EPISODE: 031 - APPROVED DOCUMENT O - OVERHEATING

BYTNAR - TALKS

EPISODE 031 - APPROVED DOCUMENT O - OVERHEATING

This episode is for people who want to know more about Approved Document Part O – Overheating.

You should like this episode if you ask yourself questions like:

• What is the Approved Document O and what does it cover regarding overheating prevention?

• How does the Simplified Method help reduce overheating in new residential buildings?

• What strategies are recommended in the Simplified Method for managing heat and solar gain?

• What is Dynamic Thermal Modelling and how does it apply to building design for overheating prevention?

• When should additional considerations be taken into account for overheating mitigation in buildings?

• What factors should be considered beyond just heat management in overheating prevention strategies?

• What information must building owners receive regarding overheating mitigation strategies?

• How can building occupants keep cool during hot weather according to the Home User Guide?

• Are there specific guidelines for communal spaces and mixed-use areas regarding overheating prevention?

LET'S TALK ABOUT YOUR PROJECT

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This is Bytnar Talks – The Engineer Takes on Construction, Episode 31.

Hi, I'm Piotr Bytnar. Each day I help my clients plan and design building projects through Bytnar Limited, a consulting chartered structural engineers practice.My biggest passion—and the cornerstone on which I've built my business—is finding clever solutions for construction projects.I am a chartered structural engineer and a budding software developer, so you can rest assured that I will strive to talk about best practices and the use of new technologies in the industry.

If you're embarking on a construction project or are involved in planning, designing, and building the world around us, you'll find this podcast useful.

Approved Document O – Overheating

Hi guys, and welcome to Bytnar Talks—your favorite podcast on all matters of architecture, engineering, and construction.It is Sunday, the 20th of October, 2024, and I'm here back with you with the 31st episode and information on Approved Document O – Overheating.

I am super happy to be back with you to deliver yet another Approved Document.We are slowly getting to the end of the whole lot, as we do at the end of the year. I cannot wait to start changing the topics and start delivering on things perhaps more lively than the dry reading of legislation and the guidance that follows it.

Fortunately—or unfortunately—the industry is full of legislation and standards, so it will often be inevitable to get back to it.But I'm looking forward to delivering on topics that are more universal.

Anyway, the last episode treated on the accessibility of buildings other than dwellings.Let's quickly go through the recap and see what was covered last week.

In the last episode, I talked about the first volume of the Approved Document—second volume of our Approved Document M: Access to and Use of Buildings, dealing with buildings other than dwellings.

Section 0 in the document brings us the overview of accessibility requirements.It tells us that this document highlights the importance of ensuring that buildings are accessible to all individuals, regardless of age, gender, or disability, by meeting functional performance criteria.It applies to both new constructions and existing buildings undergoing significant changes, with a focus on compliance with the Equality Act 2010 to eliminate physical barriers for those that use it—whether they are external users or people that actually operate the building.

Section 1 deals with access to buildings.This section provides detailed requirements for creating accessible routes from site boundaries or parking areas to building entrances, ensuring they are firm, level, slip-resistant, and free from hazards.Specific dimensions for pathways, ramps, and other access features are outlined, with references to best practices for accessibility, such as the Inclusive Mobility guide.

In Section 2 – Access into Buildings, we are shown how the consideration of entrances should be met.They must be safe and easy to use for everyone, including people with disabilities—with clear signage, level thresholds, and appropriate door widths.Both manual and power doors must be user-friendly and safe, and alternative accessible entrances must be available for revolving doors, with clear visibility for glass doors to prevent accidents.

Section 3 deals with horizontal and vertical circulation in buildings.This section focuses on ensuring smooth movement for all users, particularly wheelchair users, within non-residential buildings.It outlines guidelines for entrance halls, corridors, stairs, and lifts—emphasizing space requirements, proper layout, and the need for compliance with safety standards for seamless navigation within the building.

In Section 4 – Facilities in Buildings—yes, you guessed it—we deal with facilities like seating, refreshment counters, and sleeping accommodations, which must be accessible to everyone, including wheelchair users.Specific guidelines ensure that wheelchair users can access public spaces, lecture halls, and sport venues without segregation.This section also covers the provision of accessible controls, switches, and communication aids like induction loops.

Section 5 deals with sanitary accommodation in buildings, where sanitary facilities—including toilets and showers—must be accessible, spacious, and equipped with support rails, alarms, and easy-to-use controls.Special guidelines cover wheelchair-accessible unisex toilets, as well as ensuring that separate-sex facilities are also suitable for disabled users, with adequate space and safety features.

