There are several different types of roof insulation, and each have different properties and characteristics that they may bring to your project. In this blog we will explain why insulation is needed, how it works and cover some of the types of roof insulation that are available. We will explore the properties that different types of roof insulation, both for flat and pitched roofs, can bring to a project and cover topics such as acoustic performance and the effects of roof insulation on the sustainability considerations for a project.
As you will expect the most suitable insulation for your project depends upon a variety of different project parameters including:
- Roof construction – flat or pitched
- Roof type – warm or cold
- New build roof or retrofit of an existing roof
- Thermal performance (U-value) requirement
- Ventilation strategy
- Moisture control measures
- Sustainability
We will start by explaining some of the basics – such as why roof insulation is needed and how it works.
Why is roof insulation needed?
If a property is uninsulated, when heating is needed during cold weather it can lose around 25% [1]of its heat through the roof. Given that hot air rises, it is not that surprising that insulating the roof to keep the warm air from escaping can be beneficial.
Roof insulation can help to increase the energy efficiency of a building and help to lower its energy bills. The EST (Energy Savings Trust) has published data[2] on the savings that are possible by adding roof insulation to an uninsulated loft.
Roof insulation doesn’t just keep the heat in; it can also help to keep heat out. When the air is hot outside in the summer, it will help to slow down the passage of heat and keep the roof cooler to mitigate the effects of overheating. Overheating has a detrimental effect on people’s lives and can cause poor health and even excessive deaths. During the hot summer of 2022 there were nearly 3,000 excess deaths caused by the effects of overheating.[3]
According to the Committee for Climate Change report[4], under current UK weather conditions, 50% of existing UK homes will suffer from overheating risk. If global temperatures were to rise by 2°C this number would rise to 90%.
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[1] energysavingtrust.org.uk/advice/roof-and-loft-insulation/
[1] energysavingtrust.org.uk/advice/roof-and-loft-insulation/
As buildings face greater risks of overheating, insulation must withstand higher temperatures and humidity levels. While effective ventilation remains essential, an insulation’s ability to manage moisture is equally important. Some materials naturally buffer moisture and help to regulate humidity, reducing the risk of condensation on cold surfaces. When combined with vapour-permeable properties that allow moisture to escape the building fabric, these characteristics make certain insulations a valuable part of a long-term, low-risk strategy. It's crucial to consider these factors alongside ventilation when specifying insulation.
How does roof insulation work?
Insulation is trying to prevent all three forms of heat transfer. In this table we look at each type and explain how roof insulation can help:
Type of heat transfer | What is it? | How does roof insulation help? |
Convection | Heat transfer by the movement of air | A layer of insulation provides a barrier to help stop the roof from losing heat by convection currents. |
Conduction | Heat transfer through a solid | Insulation that has prevented escape of warm air will get warm. The slower the heat conducts through the insulation the better the insulant |
Radiation | All warm bodies radiate heat through electromagnetic waves | Roof insulation with shiny surfaces can reflect radiant heat back into the roof space to help to keep it warm |
Like all insulating materials, roof insulation works by slowing down the passage of heat. By forming a barrier layer over and/or between roof joists and rafters, it helps reduce heat loss by convection currents caused by rising warm air.
Once the heat reaches the roof insulation, the slower the heat passes through the insulation the better. This is known as the insulation’s thermal conductivity, or lambda value. The lower the thermal conductivity or lambda of the roof insulation, the better the insulant.
The lambda value insulation achieves will depend upon several factors including:
- Density
- Structure of the material
- Properties of any gas trapped within the structure of the insulation as we explain in this table:
Insulating method | Typical roof insulation | Notes |
Trapped air |
| Roof insulation such as glass and mineral fibre will trap air between its fibres to slow down heat transfer. Wood fibre insulation boards also trap air within its solid cellular structure to slow down heat transfer |
Trapped gas |
| Roof insulation such as rigid PIR and Phenolic boards have trapped gas within their cell structures. This gas is a better insulator than air so less insulation is needed to provide the same thermal performance than roof insulants using trapped air alone.
|
Can reflective surfaces enhance thermal performance of an insulated roof?
