Independent Hygrothermal Studies

We commissioned QODA Consulting Limited to undertake four separate studies, designed to assess the installation of spray foam when applied directly to high-resistance bitumen felt roof membranes. Carried out using WUFI Pro software to create a dynamic hygrothermal analysis, the results help to assess risks and identify management options that can de risk installations. This is important for spray foam installers when striving to achieve compliance whilst building surveyors and mortgage lenders can utilise the modelling to implement their own assessment of risk. QODA’s hygrothermal modelling assesses the results against known building physics such as the threshold for moisture in buildings becoming a risk to the structure.

Model 1: General Assessment Of Spray Foam Insulation Installed To Bitumen Felt Roof Membranes Showing Air Changes Required To Reduce Moisture Risks.

In UK homes, an average moisture load generally represents a 40-60% relative humidity range. When showering, bathing, and cooking activities are performed, this can increase the moisture load to what would be deemed high, leading to a relative humidity level of up to 100%. At this level, the moisture load increases and damage to the structural fabric of the building may occur through interstitial condensation that cannot suitably escape through ventilated openings. The threshold for what would be deemed a high-risk moisture content is breached when 20 kilograms of water per cubic metre penetrates the outer 1cm insulation layer at roof level.

Sucraseal Open Cell was assessed for its performance directly to high-resistance roof membranes based on the number of internal air changes per hour required to remove the moisture from the insulation. Air changes per hour, abbreviated to ACH is a calculation of how many times per hour the entire volume of air in each space is replaced with supply and/or recirculated air. Considering the typical building will reach 4-5 air changes per hour, the implementation of airtight insulation requires increased air changes to reduce moisture, as shown in then graph below:

  • Represented by the dark blue line, during autumn/winter, 0 Air Changes Per Hour resulted in more than 90% water content within the outer 1cm layer of the insulation. During spring/summer, water content is reduced to less than 5% but may not fully dry out when the next autumnal cycle commences.
  • Represented by the dark green line, at 10 Air Changes Per Hour, the water content in the outer 1cm of insulation reduced to around 50% in peak autumn/winter months and fell to less than 5% content during spring/summer months. The 50% content remains well above the 20 kg/m3 threshold.
  • Represented by the dark red line, when the number of Air Changers Per Hour reaches 25 during peak autumn/winter, the water content in the outer 1cm insulation layer peaks at 17% and remains below the 20 kg/m3 threshold, reducing to close to 0% in spring/summer months.
  • In all scenarios, winter water content dries out during summer months and returns to almost zero. This could be interpreted as a low risk to the building assembly when considered over a 12-month period.

Considering the average building will naturally experience 4-5 air changes per hour, the data shows the need to dramatically increase this to 25 ACH to reduce water content from sitting within the insulation for prolonged periods, if “no-risk” is preferred. By removing natural ventilation in the roof soffit, eaves and apex through airtight insulation, heating and cooling costs may reduce but the moisture load is likely to increase, requiring mitigation depending on how the risk is interpreted. Whilst the evidence of timber rot can take many years to manifest and become apparent, ongoing cycles of excess moisture coupled with drying out can reduce their lifespan which is why “worst case” modelling can help to eliminate risks. Considering that between 0 and 2 ACH, water content sits above the 20kg/m3 threshold for almost half of the year, unless it can be determined the building is able to provide air changes that exceed these levels, further mitigation of moisture on the insulation assembly will be required. At 10 ACH, although the 20kg/m3 threshold is breached in peak winter, the water content may be present within the insulation assembly for up to three months before returning below the threshold. Fundamentally, the more air changes a building undergoes, the greater the requirement on heating and cooling costs which can negate the performance of the insulation, therefore, it is not feasible to dramatically increase the number of air changes on the warm side of the insulation.

