ThermoFoam Open Cell Best Practice Guide

With the Inspection Protocol in place and several mortgage lenders willing and able to lend to residential buildings with spray foam insulation, homeowners can consider this type of insulation if they want long-term, durable, and airtight performance. We’ve compiled a detailed overview to cover everything about spray foam insulation, from information about the chemistry behind the product to the expected retail value and sales processes that spray foam installers should abide by. Homeowners can make the correct decision for their property and its occupants only with information and knowledge. It is essential not to be put off using spray foam insulation. Remember that when solar panels were first introduced in the UK, the industry was unregulated and often shoddy; additionally, many mortgage lenders wouldn’t lend on timber-framed buildings, but now, the situation is entirely different. Some lenders even accept buildings for a mortgage where Japanese Knotweed is present. Fundamentally, when installed correctly, backed by an approved design, and using BBA or KIWA Certified products, spray foam insulation is unlikely to cause a problem for a homeowner.

Preferred Best Practice: Card Rafter Slide Ventilated Cavity

Our recommended option is to install spray foam insulation directly to a card rafter slide spacer between the roof rafters, leaving an approximate 50mm gap between the roof membrane and the spray foam. Card rafter slides should run from apex to eaves, ensuring that the 50mm air gap above is suitably ventilated at the ridge and through the soffit board for uninhibited overflow of the roof pitches. Felt lap vents should be considered as a further measure to increase the ventilation flow in the 50mm air gap. This process maximises the life of the spray foam insulation, ensuring good access to the roof above for repairs or replacement. As the primary roof membrane is not coated with spray foam insulation, it can be replaced quickly and without damaging the spray foam insulation or counter-battening it across the top side of the existing membrane.

Once fitted, card rafter slides will take up considerable space within the roof rafter assembly. With most existing roofs comprising 100mm rafter depths, this would only allow for a remaining 50mm of spray foam insulation between the rafters. The BBA certificates for Nexseal LE closed cell and Sucraseal open cell both state that they can be applied between and under rafters, and although this may be a concept that ultimately limits the ability to inspect the condition of the roof rafters, provided the moisture content within the rafters is lower than 18%. The rafters are in good condition and free from rot and woodworms; there is no scientific reason why roof rafters cannot be encased. Part L of the Building Regulations needs to be reviewed – airtightness, limiting thermal bridging elements and reducing heat loss are the fundamental objectives of the regulations. Wherever possible, insulation assemblies should be designed accordingly. In addition, applying spray foam between and under rafters can achieve a greater insulation depth. At a total insulation assembly thickness of 225mm, the Sucraseal open cell can meet U-Value targets without needing attic floor insulation. This increases the ability for the attic to be used for storage whilst making it a much safer environment for occupants to enter.

  • Install card rafter slide spacers between the roof rafters, leaving a 50mm spacer between the roof membrane and the spray foam.
  • Please ensure the card rafter slide is inserted into the eaves to draw ventilation from the soffit while creating an airtight internal insulation seal to the wall junction.
  • Ensure the card rafter slide meets and follows under the ridge beam (if unventilated) for maximum over-roof ventilation flow. Where ridge beams restrict airflow, consider using felt lap vents to increase ventilation volume (seek advice from ThermoFoam where necessary)
  • Please ensure the roof membrane is in good condition and free from perforations or tears (repair any minor tears or imperfections).
  • Please ensure the moisture content in timber rafters is below 18% in ALL rafter sections, testing the rafter faces and sides.
  • Please make sure the timber rafters are free from rot and woodworms.
  • Ensure the rafters are free from dust and debris so that the spray foam adheres and doesn’t shrink back or delaminate.
  • Spray-apply Sucraseal open cell or Nexseal LE closed cell to the required depth between the remaining rafter space.
  • Please ensure the spray foam is hermetically sealed rafter to rafter with no shrinkage or delamination from the inner rafter faces.
  • Spray-apply the insulation under the rafter to the prescribed thickness to meet the U-Value requirement (if required).
  • Where a condensation risk analysis shows remaining evidence of interstitial condensation, spray-apply an Intelligent membrane of vapour control layer directly onto the underside of the insulation.