Where an air space occurs between two reflective layers, such as aluminium foil, the insulating value of the trapped air is increased. It is referred to as a low emissivity, or ‘low e’ air gap.
Insulation such as multifoil is made up of multiple layers of reflective foil to reflect radiant heat. It requires that air spaces are left, to enable low-e spaces to be created. In between the reflective foils of multifoils, are layers of wadding to help to trap air and add to the overall insulating effect.
Multifoils are usually used together with other roof insulation types to achieve the desired U-value. A U-value is the thermal transmittance of a structure, in this case the build-up of the roof. It indicates the rate that heat will be transferred through the roof. Therefore, the lower the U-value the better, as this indicates heat is being lost more slowly. A roof structure with a low U-value is better at insulating than one with a higher U-value.
Other insulation types such as PIR and Phenolic also use reflective foils on their surface to give the opportunity to create low-e gaps when used to insulate roof spaces and to enhance the overall U-value of an insulated roof construction.
What are the different types of roof insulation?
Here are some common types of roof insulation with a summary of their properties:
Roof insulation type | Form | Vapour open or closed? | Density range (kg/m3) | Typical lambda values (W/mK)* |
Mineral wool | Rolls or batts | Open | 30 - 100 | 0.032 to 0.044 |
Glass wool | Rolls or batts | Open | 10 - 20 | 0.032 to 0.044 |
Wood fibre | Batts or boards | Open | 50 - 270 | 0.036 to 0.048 |
Wood fibre | Air injected | Open | 35 – 42 | 0.036 to 0.039 |
PIR | Rigid board | Closed | 30 - 32 | 0.020 to 0.027 |
Phenolic | Rigid board | Closed | Approx. 35 | 0.018 to 0.023 |
*Values stated in the table are typical and may vary from manufacturer to manufacturer – always check the declared product performance with the manufacturer. Values can vary due to different densities, formulations and manufacturing techniques.
What properties of roof insulation affect its suitability for projects?
Different roof insulation types have different properties and how these are prioritised will depend on a number of factors around the type of roof, its intended use, the required thermal performance, moisture control considerations and the project's sustainability goals.
We will run through some of the key properties of roof insulation to see how they can help decide which one is the best option for a project.
How does the physical form of roof insulation affect its suitability?
The physical form of roof insulation may determine its suitability for different projects.
Flexible rolls of insulation are commonly used to insulate between and above joists at ceiling level. There is no requirement for the insulation to support itself or to provide a supporting surface.
Insulation batts are often used between rafters in a pitched roof or joists on a flat roof. Their flexibility allows them to be squeezed between the rafters or joists to hold them in position.
Rigid insulation boards can be friction fitted between rafters and joists and mechanically fixed in place from above or below the roof to help further increase its thermal performance. The layer of insulation below the rafter or joist can be applied in the form of a plasterboard laminate. This provides both an insulation and interior finishing layer in one application.
For insulation above a flat roof deck, the compressive strength of rigid boards makes them a suitable option. The boards provide a flat surface for the waterproofing layer and have sufficient strength to help resist foot traffic across the roof. [1]
Vapour open or vapour closed roof insulation?
Ensuring measures are taken to control moisture is important when installing roof insulation. Using vapour open insulation such as mineral fibre or wood fibre insulation can allow the fabric of the roof to breathe. This may be best suited to older properties to match existing and established patterns of moisture movement, although it can equally be used as insulation in new build roofs.
The use of vapour closed insulation such as PIR, introduces a barrier to the movement of moisture vapour. These are common in existing modern roofs and when creating new build roofs. However, vapour closed roof insulation may not be the first choice for older properties, where breathable roof insulation may be considered a better match.