In this episode, I will move to Approved Document O – Overheating, so without further ado, let's dive straight into this.

In this episode, I will talk about the statutory guidance contained within the Approved Document O – Overheating. The document covers the requirements of Part O and Regulation 40B, and it is contained within four sections. Sections 0 to 3 deal with overheating, and Section 4 covers the necessity to provide information to the building user.

So, let's get into the nitty-gritty of this Approved Document.

All right guys, starting with the text of the legislation, I will do both parts here—so the requirement O and Regulation 40B.

Coming straight to Part O – Overheating

Overheating Mitigation Requirement O1

Paragraph 1:Reasonable provision must be made, in respect of a dwelling, institution, or any other building containing one or more rooms for residential purposes (other than a room in a hotel or residence), to:a) limit unwanted solar gains in summer, andb) provide an adequate means to remove heat from the indoor environment.

Paragraph 2:In meeting the obligations in paragraph 1:a) account must be taken of the safety of any occupant and their reasonable enjoyment of their residence, andb) mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it.

And off to Regulation 40B – Providing Information

Information About Overheating – Requirement 40B

Paragraph 1:This regulation applies to building work in respect of a building where Part O of Schedule 1 applies—where Part O imposes a requirement in relation to building work.

Paragraph 2:The person carrying out the work must, no later than 5 days after the work has been completed, give sufficient information to the owner about the provision made in accordance with Part O, so that the systems in place pursuant to Part O can be operated in such a manner as to protect against overheating.

That went quickly, didn’t it? This Approved Document is pretty straightforward.

So, Section 0, as ever, gives us an introduction to the document.This requirement applies to new residential buildings only—so dwelling houses and flats, institutional overnight accommodation like schools, care homes, or any living accommodation for that matter (except hotels). Shared communal rooms or spaces for more than one residential unit also fall under the consideration of this requirement.

As for the spaces that are partially used for work—some of these live/work units may need an agreement with Building Control on how to treat it, as there is no particular definition of what is considered a "substantial proportion" of the building. And that’s how we are supposed to judge it—so bear that in mind.

In other clearly mixed-use scenarios, the requirement only applies to the residential part and its corridor or, otherwise, the access route.

As is the case with other Approved Documents, we may show compliance in ways other than those suggested, but agreement on that may need to be made before the building is executed.

Some interactions with other requirements are noted here—like with Part B, where the addition of escape windows may lead to overheating. Or, the amount of window opening for overheating will be higher than the purge ventilation requirements of Part F. Or, the additional ventilation may influence the use of combustion appliances—that’s Part J.

Part L, on the other hand, may work in opposition: when solar gain is considered for heating gains in winter, the same may cause overheating in summer.

Naturally, for controls, these should be accessible in line with Part K and M requirements. But Part O will have higher provision for the protection from falling through openings than that of Part K.

The security of the doors and windows should also be considered in line with Part Q requirements.

In the gist, Requirement O treats the reduction of high indoor temperature occurrences, and that should be done by:

1. Limiting unwanted solar gains in summer, and

2. Providing adequate means of removing excess heat from the indoor environment.

And—it’s all about the health and welfare, not necessarily the comfort, of the occupants.

The Secretary of State identifies two methods for achieving compliance, called:

• The Simplified Method, and

• Dynamic Thermal Modelling.

These are dealt with in the next two sections.

In Section 1, we treat the Simplified Method.A checklist is available in Appendix B that takes us through all the building control approver wants to see.

You’ll see it’s three parts:

• Part One states the building and design details.

• Part Two indicates site details—like location and building category chosen, the designed overheating mitigation strategies (like maximum area of glazing, maximum area of glazing in the most glazed room, shading strategy, total minimum free area, bedroom minimum free area), and the designer’s declarations.

• Then Part Three is for the builder and the building control body inspector to complete, to confirm that the building followed the strategy.

So, first, we categorize the building—where it is located and whether it has cross-ventilation or not.

For the location, buildings in England will generally be considered moderate risk, apart from the highly dense urban areas of London, where the risk is considered high.There is a map and a list of postcodes within Appendix C that we can refer to.

Cross-ventilated houses or flats will be those that have openings on opposite sides, and each part of the building should be treated separately in that regard.

Once we know these two parameters, we choose how we will mitigate the gain and how we will remove the excess heat.

To limit the solar gain, we follow the consideration of the maximum glazing areas in the facade that is glazed the most. For that, we have handy tables with prescribed percentages of the floor area for glazing.

