Which Wall Material has the Best U-Value?

When constructing a building, one of the critical considerations is the thermal performance of its walls. The ‘U-value’ is a key metric here, measuring how effectively a building element, such as a wall, roof, or floor, can prevent heat from escaping. A lower U-value signifies better insulation and energy efficiency. In this blog, we’ll explore various wall materials and their respective U-values to understand which offers the best thermal performance.

What is a U-value?

The U-value, also known as the thermal transmittance, is a measure of how well a building material can conduct heat. It represents the rate of heat transfer through a material, with units of Watts per square meter per Kelvin (W/m²K). In simpler terms, it tells us how good a material is at insulating.

The U-value is critical in determining a building’s energy efficiency. Lower U-values mean better insulation and less heat loss, leading to reduced energy consumption for heating or cooling. This is especially important in regions with extreme weather conditions. Buildings with materials having low U-values maintain more consistent indoor temperatures, enhancing comfort for occupants. It prevents issues like cold drafts in winter and overheating in summer.

Effective insulation, indicated by lower U-values, can significantly reduce heating and cooling costs. This is a long-term financial benefit for homeowners or building operators. Lower U-values contribute to reducing a building’s carbon footprint. By using less energy for heating and cooling, there’s a corresponding decrease in greenhouse gas emissions.

Many countries have building regulations that specify maximum U-values for different parts of a building to ensure energy efficiency. Complying with these regulations is not only legally necessary but also beneficial for energy conservation and environmental protection. Materials with appropriate U-values help in reducing the risk of condensation within the building structure. This is important for the durability of the building materials and for maintaining a healthy indoor environment.

UK Building Regulations

UK building regulations set specific requirements for U-values in both new builds and retrofit projects to ensure energy efficiency and reduce carbon emissions. For new builds, Part L of the Building Regulations in England stipulates the U-value requirements. The exact values depend on the type of building (domestic or non-domestic) and the specific building element (walls, roofs, floors, etc.). For example, for new dwellings, the typical U-value requirements might be around 0.18 W/m²K for walls, 0.13 W/m²K for roofs, and 0.22 W/m²K for floors.

For retrofits, the requirements are outlined in Part L of the Building Regulations for England and Wales. The U-values for retrofits are generally less stringent than for new builds, recognising the challenges in upgrading existing structures. In general, the value required is 0.30W/m²K for the whole thermal envelope.

Types of wall material and the U-values

Brickwork

Brickwork, as a traditional and widely used construction material, plays a significant role in the architectural landscape, particularly in the UK. Its popularity stems from its durability, aesthetic appeal, and the ease with which it can be handled and constructed.

Brick walls are typically constructed from either solid bricks or a cavity wall structure. The latter consists of two layers of bricks with a space (cavity) in between, often filled with insulating material.

1. Solid Brick Walls: In older buildings, solid brick walls are common. These usually have higher U-values, indicating poorer insulation properties. A typical solid brick wall might have a U-value of around 2.0 W/(m²K), which is not ideal for thermal efficiency. This can lead to higher heating costs in winter and could result in overheating during summer months.
2. Cavity Walls: Modern brick constructions often employ cavity walls. These improve the thermal performance significantly. The cavity, when filled with insulation, can drastically lower the U-value. For instance, a brick cavity wall with good insulation can achieve U-values as low as 0.30 W/(m²K), making them much more energy-efficient than their solid brick counterparts.

Blockwork

Blockwork, a widely used construction method in the UK, typically involves the use of concrete or aerated concrete blocks. It’s a popular choice due to its strength, durability, and versatility in various building projects.

1. Concrete Blocks: Standard concrete blocks are known for their structural strength but have relatively high thermal conductivity, meaning they are not very efficient insulators. The U-values for walls built with these blocks can range from 1.75 to 2.25 W/(m²K), depending on the block thickness and density. These higher U-values indicate poorer insulation, leading to greater heat loss.
2. Aerated Concrete Blocks: Aerated or cellular concrete blocks offer significantly better insulation. These blocks are lighter, contain many small air pockets, and have lower thermal conductivity. Consequently, walls built with aerated concrete blocks can achieve much lower U-values, sometimes as low as 0.15 W/(m²K), especially when combined with additional insulation.

Insulated Concrete Form (ICF)

Insulated Concrete Forms (ICF) represent a modern and highly efficient building technology, particularly notable for its thermal insulation properties. This method involves using hollow foam blocks or panels as formwork for reinforced concrete. Once the concrete sets, the foam remains in place, providing high levels of insulation.

1. Structure: ICF systems consist of two main components – expanded polystyrene (EPS) foam panels and the concrete core. The foam panels are lightweight and interlock to create a formwork for the concrete, which is then poured in. This combination of concrete and foam offers both strength and insulation.
2. Thermal Performance: The U-values for ICF walls are exceptionally low, typically ranging from 0.20 to 0.25 W/(m²K). This high level of insulation is due to the EPS foam panels which form a continuous insulation layer on both sides of the concrete. The lack of thermal bridges in ICF construction significantly reduces heat loss, making these buildings more energy-efficient.

