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Roofing
Insulating below a 'cold' flat roof and retaining the existing ceiling
Anonymous user 3 February 2015 - 11.37 AM
I want to Insulate below a flat roof without removing the existing ceiling. The existing ceiling is sound but provides no insulation. There is only a small gap between this ceiling and a concrete roof (cold roof, felt covered). I've built a timber frame to support a new plasterboard ceiling below the existing one. I intend to fill between my joists with foil backed insulation board (75mm) with a thin layer of insulation on top of the joists. I intend to place a vapour barrier between the new plasterboard and joists. I am concerned about condensation. There will be a gap (of at least 5cm) between to old ceiling and the new insulation and joists and ventilation to the outside. Do i need to take the old ceiling down? if i need to provide ventilation above this old ceiling to stop any condesation build up can i simply cut some holes in it to allow outside air to circulate? I would really like to keep the old ceiling in place and avoid the mess of rmoval if possible but am i creating a future problem?
3 answers from MyBuilder tradespeople
Good afternoon rob. It sounds like you have done you homework which Im glad to see and makes a refreshing change. As you know by insulating below the roof you move the dew point (the area were your warm moisture air of the inside meets the cold air of the outside) this is were any moisture will condensate. I would suggest that you try to ventilate above your insulation and below your existing celling to allow any moisture getting through your vapor barrier to dissipate. You should have gap of 50 mm to allow good cross flow ventilation. Cutting hole should work as long as they are at regular intervals and slow for cross flow. I hope this helps Regards Richard
Answered
4 February 2015
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You may be able to drill holes in the fascia to allow you to insert vents. The holes should be at both ends of each joists to allow cross flow. Its no good venting along the length of a joist as you would only be venting that small area. Ideally your vents should be within the 50mm gap. Theres lots of detail drawings on the net. Try the Government planning portal. Sometimes its easier and cheaper to install a warm roof. Especially if your flat roof needs replacing in the next few years....Hope this helps
Answered
5 February 2015
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Flat concrete roofs are generally not ventilated since the ceiling internally is generally a plaster and skim coat directly to the underside of the cold concrete. Creating a void between the old ceiling and new is better but ideally you need to eventually consider insulating the top of the concrete roof with solid insulation and a better covering (GRP or EPDM). Using the correct insulation and correct detailing / thickness is the only way of guaranteeing zero condensation. Regards Andrew @ Fibretechs - Gloucestershire
Answered
19 February 2015
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