What is a U-Value

A U-value measures the rate of heat transfer through a building element such as a wall, roof, floor or window. It is expressed in W/m²K (watts per square metre per degree Kelvin) and indicates how much heat is lost per square metre of surface area for every degree of temperature difference between the interior and exterior.

Natural fibre insulations and other breathable materials often combine low thermal conductivity, high decrement delay, and the ability to manage radiant heat effectively.

Ideally, you want a good U-value with sustainable insulation which has good breathability. Breathable insulation helps regulate moisture and reduces the risk of condensation or mould. Some options include:

• Wood fibre boards: Very breathable and provide good thermal mass.
• Sheep’s wool: Naturally regulates humidity and absorbs moisture without losing insulation performance.
• Hemp: Highly breathable and resistant to mould.
• Recycled cellulose: Breathable if installed correctly and made from recycled materials.

These materials not only provide effective insulation but also support sustainable building practices by being renewable, recyclable, or low in environmental impact.

 

How important are U-Values

U-values are important, but they do not tell the whole story. Another key factor is decrement delay, which measures how long it takes heat from outside to pass through the building and reach the interior. A solid brick wall (225mm thick) typically has a phase shift of about eight hours, so heat absorbed during the day reaches the internal face in the evening, making rooms uncomfortably warm. Adding 60 mm of Isolair Multi wood fibre insulation increases the phase shift to around twelve hours, so the heat is released after midnight, when it is cooler, keeping the home more comfortable.

Radiant heat also plays a role. This is heat coming directly from the sun or heating systems. Materials with good thermal mass can absorb and store this heat, then release it gradually, improving comfort and efficiency. Natural fibre insulations therefore offer a balance of competitive U-values, high decrement delay, and useful thermal mass, making them effective for both energy saving and comfort.

So imagine a solid brick wall in an older house. Without insulation, it has a U-value of about 2.0 W/m²K, which means it loses heat quickly in winter and gets hot fast in summer.

If you add 60mm of wood fibre insulation to the inside, the U-value can drop to around 0.4 W/m²K. This means much less heat escapes in winter, so the house stays warmer.

At the same time, the wood fibre has a good decrement delay. On a hot summer’s day, the sun’s radiant heat takes longer to pass through the wall, so the room inside doesn’t overheat as quickly. By the evening, when it’s cooler outside, the stored heat is slowly released without making the room uncomfortable.

This shows how the right insulation can both cut heat loss by up to 80% in winter and reduce overheating in summer, making the house more comfortable all year round.

 

How to Calculate U-Values

Working out U-Values can involve a complex set of calculations, and it’s certainly fair to say that calculating ‘overall U-values’ (for example, an entire wall build up including bricks, render, timber frame, insulation, plaster etc) requires specialist knowledge and software. At Unity Lime, one of our directors is a chartered architectural technologist and leads our technical department, so we are able to calculate overall U-Value ranges for our specially designed insulation systems.