Before installation, the roof membrane should be in good condition with no evidence of degradation, penetrations, or tears. The condition of the membrane is crucial in ensuring that the insulation assembly remains protected from driving rain and external moisture.

The moisture content of the roof rafters should be less than 18%, and extensive testing should be conducted throughout the attic space. Roof rafters should be thoroughly assessed to ensure they are free from evidence of rot and woodworm. Where moisture content is higher than 18%, the installation must only proceed once all timber elements show a moisture content below the maximum threshold.

A card rafter slide is fitted between the roof rafters to create a 50mm ventilation gap between the spray foam and the membrane. This also enables easy access to the membrane for replacement during a roof refurbishment. The card rafter slide should follow under the ridge beam or felt lap vents to promote airflow on either side of the roof pitch. In this image, the rafter slide stops at the ridge beam because there is a ventilated ridge above with vents present through the roof membrane.

The image shows a felt lap ventilator that creates a wider opening in the overlaps of the felt to promote airflow between the roof membrane and the rafter slide. When placed in every rafter bay, over-ridge ventilation is not required, given that there is plenty of natural airflow for any moisture to escape or evaporate from the void. Felt lap ventilators should be placed in every felt overlap in every rafter bay for maximum ventilation.

The soffit board is ventilated with the appropriate continuous ventilation openings. The general rule of thumb is that soffit vents should be present every 200-300mm apart, so approximately 2-3 per linear metre, depending on the type of vent used. Once fitted, this will enable airflow to travel into the eaves and behind the card rafter slide to remove any moisture that accumulates within the 50mm cavity between the insulation and the bitumen felt.

The spray foam is installed directly to the card rafter slide rather than the proprietary roof membrane. This enables easy access to roof repairs or refurbishments from above and allows any existing roof membranes to be removed and upgraded without damaging the spray foam. The card rafter slide should be fitted consistently within the rafter bay from apex to eaves so that the overlap opening does not allow moisture to run down the back of the card and into the insulation.

The spray foam is applied between the remaining rafter space and is hermetically sealed bay to bay. The product BBA certification also allows the spray foam to be installed between and under the rafters. Although under-rafter insulation prohibits the ability for the rafter faces to be assessed in the future, provided the pre-install survey identifies that the roof rafters are in good condition, free from moisture, rot and woodworm, there should be no concerns for encapsulating roof rafters as this will remove cold thermal bridging.

The image here shows open cell spray foam installed between the roof rafters, directly to the card rafter slide, with a further 100mm layer under the rafters. The rafters are fully encapsulated; however, when the spray foam was installed, the moisture content was well below 18%, and the pre-install survey report identified no evidence of rot or woodworm in the rafters. The monolithic insulation layer prevents surface condensation from forming, whereas should any interstitial condensation pass through the spray foam, it will reach the ventilated cavity and evaporate accordingly.

Acceptable Alternative: Spray Foam Directly To High-Resistance Roof Membranes (Including Timber Sarking) With Vapour Control Layer

Whilst not preferred due to limiting easy roof replacement in the future, it is acceptable to spray apply the foam directly to high-resistance roof membranes provided a vapour control layer is installed on the warm side of the insulation. This process is stated in our Sucraseal High-Resistance Pitched Roof BBA Certificate and endorsed within the QODA Independent Study. Applying spray foam directly to the roof membrane makes it more challenging to replace it in the future without damaging the insulation. Existing roof membranes can be left in situ, but to install a new membrane at the time of roof renewal, counter battens must be laid above the existing membrane to create a ventilated space. This can increase the height of the roof and may require guttering, fascia, and soffits to be realigned to meet the raised roof. In theory, spray foam installers should only seek to issue a product/workmanship warranty for the expected lifespan of the roof membrane onto which the insulation is applied. This might be difficult if there is little or no evidence about the age of the existing roof membrane at the time of installation.