When Spray Foam Insulation is installed directly to high-resistance roof membranes without any additional moisture vapour control or adequate ventilation, there is a pronounced risk that water content is held within the outer 1cm layer of insulation and may not completely evaporate during the summertime drying. When adequate spring/summer drying does not entirely remove water content, there can be a compound, worsening level of retained moisture held within the insulation, which may, over time, degrade and rot structural timber. Building moisture can be managed by providing ventilation in the crucial “wet zones,” toilets, bathrooms, utility rooms, and kitchens. Where older buildings may rely upon outdated ventilation strategies such as ventilated cavities or open attic eaves, the deployment of mechanical extraction ventilation is generally robust enough to handle increased moisture loads. In most homes, extractor fans will be present in bathrooms, and kitchens will have cooker hoods to ventilate excess moisture generated through cooking.

Model 2: General Assessment Of Spray Foam Insulation Installed To Bitumen Felt Roof Membranes Incorporating A Vapour Control Layer(s)

When considering an installation spray-applied to high-resistance membranes where no air changes occur or vapour control layers are present, water content in the outer 1cm layer of the insulation peaks at 95kg/m3 as defined by the dark blue line. As different vapour control layers and intelligent membranes are assessed and implemented, the potential risk is vastly reduced below the 20kg/m3 threshold and does not come close to or exceed the threshold at any given time during the year. QODA tested the impact of three different types of vapour control layer when installed on the warm, internal side of the insulation assembly.

Low Moisture Load – Light Blue Line

It cannot be guaranteed that a property is subject to a low moisture load. However, implementing robust mechanical extraction can at least reduce moisture loads on structural elements. In existing homes, automated or passive extraction can be installed. However, this increases the opportunity for heat loss and therefore, should occupants choose to cover or switch off extraction, the moisture load may increase. In addition, it may be possible to assess the moisture load on the property through occupancy levels. However, it is essential to note that occupancy levels may change. Therefore, it would be unreasonable to assume a static moisture load based on current occupancy, given that the intended lifespan of the insulation system is a minimum of 25 years. During the winter, a low moisture load can still breach the dew water threshold of 20kg/m3, reaching as high as 30kg/m3. Whilst the spike in moisture content is only marginally above the threshold, prolonged periods at levels above the threshold can affect and damage the roof’s structural elements. As a low moisture load cannot be determined as a consistent status within the property due to the potential for variability, it is not recommended that a low moisture load alone is adequate protection against water content passing into the insulation assembly. As insulation materials rely upon low thermal conductivity to reduce heat transfer, any moisture within the insulation will limit its effectiveness against heat loss.

Pro Clima Intello Plus Intelligent Membrane – Green Line

Pro Clima is a leading manufacturer of Intello membranes that react to humidity to promote bi-directional vapour migration through building assemblies. Compared to traditional vapour control systems, Intello has been designed to enable moisture to pass through the membrane based on the humidity inside and outside the property. During winter, when the moisture load is high, the membrane will alter its resistance to prevent moisture from penetrating the insulation and roof assembly. In warmer summer months, the membrane will open to release any moisture that may have accumulated within the insulation assembly through imperfections in roof coverings. This is known as bi-directional drying, and it limits the length of time that moisture sits within structural elements to maximise their lifespan. A pre-manufactured membrane’s negatives generally come through its ability to fit under the insulation seamlessly, monolithically, with penetrations sealed sufficiently. In trussed roofs, fitting a pre-manufactured membrane on the warm side of the insulation can be challenging. Based upon a water resistance Sd factor of 0.6 to 34m, Intello Plus can provide adequate protection against moisture content, as shown in the chart above, defined by the green line. During peak winter, moisture content peaks at around 4kg/m3, well below the 20kg/m3 threshold.

