Garden office/shed

[John D v2.0]

So hopefully the OSB will be inside the insulation otherwise the breather membrane won't breathe

Personally depending on budget I'd either full fill with wool, or 100mm rigid. The rigid is usually cheapest per m3 in 100mm thickness but still around 10x wool but only twice as good. With wool you'll still need a fair amount of heating input in winter.
Not sure if adding 100mm "battening" out and putting another layer would be possible for you though. It would certainly be cheaper.

 

[Mr_T25]

Nice. Bit of a ski slope shaped roof? Is Eddie the Eagle coming around?

 

[motorbiking]

I'd use the cheapest option as most of your heat will go out the roof and window , so over doing the insulation won't help.

That roof looks a bit high given the proximity to the boundary?

 

[Gazman16]

Nice. Bit of a ski slope shaped roof? Is Eddie the Eagle coming around?

Somewhere for my boy to go sledging in the winter
Its as big as I could go without building regs or planning. Perfectly blocks off the rear neighbors view into my garden giving me back my privacy with the added benefit of a big loft storage area.

John D. OSB is outside of the frame with breathable membrane stapled to it. Is this not what everyone does?
I have seen it done this way hundreds of times I'm sure.
Although I do get your point of the OSB making the breather membrane almost useless other than being a water proof barrier and ventilating the outside edge of the OSB

 

[Gazman16]

I'd use the cheapest option as most of your heat will go out the roof and window , so over doing the insulation won't help.

That roof looks a bit high given the proximity to the boundary?

4m ridge dual pitched roof with 2.5m eaves and 2m from the boundary's. Max allowed under PD.
Neighbors have already complained to planning and they have been out and are fine with it. I was expecting an argument over ground levels from planning but they never mentioned it and were fine.
I had it all printed out and highlighted so I could tell them to do one lol

 

[Gazman16]

I'd use the cheapest option as most of your heat will go out the roof and window , so over doing the insulation won't help.

That roof looks a bit high given the proximity to the boundary?

The windows and bifolds are all double glazed to modern standards (I have a double glazing company).
As I said ceiling has 25mm Celetex on the under side and will have 170 of wool in-between the joists. I will also insulate the roof but rather than buying a load for that now I will use bits that are left over from future jobs and add it over time.

 

[John D v2.0]

John D. OSB is outside of the frame with breathable membrane stapled to it. Is this not what everyone does?
I have seen it done this way hundreds of times I'm sure.
Although I do get your point of the OSB making the breather membrane almost useless other than being a water proof barrier and ventilating the outside edge of the OSB

Well maybe it's not optimal but it might be OK, you never know! I'm assuming you're not planning to have a hot tub in there

 

[Gazman16]

How should it be then? Have you got a link to a diagram?

https://www.google.co.uk/imgres?img...XWAhUFZlAKHVMsBkMQMwjGASgBMAE&iact=mrc&uact=8

I thought all timber frame walls were done this way. Pretty much everything on the internet seems to come up with the way I have done it

 

[John D v2.0]

Interesting! Thanks for the link, maybe the vcl on the inside is enough. I suppose the official way is a condensation risk analysis, but if osb outside is usual with a VCL don't let me stop you! Sorry to make the confusion

 

[Gazman16]

Interesting! Thanks for the link, maybe the vcl on the inside is enough. I suppose the official way is a condensation risk analysis, but if osb outside is usual with a VCL don't let me stop you! Sorry to make the confusion

Thats ok you just had me doubting myself for a minute.
Other than to protect the OSB from any water ingress that gets past the cladding I cant see much point in it being breathable either.

And yes I might move my Lazyspa hotub in there during the winter lol.

From peoples posts above if rigid insulation is really anywhere near 10x as efficient as wool then thats going to be the way forward I think.
Now the question if 100mm thick with a 50mm void full fill?

Some people and google search's seem to suggest a void is needed between the insulation and the inside face of the external OSB. Others say full fill.......

 

[John D v2.0]

10x the cost not efficiency! It's only twice as good for the thickness.