Bit of advice and guidance needed - if you don't mind.
The top floor of our 19th Century house is freezing cold. It has a cathedral ceiling and a slate tiled roof. Having inspected the insulation and roof from within the house - i.e. removed a bit of the spray foam insulation; it would appear that someone has applied insulation spray foam directly on the slate tiles - but has left the underside of the rafters exposed. Currently, the roof does not leak.
To increase the warmth of the room, I have thought about overboarding the spray foam insulation with additional board insulation, i.e. fixing it to the exposed rafters. It seems such action could end in disaster, i.e. build of moisture leading to rotting timbers, resulting in needing new roof timbers - in other words a whole new roof. Is this true or have I misunderstood overboarding spray foam insulation?
If I'm correct, i.e. overboarding spray insulation is a recipe for disaster, then I guess the task ahead of me is to remove all of the spray foam insulation, remove roof slates, fit sark, fit batons and re-fix slates. Then insulate room by fixing insulation boards between rafters. Is this correct?
We are roofers who also install spray foam insulation, so may be able to help.
First of all the rafters being left exposed is the correct way to apply the foam, a blanket covering of foam will not allow the timber to breathe and eventually rots the wood. However spraying the application directly onto the tiles makes it very difficult for you to change the tiles or battons in the future. A waterproof membrane should have been fitted underneath the tiles before spraying.
The foam should be efficient enough on its own. There should be no need for extra insulation, unless it has not been applied to the correct thickness. If you really think overboarding is the way forward just make sure there is an airgap of at least 2 inches between the foam and new layer of insulation. But be aware you are going to hide any future problems should the tiles or rafters start to leak at some point.
It may be that the coldness is due to a lack of insulation/dampness in the walls or windows and ineffective heating system. Spray foam usually does a very good job and shouldnt need the work you are proposing