Over Rafter Insulation using SF40BB


A SuperFOIL Solution to a Spray Foam Dilemma


Project Type



Building Type

Retrofit Pitched Roof




SuperFOIL Products Used


Project Overview

Spray Foam insulation, once thought to be a good solution, was in fact causing a huge issue for this property. Due to the high risk of condensation and rotting timbers, the valuers decreased the mortgage value of the property to £0, effectively making the house worthless. This means the roof needed a better insulation solution with great thermal performance, but also something that would maximise the value of the property. Not only that, further upgrades were implemented to the roofing and guttering systems to restore the property to its full potential.

On another note, this was also a very special project for us, as it is the current home of SuperFOIL leaders, William and Kate Bown.

The Problem

The decision to re-roof and insulate this pitched roof project came from difficulties remortgaging the property due to spray foam insulation in the roof. 


During the Covid lockdown, the Bown family attempted to sell their property. A sale agreement was reached, but when the buyers sought a small mortgage over the property, they discovered that valuers listed the property as having £0 value entirely due to the spray foam insulation in the roof. 


The roof was previously insulated using 100mm of closed cell spray foam between the rafters, which made it difficult for the roof to ‘breathe.’ The setup was putting the timbers in their roof at risk of rotting without the Bowns ever knowing it.


While closed cell spray foam is effective as an insulator, its rigid nature creates a barrier that traps air and moisture, leading to condensation and potential decay in timber-framed roofs such as in this case. For years, mortgage lenders have been cautious about remortgaging properties with spray foam insulation installed due to concerns about structural damage, massively decreasing the value of properties insulated using spray foam.


As the property’s mortgage renewal approached, the only option for the Bown family was to let the existing mortgage roll over into an amount determined by the bank upon completion of their fixed term. With current rates, this could mean their mortgage costs doubling or even tripling!

The Solution

The technical team at SuperFOIL analysed the project and recommended a duet solution, which used SuperFOIL SF40BB over the rafters and SuperFOIL SF40 under the rafters. This solution required no change to the existing rafters or spacing, did not affect the roof height, and provided plenty of usable space in the loft. 


The new solution provides a vapour barrier (SuperFOIL SF40) below to avoid moisture from reaching the rafters, with a breathable (SuperFOIL SF40BB) layer over the top to allow any moisture to escape. But there were more considerations behind choosing this specific duet system! 


In accordance with the latest Building Regulations standard, when more than 50% of the roof is being redone, it is a requirement to upgrade the insulation at rafter level to achieve a U value of 0.18 W/m2K. In this case, the insulation improved from a U-value of 0.35 to 0.14, meaning the new performance of SuperFOIL is more than twice as effective as the previous Spray Foam solution. The roof is now loft boarded and provides fully equipped storage space for the family, without insulation getting in the way. 


The result? By redoing and improving the insulation to a higher standard, the mortgage issues were entirely resolved, significant performance improvements were achieved, and an airtight finish was created. Even better, this meant the Bown family now has a brand new roof that will last another 25+ years. 


Taking advantage of the opportunity to enhance the curb appeal and feel of the house, the Bown family also upgraded the guttering, improving the roof’s efficiency and house’s overall look.

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