Build Stud walls before boarding ceiling?

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In our single storey extension, I have the floor insulated and boarded. I now want to build the stud walls.

The BC said there has to be 100mm Insulation board between the rafters, ensuring there is a 50mm air gap. Also there has to be another 40mm insulation board laid across the ceiling over the rafters. Then the plaster board over the top.
Is the 40mm insulation a specialist board or just more EcoTherm board at 40mm?
The ceiling is not flat, it is vaulted. (Hope that's the right term)

I main question is, Do I have to carry out the ceiling work before I can build my walls? Or can I build them and then work around insulating and boarding the ceiling?

I wanted the timbers for the stud work in so the Electrician can run his cabling.

Many thanks for your assistance.
 
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You would generally carcass out first - i.e. build the stud wall etc then insulate your ceiling.

We would weigh up what was likely to cause more grief - having to work around a stud wall whist fitting the (difficult) ceiling insulation detail or having to work around the electricians cables that he has left trailing ready to be fitted into the stud wall.....?

My guess is that we would build the stud wall first unless the stud wall posed problems other than those mentioned above, like having full access to the space etc.
 
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i think personally I'd insulate the ceiling first, but I'd do it slightly differently to what you're proposing. Fit boards between the rafters, cut the edges at a slight angle (a table saw with tilt facility is good for this) and wedge them in so the top is wider. Foam up the gap using expanding foam to bond the insulation in place and improve the seal. Lay 40mm boards the other way (long edges span multiple rafters) and use a long screw with a big washer (I used 4 inch squares of OSB as washers) to hold the board up temporarily. Mark the rafter positions on the boards, and proceed in getting them all up, foamed together at the edges and taped so aluminium tape. Lay a system of 50x25 battens along the lines of the rafters, screwed at 200 mm centres with 5x100 screws. You can attach your stud walls to the rafters by using 150mm screws, and you can attach your plasterboard to the battens. This gives you an air gap next to the foil face of the board, improving your roof I value. It also gives you a service void to run cables for lighting; hold cables in place with th per big plastic staples you get for underfloor heating pipes. If your rafters are at 600 centres use 15mm plasterboard, if at 400 use 12.5. If you want to se 12.5 board on 600 centres then you'll have to fasten the board edges together. Doesn't necessarily have to be a noggin- a 4 inch X 8 inch cutting of 9mm OSB will do, just something to keep the board edges so they move in the same direction when pushed

If you'll have flush down lighters you can choose either to extend your service void size or mine out the insulation. Recommend using led bulbs tho as halogens might well get too hot too close to insulation, and mining too much of it out destroys the point of fitting it in the first place. Consider dangly wire lights or surface mounted down lights instead of recessed

Consider also investing in a plasterboard lifter, especially if doing it on your own, and either rent a collated screw gun, or at the very least get a depth stop for your impact driver
 
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Fix the top plate to the ceiling, do the ceiling, then put the wall in afterwards.
 
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