Phase Change Material (PCM)

What is a phase change material?

A phase change material (PCM) is a substance that stores and releases large amounts of heat as it changes between solid and liquid states, at a nearly constant temperature. In buildings, PCMs are built into wall linings, floor screeds or panels so that a lightweight structure gains some of the heat-storing capacity that a heavy masonry house has naturally. The most common building PCMs are paraffin waxes and salt hydrates, both chosen for melting temperatures close to the indoor comfort range.

The product category has grown alongside lightweight construction, where the house starts with less built-in mass. It sits next to thermal mass, the traditional way buildings buffer temperature swings, and the two are often confused. This article explains how PCMs work, where they fit in a timber house, and why they are a specialist option in Slovakia rather than a default one.

How does a PCM store heat?

Ordinary building materials store heat as sensible heat: their temperature rises as they absorb energy. The amount stored depends on the mass, the specific heat capacity of the material and the size of the temperature change. A PCM adds a second store. When it reaches its melting point it absorbs heat while staying at roughly the same temperature, because the energy goes into changing the structure of the material. This is called latent heat. When the room cools at night, the material solidifies again and releases that heat back into the room.

This is why a PCM can hold a large amount of heat within a narrow temperature band, far more than a plaster or timber panel of the same thickness. The catch is that the effect only works in the band where the material changes phase. Outside that band, the PCM behaves like an ordinary, fairly light material.

Where are PCMs placed in a timber house?

In a drevostavba (timber-frame house) the heavy layers that normally provide mass are missing, so PCM products are usually fitted to interior surfaces that already exist. The three common locations differ in how much surface they expose and how hard they are to add later.

Location	Typical form	Advantage	Drawback
Plasterboard (sadrokartón)	Board with the PCM sealed in microcapsules or laminated in	Fits a normal dry-lining sequence and offers a large exposed area	Adds cost per square metre, and the boards need careful handling and fixing
Floor screed (poter)	Granules or pellets mixed into the screed	Heat reaches the material from the floor as well as from the room	Changes how the screed behaves, so it must be specified before pouring and is hard to change later
Glazing or shading elements	Panels or interlayers built into glass units	Can act on sunlight before it enters the room	Specialist product, usually more expensive and rarely used in housing

Plasterboard is the most common route in lightweight houses because it is a familiar dry-lining product that fits into the normal building sequence. Screed and glazing options are used less often and need the designer and the supplier to agree the details before construction starts.

How is a PCM different from thermal mass?

Thermal mass is the general property of heavy materials such as concrete, brick and stone to absorb heat and release it slowly. A PCM is a specialised material that achieves a similar buffering effect with much less volume, but only near its melting point. The table shows the practical differences that matter when choosing between them.

Property	Conventional thermal mass	Phase change material
How it stores heat	Sensible heat across the whole temperature range	Latent heat at a narrow, fixed melting point
Volume needed	Large, usually structural masonry or concrete	Small, as a board or screed layer
Behaviour across the year	Works in every season once the mass is in place	Works only when temperatures cross the melting point
Fit with a timber frame	Needs a separate heavy layer added to the frame	Can be added to the existing dry lining
Cost and supply in Slovakia	Mature, familiar supply chain	Niche product with higher cost and a smaller supply base

A PCM does not replace thermal mass in a house that is already heavy. In a lightweight timber house it can add some of the buffering that is otherwise missing, but a well-shaded, night-ventilated house usually does more for summer comfort with simpler means.

Is a PCM worth it in Slovak housing?

For most Slovak houses, a PCM is not a first measure. It is a niche and costly option that is rarely specified in standard practice. Summer overheating, the problem a PCM is meant to address, is more pressing in lightweight construction with large glazing, and the cheapest and most reliable fixes are external shading, sensible window orientation and night-time ventilation. These measures work whatever the house is built from, and they stop heat before it enters the room rather than storing it afterwards.

A PCM makes more sense in a narrower set of cases:

• a lightweight house with large south or west glazing where exterior shading cannot be fitted, for example on a protected facade;
• a top-floor room or attic conversion where a dry-lining upgrade is already planned;
• a design where the overheating assessment shows that exterior measures alone are not enough.

Treat the PCM as one line item that needs a justification. Ask the supplier for the melting range, the behaviour over repeated cycles and the fire classification in writing before deciding.

What are the common misconceptions about PCMs?

The first misconception is that a PCM is an insulator. It is not. It stores heat rather than slowing its flow, so it does not replace a properly insulated envelope.

The second is that a PCM works on its own. It absorbs heat during the day and needs a cool period at night to solidify again. If the room stays warm overnight, the material never recharges and quickly stops helping. Night ventilation is therefore part of the design, not an optional extra.

The third is that a PCM removes the need for shading. Shading stops solar gains at the window, while a PCM only holds the heat that gets in. Once the storage is full, extra heat simply stays in the room. The two measures are complementary, not interchangeable, which is why the summer overheating risk should be assessed before any PCM is specified.