warm deck flat roof and toilet extractor fan

23 Jan 2009
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United Kingdom

I'm working on getting my cold deck flat roof converted to warm deck. Latest (and hopefully last) complication is there is currently an extractor fan in a toilet that vents into the cold flat roof void. I guess the neutral airflow up there then carries away the extracted air...

Anyway, with the conversion to warm deck, i believe i need to duct the air to outside. options seem to be either put a mushroom capped vent in the flat roof, or go between the joists and out through the wall/fascia.

Currently preferring the option of going through the fascia as it avoids having more seals on the roof. plus i want to put flat roof mount solar panels on, and it would be in the way.

what i don't understand is how does the insulation of the warm deck and this big vent hole work. I've seen on similar questions that the air inside the duct should also be at room temperature, but what stops it cooling at the vent/grill to outside? what stops the wind blowing down the ducting and therefore bringing cold air with it?

Advice needed please.
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Thanks Noseall. Thats useful for how to do a through the roof job. What about through the wall/fascia? Or are you suggesting the same solution applies?

What about the insulation aspect of it all?

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A warm deck principal ensures that all elements below the deck are (usually) at a higher temp than the outside.

By the short time it takes to pass through this barrier the moist air will be out into the atmosphere.

I'm not keen on fascia mounted vents but this is an option. Same principal applies to this method regards insulation.

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