Insulating Dormer Window with Celotex - Condensation probs

[DIYerHELP]

We are currently insulating our dormer window in our bathroom. We have removed the plasterboard on the external vertical timber frame and padded out the void with Celotex, there is no air gap.

Will this be ok or will condensation occur? Should I use another product to form a barrier before I plasterboard?

 

[Deluks]

Does the bathroom have an extractor fan?

Clue: the correct answer is Yes.

 

[DIYerHELP]

Hi, no the bathroom doesn't have an extractor fan. No way of getting one in there as it's a flat roof and tiled externally.

 

[JohnD]

1) you need the vapour barrier on the "warm" side of the insulation

2) your bathroom will be wet and mouldy if you do not install an extractor fan or leave the window open. If you mean you have a tile-hung wall instead of brickwork, you can still fit one but you may need a different kind of terminal to blend in. Is it red clay tiles or concrete?

p.s. I don't understand how you fit a dormer window in a flat roof.

 

[DIYerHELP]

Hi John D, would foil plasterboard work as a vapour barrier or would I need a product like Monarflex?

We have a large window and opener (1960's house) sorry when a say dormer I meant chalet style. The bathroom faces directly south if that help?

Concrete hung wall tiles. I appreciate all your help on this one!

 

[JohnD]

Foil backed is good, but there will be odd gaps. If the cold side is ventilated it will probably be OK.

 

[^woody^]

The foil on celotex will act as a vapour check, and so no additional one is required

 

[DIYerHELP]

Thanks Woody, I decided to add a polythene as a vapor control layer before fitting the foil plasterboard. Might be an extreme but better to be safe than sorry!