Climate & Cities
Insulating a building can make it overheat
Fabric improvements were designed to keep heat in. In a warming climate they also keep it in during the summer.
This is written to be used rather than admired. Each section below is a decision about summer overheating, and each one has a default.
Before you start
- Solar gain through glazing is the dominant driver of overheating in most dwellings.
- External shading is far more effective than internal blinds.
- Regulations addressing overheating are recent and inconsistent between countries.
Two problems with one fabric
Insulation and airtightness reduce heat flow in both directions, which is entirely beneficial in winter. In summer, heat that enters through windows or is generated inside has fewer routes out.
A highly insulated dwelling that overheats stays hot for longer than a leaky one, because the fabric holds the heat. This is not an argument against insulation; it is an argument that summer performance has to be designed as well.
Solar gain is usually the source
Glazing admits solar radiation that is absorbed inside and re-radiated at wavelengths the glass does not let out. Large windows on south and west orientations are the common culprits, with west being worse because the sun is low and the day is already hot. Glazing ratio, orientation and shading together determine most of the overheating risk before any system is specified.
Measured properly, increases in glazing area for daylight and marketing reasons have run directly against this.
External shading beats internal blinds
Stopping solar radiation before it passes the glass is far more effective than intercepting it after it has entered. Overhangs, brise-soleil, shutters, external blinds and deciduous planting all work on this principle. Internal blinds absorb the energy inside the room and re-radiate much of it, which is why they help far less than people expect.
In practice, external shading is common in hot-climate building traditions and has been widely omitted in temperate ones that are now warming.
Ventilation has to be usable at night
Purging accumulated heat overnight relies on being able to open windows to cooler outside air. Security, noise, pollution and insect concerns all reduce whether occupants actually do this, particularly at ground floor and on busy streets. Secure night-vent openings, acoustic ventilators and single-aspect avoidance address these and are rarely specified in ordinary housing.
Where windows cannot be opened at night, mechanical cooling becomes the only remaining option.
Single aspect makes everything worse
A dwelling with openings on one side cannot cross-ventilate, so purging heat depends on weak stack effects alone. Combine that with a west-facing aspect, large glazing and a top floor location and the overheating risk compounds.
These combinations recur in the same building types, which makes the risk predictable at design stage. Assessment methods exist that model overheating across a design summer, and their use is required in some jurisdictions and not others.
Local statute governs most of this, and it varies street by street.
The heat health risk falls unevenly
Older people, people with chronic conditions and people who cannot leave home are most exposed to indoor heat. Excess deaths during heatwaves concentrate among these groups and occur largely indoors rather than outdoors. Housing for older and vulnerable residents therefore needs the strongest overheating protection and frequently has the weakest.
Anyone with a medical condition affected by heat should discuss it with a qualified professional rather than relying on general guidance.
The takeaway
Ask how the building loses heat in August, not just how it keeps it in January.
The design decision is visible long after the people who made it have gone.
Questions readers ask
Do I need to choose between a warm winter home and a cool summer one?
No. Insulation plus external shading, adequate ventilation and sensible glazing areas delivers both. The problem arises when fabric is improved and summer performance is not considered.
Are internal blinds any use against overheating?
They help with glare and much less with heat, because the energy has already passed the glass. Anything that blocks sun outside the window is considerably more effective.
