Timber frame wall structure for lean to utility/conservatory

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Hi, I'm looking for some words of wisdom with regard my project.

I'm planning to construct a utility room between my garage and house. Currently this is a concreted courtyard. Area measures 2.4m x 6m.

Rightly or more likely wrongly I'm treating this as a temporary structure/conservatory in terms of PP and BC consents as eventually the garage/courtyard area will have an extension built on it, and a new seperate garage built in the garden (STPP). This is atleast a couple of years off so I have to do something temporary in the mean time as we are desperate for space.

The room will be seperated from the house by an external door, it will be used like its part of the house, so need to be warm and finished like a room. There will be a window front and back, and atleast a door at the rear, possibly the front too. Ideally it will house the washing machine, tumble dryer, oil fired boiler, utility sink, and kitchen units for storage.

I plan to use a warm deck flat roof with EPDM covering. using 100mm celotex insulation. I plan to construct this first as it will be fully supported by the existing house and garage walls. It will then give me a dry store and work area during the grotty winter months when I will be constructing it.

Floor - depending on the levels/joist sizes, it will either sit directly on top of the existing concrete with a damp proof sheet laid on the concrete first, or it will be on hangers between the house and garage walls. Celotex between joists (75mm? 100mm?) then flooring (chip, ply or t&g) laid on top.

Walls front and rear-
This is where I'm not sure quite what to do. What is the "normal" way of constructing these?
originally I was thinking stud frame with celotex between studs, OSB or ply on outside, membrane, batons, timber cladding. Now I'm wondering if it should be more like the warm deck roof construction with insulation on the outside of the structural timbers?
What size timbers would be best? It will be none supporting, but will house window/door
For the membrane on the outside of the walls can I use the DPC sheeting folded straight up from the floor in a single sheet or does it need to be breathable?

Wall house side
As this is external cavity (insulated) wall of the house I just plan to paint, or plasterboard and paint.

Wall garage side
This is just single skin to the garage. When I converted a garage in the past to make a playroom/office I used frame fixings to attach studs to the brickwork, insulated between, then plasterboarded over. Is this the best route, or can I use continuous celotex fixed straight to the wall and plasterboard over?

All helpful guidance, advise, constructive criticism's, and ideas most greatefully received!

many thanks
 
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originally I was thinking stud frame with celotex between studs, OSB or ply on outside, membrane, batons, timber cladding. Now I'm wondering if it should be more like the warm deck roof construction with insulation on the outside of the structural timbers?

Either / or

What size timbers would be best? It will be none supporting, but will house window/door

Structurally you can probably have them as thin as 50mm, anything more for the studs will be for insulation (if you insulate between them).



For the membrane on the outside of the walls can I use the DPC sheeting folded straight up from the floor in a single sheet or does it need to be breathable?

Warm side vapour barrier, cold side breather membrane.
 
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Either / or

But is one preferred these days? as with a warm deck roof? Is one particularly more efficient than the other?

Structurally you can probably have them as thin as 50mm, anything more for the studs will be for insulation (if you insulate between them).

CLS it is then... I already have a stack!


Warm side vapour barrier, cold side breather membrane.
Where exactly would each mebrane go? breathable under battons? vapour behind plasterboard? or if putting the insulation external to the stud I assume vapour could go on outside of the stud, before fitting insulation?

I'm assuming celotex is the way to go in the floor and walls as well as the roof? What sort of thickness should I be looking at? It will likely be 100mm on the roof, but I assume floor and wall can be less, without any detrimental affect as roof would loose the most heat, so 75mm or 50mm?
 

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