Building Science & Moisture Management

Buildings need to breathe – this has always been the case. Historically, buildings in the UK were constructed with ventilated wall cavity voids, roof soffits, eaves, and ridges to promote natural airflow. Reducing carbon footprints wasn’t really on the agenda before the turn of the 21st century. Also, energy was cheap. Therefore, heat loss from buildings wasn’t a big deal, so natural ventilation served a purpose. Fast forward 20 years and energy conservation is imperative from an environmental and carbon footprint perspective to prevent the wastage of expensive energy. In a quarter of a century, how modern buildings are constructed is often totally different to those built in the 20th century.

If the UK cannot sufficiently upgrade the thermal performance of existing dwellings, it will continue to lag in the league tables for carbon emissions. With existing residential buildings responsible for around 20% of all carbon emissions in the UK in 2021, the Commons’ Environmental Audit Committee has warned that the sector will need to reduce this drastically to meet net-zero targets. A wide range and varied insulation market is key in achieving net-zero targets which is why the UK must take a pragmatic, optimistic and proactive view on the acceptance and use of different materials.

Understanding basic building science is important for building surveyors, specifiers and those involved in the installation of insulation materials. The modern mandate is to insulate tight and ventilate right as reinforced by Part F and Part L of the Building Regulations and spray foam expands and seals into every inch of the void into which it’s installed to help achieve thermal efficiency. To offset the airtightness achieved, it is essential to consider how moisture will escape from the building via vapour migration through the structural envelope or the provision of mechanical ventilation.

Condensation

Two types of condensation must be considered in buildings: Surface and Interstitial Condensation. BS 5250 is the code of practice for the control of condensation in buildings. It provides building designers with guidance on safeguarding the health of occupants by considering likely sources of moisture, avoiding the build-up of excessive humidity, and preventing mould growth and condensation. Spray foam manufacturers and installers are responsible for designing the insulation mandate by following BS 5250.

Surface Condensation

In most circumstances, surface condensation forms when there is a differential between the internal temperature of the building and the external air temperature. This can often be seen during winter when windows become damp with water droplets due to the variance in surface temperature where two competing temperatures collide and expel moisture. When spray foam insulation is installed on a cold surface such as roofing felt, it adds a thermal barrier that can stop two competing temperatures from colliding to create surface condensation.

It can be incorrect to assume that Building Surveyors preferred option of mineral wool insulation laid at ceiling level won’t cause condensation to build up in the roof void. The images below show examples of surface condensation build-up on both high-resistance and low-resistance roof membranes. This can occur when mineral wool insulation meets the lower pitched roof area and seals the eaves, preventing airflow from the soffit board entering the attic space and relieving moisture. Alternatively, where the insulation at ceiling level is patchy and not airtight or there is none present, heat from within the main living spaces can rise and meet the cold pitched roof surface where condensation may form. Excess levels of surface condensation can eventually lead to timber decay.

Hermetically Sealed Insulation

When hermetically sealed between roof rafters, Spray Foam Insulation can limit the opportunity for surface condensation to form since there is no opportunity for warm internal air to reach the colder external roof surface. It is vital that spray foam insulation is processed correctly so that it doesn’t delaminate and shrink back from roof rafter edges. In these scenarios, surface condensation can become an acute problem whereby internal heat tries to escape through a smaller surface area, reaching the cold exterior and manifesting in the form of excess surface condensation. Depending on the type of spray foam used, the extra surface condensation can penetrate the foam, become trapped and eventually degrade the roof assembly in the form of rafter rot. Specifying the correct insulation thickness and ensuring it’s hermetically sealed is one of the most important processes during the installation. This does not just apply to spray foam insulation; any type of pitched roof insulation requires a hermetic seal or the addition of airtight sealing to prevent heat from reaching colder exterior surfaces.

Common causes of Spray Foam shrinkage at the inner rafter face include:

  • Too much moisture in the rafter (during application) causes the foam to contract and shrink.
  • Sprayed inappropriately without proper joining to the inner rafter faces.
  • Off-ratio Spray Foam, which includes Iso-rich or resin-rich.
  • Inadequate framing of rafter bays using inappropriate spraying techniques.

