Preventing condensation with steel roof

[hanvyj]

Hi,

I've decided to replace the existing sagging bitumen sheeting mounted on OBS3 and replace with box profile steel.

Would this arrangement be adequate for reducing the condensation on the steel?

I'd be using the existing roof board & rafters, basically just attaching 2x2s to raise the steel up and provide an air-gap that can be ventilated to the exterior of the roof.

I'm on a budget, and the cheapest steel sheets I can find come to £250. Getting the "anti-condensation coated" sheets would be much more expensive. I'm hoping just decent ventilation would be sufficient?

Thanks

 

[tony1851]

Would this arrangement be adequate for reducing the condensation on the steel?

No it won't - it's a recipe for disaster.

Think of water vapour rising up through gaps in the insulation to the boarding. On a cold winter's night with still air, you will get condensation on the boarding and the underside of the steel.


You need a well-sealed vapour barrier on the warm side of the insulation, to stop vapour getting through in the first place.

 

[hanvyj]

Would this arrangement be adequate for reducing the condensation on the steel?

No it won't - it's a recipe for disaster.

Think of water vapour rising up through gaps in the insulation to the boarding. On a cold winter's night with still air, you will get condensation on the boarding and the underside of the steel.


You need a well-sealed vapour barrier on the warm side of the insulation, to stop vapour getting through in the first place.

Not sure I understand. Your saying the warm air with vapor is going to go through the insulation, and OSB, and condense on the steel?

So it needs some kind of water proof barrier (vapor barrier like this: http://www.wickes.co.uk/Wickes-Green-Polythene-Vapour-Barrier-2-5x20m/p/153230) to prevent the warm air from getting into the outside temperature vented area? By warm side, you mean under the rafter insulation, not just on the underneath of the osb?

Thanks for the help.

 

[tony1851]

Your saying the warm air with vapor is going to go through the insulation, and OSB, and condense on the steel?

No. Vapour will rise up through the inevitable gaps in the insulation and condense on the cold side of the insulation, ie directly under the OSB, causing rot. The void above the OSB will not clear the vapour - think of cold, clear nights in winter where there is no air movement.

You need a polythene vapour barrier on the warm side of the insulation. In your case, put the insulation board (eg Kingspan) level with the bottom of the joists, tack the vapour barrier to the underside of the joists, taping any joints, and then plasterboard.

Google 'cold roofs', there's plenty of info out there.

 

[hanvyj]

Your saying the warm air with vapor is going to go through the insulation, and OSB, and condense on the steel?

No. Vapour will rise up through the inevitable gaps in the insulation and condense on the cold side of the insulation, ie directly under the OSB, causing rot. The void above the OSB will not clear the vapour - think of cold, clear nights in winter where there is no air movement.

You need a polythene vapour barrier on the warm side of the insulation. In your case, put the insulation board (eg Kingspan) level with the bottom of the joists, tack the vapour barrier to the underside of the joists, taping any joints, and then plasterboard.

Google 'cold roofs', there's plenty of info out there.

Thanks, makes sense. Searching "Cold roofs" shows a lot of info, cheers

 

[Guitar_man]

Is this standard pratice on upper stories now when say putting up or replacing the ceilings, ie put a vapour barrier up first then plasterboard?
Can one substitute a foil backed plasterboard in this case instead?
Ta,

 

[tony1851]

Can one substitute a foil backed plasterboard in this case instead?

It helps a little, but vapour could still get up through the joints and it might still give problems above bathrooms and utilty rooms etc,
A poly-sheet with taped joints is best, though even that can be compromised by breaks for wiring and fixings etc.
All you can do is to try and keep the vapour out as well as you can.