Whether you choose vapour-open or vapour-closed materials, proper installation is critical to ensure the system performs as intended. Moisture movement through the building fabric is inevitable and the long term management of moisture can make or break the long-term health of the structure. Both vapour-open and vapour-closed insulation systems can deliver robust solutions, but if they’re poorly installed, they can lead to trapped moisture leading to potential mould growth, and even structural damage.
How important is a low lambda value when choosing roof insulation?
A low lambda value of roof insulation means that less insulation is needed to achieve the required thermal performance, or U-value, of the roof. This can be important where, for example, a room-in-roof is being created in a pitched roof. This usually requires insulation between the existing rafters and underneath the rafters. The latter insulation layer then takes up valuable room space, so low lambda is an important criterion for roof insulation in this case to maximise head room when renovation can only be carried out from the inside.
However, if the roof was, for example, part of a heritage building, having insulation that could breathe and protect the integrity of the roof construction may be considered a higher priority.
Installing breathable wood fibre boards that can help to meet a heritage project’s sustainability goals at rafter level may, in fact, be the most suitable choice.
Low U values have typically been the driving force for which insulation type should be used. However, with increased understanding of issues associated with overheating and requirements for sustainability goals, a balance between these factors is often a more pragmatic approach.
How does the intended use of a roof space affect the choice of roof insulation?
If an existing pitched roof requires insulating, there are two main options - either insulating at joist or rafter level - that affect the type of roof insulation that will be most suitable.
The choices are usually based on intended usage. If the roof space is just being used for storage and not as an inhabited space, then the insulation needs to run along the ceiling line. If it is being converted to a heated habitable space, then insulation is required along the existing rafter line.
Insulate at ceiling joist level and leave a cold ventilated space above
A common way to insulate at ceiling level is to use either glass or mineral wool rolls of roof insulation. They can be rolled out between the ceiling joists with another layer rolled on top. Care should be taken to ensure that existing eaves ventilation are not blocked up so that the roof space can be adequately ventilated to remove moisture and prevent any build-up of condensation.
Insulate at rafter level
Insulating at rafter level to create a warm roof can be done either from below or from above the rafters.
Insulating from below the rafters
Insulating from below the rafters is usually carried out in a refurbishment scenario where the existing roof structure is to be left in position. Here, rigid batts or boards can be fitted between the rafters and give the desired thermal performance improvement. Adequate ventilation space will need to be left between the existing roof membrane and the insulation to allow moisture to be removed. A secondary layer of roof insulation – usually with a low lambda insulation board, will normally be needed to achieve the required U-value. This has an added benefit of helping to reduce thermal bridging from the rafters.
Insulating from above the rafters
Insulating from above the rafters is usually carried out on a new build roof or where the external roof covering has been removed as part of the refurbishment to an existing building. Insulation in the form of rigid boards or batts is usually fitted between the rafters and additional rigid insulation boards over the rafters. This helps to improve the thermal efficiency of the roof and to reduce thermal bridging. Insulation between the rafters can be omitted and all the insulation can be fitted over the top to create a warm roof. Where vapour open insulation, such as rigid wood fibre boards, is used this creates a warm roof that allows water vapour to escape the building fabric and therefore significantly reduce the potential for condensation next to the new or existing timbers.
What is the most suitable insulation for a flat roof?
The most suitable insulation for a flat roof depends on the type of project being undertaken.
When insulating an existing flat roof where the waterproof covering does not need replacing, insulating from below is an easier option than removing the roof covering, insulating from above and then fitting a new waterproof roof. Mineral wool batts or rigid roof insulation such as wood fibre boards or batts are fitted between the joists of the flat roof. It is essential to leave a ventilation space between the underside of the roof deck and the insulation. Moisture entering the roof void must be controlled by ensuring that an internal vapour control layer is fitted. A ventilation strategy should also be in place to remove any moisture that does enter the void, or it will condense on the cold surface of the roof deck and could eventually cause structural problems through rotting timbers.
If the flat roof is being newly built, or an existing flat roof needs to have its waterproofing layer removed, it can be insulated by using rigid insulation boards such as wood fibre boards or dense mineral fibre batts on the structural deck.
This method also has advantages when it comes to helping to mitigate the effects of overheating. Boards such as wood fibre insulation have a combination of density, thermal conductivity and specific heat capacity that acts like a buffer in summer. The heat of the day can be stored in the roof insulation material for hours. A large proportion of the heat does not reach the interior at all and can be dissipated outside again during the colder nights. This may help to mitigate against the effects of overheating and could support a reduction of internal temperature fluctuations for a more comfortable internal climate.
Flat roofs have traditionally been one of the most difficult areas to insulate as they are more prone to water ingress from the external side and creating suitable ventilation is more complicated. The suitability of a given approach should always be verified and checked with the relevant system or insulation provider.
How does roof insulation help with a building’s sustainability considerations?
Roof insulation can help participate in improving the energy efficiency of a building and help to lower its energy use. That in turn can reduce its operational carbon dioxide emissions that are the one of the sustainability drivers for many construction developments.
Suitably specified and well installed roof insulation will help control moisture management and assist in the protection of the roof structure from condensation issues. This can help to prolong the service life of the structure and may limit the instances of roof repairs and replacements. This can assist in reducing the embodied carbon and operational carbon that is associated with such maintenance activities. In this way correctly specified and installed roof insulation can contribute to improving the sustainability of projects where these carbon metrics are used as long term key performance indicators of project sustainability.
Where projects look at the carbon contribution of construction materials as a measure of their sustainability, the Environmental Product Declarations (EPDs) of different products can be used in the specification selection process.
The EPDs for wood fibre insulation boards take into account that the wood used in their manufacture binds carbon dioxide equivalent CO2eq within the structure of each board.
Wood-based construction products effectively extend the duration of the naturally sustainable biogenic carbon cycle which can contribute to climate protection and help to meet sustainability goals.
Due to their durable nature and having wood as the major constituent, wood fibre insulation boards are well placed to be reused or re-purposed on site before they are sent for recycling.
However, as trees are the raw material for the manufacture of new insulation boards, they have bound more biogenic carbon from the atmosphere. Therefore, wood-based products are part of a short-term carbon cycle. Their major raw material does not add to long term carbon emissions unlike insulation based on raw materials that are extracted from reserves of minerals or petrochemicals that are not naturally replenished.[2]
Using wood fibre insulation also adds thermal mass to the roof, helping it to keep the building cooler in the summer and reducing the need for air conditioning that uses energy and therefore adds to the carbon footprint of the building.
Which roof insulation can help with acoustic performance?
Although predominantly specified for their thermal insulating properties, roof insulation can also have a real time impact on acoustic performance by reducing noise transfer from outside. Using a combination of insulations with differing densities is often the best approach as more sound waves of different frequencies are absorbed. This is particularly relevant in pitched roof applications where both flexible and rigid insulants can be utilized and hence external noise from air traffic or rainfall can be reduced.
Where can I find out more information about wood fibre roof insulation solutions?
STEICO makes wood fibre insulation that can help to contribute towards the sustainability goals of construction projects. Our STEICOflex 036 has a density of 50kg/m3 and lambda value of 0.036W/mk. It can be friction fitted between joists and rafters to provide a vapour open roof insulation. As well as helping to improve thermal performance and hence helping to reduce the use of carbon emitting heating systems, each cubic metre of STEICOflex 036 binds up to 73kg CO2eq*and can thus contribute to climate protection.
STEICOuniversal , STEICOuniversaldry and STEICOspecial dry are all T&G profiled boards that can be used over or under rafters in conjunction with STEICOflex 036 to further improve a roof's thermal performance. STEICOsafe is a T&G board that has a directly bonded membrane for increased weather protection in new builds and renovations.[3]
To enquire about using our wood fibre insulation on your next project, please contact our expert team who will be happy to help.
*Calculation according to EN 16449, life cycle stage A1 according to EN 15804