMENT

March 2019

Preliminary advice regarding where mitigation measures may be required to combat overheating in residential developments.

Overheating

Overheating: Technical Bulletin

stroma.com/built-environment BUILT ENVIRONMENT

Introduction

The overheating criteria for dwellings are now based on guidance in the UK industry wide standard document ‘CIBSE TM59: Design Methodology for the assessment of overheating in homes’ and supplemented by methodology within ‘CIBSE TM52: The Limits of Thermal Comfort’.

CIBSE TM59 has been published to specifically address overheating risks in dwellings, as opposed to TM52 which was developed to address thermal comfort in workplaces. The same underlying calculation methods of TM52 are used, but the criteria are superseded with standards more relevant to residential occupation.

The methodology defined is based on the theory of adaptive comfort. This states that the acceptable temperature for occupants in a space “tracks” the mean indoor temperature over a preceding period. Due to the correlation between indoor and outdoor temperature in free-running buildings, this means that comfort temperature also varies with outdoor temperature. Technical details of the calculation methodology, including the equations used to determine the running mean and maximum acceptable temperatures, can be found within CIBSE TM52.

CIBSE TM59 lays down different criteria for naturally and mechanically ventilated dwellings.

There are 2 criteria for the assessment of overheating on predominantly naturally ventilated homes. These criteria are as follows:

In order to demonstrate compliance with the TM59 criteria, both criteria 1 and 2 must be passed for all applicable rooms.

In the case of predominantly mechanically ventilated homes, meant to assess those where there is no or very limited opportunity to open windows, the following criterion applies in lieu of the above:

1 - Criterion 1 – Fixed Temperature Method

(Applicable to living rooms, kitchens and bedrooms)

All occupied spaces should not exceed an operative temperature of 26°C for more than 3% of the annual occupied hours.

It is important to note that should this fixed temperature method be followed for a development in London, active cooling will be required to achieve compliance. This is due to the weather parameters stipulated for the assessment. Within this weather data the external temperature is above 26°C for 3.3% of hours, exceeding this threshold, and making compliance without active cooling or air tempering impossible to achieve.

The following design advice is therefore only applicable to dwellings assessed via the ‘Naturally Ventilated Homes’ criteria.

1 - Criterion 1 – Hours of Exceedance (He)

(Applicable to living rooms, kitchens and bedrooms)

The number of hours the predictive operative temperature exceeds the maximum allowable operative temperature (θmax) by 1°K or more, must not exceed 3% of the total occupied hours, during the five summer months (May-September).

2 - Criterion 2 – Overnight Comfort

(Applicable to bedrooms only)

To guarantee comfort during sleeping hours the operative temperature in bedrooms from 22:00 to 07:00 shall not exceed 26°C for more than 1% of annual hours. (Note: 1% of the annual hours between 22:00 and 07:00 is 32 hours, so 33 or more hours above 26°C shall be recorded as a fail).

Overheating: Technical Bulletin

stroma.com/built-environment BUILT ENVIRONMENT

Overheating

Design implications – Naturally ventilated homes

1. Controllable Shading

The TM59 assessment criteria cannot be achieved without some form of controllable shading. It is up to 75% more efficient to fix this shading externally, eg. shutters or external roller blinds.

If external shading is not feasible, it is highly recommended that integrated (mid-pane) blinds are specified over internal blinds. A preliminary review of a basic white venetian inter-pane blind demonstrates an effect of reducing the window G-value from 0.60 to approx 0.13.

Internal blinds shall necessitate a far lower glazing G-value being required, resuting in a higher cost and tinted glass. This is because, by the time the heat hits the blind, it has already entered the dwelling. This can result in a radiator like effect from the blind in high-summer. Internal blinds should therefore be considered as a last resort. However, they must be installed if no other options are feasible.

Note that the TM59 assessment mandates that blinds are installed by the developer if it is part of the mitigation strategy. It is not acceptable to assume occupants will install blinds.

Note that whatever shading strategy is chosen, it shall be required to all windows, glazed doors, and any full height fixed glazing.

2. Window Design

A good rule of thumb is for a minimum of 50% of bedroom glazing area, and 40% of living room glazing area be fully openable for purge.

Large areas of glazing are not particularly opposed, however large fixed panes should be avoided. Large opening windows are actively encouraged, as this both aids ventilation and improves daylight levels. Living areas should include windows in addition to balcony doors, to allow night ventilation.

Windows should be hinged to allow maxiumum free area. If inter-pane blinds are proposed, top hung windows should be favoured, as this allows partial shading whilst the windows are open. If louvred shutters are proposed, windows should be inward opening, in order to allow ventilation whilst the shutters are closed.

3. Window Specification

Solar control glass should be specified to the South and West façades. However, this also affects the SAP calculations, making compliance more difficult.

The extent of the solar control will depend on the shading strategy undertaken:

• If external shutters are proposed, solar control glassshould not be required.

• If inter-pane blinds are proposed, glass of betweenG = 0.5-0.6 should be sufficient, to the South andWest façades depending on the blind specification.

• If internal blinds are proposed, solar control glasscould require G-values down to 0.3, dependent onorientation. Solar glass is also likely to be requiredto the South, West and East façades. This has largeknock-on effects to the emissions calculations andshould be avoided where possible.

4. Night Ventilation

A key requirement for compliance with criterion 2 is the ability to leave bedroom windows open overnight. This is not generally an issue on upper floor apartments. However, this can present problems where windows are accessible at street level.

Ground floor bedroom windows (and any other accessible windows) must therefore include security devices to allow the window to be left open overnight.

Note that a standard 100mm window restrictor is not sufficient and will not pass the criteria. It is important that windows are able to open at least halfway overnight, securely. This can be achieved either with secure louvred shutters, security bars, or other fixed measures.

Please also note that it is not possible to avoid this by installing mechanical ventilation, as in order to provide sufficient airflow, over 100 l/s would be required to each bedroom, which is not feasible from an energy, cost, acoustic or technical perspective.

It may also be possible to achieve compliance to these spaces with additional ventilated louvres to the bedrooms. However, these would have to be large enough to provide similar ventilation levels as a half-open window, which would likely not be architecturally feasible.

Overheating

Overheating: Technical Bulletin

stroma.com/built-environment BUILT ENVIRONMENT

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Preliminary advice regarding where mitigation measures may be required to combat overheating in residential developments.