Considering that one of Spray Foam’s primary benefits is its airtightness, installations must promote suitable sealing of the entire roof void, apex to eaves, to ensure that heat loss does not escape. When small gaps are present in the roof structure, heat loss from the living areas may reach the roof surface and attempt to run to the cold side. Heat and vapour will always seek to pass through the path of least resistance; therefore, any gaps in the insulation assembly can become hotspots for heat loss. Suppose the gap between the roof rafter and spray foam continues to the roof membrane. In that case, the heat within the airflow can reach a cold external surface and cause surface condensation, which may sit within the spray foam, particularly during winter.

Hermetical sealing of roof voids is critical to implement a thermal barrier to prevent heat loss – the airtight performance that spray foam insulation provides can be offset entirely when proper sealing is not achieved. Where traditional mineral wool insulation is laid at attic ceiling level, its lack of airtightness and potential shrinkage or degradation can lead to surface condensation on the underside of roof membranes. British standard BS5250 discusses the importance of managing surface condensation risks, and when installed correctly, spray foam insulation can eliminate the risk of surface condensation.

Where installations are not hermetically sealed, the following remedial steps are recommended:

  • Damp probe the void/gap between roof rafters and spray foam insulation to check moisture content within the insulation assembly.
  • Where surface condensation has accumulated at the roof membrane or within the spray foam, wait for warmer weather to commence the drying-out process.
  • If the roof membrane and spray foam are free from moisture or the content is below 18%, the edges should be sealed so that gaps are not present.
  • Do not hermetically seal where there is a risk of trapping excess moisture within the roof assembly; let it dry out first and then remediate.

Where installations are not hermetically sealed, the following remedial steps are recommended:

  • Damp probe the void/gap between roof rafters and spray foam insulation to check moisture content within the insulation assembly.
  • Where surface condensation has accumulated at the roof membrane or within the spray foam, wait for warmer weather to commence the drying-out process.
  • If the roof membrane and spray foam are free from moisture or the content is below 18%, the edges should be sealed so that gaps are not present.
  • Do not hermetically seal where there is a risk of trapping excess moisture within the roof assembly; let it dry out first and then remediate.
The Spray Foam is not Hermetically sealed, and there are gaps for air leakage to penetrate.
The Spray foam is Hermetically sealed with no penetrations for air leakage which limits the opportunity for surface condensation to form.

Interstitial Condensation

This is a less obvious form of condensation but can damage the building fabric. An example of interstitial condensation is when we boil our kettles; steam rises from the spout and then disperses into the room, becoming invisible after a few seconds. As the steam cools, it turns into water vapour, which will migrate from hot to cold, hot being the inside of the building and cold being the outside. When buildings are insulated to airtight standards, the vapour can no longer migrate freely to the nearest natural ventilation point and may accumulate within the structural building envelope. When the interstitial condensation levels become too high during winter, the building fabric may not fully dry out during the summer, compounding the moisture levels that remain trapped within the structural assembly.

Understanding interstitial condensation is particularly important for those retrofitting insulating into existing homes. Historically, high-resistance, bitumen-felt membranes were used as the primary roof tile underlay, usually visible from the loft space as a black sheet between the roof rafters. Such membranes are non-breathable and, therefore, restrict the ability of vapour to pass through them to the exterior of the building. When insulation is installed into buildings with a non-breathable building assembly, interstitial condensation may sit within the insulation material and potentially rot structural elements such as roof rafters.

When referring back to the scenario of steam from the kettle, instances of interstitial condensation are more extensive and include:

  • Vapour from cooking
  • Vapour from bathing/showering
  • Vapour from breathing
  • Vapour from drying clothes indoors
  • Vapour from washing machines/tumble dryers.

The risks from interstitial condensation also relate to the size of the building and the number of occupants. The more occupants within a smaller space, the more interstitial condensation is produced. Referring to uninsulated homes with open soffits/eaves and cavity walls, draughty homes effectively eliminate interstitial condensation, and it’s unlikely to become a problem. Heat loss will be reduced when we seal our buildings with high-performance insulation materials, but the impacts of interstitial condensation will increase. We shouldn’t be afraid to achieve airtight, thermal performance, instead, we should design the insulation specification in accordance with manufacturers guidelines, BBA/KIWA certificates and BS5250. The spray foam installer is responsible for designing and installing insulation specifications that consider risks, consequences, and mitigation.

