July 9th 2019

Building Bulletin 101: Guidelines on ventilation, thermal comfort and indoor air quality in schools (BB101 2018)

Building Bulletin 101: Guidelines on ventilation, thermal comfort and indoor air quality in schools (BB101 2018)

IES welcome the release of the BB101 guidelines published by the government following an extended period of consultation within the industry. BB101 provides guidance for ventilation, thermal comfort and indoor air quality in schools; with the guidelines updated to align with the most recent industry best practice and health & safety standards. Its focus is on the health and wellbeing of students and teachers to promote learning.

IES Consulting provide dynamic simulation and CFD modelling expertise to investigate and interpret the risk on your building design. We can work with you from the concept stage onwards to feedback on the building design performance, helping quickly identify potential issues and test design solutions toward meeting BB101 requirements.

Thermal Comfort

BB101 guidance and requirements on thermal comfort covers summer overheating and also winter discomfort to ensure occupants are suitably warm. Guidance on room temperatures and cold draughts is included to provide a comfortable environment suitable for learning year round.

Adaptive thermal comfort calculations are introduced to prevent summertime overheating based on the latest research on how people adapt to higher temperatures. These calculations use variable maximum indoor temperatures that depend on the outside temperature. This helps to avoid the unnecessary use of air conditioning by using passive measures such as night cooling and thermal mass to cool spaces in summertime.

Operative temperature now replaces dry resultant temperature, as it combines the effects of air temperature and mean radiant temperature within a limited range of air velocity and humidity.

Overheating in classrooms and over-glazed larger spaces such as libraries and learning resource centres frequently occurs. This problem is often reported in post occupancy and staff surveys. DfE has adopted the adaptive thermal comfort method from BS EN 15251 to deal with this problem. The adaptive comfort criteria can only apply to free running buildings, i.e. those without mechanical cooling, and with means for the occupants to locally alter conditions, i.e. to increase the ventilation rate by means of opening windows and local room controls. Most schools are free running outside the heating season.

Dynamic thermal modelling should be used to assess buildings for overheating and to size ventilation openings. The modelling should use the CIBSE DSY1 2020 weather file most appropriate to the location of the school building. Simulation provides modellers the open capability to analyse against a local file for comparison should there be microclimatic concerns.


BB101 requires as a minimum achieving Criterion 1 - Hours of Exceedance (He) for the overheating risk assessment. Criterion 2 – Daily Weighted Exceedance (We) and Criterion 3 - Upper Limit Temperature (Tupp) are primarily measures of short-term discomfort and for information reporting only. If a school design fails to meet Criterion 2 or Criterion 3 then designers should consider potential overheating mitigation measures and indicate which are viable for the project. The use of these three performance criteria together aims to ensure the design is not dictated by a single factor but by a combination of factors that will allow a degree of flexibility in the design.

The criteria should be assessed Monday to Friday 09-16h, from 1st May to 30th September, including the summer holiday period as if the school was occupied normally through the summer. A 12pm to 1pm lunchbreak with no internal heat gains is allowed for classrooms.

Summarising the 3 criteria are:

  1. Number of hours for which an adaptive thermal comfort threshold temperature is exceeded (total hours of exceedance).
  2. Degree to which the operative temperature exceeds the adaptive thermal comfort threshold temperature (daily weighted exceedance).
  3. Maximum temperature experienced at any occupied time (upper limit temperature).

Note the calculation includes August, which is not how schools are currently used and therefore provides an element of future proofing against climate change. Design teams are however free to explore future proofing further by modelling against the future weather files now commonly available. The range these files offer can be quickly simulated to demonstrate a corridor of risk and how far the design stretches to cover this.

In addition, BB101 notes the asymmetric radiation from hot ceilings in single storey teaching spaces should be less than 5K in summertime. This means preventing trapped hot air at ceiling level by an adequate means to remove the hot air, which could include a cross-ventilation solution.

Winter Discomfort

Updated guidance for underfloor heating is added to prevent a floor that is too hot and lead to thermal discomfort. BB101 notes the floor temperature rather than the floor material is the most important factor for foot thermal comfort and advises floor temperatures higher than 26 °C should be avoided. This is particularly important where there are nursery-age pupils or pupils with complex health needs, where there is low activity and said pupils are likely to be sitting on the floor. In these cases, one solution is a self-regulating underfloor heating system set to ~ 24oC maximum surface temperature with a supplementary heating system.

With our dynamic simulation and CFD capability IES Consulting can analyse various modelling scenarios and openly communicate their performance.

Ventilation & Indoor Air Quality

In teaching and learning spaces, carbon dioxide is the key indicator of ventilation performance for the control of indoor air quality. There is a range of ventilation strategies available to meet BB101 requirements, ranging from a completely natural system to a completely mechanical system. For classrooms and practical spaces, design constraints typically determine the ventilation strategy used, with current designs the majority of teaching spaces use hybrid or mixed mode systems that employ a mix of MV and NV.

BB101 states where MV is used, or when hybrid systems are operating in mechanical mode, then sufficient outdoor air should be provided during the occupied period to achieve a daily average CO2 concentration <1000 ppm. The maximum concentration should also not exceed 1500 ppm for more than 20 consecutive minutes each day.

Where natural ventilation is used, or when hybrid systems are operating in natural mode, then sufficient outdoor air should be provided to achieve a daily average CO2 concentration <1,500 ppm. The maximum concentration should also not exceed 2,000 ppm for more than 20 consecutive minutes each day. The ventilation solution should be designed to achieve a CO2 concentration for the majority of the time <1,200 ppm for a new building (800 ppm above the outside CO2, taken as 400 ppm) for the majority of the occupied time during the year. This increases to 1,750ppm for a refurbished building.

Continuous Monitoring

Monitoring ensures designs achieve an acceptable standard of indoor air quality and thermal comfort in each teaching space with information fed back to the designers. 

With modern building controls it is easy to monitor the indoor environment by recording temperature and CO2 as well as energy consumption. This provides the building occupants and facilities management team a greater knowledge and control over their environment. Performance in use should be monitored throughout teaching and key spaces including atria, dining spaces, libraries, learning resource centres, admin and head teacher’s offices, server rooms and reprographics rooms.

BB101 summarises demonstrating spaces achieve temperatures within the acceptable range when windows, fans and ventilation systems are operated to reduce summertime temperatures, and the space has the intended number of occupants, numbers and types of computers, data projectors and other ICT equipment. The monitoring approach can be utilised to ensure winter discomfort is also prevented by ensuring start up periods are acceptable and plant is suitable to meet heating set-points.

IES have developed significant experience using our in-house technology to interrogate operational data. Through building tuning IES regularly identify savings opportunities in measured data due to fault detection/troubleshooting. Our services extend into taking control of your BMS data to understand your building’s operation and post occupancy evaluation.

Full details of BB101 are available online: