Roof Insulation

SOPREMA offer a variety of insulation products suitable for flat roof applications.

Flat roofs are a less common sight in the UK owing to their typically higher maintenance requirements when compared to other forms of roof construction, although they can be a versatile option.

Achieving the required thermal performance in flat roof constructions can sometimes be challenging, whether insulation is placed at joist level in a cold roof (where joist depth may be limited) or above the roof deck in a warm roof (where overall roof height needs to be considered).

SOPREMA manufacture and supply rigid polyisocyanurate (PIR) boards that can be considered for a range of applications in flat roofs, whether insulation is located at joist level or above the roof deck.

All ground floor constructions, unless very large commercial floors, generally require insulation within the floor zone to meet Building Regulation requirements. If the ground floor is a suspended timber construction, whether this is a renovation or new-build project, the insulation is normally installed in the joist zone - that is, between the joists.

A rigid polyisocyanurate (PIR) board, such as SOPRATHERM GA4000 or XR4000 (previously Celotex GA4000 and XR4000), can be considered for installation between the joists, provided it is cut accurately and fitted carefully to minimise air gaps. Thinner PIR boards, such as SOPRATHERM TB4000 (previously Celotex TB4000), can be considered for perimeter upstand insulation.

The floor would typically be finished from above with a vapour control layer (VCL), if required, and a suitable timber floor deck.

Pitched roofs are a common sight in the UK owing to their versatility and typically lower maintenance requirements when compared with other forms of roof construction. Another consideration when specifying a pitched roof is the ability to achieve a high level of thermal performance in these roof types, whether you’re looking to insulate at joist (ceiling) level for a cold pitched roof or at rafter level if a living area within a warm roof construction is desired.

Here at SOPREMA, we manufacture and supply rigid polyisocyanurate (PIR) boards that can be considered suitable for a wide range of applications across pitched roofs, whether you’re looking to insulate a loft at joist level or to insulate at rafter level within a pitched roof construction.

Loft insulation refers to material installed within the joists of a pitched roof space to help reduce heat loss from the building below.

We offer a range of SOPREMA products suitable for installation in a loft.

Some older loft insulation installations may include only a single layer of around 100mm of insulation between the ceiling joists. A common approach to improve thermal performance is to place additional insulation over the top of the existing layer within the ceiling joists. However, if the existing material is in particularly poor condition, replacing it with new insulation may be more appropriate.

If the loft area is intended to be used for storage, the loads applied to the roof structure should be carefully considered to ensure the construction is suitable for this purpose.

Assuming the roof construction is able to deal with the intended loads then SOPREMA rigid polyisocyanurate (PIR) insulation boards can accommodate typical storage loads without compressing. Timber boarding should be laid over the boards to provide protection and maintain their performance. Screw fixings are typically all that is required to secure a product like SOPRATHERM GA4000 or XR4000 (previously known ask Celotext GA4000 and XR4000) in position above the joists.

The insulation is lightweight and does not impose significant weight on existing structural timbers. The rigid boards can be laid over the entire floor of the loft; or they can be used only in the area required for storage.

PIR insulation is thermally efficient and can be used as part of a complete refurbishment of the loft floor, or in conjunction with existing insulation laid between the joists, to help improve heat loss from the property and, where necessary, meet the requirements of national building regulations.

Advice should be sought on correct installation practices, including ensuring a well-sealed ceiling below the loft joists. Continuity of insulation at the wall/roof junction should be achieved without blocking eaves vents and while ensuring adequate ventilation of the loft space.

Taking a ‘fabric first’ approach to a building’s design involves focusing on the thermal efficiency of each element and using a thickness of insulation that is practical to install to help achieve low U-values.

Fixing insulation externally can provide the greatest reduction in heat loss, although this can be challenging in practice, particularly on pitched roofs.

New-build projects generally have fewer constraints and more space to accommodate insulation above the rafters, while large-scale refurbishment projects, especially those involving complete re-roofing, may provide a similar opportunity.

An over-rafter insulation layer can help to achieve a well-insulated, airtight building with a useful and comfortable habitable room in the roof space. Installing insulation over rather than between the rafters eliminates concerns about thermal bridging and, overall, saves on the total thickness of insulation used.

The key is good detailing. It’s important to make sure the roof insulation layer connects with the insulation in the wall to achieve a continuous thermal envelope. Any breaks and the benefit of fixing the insulation in this way are quickly lost, in the form of heat escaping through the gaps at the junction.

Of course, combining the depth of both the rafters and the insulation can result in a substantial roof construction. Getting the detailing right also makes sure the total thickness has been accounted for at design stage and doesn’t cause any nasty surprises when it comes to building the roof. Low-energy construction does not have to mean compromised aesthetics.