When applying directly to a high-resistance roof membrane, the installer should consider the following processes:

  • Please ensure the roof membrane is in good condition and free from perforations or tears (repair any minor tears or imperfections).
  • Please ensure the moisture content in timber rafters is below 18% in ALL rafter sections, testing the rafter faces and sides.
  • Please make sure the timber rafters are free from rot and woodworms.
  • Ensure the rafters are free from dust and debris so that the spray foam adheres and doesn’t shrink back or delaminate.
  • Spray- Apply a light flash coat to the membrane to create a bonding layer and maintain a downward drape.
  • Spray-apply Sucraseal open cell or Nexseal LE closed cell to the required depth between the remaining rafter space.
  • Please ensure the spray foam is hermetically sealed rafter to rafter with no shrinkage or delamination from the inner rafter faces.
  • Spray-apply the insulation under the rafter to the prescribed thickness to meet the U-Value requirement (if required).
  • Regardless of any third-party or in-house hygrothermal analysis, a vapour control layer or intelligent membrane must be installed on the warm side of the insulation.

The roof membrane should be in good condition before installation with no evidence of degradation, penetrations or tears. The condition of the membrane is crucial in ensuring that the insulation assembly remains protected from driving rain and external moisture.

The moisture content of the roof rafters should be less than 18%, and extensive testing should be conducted throughout the attic space. Roof rafters should be thoroughly assessed to ensure they are free from evidence of rot and woodworm. Where moisture content is higher than 18%, the installation must only proceed once all timber elements show a moisture content below the maximum threshold.

The spray foam should be installed directly on the roof membrane using a flash coat bonding layer. This ensures that the spray foam’s adhesion is consistent and that there is no membrane delamination. The second layer of spray foam can be installed on the flash coat to the required insulation depth. The spray foam should be consistent in colour and density, paying close attention to ensure it is not off-ratio, crumbly, or tacky.

The image here shows open cell spray foam installed between the roof rafters, directly to the high-resistance roof membrane, with a further 100mm layer under the rafters. The rafters are fully encapsulated; however, when the spray foam was installed, the moisture content was well below 18%, and the pre-install survey report identified no evidence of rot or woodworm in the rafters. The monolithic insulation layer prevents surface condensation from forming, whereas should any interstitial condensation pass through the spray foam, it will reach the ventilated cavity and evaporate accordingly.

The image here shows an example of a vapour barrier being fitted on the warm side of the pitched roof insulation. Vapour barriers should be overlapped at the joins and then taped at the seams to create a continuous layer that prevents interstitial condensation from entering the insulation assembly. Many vapour control layers or barriers are available, ranging from plastic to insulating foil. A vapour barrier must have an adequate SD value to prevent moisture from penetrating the insulation. Alternatively, intelligent membranes can be used to promote bi-directional vapour migration. Check out our Independent Hygrothermal Study page to learn more about the importance of vapour control layers:

It may be challenging to install a traditional sheet plastic vapour control layer, particularly where the roof rafters are fully encased, and there is limited opportunity for fixings. We recommend using a spray-applied ThermoFoam-approved Passive Purple vapour control layer or a Vara Bi-Directional Intelligent Membrane. The spray-applied vapour control coating should be a different colour from the spray foam insulation so that it is possible to identify areas where a monolithic seal has yet to be achieved. The installer has used passive purple on spray foam insulation, as shown in the image below. A seamless, monolithic vapour control layer has been completed.

