The spray foam insulation cost in London depends on the area to be installed (attic, wall shed, etc), the depth of insulation required, whether it is an open-cell or closed-cell and if there are any access issues. For example, the cost of spraying an attic or loft using open-cell spray foam insulation includes the cost of the breathable vent card required under building regulations to create a 50mm air gap between roofing felt and spray insulation material, which allows your timbers to breathe. Always ask your contractor if a breathable vent card is required.