Passive Purple Vapour Control Layer (Spray-Applied) – Turquoise Line

Passive Purple is a leading spray-applied vapour control product supplied by Intelligent Membranes. It has a vapour resistance Sd factor of 76m and can add suitable protection to the insulation assembly during winter months. The spray-applied nature of Passive Purple makes it an ideal solution to stop moisture from migrating into the insulation from the warm side (internal living areas). Unlike pre-manufactured membranes, when installed at the correct thickness, Passive Purple will seal every inch of the insulation to provide a seamless, monolithic barrier. This is particularly useful when the roof structure is trussed or has awkward angles and limited space. It has a solid adhesion to the insulation material and is not subject to mechanical fixings or failures within those fixings that may lead to reduced performance over time. As defined by the turquoise colour on the chart above, Passive Purple offers the most excellent protection against moisture transfer into the insulation assembly, barely breaching a water content of 2kg/m3 in peak winter months. However, it does not offer bi-directional drying, promoting a vapour-closed roof assembly. This means that any imperfections in primary roof coverings may allow moisture to penetrate the insulation from above, becoming trapped within the layers, which may lead to a heightened risk of structural degradation. Passive Purple is a recommended solution in property subject to a high moisture load, and provided the external roof structure is watertight, and the roof membrane is in good condition; it will offer the most protection of any variable tested in the QODA study.

Vara Intelligent Membrane (Spray-Applied) – Pink Line

The spray-applied Vara Intelligent Membrane has the benefits of Passive Purple from an installation perspective. With similar bi-directional drying properties of Intello Plus, Vara has a vapour resistance factor of Sd 0.15 to 16m to be defined as an intelligent membrane. Defined by the pink line on the chart above, it offers the least protection when compared to Intello Plus and Passive Purple; however, during peak winter months, although it may enable up to 7kg/m3 of water content to penetrate the insulation, this returns to almost 0kg/m3 during summertime. This is well below the 20kg/m3 dew water threshold. Therefore, Vara becomes a safer proposition from an installation perspective, coupled with its ability to release moisture from the insulation assembly during drying-out cycles. Much like Intello Plus, Vara senses humidity within the property, closing its structure to reduce moisture from penetrating the insulation. During the summer, the membrane opens to release residual moisture into the property, which will dry accordingly based on temperature and airflow. This is known as bi-directional drying and represents the safest way to protect the structural elements of new and existing buildings.

Model 3: Assessment of Risk of Timber Rot

The modelling scenarios developed for this section aimed to assess the impact of each improvement option on the relative humidity levels on the internal surface of the bituminous felt. The hygrothermal conditions at this location can provide a means of assessing the risk of rafters decaying due to rot. The relative humidity threshold to evaluate this risk is related to the moisture storage function and the porosity of timber. It is linked to the threshold of 20% mass, which should not be exceeded for prolonged periods.

Chart 1: Winter

Chart 2: Summer

QODA assessed the “risk” of timber rot in the roof assembly, as depicted in the charts above. This analysis showed that all improvement options positively impact the risk of timber decay due to rot. In existing installations with no further improvements, the threshold is exceeded in most of a simulation year, whilst all improvement options significantly reduce this period. The most significant improvement is noted in the case of the installation of Passive Purple, as the threshold is exceeded periodically only in the winter months. Just about all improvement factors reduced the relative humidity below the threshold for much of the year. Therefore, the risk of timber rot is significantly reduced. It is important to note that when the threshold is breached, timber rot doesn’t instantly occur; it generally takes many months of sustained moisture load for structural timber to suffer damage.

Model 4: Ventilated Cavity (Rafter Slide Separator) Above The Insulation

In addition to the above, QODA modelled the impact of creating a ventilated cavity above the insulation and below the high-resistance roof membrane. A ventilated cavity is formed by implementing a separating layer between two elements using a low-resistance membrane. Spray foam installations generally use a card rafter slide stapled to the roof rafters’ inner face to create a recommended 50mm void. This provides a stable layer to which the spray foam insulation can be applied uniformly and consistently without any risk of delamination from the substrate. Some sections of the spray foam industry also fit a low-resistance roof membrane to create the separating layer. In principle, this is also acceptable; however, due to the potential for the membrane to crumple under the pressure of the spray foam during application, the membrane must be pulled taut to the rafter edges.