Acceptable Alternative: Spray Foam Directly To Low-Resistance Roof Membranes With Or Without A Vapour Control Layer

The product certification for our Sucraseal open cell and Nexseal LE closed cell spray foams show that they can be spray-applied directly to low-resistance roof membranes without the need for a vapour control layer (unless a hygrothermal analysis states otherwise). A default vapour control layer will be required when the attic space is used as a habitable space, such as a loft conversion, as stated in section 3.2.6 of the product BBA Certificate. Installing open-cell spray foam to low-resistance membranes is similar to the policy for high-resistance roof membranes for these applications. Unlike high-resistance bitumen felt membranes, thinner low-resistance layers are subject to pressure from the spray foam application process and the membranes can become crumpled or pushed upwards. It is essential to retain any downward drape in the roof membrane to ensure that any external moisture reaches the top side of the membrane and escapes sufficiently. This is achieved by applying a thin flash coat to the roof membrane with low pressure, creating the bond whilst maintaining the downward drape. We prefer installing a card rafter slide in all scenarios, as this enables more accessible access to the roof from above to conduct repairs and roof renewals.

When applying directly to a low-resistance roof membrane, the installer should consider the following processes:

  • Please consider conducting a hygrothermal assessment of the roof assembly before installing the spray foam insulation to ensure it passes without a vapour control layer, particularly in northern regions of the UK.
  • Please ensure the roof membrane is in good condition and free from perforations or tears (repair any minor tears or imperfections).
  • Please ensure the moisture content in timber rafters is below 18% in ALL rafter sections, testing the rafter faces and sides.
    Please make sure the timber rafters are free from rot and woodworms.
  • Ensure the rafters are free from dust and debris so that the spray foam adheres and doesn’t shrink back or delaminate.
  • Spray—Apply a light flash coat to the membrane to create a bonding layer and maintain its downward drape.
  • Spray-apply Sucraseal open cell or Nexseal LE closed cell to the required depth between the rafter space.
  • Please ensure the spray foam is hermetically sealed rafter to rafter with no shrinkage or delamination from the inner rafter faces.
  • Spray-apply the insulation under the rafter to the prescribed thickness to meet the U-Value requirement (if required).
  • Unless a hygrothermal analysis states otherwise, a vapour control layer is not required when installing low-resistance roof membranes.
  • A vapour control layer should be used if the attic space is converted into a habitable space, such as a bedroom or office.

Before installation, the roof membrane should be in good condition with no evidence of degradation, penetrations, or tears. The condition of the membrane is crucial in ensuring that the insulation assembly remains protected from driving rain and external moisture. Note that low-resistance roof membranes are subject to crumpling under pressure. Therefore, it is essential to spray-apply a light flash coat bonding layer to maintain the downward drape of the membrane and prevent it from being pushed up against the roof tiles by the pressure of the spray foam insulation.

Irrespective of the roof membrane vapour permeability, the moisture content of the roof rafters should be less than 18%, and extensive testing should be conducted throughout the attic space. Roof rafters should be thoroughly assessed to ensure they are free from evidence of rot and woodworm. Where moisture content is higher than 18%, the installation must only proceed once all timber elements show a moisture content below the maximum threshold.

The open-cell spray foam is applied between the roof rafters, hermetically sealed from rafter to rafter with no gaps or shrinkage. When spray-applying to low-resistance roof membranes, care should be taken to retain any downward drape whilst ensuring the membrane is not pushed up against the roof tiles during the treatment. Applying a flash coat bonding layer to the membrane is essential before carefully building further layers to the final thickness.

Conducting a hygrothermal assessment is recommended even when spray-applying open-cell insulation to low-resistance roof membranes. If unacceptable moisture levels are detected, particularly in Northern regions of the UK, a vapour control layer must be used. Most plastic membranes have the appropriate SD value to reduce moisture ingress into the insulation assembly. In contrast, spray-applied membranes such as Passive Purple have an SD value of 76 (U), making them robust enough to withstand interstitial condensation in all locations of the UK. Alternatively, an intelligent membrane can be used instead of a vapour control layer. It offers bi-directional vapour diffusion that reduces moisture ingress to the insulation assembly whilst allowing moisture to escape should it enter through imperfections in the primary roof membrane. To learn more about hygrothermal assessments and vapour control layers, please visit our Independent Hygrothermal Study here: