Bi-fold or sliding doors: the overheating cost

Choosing between bi-fold and sliding doors on a rear extension is usually framed as a question of looks and budget. The part that catches people out is heat. A fully glazed south-facing rear wall can push a single-storey extension over the overheating limits in Approved Document O of the Building Regulations, and the fix is almost always cheaper to design in now than to retrofit later. Both systems can be made to work. They fail in different ways.

Planning rarely cares which system you pick. Building control does, and so does your comfort in August.

How the two systems behave in use

A bi-fold folds its leaves into a stack at one or both ends, so you can open most of the opening. A slider runs panels past each other on tracks, so a two-panel slider opens roughly half the width and a three-panel slider a little more.

That difference matters less than householders expect. Most people open a rear door fully perhaps a dozen times a year and use one leaf the rest of the time. Bi-folds need a traffic leaf specified deliberately, otherwise every trip to the bin means folding the stack. Sliders usually have one panel that simply slides.

The stack also lands somewhere. A six-leaf bi-fold parks roughly 600mm of folded frame against the reveal, which eats into the opening you thought you had and gets in the way of furniture or a bin store outside.

Frame area, sightlines and the view when shut

Count the vertical frames. A four-leaf bi-fold has four meeting stiles across the opening, each of which is two frame sections thick. A three-panel slider has two interlocks. For most of the year the doors are closed, and what you look at is frame.

Slim aluminium systems narrow the difference but do not remove it. If the view out matters, the slider wins when closed and the bi-fold wins when open, which is a genuine trade-off rather than a marketing point.

Frame area affects heat loss too. Aluminium frames with a polyamide thermal break perform respectably, though the whole-door U-value is almost always worse than the wall it replaced. More frame means more of that worse element.

Solar gain and summer overheating

Approved Document O applies to new dwellings and to some changes of use, not to an ordinary householder extension. That is a real distinction and worth stating plainly. But the physics does not check which part of the Building Regulations you fall under, and the simplified method in Approved Document O is the clearest free way to sanity-check a glazed rear wall.

The simplified method works on two things: the area of glazing facing within a range of south, and the area of free opening you can achieve for cross ventilation. It sets maximum glazing areas by orientation and by whether the dwelling is cross-ventilated. A rear extension with glass across the whole back wall, no openable window on the opposite side and a south or west aspect is exactly the case the method is designed to catch.

A room that gets 3kW of solar gain through the glass at 3pm has to lose it somewhere. Opening a door helps only if air can leave by another route.

So before you choose a door system, work out the orientation and whether the room has ventilation on more than one side. A north-facing rear wall is a different problem from a west-facing one, and west is usually the worst because the late-afternoon sun is low and shines straight in under any overhang.

Shading that keeps the opening usable

The cheapest shading is geometry. A 600mm overhang, a deep reveal or a first-floor extension above cuts midday summer sun without touching the winter gain, because the sun sits far higher in July than in December.

External shading beats internal blinds by a wide margin, because internal fabric absorbs the energy after it has already entered the room. Options that suit a rear extension include a brise soleil fixed above the head, an external roller blind on a guided track, a pergola with deciduous planting, or a projecting canopy forming part of the extension roof.

Glass specification is the other lever. A solar control coating with a g-value around 0.4 cuts the transmitted heat by well over a third compared with standard clear double glazing at roughly 0.6, with a modest reduction in visible light. Ask the supplier for the g-value in writing, not just the U-value. Most quotations give only the U-value, which tells you about winter heat loss and nothing about summer gain.

Rooflights deserve a warning. A glazed rear wall plus a large flat rooflight is the combination that produces the complaints, because a horizontal rooflight sees the sun almost all day and no overhang shades it.

Thresholds, drainage and level access

Level thresholds are easier to achieve with sliders, which commonly run a low-profile track with a drainage channel in front. Bi-folds can be done level, though the track detail is more complex and more prone to collecting grit.

Whichever you choose, the detail that matters is what happens to water on the outside. You need a drainage channel immediately outside the threshold connected to a soakaway or surface water drain, and a paving fall away from the house. Skimping here is the single most common cause of water tracking under the track in winter.

Keep the external paving at least 150mm below the damp-proof course where you can, and where a level threshold means you cannot, use a channel drain and a proprietary threshold designed for the job. If you are also altering eaves height near a boundary, check the two details together, because the roof edge and the threshold often get designed by different people.

Security, ironmongery and the practical details

Both systems can meet PAS 24 and most reputable suppliers offer it as standard on new work. Ask for the certification by reference rather than accepting a claim that the doors are secure.

Bi-folds have more moving parts: hinges, running gear, shoot bolts on each leaf. Sliders have fewer, and a lift-and-slide mechanism is generally the most durable. Over a 20-year life that difference shows up in maintenance calls rather than in failures.

One small thing people forget. Bi-fold leaves swing, so you need clear floor space inside or outside for the stack, and anything on the patio within that arc has to move every time.

Where neither is the right answer

If the room is small, a pair of French doors with a fixed sidelight often gives more usable wall, better winter performance and a lower cost than either system. If the opening is very wide and the view is the point, a large fixed pane with a single opening door beside it does the job with the least frame.

If the extension is to a listed building or sits in a conservation area, start with the heritage position rather than the product catalogue. A wide aluminium glazed wall on a rear elevation visible from a public vantage point usually needs a design argument, and that belongs in your Heritage Statement alongside the application. The same reasoning sits in a Design and Access Statement for larger or more sensitive schemes.

Illustrative example: a rear extension on a 1930s semi, 4m deep, west-facing, with 7m² of glazed door and a 1.2m² flat rooflight. Swapping standard clear glazing at a g-value of about 0.6 for solar control glass at about 0.4, adding a 600mm overhang and making the rooflight openable changes the room from one that needs a fan in July to one that does not. None of those three moves changes the planning position. All three cost less when specified before the doors are ordered. The figures here are an illustration of the method, not a prediction for your house.

Decide orientation first, shading second, product third. That order saves money.

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This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.