Rafter Ventilation System

When installing spray foam insulation to high-resistance roof coverings, there is an increased chance that internal moisture, known as interstitial condensation, will attempt to pass into the insulation and accumulate in the outer 1cm layer against the membrane. During the winter, internal humidity can increase which means heated, moisture-laden air will attempt to escape from the interior to the colder exterior surface. As the air migrates to the colder exterior surfaces, it will pass through the insulation and become trapped at the high-resistance membrane. Over time, moisture can saturate the insulation and cause structural decay of the timber roof structure. Spray foam can sometimes be rendered as the cause of such damage when in fact, any type of pitched roof insulation may suffer similar problems.

Whilst the insulation may become saturated with moisture during the winter, as the external temperature rises in the summer, the interior humidity can drop whilst external humidity increases. This creates a reverse vapour drive where any moisture held within the insulation will escape within the property, a phenomenon known as summertime drying. In instances where low-resistance roof membranes are in place, the bi-directional vapour flow can be an acceptable way to manage moisture, however, when it comes to high-resistance roof membranes, insulation installers should seek to limit the impact of moisture on structural assemblies by following BS 5250.

Rafter Ventilation System

One method of mitigating risks comes in the form of a rafter ventilation system that creates a separating layer between the insulation and the high-resistance roof covering. Provided the cavity void is ventilated through the soffit, eaves and ridge or felt lap vents can be introduced to increase airflow within the cavity void, this system can act as a viable method of reducing moisture risks and increasing ventilation should internal moisture form within the insulation. The rafter ventilation system is a made from card and can be pushed in between roof rafters to leave a 40mm airgap before the insulation is installed between or between and under the remainging roof rafter space. This system is particularly beneficial for spray foam insulation as it will bond to the rafter ventilation slide and continue to provide an airtight seal to the inner rafter faces.

Additionally, when it comes to replacing the roof in the future, roof tiles and existing membranes can be replaced at ease without damaging the insulation. Where the lifespan of spray foam insulation may previously be determined by the remaining life of the roof membrane onto which it is applied, with the rafter ventilation slide, it can remain in place, maintaining the 40mm air gap for potentially the life of the building. The rafter ventilation slide is simple to install and provides added assurance that moisture within the building will not saturate and sit within the insulation for excessive periods of time. Not only does this reduce the effectiveness of the insulation but can also lead to structural decay. Should there be any imperfections within the primary roof coverings which leads to moisture ingress, the ventilated cavity can also play a role in managing moisture.