For cross-ventilated units, we’re thinking between 22% and 37% of the floor area of the room—maximum—for the most glazed room in the building, and between 11% to 18% for the maximum glazing area across the floor area overall.Anyway, have a look at the tables—the units without cross-ventilation will obviously have smaller limits.

In addition, the maximum glazing areas in high-risk locations (between northeast and northwest, via the south) should either:

• Have external shutters with ventilation options, or

• Glazing with a maximum G-value of 0.4 and minimum light transmittance of 0.7, or

• Have overhangs with a 50° altitude cutoff, on due south-facing facades only.

Removing the excess heat is done by means of opening the windows wide, and a slightly different size of free area of the opening will be required depending on whether the building is in a high- or low-risk type of situation.

The percentages are given as minimums of the room floor area and glazing area.So, for bedrooms, this will be:

• 13% and 4% of floor area, and otherwise

• 6% to 12% of floor area,

• And between 55% to 95% of the glazing area.

We can calculate the equivalent area by following BS EN 13141-1, or by following Appendix D.

It is generally considered that this expectation will exceed that of purge ventilation in Document F.

We need to mind, though, that buildings using distributed heat and hot water systems may have extra heat load coming from the piping—so the Simplified Method will not be suitable here.

This leads us to Section 2, which covers Dynamic Thermal Modelling.

This methodology is suitable for all residential buildings and may lead to more flexibility—especially when:

• The building is airtight,

• There are higher levels of insulation,

• We want to use the particular location of the building in consideration, or

• The buildings are highly shaded.

The Dynamic Thermal Modelling method should follow the methodology of CIBSE’s TM59 publication, but it must be limited to the choices described in this document and reported using a checklist—as was the case with the Simplified Method.

The limiting parameters are:

• For the daytime use (between 8:00 a.m. and 11:00 p.m.), if the indoor temperature goes above 22°C, the window should start to open, and reach full opening by 26°C indoors—and vice versa.

• For the night-time (between 11:00 p.m. and 8:00 a.m.), the opening should be modelled as fully open when on the first floor or above and not easily accessible, and the internal temperature exceeds 23°C.

• When the ground floor or easily accessible rooms are unoccupied, they should be modelled as open during the day if they are otherwise secured—and should be considered closed at night.

• Entrance doors should be considered closed at all times.

The following strategies should also be considered:

• Limit solar gain by the use of fixed shading devices, like shutters, external blinds, overhangs, and awnings.

• Consider glazing design, taking into account its size, orientation, G-value, and the depth of the window reveal.

• Also, think about the building design in general—with its different elements like balconies, articulations, and features, as well as shading provided by other parts of the  permanent buildings, structures, or landscaping.

Foliage and internal blinds are excluded from the consideration. But for more guidance on what to include and what is effective, take a look into BRE BR 364 – Solar Shading of Buildings publication.

Removing excess heat is typically done by opening windows, using ventilation louvres in external walls, mechanical ventilation, or cooling systems. The preferred methods are naturally achieved by passive means, and it will need to be demonstrated to Building Control that such methods are insufficient before specifying mechanical solutions.

Section Three

It may be the case that, for some reasons, the simplified method of achieving an appropriate level of heat mitigation is not possible. These reasons can be varied.

For example, we should consider noise levels at night, as this may preclude us from using openable windows. Generally, if the average noise is over 40 dB over 8 hours (between 11:00 p.m. and 7:00 a.m.) or at least 10 instances per night above 55 dB during the same hours, we will need to be a bit more clever than just using windows for our heat dissipation strategy.

When external air is not suitable for use in the strategy, we may need to ventilate and filter the air.

Security should be achieved by either:

• Not providing overnight-opening windows in positions of easy access, or

• Protecting them with grilles, railings, or louvered shutters.

Protection from falling tells us to protect open windows where needed. If a window can open by 100 mm, it can be considered part of the strategy—but if they open outwards, the handles must not be further than 650 mm away from the inside face of the wall when the window is at its maximum opening position.

Guarding heights depend on whether the drop is higher than 600 mm:

• For lesser drops, we are referenced to Document K.

• For higher drops, we must provide guarding up to 1.1 m height, and the guarding should not be easily climbable.

For large openings, the guarding may either be fixed or be formed of shutters with child-proof locks.

We also need to consider protection from entrapment, as is the case in Document K.The 100 mm clear opening limit prevails when thinking about babies’ heads getting stuck.We should also think about fingers—if there is an 8 mm gap, then it should be at least 25 mm and must not taper, so as to avoid trapping a finger at any point.

