There are 2 main types of spray foam installations, and the main factors of these installations are as follows:

Closed Cell Polyurethane Sprayed Foam

This is applied directly to the underside of tiles and slates or to underfelts within domestic pitched roofs. It can be used to stabilise failing roof coverings e.g. nail fatigue. It has a minimal value for improving the thermal performance of the building. It is usually applied as a relatively thin layer of approximately 20-40mm.

It has some vapour permeability but vapour transfer through the material is typically quite low. It has very strong adhesive properties, bonds strongly to substrate, is relatively dimensionally strong and resists compression. The product is essentially waterproof resisting the passage of free water. When used to stabilise failing roof coverings it should be applied to a “dry” (leak-free) and defect free roof. The use of adhesive PU foams may work to delay the inevitable renewal of roof coverings. Sadly it may render roof coverings (slate, tile and stone) unrecyclable. Foam removed from roof coverings presents a waste disposal / recycling problem at the point when roof renewal is inevitably needed. The presence of sprayed foam means that simple visual inspections to establish the condition of roof timbers is impossible.

Investigations performed to establish the condition of roof timbers will be difficult and limited. Where evidence of roof leaks, timber decay or insect infestations are identified, the full extent of the problem and the extent of any repairs, treatment and drying can only be undertaken if the foam is removed. Where timber decay or insect attack is discovered, or is suspected, in a roof that has been coated in closed cell foam that has been applied directly to the roof coverings, it is likely that this will result in the full strip and recover of the roof as the foam will be extremely difficult to remove from localised areas. The need to remove well bonded high-density foam from tiles, slate and stone will probably prevent their reuse.

Open Cell Sprayed Polyurethane Foam Insulation

This is used to improve the thermal performance of the building, and is applied to a thickness of 100-150mm. It is relatively soft, easy to squish, and can push a probe into it easily. It has a varying degree of vapour permeability, and may be applied to roofs with vapour permeable and non-permeable underfelts. There is however some risk of vapour transfer. Hygrothermal modelling is needed to confirm that condensation risk is managed. It can be made wet but will not significantly restrict the passage of free water. This product is usually adequately vapour permeable and will allow vapour transfer. Open cell spray foam is not recommended for applications directly to the underside of tiles, slates or other domestic roof coverings. This foam should only ever be applied to the underside of felt.

Note - Problems could occur if open cell foam is applied to underlays that are not vapour permeable. The inappropriate use of reinforced, highly vapour resistant plastic underlays was relatively common where roofs were being replaced during the 1960’s to 1990’s. Open celled PU foams are relatively dimensionally weak and although they bond to substrates effectively and hold their shape permanently, they have little or no value as an adhesive or bonding agent. Hence, they are not commonly, or correctly, used for stabilising failing roof coverings. Vapour conductivity of ceiling, occupation level and the ventilation regime in the occupied parts of the house affect condensation risk. A vapour control layer (VCL) may sometimes be needed (recommended after a hygrothermal evaluation) on the warm side of open cell spray foam insulation. This is required if there is a risk of interstitial condensation. The presence of sprayed foam means that a simple visual inspection to establish the condition of roof timbers is impossible. Investigations performed to establish the condition of roof timbers will be difficult and limited. Where evidence of roof leaks, timber decay or insect infestations are identified, the full extent of the problem and the extent of any repairs, treatment and drying can only be undertaken if the foam is removed.

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