Timber and celotex ceiling in outbuilding

[OldKettle]

Hi,
I am looking to better insulate an outbuilding, which will eventually be fully renovated.

The building is double bricked and, as far as I know, has cavity wall insulation, but has a flat roof, and seem to cool down quickly. The building is approximately, 9m x 3.5m.

It has a concrete floor, on which I intend to lay laminate, over moisture resistant underlay.

For the flat roof, I had the idea of running timbers, at 600mm intervals from joist hangers, and then attaching celotex boards to those timbers, to create the ceilings.

As this is a temporary solution (perhaps one to two years) the aesthetics of a unfinished celotex ceiling aren't a problem, and I would intend to anchor any lights to the timbers.

The reason for using joist hangers, rather than directly anchoring to the ceiling, is that the flat roof may contain asbestos, so I would prefer to leave it untouched.

I wonder if anybody had any thoughts on the above. Any input would be much appreciated.

 

[Notch7]

that would work well.

I think maybe 50 x 25 counterbattens will be needed to hold the celetex in place.

Itll be worth foil taping the insulation joints as a vapour barrier.

Note that if its an unheated space, the foil face might cause condensation? It may not as its on insulation, I dont know

 

[motorbiking]

If I understand you correctly you are not planning to plasterboard over the celotex? Given the small additional cost ~£60 and the massive fire risk of unprotected celotex, I'd think again.

 

[OldKettle]

If I understand you correctly you are not planning to plasterboard over the celotex? Given the small additional cost ~£60 and the massive fire risk of unprotected celotex, I'd think again.

Thanks. Hadn't considered that at all.

I believe 12.5mm for a ceiling would be the right thickness.

Total depth is a concern, because the ceiling is not particularly high.

I'm thinking that, to span the approx 3.5 metres, and support the 12.5mm plasterboard, plus the insulation underneath, I could use 2x4 C24 at 600mm intervals, for a total depth of about 110mm.

But, I wouldn't know where to start with the calculations. Does the above sound OK?

 

[motorbiking]

It depends on the construction of the roof, I'd expect there to be existing joists - why can't you insulate between those and just add the plasterboard after? It might be easier to use a loft roll rather than celotex if you can get to the existing joists. Rockwool is fire resistant.

 

[Notch7]

Is there that much of a fire risk with celetex?

anybody that has tried to set light to it knows it doesnt burn easily.

polystyrene on the other hand is a different kettle.

 

[motorbiking]

Its fairly resistant, but its in the roof, so the concentration of heat would be higher. Personally I'd be looking at loft roll and plaster board on the existing beams. It would be cheaper and 50% as good.

The problem with uninsulated buildings is you have to be realistic about the U-value you can achieve. It may have a U-value of 2.5 currently so bringing it down to 0.5 or 0.75, while still very poor (by current standards), would make a big difference.

 

[John D v2.0]

Is there that much of a fire risk with celetex?

anybody that has tried to set light to it knows it doesnt burn easily.

polystyrene on the other hand is a different kettle.

It doesn't burn in isolation but it has a huge amount of energy that it can release if kept alight in a suitable enclosed space
They've been discussing this exact topic this past week at the Grenfell inquiry, why did the celotex burn so well in many areas.
Seems the cladding may have helped keep the celotex burning and the celotex released even more heat.