DKG

@Iceverge - Is this more what you had in mind Cedral Lap instruction state a minimum airgap of 30mm, will a 25mm batten be sufficient? Do I require an airgap on the cement base board? Any comments regarding the 50mm thick EPS upstand around the concrete slab perimeter, do I perhaps need cement particle board outside of that EPS to separate the EPS from the pea shingle and to stop the EPS coming away from the sides of the concrete base and leaving an airgap

What are your views on stud sizes 95mm or 120mm?

Thanks for your insights I will give it some thought tomorrow I have been reading many of your comments with interest in a previous similar discussion earlier last last

I have amended some details Excuse the poor drawing (I'm experienced in Autocad but currently so using a Free cad program) I have increased studs to 120mm wide, reduced PIR to 60mm between studs and 20mm over (internally) I now have a cold bridge where the concrete slab extends past the building structure, so either need to bring the edge of the slab in so the bricks sit on the edge or accept the cold bridge or come up with a way to solve the cold bridge issue

I was not planning on leaving the bottom cladding board unsupported but hadn't worked out the details of how to support it. It was originally brought down lower to protect the PIR, so if I now move that to the inside the lower cement board will no longer be required

Hadn't considered 5" x 2", but will certainly give that some thought

I was also concerned with screwing through the PIR. I was looking to avoid cold bridging but think I'll bring it back to the internal side of the studs I think perhaps 80mm between and 30mm over the studs is excessive, so might look at reducing that to perhaps 50-60mm between and 20mm over the studs

Yes you are correct, the arrow is incorrectly positioned I'll take a look at those strap, although look a little expensive

The other option might be to put the PIR on the inside of the building, but I opted to put it on the outside down to the ground to prevent cold bridging where the concrete slab extends beyond the walls, which because I was concerned about having the threaded bolts too close to the edge of the concrete slab I wonder if PIR between the studs and OSB board would work or if there is any concern that the OSB boards wouldn't then provide the necessary rigidity

I was planning on fixing the vertical battens over the top of the PIR to keep a continuous airgap I may need to consider how well PIR will withstand the British weather when external to the OSB but protected by the cladding I also haven't shown the breather membrane, which I'm unsure where is the best place to put it

I am looking to lay foundations for a large garden room in the next month and so have done some research and knocked together a drawing on what I have in mind currently Size 5.7m x 5.25m I wish to maximise internal headroom as I am limited to the 2.5m maximum and would prefer to avoid applying for planning permission, as it's relatively large I would also like to keep the roofline height low relative to the neighbour garden to avoid it looking too dominating as it's only 1m from the boundary and so hoping to have the top of the concrete slab approximately level with the outside ground, although realize this isn't ideal and plan to have a French drain around the perimeter and ensure any timber is at least 150mm above ground level I would appreciate it if more experience members here could provide any constructive comments on the design with any potential problems