What is a phase change material (PCM)?
A PCM or phase change material stores heat by melting at a fixed temperature and releases that heat again as it solidifies. It therefore acts as thermal mass, without the weight of concrete or masonry.
When a material melts it absorbs a great deal of heat without itself getting hotter; this is called latent heat. Water is the familiar example: ice stays at 0 °C until the last crystal has melted. For buildings you choose a PCM whose melting point sits in the comfort range, usually somewhere between about 21 and 26 °C.
How it works in a home
During the day the indoor temperature rises. Once the melting point is reached the PCM starts to melt and takes heat out of the air, so the room warms up far more slowly. At night, with night ventilation or lower outdoor temperatures, the material solidifies again and gives the heat back. The cycle has to complete every 24 hours: if the PCM cannot solidify overnight, it is “full” the next day and does nothing.
Types
- Paraffins — organic, chemically stable, non-corrosive, but combustible and with a relatively low density.
- Salt hydrates — inorganic, higher storage capacity per litre and non-combustible, but prone to supercooling and phase separation after repeated cycling.
Both are encapsulated: as microcapsules mixed into plaster or plasterboard, or as macrocapsules — pouches, mats or aluminium profiles — placed in a suspended ceiling, behind a wall lining or within a floor build-up.
Where it makes sense — and where it does not
PCM is mainly of interest in lightweight construction with little thermal mass of its own: timber frame, loft conversions, attic rooms, tiny houses and light office refurbishments. In a house with thick concrete floors and masonry internal walls the gain is small — that mass already does the job.
Key conditions: the PCM must be in contact with the room air (not buried behind insulation), the melting point must match the use, and there must be a way to discharge it overnight. Without external solar shading and purge ventilation the effect stays limited — PCM does not replace good shading, it supplements it.
Checks before applying PCM in a room
- In contact with the room air: Not buried behind insulation.
- Melting point matches the use: For buildings usually somewhere between about 21 and 26 °C.
- Overnight discharge is possible: If the PCM cannot solidify overnight, it is full the next day and does nothing.
- External solar shading and purge ventilation: Without them the effect stays limited.
Related terms
- Thermal mass
- Night ventilation
- Insulation value
- Overheating
- Timber frame construction
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