Part O of the Building Regulations covers overheating in new dwellings, and it comes up in rooflight conversations more often than it applies. This page sets out what it actually covers, who it applies to, and what governs an extension instead.
Why Part O exists
Houses have got better at retaining heat, which is the point of the last thirty years of building regulation. A well insulated, airtight dwelling that is difficult to heat in January is also difficult to cool in July.
Add larger glazed areas, which people want, and the result was new homes that overheated badly enough to be a health concern rather than a comfort one. Part O was introduced to address that at design stage rather than leaving occupants to deal with it.
It is worth understanding the intent, because it explains why the requirement is shaped the way it is.
What it asks for
Two things, and both have to be satisfied.
Limit unwanted solar gains. Control how much solar energy enters in summer, through glazed area, orientation, shading and glass specification.
Provide a means of removing excess heat. Ventilation capable of taking heat out of the dwelling when it has arrived anyway.
The second matters as much as the first, and it is the half that gets forgotten. Reducing gain without providing a route for heat to leave is half a solution.
Who it applies to
| Situation | Part O? | What governs instead |
|---|---|---|
| A new house | Yes | |
| A new flat | Yes | |
| A building converted to dwellings | Generally yes | |
| An extension to an existing house | Not in full | Glazed area limits and thermal elements |
| A loft conversion | Not in full | The same, plus ventilation under Part F |
| Replacing a rooflight | No | The limiting U-value for a replacement |
Most of the work we do falls in the bottom three rows, which is why Part O is a page about something that usually does not apply.
What applies to an extension instead
An extension to an existing dwelling is subject to the requirements for the new work, and for glazing that means two things in practice.
A limit on the total area of windows, doors and rooflights, expressed as a proportion of the floor area of the extension, with allowances for the openings being replaced or covered up. And a limiting U-value for each element, which any current rooflight clears comfortably.
There are compliance routes that allow more glazing where the overall energy performance is demonstrated to be no worse. Which route a project takes is agreed with Building Control, and it is the designer’s job rather than the rooflight installer’s.
Why the physics does not care
This is the point worth taking away, and it is the reason we raise overheating on jobs where no regulation compels it.
A large unshaded south or west facing rooflight over a small single storey extension will make that room uncomfortable in July whether or not Part O applies to it. The regulation decides who has to demonstrate what. It does not decide how the room feels.
So on an extension we will talk about orientation, glazed area, glass specification and ventilation regardless, because the client is going to live in it.
Where the risk actually concentrates
Overheating is not evenly distributed, and a few characteristics account for most of it.
- Single storey with a large roof area relative to floor area. A rear extension is mostly roof, so overhead glazing is a large proportion of its envelope.
- Single aspect. One set of doors and nothing on the other side, so no cross ventilation path.
- West facing. Peak gain in the late afternoon onto a room already warm.
- A kitchen. Generating its own heat from cooking and appliances on top of everything arriving.
- Hard surfaces throughout. Tile, stone, glass and plaster store and re-radiate rather than absorbing.
A kitchen extension is frequently all five at once, which is why it is the room this comes up about.
The order the levers come in
- Glazed area. The largest lever and the one only available at design stage. Less glass is less gain, and this is where the conversation should start.
- Orientation and position. Where the glazing goes on the roof, and which way any pitched surface faces.
- External shading. Stops energy before it reaches the glass, which is why it outperforms anything internal.
- Glass specification. Solar control, chosen against the orientation.
- Ventilation. Removing heat that arrived anyway, at the highest point of the room.
- Internal blinds. Glare and privacy. Not a heat solution.
Most conversations start at four and skip one, two and three, which is why they end in disappointment.
The ventilation half
Warm air collects at the highest point of a room, which on an extension is the ceiling, which is where the rooflight is.
An opening at that point removes heat far more effectively than an open door at floor level, and paired with a low opening on the opposite side it produces a genuine flow path rather than a stagnant room with a draught at the bottom.
Which makes an opening rooflight one of the more useful things in an extension that might overheat, and it is a decision made at the ordering stage because a fixed unit cannot be made to open later. The wider ventilation requirements are Part F and have their own page.
Night purge
A technique worth naming because it is what makes a heavy, well insulated extension work in summer.
The fabric of a room absorbs heat through the day and releases it at night. Opening up overnight, when the outside air is cooler, flushes that stored heat out and leaves the room starting the next day cool rather than already warm.