Looped cords must be fitted with child safety devices.

Section Four – Provision of Information

It’s all about giving information to the owner of the new building so that they can use the building effectively.

So, what do we need to provide?

It is a non-technical instruction manual on how to:

• Use the overheating mitigation strategy, and

• Understand the maintenance requirements.

This includes:

• The overall overheating mitigation strategy,

• The location of all elements of the strategy,

• Instructions on how to use these elements,

• The time of day, and season for use,

• Manufacturers’ contact details,

• The location of controls and instructions for setting them,

• The location of sensors, and how to recalibrate them,

• Cleaning and maintenance instructions.

This should be included within the Home User Guide, in a section called "Staying Cool in Hot Weather."

We are also reminded that some of the systems may be shared with other considerations—such as for ventilation or conservation of fuel and power—and these should be treated separately within the manual.

So, this is it folks—Approved Document O: Overheating.

Let’s sum it up section by section:

Section 0 – Introduction

This section outlines the scope of the document, which applies to new residential buildings, including:

• Dwelling houses,

• Flats,

• Institutional accommodations (like care homes and schools).

It addresses communal spaces and live-work units, requiring agreements with Building Control in mixed-use situations.

The section highlights interactions with other Parts:B, F, J, L, K, M, and Q—emphasizing the balance between safety, ventilation, and overheating prevention.

Section 1 – Simplified Method

The simplified method provides a checklist in Appendix B for mitigating overheating in residential buildings.It categorizes buildings by location and ventilation type, with high-risk areas identified for special treatment.

Strategies include:

• Limiting solar gain through glazing restrictions and shading,

• Removing excess heat via window openings.

The method is straightforward, but not suitable for buildings with high heat loads from distributed systems.

Section 2 – Dynamic Thermal Modelling

This introduces Dynamic Thermal Modelling, offering more flexibility—especially for airtight or well-insulated buildings.

It follows CIBSE TM59 methodology and allows for detailed consideration of:

• The building’s location,

• Shading strategies,

• Focusing on solar gain limitation and natural ventilation,

• With mechanical systems as a last resort.

Section 3 – Additional Considerations

When the simplified method is not feasible, factors like:

• Noise,

• Unsuitable external air,

• Security,

• And fall protection must be addressed.

This includes:

• Secure ventilation options,

• Protecting open windows from fall hazards,

• Ensuring child safety by preventing entrapment in window mechanisms.

Special attention is required for buildings in noisy or high-risk areas.

Section 4 – Provision of Information

Owners must be provided with a non-technical manual outlining the overheating mitigation strategy.

It must include:

• How to operate and maintain each element,

• When and how to use them,

• Manufacturer contact info,

• And maintenance guidance.

It should be part of the Home User Guide, in the “Staying Cool in Hot Weather” section, and be considered alongside ventilation and energy conservation systems.

In the next episode, I will move on to Approved Document P – Electrical Safety.

I hope you enjoyed this episode and that the considerations of Approved Document O – Overheating are a bit clearer for you now.

If you have any questions, reach out to me on LinkedIn, or send me an email—I’m more than happy to help you out.

At Bytnar, we deal with planning, designing, and managing your projects, and we’re always glad to facilitate a free initial consultation to steer you in the right direction.

Visit www.bytnar.co.uk and reach out to us. Whether your question is, “Can you help me with my project?” or, “What should I do?”—we’ll be able to give you a piece of non-obligatory advice.

At Bytnar, we help our clients design and execute their dream homes or investments.If your building is falling apart, we can also help investigate the reasons behind it, and provide you with an appropriate strategy, design, and specification for the repair.

Thank you again for listening. Please voice your opinions—I’m waiting for you on LinkedIn, and I want to hear from you.

See you next week.

Bytnar Designs the World Around You.Toodloo!

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Piotr Bytnar BEng (Hons) MSc CEng MIStructE

Chartered Structural Engineer who deals with the Architecture of buildings. His Master's Studies led him to an in-depth understanding of risk and contract arrangements in construction as well as specialist knowledge in soil mechanics.

He and his team help homeowners and property developers to design and deliver construction projects reducing waste in time and the cost. He believes that the construction project is an iterative process that can be well managed and it is best managed if all the aspects of the project definition and management are dealt with in-house or coordinated by one organisation. His team works to all stages of RIBA and ISTRUCTE stages of work and enables contractors to deliver projects on-site providing risk evaluations, methodologies for execution of works and temporary works designs.

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