Insulating slopping roof - who has done it?

5 Jul 2006
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United Kingdom
I know this has been covered many times on here and I have a pretty good idea of my options.

I have a bathroom in a Victorian terrace that is located in the rear extension, the roof is pitch but internally it has a half slopping roof. So from inside the ceiling boards are nailed directly to the roof rafters, but half way up this levels off to leave a flat portion of the ceiling. this creates a loft space above which I can just about squeeze into from the main attic.

I have just discovered that there is no insulation in the part of the roof and I would like to add something. I will lay standard Knauf 200mm insulation on the flat section which is about 50% of the space, but the question how to insulate the slopping section.

the slopping section is only around meter long and I plan to slide Celotex 50mm boards into the gap between the Ceiling and felt/tiles above. the membrane is old none breathable style and I have no vents in the eves or elsewhere. this roof space is 100mm so there will be a 50mm air gap.

so my question is has anyone else done something similar and since had problems with condensation? my only concern is that the roof space isn't vented. the fact that I will be leaving 50mm air gap with the large air gap above the flat section of the roof I would hope still provide the same amount of ventilation as previously. in other words will reducing the 100mm air gap to 50mm be an issue?

any feedback is appropriated.
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It will largely be a wasted effort, tbh. There's no way you'll get an adequate airtight seal between the celotex's and the plasterboard, so rather pointless. Use a magnet to mark out where the existing fixings are then fix a new bit of insulation backed plasterboard to the sloping section, plaster and paint it. Alternatively, remove the plasterboard from the slopi section, fit celotex's from below, using expanding foam to seal it in place, then reinstate he ceiling, either with normal or (better) insulation backed plasterboard, and make good
Just to add to the above; if using insulated plasterboard, use the type with an integral vapour barrier eg Kooltherm. Don't use polystyrene-backed board - a client of mine used that as a cheaper alternative and paid the price when winter came along.

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