Small lean to extension - Spray insulation

booooonzo

My parents are having a small lean too extension built.

The wood framing is up and a trocal roof on at present and that's it.
(no slab poured yet, just foundations for the walls)

They were advised to get the roof open cell spray foamed for insulation and there has been a 50mm gap left for this.

When the guy was out he priced to spray the walls also.

So afaik it would be wood cladding -> battens -> breathe membrane -> osb -> spray between the wooden frame -> plaster board.

Its just a solo builder doing all the work so i just wanted some advice as to weather this would be a good approach or would 100m pir between studs and possibly 50mm insulated plasterboard over this be a better job?

thanks

BryanF

Walls: I went with cellouse - (from external: ) cladding, double batten vent space, breather membrane, ply racking board, 225mm space to fill between staggered studs, vapour barrier, service cavity inside.

Ceilings: Insulated above the rafters to avoid ventilating the roof space.

hesker

booooonzo wrote: »

My parents are having a small lean too extension built.

The wood framing is up and a trocal roof on at present and that's it.
(no slab poured yet, just foundations for the walls)

They were advised to get the roof open cell spray foamed for insulation and there has been a 50mm gap left for this.

When the guy was out he priced to spray the walls also.

So afaik it would be wood cladding -> battens -> breathe membrane -> osb -> spray between the wooden frame -> plaster board.

Its just a solo builder doing all the work so i just wanted some advice as to weather this would be a good approach or would 100m pir between studs and possibly 50mm insulated plasterboard over this be a better job?

thanks

Not a great idea to have OSB on the cold side of the insulation. Do a search on this. Risk of condensation and rot. There is a breathable version of OSB but not easy to source for small jobs.

Lumen

hesker wrote: »

Not a great idea to have OSB on the cold side of the insulation. Do a search on this. Risk of condensation and rot. There is a breathable version of OSB but not easy to source for small jobs.

Yes, the Americans seem to do it a lot though, e.g. OSB as a roof sheathing.

hesker

Lumen wrote: »

Yes, the Americans seem to do it a lot though, e.g. OSB as a roof sheathing.

Yeah it might be fine and it might not. But if you have the opportunity to avoid the risk surely it’s worth giving it some serious thought.

booooonzo

hesker wrote: »

Not a great idea to have OSB on the cold side of the insulation. Do a search on this. Risk of condensation and rot. There is a breathable version of OSB but not easy to source for small jobs.

Thanks, the builder has gone and got ply today over osb

still unsure what insulation they are going with. It just has 4 x 2 studs up at present.
i'll try do some research tonight but in general would 100mm rigid insulation have a better u value over open cell spray or is it more to do with the detailing that matters?

cheers

BryanF

Who is signing off on building regs? And who provide drawings to the builder?
Neither spec’s you propose are a great idea with timber frame.

hesker

Lumen wrote: »

Yes, the Americans seem to do it a lot though, e.g. OSB as a roof sheathing.

Yeah it might be fine and it might not. But if you have the opportunity to avoid the risk surely it’s worth giving it some serious thought.

booooonzo

BryanF wrote: »

Who is signing off on building regs? And who provide drawings to the builder?
Neither spec’s you propose are a great idea with timber frame.

No one I would imagine!
It's a small extension at the back of a country house and I'd imagine they just got the builder they've been getting the last 30 years in to throw it up.

Far from ideal i'd agree but i'm just trying to prevent them ending up with a cold waste of space and money.

Any steer towards a better alternative at this point would be much appreciated.

BryanF

I’ve given my opinion and spec for my own extension above. Best advice would be get an arch tech or surveyor to visit the site and agree remaining envelope spec with the builder.

As an absolute minimum standard insure this is complied with: https://www.housing.gov.ie/housing/building-standards/tgd-part-l-conservation-fuel-and-energy/acceptable-construction-details-4

booooonzo

BryanF wrote: »

I’ve given my opinion and spec for my own extension above. Best advice would be get an arch tech or surveyor to visit the site and agree remaining envelope spec with the builder.

As an absolute minimum standard insure this is complied with: https://www.housing.gov.ie/housing/building-standards/tgd-part-l-conservation-fuel-and-energy/acceptable-construction-details-4

perfect, appreciate the help!