studio insulation conundrum

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I am designing and pricing to build a studio that is under permitted development , it will be timber framed. 6x2 floor joists 4x2 walls and 5x2 roof joists "flat roof". I have designed it to receive 100mm celotex in the walls floor and ceiling. I a now wondering if this will be enough insulation?

I don't know who to ask hence asking on this site or how to work it out.

The studio has a total area of 31.25 cubic metres. The joists will be suspended on concrete piers 50mm off of the ground. The walls will be 3mm skim on 12mm plasterboard on 4x2 C16 studs 100mm celotex with 12 mm WBP ply then a tyvek membrane 2x1 treated baton then vertical cedar cladding. The roof is 3mm skim 12mm plasterboard 5x2 C16 joists 100mm celotex 25mm vent gap 18mm OSB then EPDM roof covering.

There is one set of UPC french door 1800 x 2100mm and 2 windows 600 x 2100 all up to current regs.

I appreciate i may be asking for more help than the norm but this is one of the biggest jobs I've priced since being SE and I just want to be surer. Thank you.[/quote]
 
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150 insulation in the roof, comprising 100 between and 50 over, then you can do away with the air gap. You can get 50mm insulation bonded to 9mm ply for an all in one roof decking solution.

Use extra insulation over the internal studs, 20mm will be alright, or use insulated plasterboard.
I'd swap the osb and ply over, 11mm OSB for the walls and 18mm wbp for the roof (If you don't go with the bonded stuff mentioned above)
 
Thank you for replying deluks, I know in a pitched roof a 50mm air flow gap is required under the felt and above the insulation. So a flat roof doesn't need venting then?

I may go for the warm roof option but that would give me a 9 inch fascia, so I may put the celotex under the rafters\roof joists.

Thank you for your advice though its very much appreciated.
 
Unless you have a minimum of 100mm of celotex above the roof joists then it may not conform to 'warm roof' standards.

We are now fitting 125mm above joists.

Be careful of hybrid type insulated roofs, i.e. the bulk of the insulation between joists and not over, as they may require venting.

Walls require 140mm i.e 100 betwixt with 40mm across the inside and a 20mm vented gap externally.
 
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Thank you, but I still need clarification on the walls:

Sooo........

I am looking at "inside out" 12mm PB then 25/40mm celotex then 100mm celotex or Glass Wool then 12mm ply then Breather Membrane, then 20mm baton for vented gap then counter baton for the cladding?

For the record I wasn't looking at a warm roof set up, I am happy to go 100mm in and 50mm over for celotex then have a vent gap, I suppose I would have to incorporate a continuous vent strip into the soffit?

Thank you.
 
For a cold flat roof, you will need a good 2" gap at each end, as well as the gap between deck and insulation.
Does this building need building regs? If so, a span over 5m may be problematic as regards rules for ventilation.
 
No there are no building regs on this job, it is being built under Permitted Development and I have spoken with the local council office just to make sure.

The span of the joists is 2.8 with an unsupported span of 2.3 mtrs so there is not problem there, a 2 inch gap is achievable as its having a 9 inch soffit to the front and a closed fascia the rest of the sides so. The rear facia will also be vented for the continuous air flow,

I worked for a builder for eight years and whenever we did cladding on timber framed buildings it would be 12mm plasterboard 40mm celotex over the studs 100mm in between then ply the breather membrane then baton then cladding, albeit this was always horizontal cladding, whereas now I am using vertical cladding which as Noseall has pointed out needs a 20mm continuous vent gap which can only be achieved by batoning and counter batoning. "To my mind"

All I care about is that I put enough insulation in that they don't freeze to death in the winter, whether that be all celotex or rock wool/sheeps wool and celotex. I suppose rock wool is damn site cheaper than celotex and would be preferable if I achieve the standard required.

Oh I seem to remember on one job, I think it was cement board we had to put building paper on the inside of the timber walls
 
For maximum wall insulation with that structure, you've probably got the best practical solution in terms of materials.
Presumably it will be intermittently heated? Electric?
 
Thank you to everyone that has helped with this job. I did not get it. It had been priced already by a friend of their family whom fell ill l prior to starting it, and suddenly made a miracle recovery when my price went in, all was not lost though, they gave me £50 and a bottle of wine for my efforts.
 

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