Ventilation

Ventilation becomes integral to the insulation strategy to mitigate the risks from interstitial condensation. There is no point in insulating a building and leaving natural ventilation. For example, why insulate a pitched roof assembly with spray foam insulation only to leave the eaves open? Heat will escape through these openings and vastly reduce the impact of the insulation, making it a pointless investment for the building occupant. Mechanical ventilation is the key to offsetting condensation risks by providing airtight insulation, and it’s becoming an increasingly important option in newly built and retrofitted homes.

Mechanical ventilation comes in many forms, most commonly cooker hood extraction and bathroom extraction fans; both are manually switched on when in use or can be automated to turn on when, for example, a light switch is pressed or increasingly through humidity sensors. During their operation, these ventilation systems will extract excess moisture from the room at source, preventing it from migrating through the rest of the building and attacking the building fabric. This can create a low moisture load on the structural building assembly and vastly reduce the risks of interstitial condensation forming within the insulation. It may be considered that a low moisture load with interstitial condensation handled at source is enough to de risk the impact of airtight spray foam insulation at pitched roof level and this may form a valid argument from some manufacturers, distributors, and installers.

General Insulation Risks

New buildings are being constructed increasingly airtight, using insulation in floors, walls, and attic spaces to seal the building envelope from air leakage and heat loss. A robust insulation strategy is integral to the comfort of occupants within the building, enabling them to control heating and cooling costs. Additionally, insulation helps to improve the sustainability of a building and, therefore, its carbon footprint. New homes must meet stringent regulations per Part F and Part L of the Building Regulations. Part F for Ventilation and Part L for Energy Conservation are intrinsically linked to each other to ensure that suitable ventilation strategies offset airtightness. Ventilation strategies usually represent bathroom, utility room and kitchen extract ventilation, where mechanical systems manage excess moisture caused by cooking, bathing, and breathing.

When undertaking retrofit insulation in older buildings, it can be misinterpreted that airflow needs to remain in the roof space through eaves soffit ventilation, and this is why it’s common to see the insulation laid at attic floor level, retaining airflow into the remaining void through the eaves. While this is one way to maintain heat within the living areas, there are many pros and cons of insulating at the attic floor level, and it can restrict occupants from using the space for storage, amongst other reasons. Attics experience extreme cold during winter, while temperatures can exceed 40c during the summer. The variance in temperature across seasons can impact the structural integrity of the roof space and any roof membranes whilst impacting the living environment below. While some may assume that insulation at floor level stops heat from escaping the living areas or summertime solar gain penetration, the reality can be very different.

Mineral wool is commonly used to insulate attic floors and can effectively slow heat loss from the living areas relatively inexpensively to other insulation materials. However, this method of insulation can, in practice, cause several problems, which include:

  • It reduces storage space.
  • The additional cost of loft boarding above the insulation should storage space be required.
  • Health and safety concerns related to safe access across the floor owing to the insulation covering the joists.
  • This type of insulation is subject to shrinkage and degradation over time.
  • This type of insulation can promote an environment for pests and rodents to nest.
  • Any movement or compression of the insulation leads to heat loss.
  • Mineral wool holds dust and debris and can impact the allergies of occupants.
  • This type of insulation offers no protection to the roof structure nor mitigates the impact of hot and cold temperatures on structural timber.
  • Over time, replacing or “topping up” mineral wool can become expensive.
  • Mineral wool may offer no primary protection to water tanks, pipework, boilers or equipment, which can freeze or overheat if placed in the attic.
  • Mineral wool offers no primary protection to stored items or belongings, which can become hot, cold or damp throughout the seasons.
  • Where mineral wool insulation isn’t fitted correctly or has degraded, heat leakage may occur, which can reach cold roof surfaces and cause dampness.
  • Mineral wool insulation lacks airtightness, even when cross-laid.

When considering retrofit insulation, the default option is to maintain strong airflow in the attic space whilst using insulation materials that may only sometimes be right for the building or its occupants. Part F and Part L of the Building Regulations reference “targets” for existing buildings. When both elements of the regulations are considered, existing homes can be insulated to modern airtight standards, provided suitable provision for ventilation is implemented. In addition, vapour control layers can be incorporated into a retrofit insulation strategy to protect the structural elements from the impact of interstitial condensation. Without a forward-thinking agenda on how to insulate the UK housing stock more robustly or the ability for insulation installers, property surveyors and other building professionals to consider how to adopt British Standard BS5250 or Part F and Part L of the Building Regulations, the UK will continue to lag behind the league tables on adequate insulation provision and carbon reduction.

