Some insulation advice on a garden office/gym/bedroom

Hi there

We are in the process of designing our garden office come gym come extra bedroom for when the family stay...

It's going to be a 6.6mx5.8 x 2.5m high timber frame building, flat roof, boarded in OSB on the walls and roof, suspended timber floor, and then covered in shiplap cladding.

It's going to be used quite a bit as a gym/study and also as a bedroom (sofa bed) if family are round. So needs to be warm, but it's not being lived in 24/7...

I keep going round and round in circles with regards the insulation. I want to keep costs down as much as possible and the biggest expense is the insulation.

Question 1 - floor insulation
I've decided to use EPS70 75mm polystyrene for the insulation for the floor - nice and cheap at around £165 for 38m2 and think this should suffice. Does this sound okay?

Question 2 - roof insulation
It's going to be a flat roof. I'm thinking of going for a warm roof construction with an EPDM membrane straight on top of the PIR boards. What is the minimum depth I can get away with to keep costs down? Its 38m2 so I need around 13 boards and want it cheap as poss but still thermally good!

Question 3 - wall insulation
It's going to be boarded OSB outside and then clad and internally plasterboarded. Again - what depth insulation should I be shooting for to make it 'good enough' for the use described? It's going to be 4x2 walls so the cavity can take anything up to 100mm but theres around 54m2 of insulation (about 19 boards) needed which very quickly adds up...

Any help, opinions etc are greatly appreciated!

Cheers
Matt

Its pretty tedious but to work out your savings see :- http://www.ambthair.com/how_to_calculate_financial_and_carbon_payback.html
So its up to you, how much you invest in foam and how much you will save. I think you have the proportions OK.
Do not speak about people sleeping in your OFFICE or you will have the Planning people and BCO breathing down your neck.
Once you have your frame in place I would wrap it in breather membrane to stop any knot holes letting water. Like wise, cover the inside frame with VCL to stop your internal moist air causing interstitial condensation.
Frank

Thanks princeof for the link. This is some very comprehensive info - maybe a bit much for me to digest as it is just going to be an occasional structure. If it were a house or extension it would be 125-150mm of boards all round.. I just wondered whether using it as a gym in winter and occasional use as a study (and by sleeping in it, I mean maybe at Christmas for the nephews and nieces if the house is full! But totally know where you are coming from re building regs).. Just don't want it a cold space that we can't heat, or boiling hot in summer where we can't use it. I know its a difficult question - just wondered whether anyone else has built something similar and has any experience with what they used?

Cheers!

Ok I can answer this one - Heat put into a building rises and disappears through gaps in windows doors or construction so spend most on good sealing windows and doors then on roof use 100mm celotex or similar - between the wall studs which should be 95mm use 70mm celotex or similar cut tight this leaves a gap for wiring installation and does the job - put soft insulation 50mm between the floor joists then use flexel underfloor heating system or similar with a 5mm layer on top of floor to stop heat loss from the mats downwards - you will be cool in summer and warm as toast in winter don't bother looking elsewhere this works. You are using the right roof covering all else is fine.