An opening rooflight at ceiling level is the ideal outlet for it, because it is where the warm air is. The obstacle is usually security and rain, and a powered unit with a rain sensor answers both, which is one of the better arguments for one.
Thermal mass, and why it varies so much
Two extensions with identical glazing behave differently, and mass is a large part of why.
A heavy floor, solid walls and a screed absorb heat through the day and slow the temperature rise, then give it back overnight. A lightweight construction with a timber floor heats quickly and cools quickly.
Mass is helpful where there is night ventilation to flush it and unhelpful where there is not, because a heavy room with no night purge simply stays warm. It is one reason a rule of thumb about glazed area does not transfer between buildings.
What overheating actually feels like
It is worth being concrete, because people describe it in different ways.
A room that is bearable in the morning and unusable from three in the afternoon. A kitchen where cooking becomes unpleasant in June. A room where the doors are open and it makes no difference, because there is nowhere for the air to go. And a room that is still warm at eleven at night, which is the thermal mass giving back what it took in.
Each of those points at a different lever, which is why the description matters at survey.
Retrofitting a solution
Where an extension already overheats, the levers available are fewer and they are not nothing.
The glazed area is fixed. The glass can sometimes be changed, where the frame is designed to be reglazed and the rebate suits, or applied film can be used with the caveats on the solar control page. Ventilation can frequently be improved, by adding an opening rooflight or an extract fan. External shading can be added. And internal blinds help with glare.
In our experience the ventilation route is the most effective retrofit and the least often considered, because everybody looks at the glass.
The trap in the calculation
Where a compliance route involves a calculation, it is worth knowing what it does and does not represent.
A calculation demonstrates compliance against defined conditions and assumptions. It is not a promise about how a particular room will feel on a particular July afternoon, and it assumes occupants operate the building as modelled, including opening things.
A design that complies on paper and provides no practical way to get heat out will overheat, and the occupant will be right and the calculation will also have been correct. Both can be true.
Vulnerable occupants, and why the bar is higher
Overheating became a regulatory matter rather than a comfort one because of who it affects most.
Older people, very young children and anybody with a cardiovascular or respiratory condition tolerate sustained high indoor temperatures considerably less well than a healthy adult does. A bedroom that does not cool overnight is the case that matters, because that is where the exposure is longest and unbroken.
On this coast, where a great deal of the housing stock is bungalows occupied by people who bought them precisely because there are no stairs, that is not an abstract consideration. A large rooflight over a single storey bedroom facing south or west is worth thinking about carefully, and it is one of the situations where we will raise the glazed area rather than the glass.
Bedrooms and the night time case
Daytime and night time overheating are different problems and the second is the one people report.
A living room that is warm at four in the afternoon is unpleasant and you can leave it. A bedroom that has not cooled by midnight is a night of poor sleep, and in a loft conversion or a bungalow under a large rooflight it is the common complaint.
What helps is night ventilation rather than glass, because by bedtime the gain has already happened and the heat is in the room and in the fabric. An opening rooflight at the highest point, openable safely and left open overnight, is the single most effective thing available. Security and rain are the obstacles, and a powered unit with a rain sensor answers both.
Where we are useful and where we are not
We install and replace rooflights. We are not the designer of your extension and we do not do the compliance calculations.
What we can do is tell you what a given rooflight in a given position will do to a room, specify the glass and the ventilation to suit it, and say plainly when a proposal looks like it will overheat regardless of what glass goes in it. Where the answer is less glazing rather than better glazing, we will say so, even though it is the smaller order.
What this coast changes
Two local factors, pulling in opposite directions.
The light here is frequently bright overcast rather than clear sun, and a maritime climate moderates summer peaks compared with inland. Both reduce the overheating risk relative to a drier, more continental site.
Against that, the coastal plain is largely unshaded, the extensions here are overwhelmingly single storey rear ones facing south or west onto gardens, and sea air is humid, which makes a warm room feel warmer. On balance it is a real risk here and not the worst in the country.
What we ask first on the glazing
We ask which way the extension faces and what the room is for before anything else, and we raise overheating on jobs where no regulation requires us to. We specify solar control against the orientation, recommend an opening unit at the highest point of the room, and say when the glazed area rather than the glass is the thing that needs to change.
Part O applies to new dwellings. Overheating applies to whatever you build, and it is decided before anything is ordered.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.