Glossary
Thermal mass and comfort in solid-wall London houses
A material's ability to absorb and slowly release heat. Masonry has high thermal mass.
A material's ability to absorb and slowly release heat. Masonry has high thermal mass.
Thermal mass is a material's capacity to absorb, store and release heat. Dense materials — solid brick, concrete, stone, screed — have high thermal mass and damp temperature swings; lightweight timber and plasterboard construction has low thermal mass and responds quickly.
Overheating. A glazed rear extension with a lightweight roof and a suspended timber floor has almost no capacity to absorb solar gain, so it becomes uncomfortable by mid-afternoon in June. Exposed concrete or a screeded floor absorbs that gain and releases it overnight.
The existing solid brick walls of a Victorian house already provide substantial mass, which is one reason older London housing is often more comfortable in a heatwave than a lightweight extension bolted onto it.
Exposed mass is the requirement — mass behind insulation and plasterboard is thermally isolated from the room and does nothing. A power-floated or polished concrete floor, an exposed soffit, or a dense blockwork internal wall left unlined all work. Night purge ventilation is needed to discharge the stored heat.
Approved Document O addresses overheating in new residential buildings, requiring solar gain to be limited and excess heat removed. It does not apply to extensions to existing dwellings, but its logic — limit glazing on south and west, provide shading, provide purge ventilation — is the right approach for any glazed rear extension.
How thermal mass plays out in Lewisham: London Borough of Lewisham (Lewisham Council) maintains 30 conservation areas, the Blackheath Conservation Area and the Deptford High Street Conservation Area among them, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Anyone dealing with thermal mass at a Lewisham address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where thermal mass still qualifies.
| High mass materials | Solid brick, concrete, stone, screed |
|---|---|
| Low mass materials | Timber frame, plasterboard, lightweight roof |
| Requirement | Mass must be exposed to the room to work |
| Paired with | Night purge ventilation |
| Overheating document | Approved Document O — new dwellings only |
| Main London risk | Glazed rear extensions facing south or west |
Very likely, without shading, exposed mass and purge ventilation.
No — the two do opposite jobs. Insulation resists heat flow; mass stores heat.
No. It applies to new residential buildings, but its principles are worth following.
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