Extract Fan Through Skylight

26 Apr 2010
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United Kingdom
I have a ground floor flat c1901 the bathroom of which is a more recent extension (1980s?). The bathroom is surrounded on all sides by other flats and my own and as such has a skylight rather than a window.

The skylight is raised above roof level and has aluminium ventilation flaps on all sides. A pole with hook is used to open the flaps. The problem is that the flaps no longer shut properly and even if they did the insulation value of the single glassed glass is poor. The bathroom is freezing and costing a fortune to heat and the skylight is an eye sore.

I have been advised that since the skylight is in a flat roof and does not leak its best left in place. I therefore plan to fit a 2nd sheet of frosted double glassed glass flush with the internal ceiling to make the room warmer and to hide the ugly skylight. Obviously i will need some form of ventilation so an extract fan is required. I plan to have the extract fan in the new glass panel pumping expelled air into the void above. I will jam the flaps in the open position to ensure the void is ventilated; an alternative idea is to run a duct from the extract fan up through the upper glass and fit a rain proof capping. The duct will need sealing to the upper sheet of glass to prevent rain water running in (it pools slightly on the upper sheet).

My questions are:

1) Will idea 1 meet with building regs and am i likely to experience a build up of mould in the void?
2) If idea 1 is out will i have problems with idea 2? Will sealing the duct to the glass be possible?
3) If both of the above are out what do i do?

thanks, James
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If you put a fan in the double glazed unit you are negating the insulation values of the glass. You are wasting your money.
I assume the glass can be made with a silver foam disk inside that the fan is then mounted into in much the same way as a cat flap.

If need be i will put in a single sheet of glass but the thrust of my question was less about the glass and more about venting into the void, is it a good idea?

Relace the rooflight with a double glazed jobby and trickle vents and insulate the flat roof while you're at it. You're just throwing good money after bad. Who said not to replace it? Your ideas are crackers.
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Replacing the whole skylight is a bit beyond my budget, access to the roof is restricted (through a window in the flat above me), and i do worry about disturbing the flat roof and causing a leak, i'd much rather just find the right extract fan solution.

I see ventaxia do a range of extract fans for glass which say they can go into double glassed units. I wonder about extracting through the first sheet of glass and ducting it out the ventilation flaps?
Well in the end I did it. I had a frosted double glased unit made with a hole for the extract fan (yes it was still sealed) and mounted this flush with the celing. I then ducted the extraced air through the void and out of one of the flaps. Seems to all work perfectly, the bathroom is toasty and the void stayed dry all winter. The only slight snag is cleaning the glass as debris does get blown in through the flaps. Total cost less than £200.

My idea may have been 'crackers' but it worked.

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