On Sun, 09 Nov 2008 23:50:26 +0000, Hugo Nebula abuse@localhost
wrote:

Mike wrote:
I'm planning on building a new large shed, hopefully with low heating
requirements and a steady temperature all year round to enable me to
use it for woodworking projects.

The shed will be around 0.5m from a shared boundary fence and at least
5m from any existing structures. Will a mainly timber frame structure
covered in fibre cement weatherboarding (with a green roof of sedums)
meet class 0 standards or will I need plasterboard on all the inner
surfaces?

When you say 'large', anything with a floor area more than 15m^2 that
close to the boundary will require a Building Regulations application.

I'd deliberately planned it to be a smidgen under the 15m2 floor area

Also check about Planning. If it were more than 1m from a boundary, it
could be up to 30m^2 and still be exempt from Building Regulations.

1m from the boundary would place it too close to an existing garage to
use the path (it's a very narrow site)

Building Control will be concerned about structure, external fire
spread, resistance to moisture, and ventilation (amongst other things).

I really don't want to involve building control at all, the nearest
approach to any existing property besides the brick built detached
garage is over 5m

Roofs within 6m of a boundary must be class AA rating, ie,
non-combustible. Vegetation can dry up and represent a risk of fire
spread.

A green roof soaked with rainwater water and covered in soil and
sedum's will be just about non combustible - that it will sit on a
flammable fibreglass laminate over OSB ought to be irrelevant - surely
normal roofing felt will easily burn?

External walls must be of 30 minutes fire resistance within 1m
of the boundary, which means in your case that you will need
plasterboard on the inside, as well as the non-combustible cladding.

Plasterboard on the inside is completely unacceptable, I want to use
it for a shed/workshop not a living space!

The thermal insulation requirements do not apply to a free-standing
building of less than 50m^2.

They might not, but I'm reluctant to pay a fortune to heat or cool
this space when a few inches of celotex will keep it snug in winter
and cool in summer.

A timber framed building should have a vapour barrier on the warm side
of the insulation.

That *normally* being the plasterboard (foil backed?)

I'll not be using that, 12mm ply as a minimum so I can hang things on
the wall.
..
The plywood or other sheathing should be on the
outside of the studs,

Well I'd not planned for anything to be on the outside except the
fibre cement weatherboarding

with breather membrane to the outside of that, and

The breather membrane surely goes under the outside layer?

a ventilated and drained cavity inside of the cladding.

If the moisture barrier is on the inside, using foil backed
plasterboard or in this case probably plastic sheet, then surely any
moisture in the cavity on the outside of the celotex permeates through
the breather layer into the atmosphere. Venting the cavity to the
inside is surely the last thing to do, it permits moisture from
activity in the shed into the cavity which may then condense on cold
insulation.


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