Rigid, lightweight, polyisocyanurate (PIR) foam insulation boards (such as SOPRATHERM (Celotex) GA4000 and XR4000) are among the most thermally efficient, commonly-available insulation products on the market. Fixed over the rafters of a pitched roof, they can help to achieve the required U-value in just a single layer.

Where the benefits of over-rafter insulation are desired but practical restrictions prove difficult to overcome, you may consider insulating between and over rafters. 

The benefits of insulating a building externally, which can help protect the structure and reduce heat loss, are widely recognised. For pitched roofs, however, adding a thick layer of insulation above the rafters can increase the overall roof depth, which may not be suitable for every site. Internal constraints, such as maintaining sufficient headroom, can also limit the options for installing insulation between and below the rafters.

Where the advantages of over-rafter insulation are desired while still providing a usable room within the roof space, it is possible to insulate both over and between the rafters. Due to the thermal bridging effect of the timber, the overall thickness of insulation required to achieve a low U-value may be less efficient than over-rafter insulation alone, but it can offer a practical compromise solution.

To protect the roof from the potential of condensation occurring unseen within the construction, it’s important to remember that the layer of insulation installed over the rafters should be equal or greater in thermal value than the layer between the rafters.

An over and between rafter insulation solution can be a feature of a new-build project. It is also suitable for retrofit with a substantial renovation of the roof. The loft space can be used as a habitable room.

Rigid polyisocyanurate (PIR) foam insulation boards (such as SOPRATHERM (Celotex) GA4000 and XR4000) are easy to cut and handle. Once up at roof level, the between-rafter boards can be cut accurately to fit the rafter spacings. Being lightweight, they do not impose significant additional loads on existing structures. Where the insulation does not fill the total rafter depth, the low-emissivity foil facings can gain additional thermal benefit from the air spaces they face into.

The benefits of insulating a building externally, which can help protect the structure and reduce heat loss, are widely recognised. For pitched roofs, however, adding a thick layer of insulation above the rafters can increase the overall roof depth, which may not be suitable for every site. Internal constraints, such as maintaining sufficient headroom, can also limit the options for installing insulation between and below the rafters.

For both new-build projects and when creating a habitable area in an existing loft, the insulation line will typically need to be within the pitched (sloped) part of the roof. This means insulation is often installed between the timber rafters to help provide protection to the newly created habitable space from external temperature fluctuations.

Rigid polyisocyanurate (PIR) foam insulation boards (such as SOPRATHERM (Celotex) GA4000 and XR4000) are lightweight and easy to handle. Once positioned at roof level, the between-rafter boards can be cut to fit the rafter spacings accurately. Where the insulation does not fully fill the rafter depth, the low-emissivity foil facings can provide additional thermal benefit from the air spaces they face into.

One of the most commonly used pitched roof solutions is installing insulation between and under the rafters, which makes practical use of the depth of the structural timbers. renovation.

Its versatility contributes to its popularity, whether being used for:

• Creating a room-in-the-roof in a new-build property where over-rafter insulation may not be desirable
• Refurbishing an existing roof where there is sufficient headroom below the ceiling, or
• Converting a loft space into a habitable room.

This approach allows insulation to be installed in a watertight roof, either after the roof covering has been fixed on a new-build project, or without disturbing an otherwise sound existing roof covering as part of a renovation.

Modern U-value targets mean installing insulation between rafters only is rarely a viable option. Using a thermally efficient material to get the maximum benefit from as much of the rafter depth as possible means the under-rafter insulation layer can be kept to a minimum, making installation easier.

Rigid polyisocyanurate (PIR) foam insulation boards (such as SOPRATHERM (Celotex) XR4000 and GA4000) are among the most thermally efficient commonly available insulation materials on the market. Depending on the specifics of the roof, it may be possible to fully fill the rafter depth or, in most cases, leave a 25mm or 50mm air gap between the top face of the insulation and the sarking membrane.

The level of thermal performance required then dictates the thickness of insulation required below the rafters - usually anything from 25mm to 65mm of PIR foam. That can be a general application board (SOPRATHERM (Celotex) TB4000 and GA4000 can be considered) followed by a separate plasterboard. Alternatively, using a thermal laminate featuring rigid PIR insulation pre-bonded to a plasterboard (such as Celotex PL4000) means fitting the insulation and ceiling lining in one operation, reducing storage, cutting and fixing.

When using SOPREMA products, you need to satisfy yourself that use of the product meets all relevant national Building Regulations and guidance as well as local, national and other applicable standards relevant for your construction or application, including requirements in relation to fire and applicable height restrictions. In addition to the product datasheet, please refer to the following product documents:

• BBA certificates - where applicable to the application
• Declarations of Performance
• Health & safety datasheets