Building regulations - overheating risk in new homes: research report

Annex A Modelling Assumptions

The following tables detail the internal gains applied to the dynamic thermal models used in this assessment.

* Losses from a communal heating system are based on average 10W/m loss over a 21m long corridor and multiplied by two for both flow and return pipes. For pipe sizing an apartment block with more than 5 apartments per floor has been assumed.

The following section details the ventilation rates applied in the dynamic thermal models.

Infiltration Rates

The air infiltration rates have been calculated from CIBSE Guide A based on the following air-permeability rates:

• Naturally ventilated mid and high-rise apartments: 0.43 ach (based on air tightness of 6 m³/m²/hr at 50Pa)
• Mechanically ventilated mid and high-rise apartments: 0.25 ach (based on air tightness of 3 m³/m²/hr at 50Pa)
• Houses: 0.30 ach (based on air tightness of 6 m³/m²/hr at 50Pa)

All unheated loft spaces in the houses have an assumed infiltration rate of 1.0 ach.

Mechanical Extract Ventilation in Apartments

Half the apartment typologies are assumed to have MEV (Mechanical extract ventilation), with supply in bedrooms and living rooms and extract from bathrooms and kitchens. The ventilation rates are taken from the Approved Document F, Table 5.1b. The following rates are assumed to be on continuously;

• Apartments – MEV – 1 bed (46m²): 13.8 l/s
• Apartments – MEV – 2 bed (72m²) corner: 21.6 l/s

Apartments modelled with MEV are assumed to also have the facility to open windows for additional natural ventilation in the same way as the purely naturally ventilated dwellings.

In line with the TM59 standard, boosted mechanical ventilation rates are not included in the assessments.

Note, in Approved Document F a 2-bed dwelling is assumed to have 17 l/s if the second bedroom is occupied by 1 person. As the 2 bed typologies both have double bedrooms an additional 4 l/s per occupant has been added. Also, a minimum rate of 0.3 l/s is required per m2 area and therefore the ventilation rate for the 1-bed and the 2-bed corner apartments have been adjusted to meet this criterion.

Natural Ventilation

The following table shows the areas of external glazing in each dwelling type as a percentage of external wall area and the percentage of openable glazing relative to floor and wall areas.

Windows and patio and balcony doors are modelled to start to open in occupied rooms when the indoor operative temperature exceeds 22°C and are fully open when temperature exceeds 26°C. Similarly, window and door openings are modelled to start closing as the internal temperature drops below 26°C and are fully closed when internal temperature drops below 22°C. The air changes achieved through openable windows are based on the openable area that windows can achieve as per architectural drawings and assuming no further restrictions apply.

Internal doors are assumed to be open all the time, except for bedroom doors which are assumed to be closed during the period 22:00 – 09:00. Bedrooms doors are modelled with an undercut in line with English Approved Document F requirements, equivalent to 1% of the door area during the night hours when shut. The external entrance door is assumed to be shut all of the time.