Warning - this question contains the "R" word..

[greenbarn]

......and the "F" word, for that matter. Sorry to touch a nerve on my first posting, but I am asking for any views on that Renotherm Foam stuff in a particular application.

My understanding from reading postings on this and other fora is that spraying gunk onto the underside of slates is a BAD THING . I understand the problem with rendering the slates unusable for the future. I think I understand that moisture could penetrate into the rafters and remain undetected until such time as the roof collapses. Hopefully I've grasped those basics, so here's my question:
I'm having a barn conversion done, and the living space will extend into the full height of the roof. The roof is felted. My architect is suggesting using the "R" stuff sprayed directly onto the felt as a means of insulation, more cost-effective than eg Kingspan. As the roof timbers have felt between themselves and the slates, does this mean that there is no longer a potential problem of water penetration and hidden rot? Or is that a naive assumption?
Any experiences or ideas gratefully received, as I'm going nuts trying to decide if this is a good idea - or am I being put off unnecessarily simply because of the "R" word?

 

[Nige F]

It`s the A word I`m worried about ...........Archie Tec

 

[Deluks]

It is a brand spanking roof or the original one?

In theory the foam shouldn't touch the slates, but if the felt has any holes or is saggy then it could push upwards as the foam expands.

What I'd be inclined to do is line every rafter with rockwool strips a few inches wide, then get them to spray the stuff. That way the foam only sticks to the felt and the rockwool keeps it from coming into contact with the timbers.

What's the price difference anyway, vs. kingspan, those foam guys charge a packet, and I'm pretty sure you can rent those foam machines.

 

[greenbarn]

It is a brand spanking roof or the original one?

It's a mixture - it's a fairly complex roof, some sections have been renewed, the rest was sound and already felted.

What I'd be inclined to do is line every rafter with rockwool strips a few inches wide, then get them to spray the stuff.

Interesting idea. Sounds as though the work involved would be as much as, maybe more than, battening work for the Kingspan solution.

What's the price difference anyway, vs. kingspan,

Good question, particularly with regard to the previous bit about the rockwool. Does anyone have any guide prices for fitted Kingspan per sq metre? If the price difference is realistic (particularly in the context of the whole conversion job) then I shall veto the architect.

I'd be interested in some thoughts as to whether it's necessary/desirable to have an air gap between the insulation and the felt, which obviously wouldn't be the case with the foam sprayed onto the felt, and why the foam sticking to the sides of the rafters could be a problem if the tops are covered by the felt? Sorry, I'm a bit ignorant on this, but at least from reading through the forum I've grasped enough to realise that I need to ask the questions!

 

[masona]

I'd be interested in some thoughts as to whether it's necessary/desirable to have an air gap between the insulation and the felt, which obviously wouldn't be the case with the foam sprayed onto the felt, and why the foam sticking to the sides of the rafters could be a problem if the tops are covered by the felt? Sorry, I'm a bit ignorant on this, but at least from reading through the forum I've grasped enough to realise that I need to ask the questions!

Well this is my opinion, do we really need any messy foam stuck to the side of the roof rafters timber? The timber need to breathe, are the foam flexible, if not, surely it will shows hairline cracked against the timber due to timber breathing. The very best way is to use insulation slabs between rafters leaving 50mm air flow at the back then use foiled plasterboards, ridge roof vent at top and soffit vent at the bottom for circulation of air. When the foam roof first came about, it was a cheap design saving having to replace the roof tiles without the roofing felt.