In new-build homes, pitched roofs are generally constructed as low-resistance to moisture assemblies, promoting the substrates’ vapour permeability. Modern breathable membranes are significantly thinner than their bituminous felt counterparts. Still, in return, they enable interstitial condensation to migrate through the layers into the ventilated gap between battens and tiles. Nexseal LE closed cell or Sucraseal open cell can be spray-applied directly to low-resistance roofing membranes. Still, hygrothermal calculations should be carried out to assess condensation risks along with the expected lifespan of the roof membrane. In addition, most newly built homes will have adequate ventilation provisions to create a low moisture load on the building assembly, so it is essential to seal up the building assembly to prevent air leakage.

Spray Foam Insulation, like any other form of insulation, can pose risks to buildings when installed incorrectly or without consideration for BS5250, Part F and Part L of the Building Regulations. It is often never the product or material that causes the problem and always usually how it has been installed. There is no doubt that Spray Foam, along with many other forms of insulation, have and can cause damage to buildings. Some of the common Spray Foam failures include:

  • Installations directly to high-resistance substrates without vapour control management or additional ventilation.
  • Installations to wet, rotting or wood-worm-infested structures.
  • Installations directly to roof tiles can trap moisture and encourage the degradation of structural elements.
  • Installations to failed or degrading roof membranes or roofs where leaks are present or flashings have perished.
  • Installations with non-hermetically sealed insulation to the inside rafter faces which promote heat to travel to cold external surfaces.
  • Installations where the chemical ratios are off-balance, causing failed or defective insulation.

Every insulation product comes with a risk if installed poorly, and every potential failure from Spray Foam is from applicator error, negligence or a lack of training and guidance, which is the responsibility of manufacturers and distributors like us to enforce. With that said, there are many bonfide and quality spray foam installers who usually always achieve compliant results, many of whom have been categorised similarly to those who conduct poor installations.

To demonstrate why spray foam insulation must not simply be accused of causing damage to the roof structure, the image below shows and example of how the mineral wool at pitched roof level has become saturated with dampness to create black mould spores. The lack of airtightness and a hermetic seal of the fibrous insulation has failed within the roof assembly. Considering the structure is a dormer roof, the damp and mouldy mineral wool may have sat in poor condition for quite some time, creating an enhanced risk of timber rot due to moisture content. Fortunately for the property owner, the black mould formed on the sloping ceilings, alerting her to a problem. Many homeowners may have cleaned off the mould and repainted the ceilings to address the issue. Upon inspection by a building surveyor who helps to assess the suitability of the property for lenders to issue a mortgage, such an example of failed insulation could easily have been missed, yet with spray foam insulation, as soon as it is seen, the building is usually marked negatively, therefore restricting its suitability for lending.

To demonstrate why spray foam insulation must not simply be accused of causing damage to the roof structure, the image below shows and example of how the mineral wool at pitched roof level has become saturated with dampness to create black mould spores. The lack of airtightness and a hermetic seal of the fibrous insulation has failed within the roof assembly. Considering the structure is a dormer roof, the damp and mouldy mineral wool may have sat in poor condition for quite some time, creating an enhanced risk of timber rot due to moisture content. Fortunately for the property owner, the black mould formed on the sloping ceilings, alerting her to a problem. Many homeowners may have cleaned off the mould and repainted the ceilings to address the issue. Upon inspection by a building surveyor who helps to assess the suitability of the property for lenders to issue a mortgage, such an example of failed insulation could easily have been missed, yet with spray foam insulation, as soon as it is seen, the building is usually marked negatively, therefore restricting its suitability for lending.

As stated above, spray foam insulation failures almost always occur due to applicator error or incorrect design of the insulation system; therefore, when negative comments appear in the media, sometimes championed by sections of the qualified building surveyor’s community who state, “spray foam rots roof rafters”, it is an incorrect statement to make if the spray foam is installed properly based upon a robust design. Of course, there can be failed spray foam installations, as there can be failed installations from any other insulation material.