Structural Insulated Panels (SIPs)

Structural Insulated Panels (SIPs) are an advanced form of construction material that has become increasingly popular in modern building projects, particularly for their high energy efficiency and structural integrity. SIPs consist of an insulating foam core sandwiched between two structural facings, commonly made of oriented strand board (OSB).

1. Structure: The core of SIPs is typically made from expanded polystyrene (EPS), extruded polystyrene (XPS), or polyurethane foam. This core is bonded between two structural boards, usually OSB, creating a sandwich panel. These panels are strong, rigid, and lightweight.
2. Thermal Performance: SIPs are renowned for their excellent thermal insulation. The solid foam core eliminates thermal bridging, which occurs in traditional stud-frame construction. As a result, SIPs can achieve very low U-values, often around 0.20 to 0.25 W/(m²K), although this can vary depending on the thickness and type of foam used. This superior insulation efficiency significantly reduces the need for additional heating or cooling, leading to energy savings.

Timber frame

Timber frame construction, a method with deep historical roots, remains highly relevant in modern building practices, particularly for its aesthetic appeal and environmental sustainability. In timber frame buildings, the load-bearing structure is made entirely from wood, creating a natural and warm aesthetic.

1. Structure: In timber frame construction, the structural frame is made from timber posts and beams. This frame supports the entire building and is typically filled with insulation material to improve thermal performance.
2. Thermal Performance: The timber itself has natural insulating properties, and when combined with additional insulation materials, timber frame buildings can achieve excellent thermal efficiency. The U-values in timber frame construction can vary significantly depending on the type and thickness of the insulation used, but they can be as low as 0.19 W/(m²K), making them highly energy-efficient.

Hemp walls

Hemp walls, a part of the growing movement towards eco-friendly and sustainable building materials, are gaining attention in the construction industry for their unique properties and environmental benefits. Made from a mixture of hemp fibres, lime, and water, hemp walls offer an innovative alternative to traditional building materials. This method, known as “hempcrete”, combines the woody inner fibres of the hemp plant with a lime-based binder, creating a material that is both strong and lightweight.

1. Structure: Hempcrete is created by mixing hemp shiv (the woody core of the hemp plant) with a lime-based binder and water. This mixture is then cast around a timber frame, creating a solid wall once it dries. The hemp fibres provide insulation, while the lime binder solidifies the mix into a stone-like material.
2. Thermal Performance: Hemp walls have excellent thermal insulation properties. They typically have U-values around 0.23 W/(m²K), although this can vary depending on the density and thickness of the hempcrete. The material’s ability to regulate temperature and humidity makes it highly efficient for maintaining comfortable indoor environments.

Straw bale walls

Straw bale construction, an innovative and environmentally friendly building method, has gained significant attention for its sustainability and excellent insulation properties. This technique uses bales of straw, typically a by-product of cereal crops like wheat, as a key building material.

1. Construction Method: In straw bale construction, walls are formed by stacking bales of straw, which are then covered with earthen or lime plasters. The bales can be used as load-bearing elements or as infill for a structural frame, typically made of timber.
2. Thermal Insulation: Straw bale walls provide exceptional thermal insulation due to the natural air pockets within the straw. They typically have very low U-values, sometimes as low as 0.13 W/(m²K). This high level of insulation means that buildings with straw bale walls require less energy to heat and cool, leading to reduced energy costs and a smaller carbon footprint.

Concrete varieties

Lightweight Aggregate Concrete

1. Composition: Lightweight aggregate concrete is made using lightweight aggregates such as expanded clay, shale, or slate, which are produced by heating these materials in a rotary kiln. This process creates a porous structure within the aggregate, reducing the overall density of the concrete.
2. Thermal Properties: The use of lightweight aggregates significantly improves the insulation properties of this type of concrete. Its U-value can be around 0.28 W/(m²K), depending on the mix and the type of lightweight aggregate used. This improved thermal performance makes it a suitable choice for building elements where insulation is a priority.

Autoclaved Aerated Concrete (AAC)

1. Composition: AAC is a lightweight, precast concrete material that contains air pockets throughout its structure. It is made by mixing sand, lime, cement, and water with a small amount of aluminium powder, which reacts with the other ingredients to form hydrogen gas. This gas forms air pockets within the concrete, giving it a lightweight and porous structure.
2. Thermal Performance: AAC’s unique cellular structure provides excellent thermal insulation. It can achieve U-values as low as 0.15 W/(m²K), making it one of the best insulating concrete materials available. This high level of thermal efficiency is particularly beneficial in reducing heating and cooling costs in buildings.

Ferrocement

1. Composition: Ferrocement is a type of thin reinforced concrete commonly composed of a cement mortar reinforced with closely spaced layers of continuous and relatively small diameter mesh. The mesh may be made of metallic or other suitable materials.
2. Properties: While ferrocement is not primarily known for its thermal insulation properties, it stands out for its strength and durability. It’s especially useful in applications where a strong, yet thin and lightweight, concrete structure is required.