Part O overheating: simplified method or modelling

Approved Document O gives you two ways to show that a new home will not overheat, and the choice is usually made for you by the site rather than by preference. The simplified method is a set of tabulated limits on glazing area and openable area. The modelling route is a dynamic thermal simulation against the CIBSE criteria. Single aspect flats in high risk locations, and anything on a noisy road, tend to end up in the modelling route whether the design team planned for it or not.

Both routes satisfy requirement O1, which asks the designer to limit unwanted solar gains in summer and to provide an adequate means of removing excess heat from the indoor environment. Neither route is a planning matter. Both change the elevation, which makes them a planning problem in practice.

The two routes and what each one asks for

The simplified method works by categorising the dwelling and then applying limits. Two categorisations drive everything: whether the dwelling is cross-ventilated, meaning it has openings on opposite or substantially different facades, and whether it sits in a moderate risk or high risk location. From there, the document gives maximum glazing areas as a proportion of floor area, split by orientation group, and minimum free areas for removing excess heat.

Free area is not the same as window area. It is the unobstructed opening available when the window is fully open, which for a side-hung casement is close to the clear opening and for a tilt-only or restricted window is a fraction of it. Insect screens, guarding and restrictors all reduce it, and the document requires that reduction to be taken into account.

The modelling route replaces those limits with a simulation. You model the dwelling, apply the specified occupancy profiles, internal gains and weather file, and demonstrate that the predicted internal temperatures satisfy the criteria. There are no glazing limits in this route, which is why it is the one used for schemes with large windows.

Where the simplified method runs out

The simplified method fails in four recognisable situations, and recognising them early saves a wasted iteration.

• Single aspect dwellings with a west or south facing elevation and a normal amount of glass, because the glazing limit for a non-cross-ventilated dwelling in that orientation group is tight
• Dwellings in the high risk locations identified in the document, where the limits tighten further
• Sites where external noise means the windows cannot be relied on at night, which removes the openable area the method depends on
• Deep plan flats where the free area available at the single facade cannot reach the required proportion of floor area

There is a fifth case that the simplified method does not address at all. Heat gains from communal corridors and risers into flats are a real contributor to overheating in apartment blocks, and the modelling methodology deals with them explicitly by requiring raised corridor temperatures to be modelled. If your scheme has long internal corridors with distribution pipework in them, the simplified method will not reveal the problem and the building will still have it.

What the modelling route involves

The modelling has to follow the methodology the document specifies rather than a modeller's preferred approach. That means the stated occupancy and internal gain profiles, the specified weather file for the relevant location, and the stated criteria.

For predominantly naturally ventilated homes there are two criteria and both must be met. The first limits the number of occupied hours in which the operative temperature exceeds the adaptive comfort threshold by one degree or more, to no more than three per cent of occupied hours between May and September, assessed in living rooms, kitchens and bedrooms. The second applies to bedrooms only and limits hours above 26 degrees between 10pm and 7am to no more than one per cent of annual hours. For homes with mechanical ventilation or cooling, a fixed temperature criterion applies instead.

The bedroom night-time criterion is the one that fails. Three per cent of occupied hours over five months is a reasonably generous allowance. One per cent of annual night hours above 26 degrees is not, particularly in a bedroom with a west facing window and limited night ventilation.

Check the weather file before the modelling starts. Approved Document O specifies a design summer year for each weather location, and running the simulation against a different file, or against a future climate scenario, produces results that the building control body will not accept even if they are more conservative.

Noise, security and the window opening problem

Both routes depend on windows being openable in practice. Approved Document O recognises that they sometimes are not, and sets out how to deal with the two situations where they are not.

Where external noise would prevent windows being opened at night, the document gives threshold noise levels and requires the design not to rely on openable windows for night cooling once those thresholds are exceeded. On a main road site that usually forces mechanical ventilation with a summer bypass, or mechanical cooling, and it also interacts with the acoustic design. A Noise Impact Assessment prepared for planning purposes will often contain the facade noise levels needed for this judgement, so get the acoustician's measurement data to the overheating modeller rather than having it measured twice.

Security and guarding produce a quieter version of the same problem. A ground floor window that cannot be left open overnight, or an upper floor window with a restrictor fitted for guarding reasons, has less free area than the drawing suggests. Resolve the restrictor strategy before the free areas are calculated.

The cooling hierarchy and mechanical cooling

Approved Document O does not prohibit mechanical cooling. It requires that passive means of limiting solar gains and removing excess heat are used first, and that cooling is not a substitute for a facade that controls gains. The practical reading is that a scheme relying on comfort cooling with a highly glazed unshaded facade will not comply, while a scheme with controlled glazing, external shading and cooling for the residual risk will.

Where a London scheme is involved, the London Plan cooling hierarchy adds a planning layer on top, and the two are assessed by different bodies at different times. The passive measures come first in both, which is convenient, and the evidence for how officers assess passive cooling features is worth reading before committing the elevation.

Mechanical cooling also brings consequences outside Part O. Plant needs space on the roof, it makes noise, it adds peak electrical demand, and on a referable scheme it affects the carbon figures in the Energy Statement. Adding cooling late is the most expensive way to solve an overheating problem.

Evidence the building control body expects

Compliance is demonstrated to the building control body, not to the planning authority, and the submission normally comprises a report plus drawings.

1. Identification of the route taken, and for the simplified method the risk category and ventilation type for each dwelling type
2. A glazing schedule by orientation group with areas as a proportion of floor area
3. A free area schedule showing the openable area actually achievable, with deductions for restrictors and screens
4. Shading geometry on drawings, dimensioned, where external shading is relied on
5. For the modelling route, the full model inputs, the weather file used, the dwellings selected as worst cases and the justification for that selection
6. Confirmation at completion that what was built matches what was assessed

Item 6 is where schemes come unstuck. Where an Overheating Risk Assessment relies on a particular glazing specification or a particular opening arrangement, substitution during procurement changes the result. Keep the performance values in the specification rather than in the modelling report alone.

Where planning and Part O collide

Three collisions are common enough to plan around.

External shading added for Part O reasons changes the elevation, and if it appears after permission it needs a non-material amendment at best. Design the shading into the planning drawings even if the modelling is not finished, and describe its purpose in the application so an officer does not ask for it to be removed on appearance grounds.

Glazing reduced for Part O reasons makes the internal daylight case harder, and on a scheme where the daylight assessment supported the quality of accommodation at planning stage, a later reduction may undermine the basis of the permission.

Mechanical cooling added for Part O reasons needs plant space, which competes with everything else on the roof, and may need its own acoustic assessment. On a scheme where the roof was already fully allocated at planning, that is a problem with no good answer.

The fix in all three cases is sequencing. Run an indicative overheating check before the planning submission, even though the formal assessment comes later. It costs a fraction of a redesign.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.