Summer Heat Protection

Why do we need Summer Heat Protection?

Summer Overheating Protection and Thermal Mass

Thanks to climate change, our summers are getting hotter, and this trend is due to continue into the foreseeable future, so when we are designing or renovating buildings, we should pay considerable attention to summertime overheating protection, as well as to winter-time heat conservation. This is particularly critical in converted attic spaces, in dormer bungalows, in top floor apartments, in lightweight structures such as timber frame buildings and in contemporary buildings with lots of south and west-facing windows designed for solar gain. Schools and office blocks are also very susceptible to overheating in the warmer weather due to an abundance of windows, and heat being emitted from office machines, lighting and people. If anyone has spent a hot summer in a caravan or mobile home, they will be very familiar with this effect. The lightweight frame heats up very quickly and it becomes extremely uncomfortable inside. The opposite happens in an old stone building with shutters on the windows. The external stonework takes many hours to heat up from the sun and then it starts to cool down when night-time comes, so very little heat transfers into the building.

Pavatex wood fibre is a unique insulation product because as well as effectively thermally insulating a building from the cold, it also keeps buildings several degrees cooler during the hot summer days. This is due to its very high thermal mass properties which are the highest of all insulation materials. The added benefit is that Pavatex wood fibre is totally natural and is a sustainable, renewable product made from FSC and PEFC certified timber off-cuts from local sawmills. It helps to stabilise the internal temperature reducing the need for mechanical cooling systems, and so further contributes to reducing carbon emissions.

What Determines Thermal Mass?

Specific Heat Capacity

Specific Heat Capacity (also known as C) is the important factor in slowing-up the transfer of heat. It refers to the amount of heat required to raise the temperature of 1 kilogram of a material by 1 degree centigrade and is measured in J/kgK. If a material has a high thermal mass, it must also have a high specific heat capacity. Pavatex Isolair Multi wood fibre roof and wall sarking boards has a Specific Heat Capacity of 2100 J/kgK which is the highest of all insulation materials.


The density refers to the mass per unit volume of a material and is measured in kg/m³. If a material has a high thermal mass, it must also have a high density which is an aspect of slowing down thermal diffusivity. Pavatex Isolair Multi wood fibre roof and wall sarking boards have a Density of 145 – 200 kg/m³ depending on the board thickness, and this is the highest of all insulation materials.

Thermal Conductivity

Thermal conductivity (also known as k or lambda λ) is a measure of how easy it is for heat to travel through a material and is measured in units of W/mK. If a material has a high thermal mass, it must have a medium thermal conductivity figure so that the material can absorb heat and release it along with the natural heating and cooling cycles of the building. Pavatex Isolair Multi wood fibre roof and wall sarking boards have a Declared Thermal Conductivity of 0.041 – 0.044 W/mK depending on the board thickness.

Benefits of Thermal Mass in Summer

During the summer months, attic rooms and top-floor apartments tend to become the hottest areas within a building due to solar heat entering through windows, which then rises. Additionally, lightweight roof coverings with low thermal storage capacity quickly absorb and transfer heat from the sun into the attic space. The large surface area of a pitched roof compared to the internal volume of the space exacerbates this heat transfer risk. Thermal mass plays a crucial role in effective passive solar heating design because materials with high thermal mass can absorb and retain heat, providing protection against summer overheating. Conversely, building fabrics with low thermal mass heat up rapidly during the hottest part of the day and lose heat quickly at night when temperatures drop.

In the warmer summer months, the internal heat from the sun is absorbed and stored by the Pavatex wood fibre and so ensures a more comfortable living or working environment. This stored heat is then released gradually at night-time into the building or out through the windows through natural ventilation, depending on personal choice. This works particularly well in the UK and Ireland where there is quite a differential between day-time and night-time temperatures, so at night the stored heat passes to the cooler areas.

By incorporating Pavatex wood fibre sarking insulation boards, the rate at which the interior space heats up during the day is slowed by 10 to 12 hours (called decrement delay), compared to the 5 to 8 hours of lag time typical with most common insulation materials. Unlike materials with low thermal mass, wood fibre insulation possesses exceptional thermal accumulation capacity. The problem with having a low thermal accumulation capacity is that the building reacts very rapidly to any change in temperature – either externally from the sun or internally from the heating system or heat from cooking appliances etc. This absence of thermal mass is often cited as the main disadvantage of lightweight structures.

This graph shows that on a hot summer’s day with a maximum temperature of 26.2ºC, the inside surface of the roof tiles heats up to nearly 60ºC whereas the surface below the Pavatex Isolair Multi roof insulation only reaches 45ºC. The insulation stores the heat and prevents its transfer to the interior so that it remains pleasantly cool at around 15ºC, even on the hottest days. Thanks to the decrement delay of 12 hours and the reduction of the temperature amplitude to 5%, it is not until after midnight that the room temperature rises slightly to approx 17ºC.

Material Specific Heat Capacity (J/kgK) Thermal Conductivity (W/mK) Density (kg/m³) Thermal Mass Effectiveness Decrement Delay (Hours)
Limestone 910 1.5 2180 High -
Brickwork – Outer Leaf 800 0.84 1700 High -
Dense Concrete Block 1000 1.63 2300 High -
Light Concrete Block 1000 0.19 600 Medium -
Cellulose Insulation 2020 0.038 27-65 Medium / Low 8.7
Sheep Wool Insulation 1800 0.038 23 Low 7.0
EPS Polystyrene Insulation 1300 0.036 15-30 Low 6.3
Glass Mineral Wool Insulation 1030 0.035 20 Low 5.9
Pavatex Isolair Multi Wood Fibre Sarking Board 2100 0.041 145 High 11.7

Benefits of Thermal Mass in Winter

The thermal mass, or ability to accumulate heat, of a building offers advantages in both summer and winter. During the colder winter months, harnessing daytime heat from solar gain and conserving heat generated by central heating, lighting, appliances, cooking and occupants is essential. Pavatex wood fibre insulation effectively absorbs and stores this heat within the building’s structure, gradually releasing it overnight as temperatures drop. This process helps maintain a warmer indoor environment and reduces the need for additional heating.

High thermal mass enables buildings to naturally adjust to temperature fluctuations by storing and releasing heat, thereby stabilizing indoor temperatures. Pavatex wood fibre boards serve as a versatile insulation material capable of fulfilling both insulation and thermal accumulation functions:

By using Pavatex wood fibre sarking boards, along with flexible insulation, in attics, roofs, and timber/metal frame walls, buildings can meet the requirements for both winter and summer performance. These boards provide thermal insulation and thermal mass, ensuring effective temperature regulation throughout the year.