Vaulted ceiling hidden condensation

[Graham Jones]

Hello,

A family room extension to our house has a vaulted ceiling. This is constructed from 150mm rafters with 100mm solid insulation between the rafters, leaving a 50mm void between the insulation and breathable roofing membrane. Insulated plasterboard has then been screwed to the rafters and skimmed, giving a nice catherdral style room. Apart from the breathable membrane there is no other ventilation to the 50mm void.

Cut into the plasterboard are six downlighters. My worry is that moisture will escape through the downlighters and condense in really cold weather on the underside of the membrane and over time soak into the timbers. My worry is further increased by the fact that I cannot inspect the void to see what is going on, without maybe removing the soffits and guttering and inspecting from the outside. Even if I fit soffit vents I still can't be sure there is no condensation.

Is there anything I can do? Or should I just not worry and maybe let it rot!

Thanks,

Graham

[Phil L]

It doesn't make a blind bit of difference how much condensation gets up there, not that any ever would, but even if it was continuous, it would have no effect whatsoever - the membrane is breathable - timbers don't rot in well ventilated conditions, regardless of the moisture levels

[Graham Jones]

Thanks for your reply, most helpful, so what you are saying is that a timber can be damp and not rot if there is sufficient ventilation?

Graham

[robgraham]

In theory I would agree with Harry about the vapour barrier, but he hasn't read the OP's posting fully and I think he missed the fact that 'insulating plasterboard was fitted, which although not a perfect membrane is quite likely adequate, and 'solid insulation' which won't absorb moisture.

Harry is right about the potential for condensation in insulation if it was open cell eg fibreglass. One solution if the OP is concerned about it is to arrange ventilation of the 50mm void.

One thing that might concern me is the heat generated by the down lighters which could melt the insulation. Rob

[Graham Jones]

Thanks Rob, I do remember overhearing the electrician swearing while he attempted to hack out insulation from the behind the cut outs he had made. So this should be OK, I might replace with LED bulbs anyway, this should overcome any potential melting issues.

Also the downlighters have a foam gasket and are fire rated, will this be enough to prevent moisture egress?

Graham

[fred]

In article , Graham Jones writes

Yes, a fire rated downlighter provides a sealed barrier between the underside of the ceiling and its associated void area.

This, combined with your insulation backed plasterboard providing a vapour barrier should remove any risk of condensation in that space. The breathable membrane to the outside should remove any possible doubt.

[Lyndsay Williams]

Perhaps OT, but I have used silica gel for years in moisture sensitive elec= tronics applications. I am currently redesigning my kitchen lighting with c= oloured mood lighting LEDS, (the colour changes in the kitchen throughout t= he day using a microcontroller ). These LEDS are low power and low heat an= d attached via neodymium magnets to ceiling. Via this thread I note the cha= llenges of inset lighting in ceilings and want to keep life simple! I hope the OP gets the problem sorted.

Lyn

[trevor]

I have same design and suffer massive condensation coming through plasterboard