Icynene spray foam between roof joists

I've used sprayfoam insulation on a recent renovation and would only specify it with a plastic card backing and a vapour control layer. By using the card, the timber gets enough exposed surface to breath.

Renovation.jpg
Sprayfoam roof.jpg
Sprayfoam roof-2.jpg

With all these issues taken on board, in my case it doesn't seem to be the way to go, or indeed for the op.

As an alternative can an open cell foam be used on the attic floor, or in my case the floor of the crawlspace and dwarf walls provided the ventilation is maintained from eaves as it is at the moment?

Would cellulose be suitable for reducing draughts or another blown in type such as rockprime?

sas wrote: »

If you've ready done this then feel free to ignore me.

Any chance you could put a sketch together (in section) of your specific build up and the issue you have? It would help people advise.

I'll give it a go. What's the easiest way to upload it as I am not a computer whizz?

Good luck with your mother's house. Hope it does the trick.

In relation to the foam, if applied at ceiling level I'm assuming the condensation problem would be overcome once the attic ventilation is maintained, as water vapour will travel up through the foam, if it's open cell and breathable, and removed by the natural ventilation the same as any other ceiling level insulation.

Interesting threat.
My 5cent worth:
If you insulate an attic at rafter level, you do turn the "cold" attic into a warm roof. It is the last line of insulation that counts as your thermal envelope.
Side effects are:
your former insulation layer (if there was any) on ceiling / joist level turns into an intermediate floor.
You heat your attic in addition to your living space, you actually heat more volume than you did before and you get an increased area of external envelope.
This might effect your BER as well as your heating bill.
It is by far easier to insulate on joist level than it is at rafter level.

Regarding the spray foam:
I`ve read through a lot of certs, all of them stating that a VCL is required on the warm side, unless exception like stated by Sinnerboy. This is actually in line with building physics and makes sense regarding risk of condensation. You wouldn´t install any other breathable insulation without a VCL, so why spray foam?
Another thing to ask you / to think about:
If breathable foam is not supposed to be sprayed onto diffusiontight bitumen felt, how do PVC trays improve this situation ?
Well, not at all... the gap created for ventilation will never kick in as moisture will not get that far being stopped by PVC trays instead of bitumen felt.

Poor Uncle Tom wrote: »

the 50mm air gap is meant to provide ventilation to the roofing timbers, not the roof space.

It still does not deal with the condensation issue you get where the foam touches the trays and thereby I do not fully understand why one would go that route.
Given there is no VCL installed on the warm side of the foam (like it is the case in 99.9%) moisture gets into the foam, cools down, condensates on the outside of the foam, is blocked by the trays, finds it way into the rafters, bypasses the trays through the rafters and is then ventilated away using a 50mm air gap which has no ridge vent and thereby no stack / chimney effect??? Would you really rely on that?
It also does not solves condenstaion issues that might arise underneath the old bitumen felt as it might happen in certain circumstances.

What thickness is required (foam) have been quoted 150mm plus 50mm for vent card.