Adding insulated plasterboard underneath flat roof

1 Oct 2004
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United Kingdom

We have a flat roofed kitchen extension, roughly 3M x 3.5M, that was built in 1980. It's by far the coldest part of the house so I'm wondering about trying to retain a bit of the heat by adding insulation.

We had it re-roofed last year (I wish I'd thought about this back then!) and there are some parts of the ceiling where nails have popped the plaster skim when being disturbed, so it needs something doing with it anyway. The roofer said it had a bit of fiberglass insulation in there, but not a full fill to leave some ventilation underneath the roofing boards. There are ventilation grilles at the ends of the roof so I'm guessing it's what you call a cold roof.

I was wondering about overboarding the ceiling with insulated plasterboard. This would tidy up the nail issue and hopefully help to keep the room warmer. But how thick do I go to make a worthwhile difference? I know it'll never be to current standards would 25mm (plus board thickness) be worth doing or is it just a waste of time unless I go much thicker?

I'd rather not pull the existing ceiling down but we can afford to lose a bit of height if need be.

Do I need to worry about condensation in the roof / vapour barriers etc?

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I'd use as much insulation as you can before the head height gets too low. You could also use separate insulation and plasterboards.

If you put 50mm below, taped the joints, then plasterboarded with 100mm drywall screws I think you would save compared to the 62.5mm insulated boards and have a better result.
Hadn't thought about separate, will have a look at prices thanks.

I assume I could just glue the insulation up temporarily to make life easier, rather than wrestling two layers of product at once? Screws obviously then going through both for a permanent fix.
You would screw it up with some of these 35mm washers:

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