The modelling scenarios developed for this section aimed to display the impact of introducing a ventilated cavity, as described previously, on the hygrothermal behaviour of future installations of pitched roof insulation regarding the risk of damage due to dew water. The calculation of the actual air changes occurring due to the felt lap ventilators within the cavity has yet to be part of this study. Studies that have attempted to quantify the air movement within ventilated cavities in buildings using a combination of unique airflow modelling and real-life measurements showed that the airflow in a ventilated cavity is 5-45 air changes per hour for external wind speeds of 0.5 to 4 m/s. These numbers can be used to provide a perspective on the airflow rates that should be expected to occur. A ventilation rate of 5 air changes per hour has been selected in the analysis as a conservative approach.

The results of the hygrothermal modelling showed that the introduction of a ventilated cavity is adequate to minimise the risk of damage due to dew water as the water content in the outer 1 cm of the insulation layer remains well below the 20 kg/m³ threshold at a peak of around 3kg/m3 during the winter. Soffit ventilation is considered in creating the ventilated cavity, and the separating rafter slide fits under the roof ridge. The introduction of felt lap ventilators can help to increase the number of air changes per hour by creating a wider opening where the roof membrane layers overlap. It is important to note that the increased air changes occur above the insulation, therefore, limiting heat loss from the interior of the building by using other methods of internally installed ventilation.


The scientific hygrothermal modelling shows the potential for an increased risk of water content within the structural roof assembly when Sucraseal Open Cell Spray Foam Insulation is applied directly to high resistance or bitumen felt membranes. Whilst the BBA/KIWA certification for some Open Cell foams may encourage using a card rafter slide between the roofing membrane and the insulation, it is essential to consider whether this void is adequately ventilated from the soffit to the ridge. The findings from QODA based on the models adopted are clear that the risk of water content within the insulation can be drastically reduced well below the 20 kg/m3 threshold when suitable Vapour Control Layers or Intelligent Membranes are introduced on the warm side of the insulation.

Where there have been varying opinions between mortgage lenders, surveyors, spray foam manufacturers, distributors, and installers, it is essential to understand and utilise scientific modelling to de risk installations whilst increasing confidence that Spray Foam Insulation is a viable retrofit solution in residential properties throughout every area of the UK. While scientific modelling may produce “worst case” scenarios, the home occupancy dynamic can change over time. What may initially seem low risk can alter over time to increase the risks to the structural building fabric. Changes to the occupation levels within the property can vary the hygrothermal dynamic considerably; for example, if a family of 2 becomes a family of 5 in the years following an installation, there could be potentially 60% additional moisture production through breathing, bathing, and cooking activities.

The simulation in WUFI Pro and the analysis of the hygrothermal performance of the proposed pitched roof retrofit solutions in existing and future installations showed that the high risk of moisture-related issues because of the direct application of the Sucraseal Open Cell foam on the bituminous felt can be significantly reduced or minimised with the proposed improvement options. Regarding existing installations, the hygrothermal modelling indicated that the installation of a vapour control layer (e.g. Passive Purple or similar) results in minimising the risk of material damage due to dew water and significantly reduces the likelihood of timber decay due to rot as far as the rafters are concerned. It should be noted that with the addition of the vapour control layer, this construction becomes effectively ‘vapour closed’ as there are several vapour-resistant layers.

The analysis has considered moisture accumulation based on typical internal and external conditions. Even if construction imperfections have been part of the models, the modelling has not included any additional moisture entry due to material failure or poor-quality installation. Under these conditions, the vapour-closed nature of the fabric could lead to unacceptable moisture levels in localised areas. Care should, therefore, be taken to ensure the integrity of the construction. The impact of exterior imperfections can be vastly reduced when using Intelligent Membranes which promote bi-directional drying.

Regarding future installations, the hygrothermal modelling showed that the introduction of a 50mm ventilation cavity, with the use of the proposed felt lap ventilators and ventilation spacers, combined with a vapour control layer, minimises the risks of both dew water damage and timber decay due to rot. Please note that the ventilation spacers must be made of a vapour-permeable material that will not restrict vapour diffusion towards the ventilated cavity.