50mm Kingspan on Kitchen ceiling?

Frere Jacques

Right, this is a bit of an on going project.
The kitchen is an add on to the main house. It has a flat roof which has a series of 6x2 rafters running long ways from where the attic joins the main house. This makes "8 mini attics" above the kitchen if you get me.
As mentioned in a previous thread, I've separated the flat roof "attics" from main attic, by blocking them with 100mm boards and vented each of the gaps between the rafters to the open air with 8 separate vents at the far end of the ceiling.
The thing is though that there is zero insulation on the kitchen ceiling now and there is plenty of ventilation above it. The ceiling is nice and dry as is the area above it which is free from leaks.
So I'm wondering what the best way is insulate it is?
Ideally if I had the cash I'd get the eight cavities filled by blown beads but I don't. My plan is to mark the joists and put up 3 sheets of 50mm backed board with some 100mm screws and washers through the existing ceiling. Then paint the washers, tape and skim the joints and paint the celing.
So I'm hoping that the job is airtight and the insulation is pretty good so there is no thermal differential between the back of the insulation and the old ceiling.
Am I going to get myself into trouble going down this route. The existing ceiling has a stipple finish so there will be lots of tiny gaps between the back of the new board and the old ceiling which obviously could be a haven for black mould but I'm hoping this won't happen due to the ventilation and the lack of a significant temp drop.
I really don't want to have to pull down the existing ceiling. Wife is expecting in a few weeks so need to move on this quickly with min disruption to the house and minimum dust. Any tips or alternatives welcome.

OBrother

Ideally you would need ventilation either end of the joist,but there should be enough air changes within the joist space to move any excess moisture. IMO putting rigid insulation below the plasterboard will be far better than blocking the air circulation between the joists.
Be careful that you don't lower the internal ground to ceiling height to less than 2.4m as this is regarded in the Building Regs guidance as the minimum allowable.
If you really wanted to limit the possibilities of mould growth, counter batten the existing ceiling with timber battens, fit the insulation between these, fit an air tight membrane or heavy gauge polythene under the battens lapped and taped, and then the plasterboard sheet on its own, this will prevent almost all moisture entering the insulation.
Just in case - adding extra weight to this ceiling which has 6x2 rafters should be checked out, but it seems you have quiet a small area.

Frere Jacques

Cheers OBrother.
My height will be down to 2.35m after the work. D'oh.
Do all these regs apply to houses built in the '70s and if so what are the consequences. Would it void house insurance?
Not looking for ways to break the law, just want to know what my options are.

OBrother

Building regs apply to all works being carried out, the 2.4m ht is a ventilation requirement, and is a guidance. What guidance means in this circumstance is that if you proove you can meet the requirement in another way such as through a British or other standard then it is ok.
Cant see how it would void your insurance, though I could be wrong.

MrsWhite2

When you say 'stipple ceiling' do you mean artek? Does that not contain some asbestos, requiring care on removal? Just check it out before removal...

Frere Jacques

Cheers folks. I think it's time to look up some building standards but it might be looking like a dpm (or tanking) on the ceiling and the 50mm board onto this. Maybe a bead of silicone an inch from the board edges on the flat and a bead in the butt joints where the boards meet.

Thanks again.
FJ