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Summer Overheating Protection and Thermal Mass with Pavatex Wood Fibre Insulation

Summer Heat Protection with Pavatex Wood Fibre Insulation


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 summer overheating protection as much 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.


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) refers to a material’s capacity to store heat for every kilogram of that material and is measured in J/kgK. If a material has a high thermal mass it must also have a high specific heat capacity. Pavatex wood fibre roof and wall sarking boards – Isolair – have a Specific Heat Capacity of 2100 J/kgK which is the highest of all insulation materials.
Density 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. Pavatex wood fibre roof and wall sarking boards – Isolair – have a Density of 240 kg/m³ which is the highest of all insulation materials.
Thermal Conductivity Thermal conductivity (also known as k or lambda λ) is a measure of the ability of a material to transmit heat through the 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 wood fibre roof and wall sarking boards – Isolair – have a Declared Thermal Conductivity of 0.047 W/mK.


Benefits of Thermal Mass in Summer

Attic rooms and top floor apartments are usually the hottest areas within a building during summer-time due to solar gain coming in through the windows, and this heat moves up to the top of the building as hot air rises. Also because the roof covering is a lightweight material with low thermal storage capacity, the heat from the sun will quickly heat it up and this heat will transfer into the attic space. In addition a pitched roof area is quite large compared to the internal volume of the space and so this increases the risk of heat transfer. Thermal mass is critical to good passive solar heating design because high thermal mass materials can absorb and retain heat and so offer summer overheating protection. When a building fabric has low thermal mass, the interior will heat up quickly at the hottest time of the day, and it will lose its heat much more quickly during the night as it gets cold.


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 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.


The addition of Pavatex wood fibre sarking insulation boards will slow down the rate at which the sun heats the interior space during the day by between 10 and 12 hours whereas most common insulation materials only have a lag time of 5 to 8 hours. As shown below most insulation materials have low thermal mass but wood fibre is an exception. 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.


Wood Fibre Insulation Summer Graph


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 60 ºC whereas the surface below the Pavatex Isolair 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.



Specific Heat Capacity


Thermal Conductivity




Thermal Mass Effectiveness

Decrement Delay







Brickwork – Outer Leaf





Dense Concrete Block





Light Concrete Block





Pavatex Isolair Wood Fibre Sarking Board











Cellulose Insulation




Medium / Low


Sheep Wool Insulation






EPS Polystyrene Insulation






Mineral Fibre Batt Insulation







Benefits of Thermal Mass in Winter

The thermal mass or thermal accumulation of a building offers benefits both in summer and winter. In the colder winter months we want to take advantage of the day-time heat from solar gain as well as saving the heat produced from central heating, lighting, electrical appliances, cooking and people. This heat is absorbed and stored by the Pavatex wood fibre within the fabric of the building and it is gradually released overnight as the temperature dips, helping to keep the building warmer and reducing the need for additional heating.


By alternately storing and releasing heat, high thermal mass enables buildings to respond naturally to changing temperatures, helping to stabilise the interior temperature.


Pavatex wood fibre boards represent a multifunctional insulation material that is able to meet both of these functions:

  • Thermal insulation (properties comparable to mineral wool and polystyrene)
  • Thermal accumulation or Thermal mass (when compared to insulation products such as those mentioned above, with the same thickness, its accumulation ability is 20 – 30 times higher).


Pavatex wood fibre boards (which can be used in conjunction with other thermal insulation materials) will provide thermal cladding structures for attics, roofs and timber frame walls which will meet the requirements for a building’s winter performance, as well as its summer performance.