Thermal Performance of Wall Materials: Block, Brick, Stone

Blocks, brick and stone conduct and store heat differently. When choosing a wall material, insulating ability, thermal mass and moisture behaviour should be judged together.

In private house construction, the wall material is often chosen by price, the builder's habit or what the neighbours used. Yet two walls of the same thickness can hold heat very differently. Understanding a material's thermal properties makes it easier to decide whether insulation is needed and how thick it should be.

Three concepts: conductivity, resistance and capacity

Thermal conductivity shows how easily a material transmits heat. The lower the figure, the better the material insulates. The more small, closed air pores a material contains, the lower its conductivity, because still air is a poor conductor of heat.

Thermal resistance is the resistance of a whole wall layer to heat flow, and depends on the material's conductivity and thickness. In other words, even a poorly insulating material offers some resistance when thick, but the wall then has to be very thick.

Heat capacity is a material's ability to absorb and store heat. Heavy, dense materials slowly soak up heat during the day and release it at night. This smooths out temperature swings, but it is not the same as insulation.

The main wall materials

Aerated concrete blocks contain many closed air pores. They are light, easy to cut and insulate well. The lower the density grade, the better the thermal performance, but strength decreases. Their porous structure takes up moisture, so they need an external finish that is vapour-open yet protects against rain.

Hollow clay blocks are made of fired clay with many vertical channels. The channels and porous clay give good insulation and reasonable heat capacity. Many of these blocks have interlocking tongue-and-groove edges and are laid without mortar in the vertical joints.

Solid brick is dense and strong with high heat capacity, but it conducts heat considerably better. A wall of solid brick alone usually does not meet modern energy requirements without added insulation.

Natural stone, such as the limestone widely used on the Absheron peninsula, varies with density. Porous stones insulate better than hard stones, but still not at the level of dedicated insulation materials. Stone's strengths are heat capacity, durability and appearance.

Cast-in-place concrete is among the most conductive load-bearing materials; uninsulated concrete walls and columns act as thermal bridges.

The fact that old houses with thick stone walls stay cool in summer is often taken as a sign of good insulation. In reality it comes from high heat capacity: the wall slowly absorbs daytime heat and passes it to the room late. In winter, the same wall holds cold just as slowly and needs long heating. Knowing this difference helps make the right insulation decision when renovating an old stone house.

Mortar joints and moisture matter too

A wall's real thermal performance does not depend on the block alone. Thick mortar joints conduct heat better than the block and create a cold strip at every course. Precisely shaped blocks laid with thin-bed adhesive reduce this problem.

Moisture increases thermal conductivity, since water conducts heat far better than air. A wall left in the rain during construction or drawing up moisture by capillary action performs worse than laboratory figures. Keeping walls dry therefore depends on three things: a damp-proof layer above the foundation, protecting the masonry from rain during the build, and a suitable facade finish.

Think of it together with insulation

In practice one of two approaches is usually chosen. The first is a single-leaf wall of thick blocks with good insulating properties: simple, with few layers, but the wall is thick. The second is a strong load-bearing wall that conducts heat more readily, with an external insulation layer on top: load-bearing and insulating functions are separated.

In the second scheme, the heavy material keeps its heat capacity on the inside of the house, which helps keep temperatures stable in summer. In a climate with hot summers and cold winters, this combination is often considered advantageous.

Questions for choosing

• How many storeys will the building have, and what load will the wall carry?
• Is external insulation planned for the wall?
• Does the manufacturer document the thermal conductivity and density grade of the chosen blocks?
• Will the masonry be laid with thin-bed adhesive or ordinary mortar?
• Is the facade finish compatible with the material's moisture behaviour?
• How will concrete elements such as lintels and columns be insulated?

Zaferoğlu İnşaat builds private houses, residential and commercial buildings to design. Balancing wall thickness against insulation is best worked out with us at sketch stage, while the plan can still change.