Flat roof insulation

[stuart.macintyre@hot]

Hi, any advice would be great.

We had a leak coming in our bathroom that sits inside the rear dormer under a flat roof the roof, felt has been replaced & leak was sorted.

I now need to replace the bathroom ceiling, it's a small room 2x2.5 m, if it was a bigger room I would get a contractor in. I would like to improve the insulation.

The house is late 60s to early 70s, Chalet style semi-detached with full length from and rear dormers. About half of the ceiling of the upstairs rooms 3 x bedrooms and a bathroom is under the flat roof, the rest is under the loft space.

I have concerns about if this is a cold or warm roof design.

Above the bathroom ceiling, there is an empty 120mm void between the joists/rafters. Above the joists, there is a layer of fibre insulation (1-2 cm thick) encased in weed-control-type material or maybe its vapour barrier. Directly above this is the flat roof deck. There are no obvious vents.

Can I fill this 120mm void ( leaving now empty space) between the ceiling and the 1-2 cm thick insulation under the deck with insulation boards then fit the new plasterboard?

Should I fit external vents and use 70mm of insulation board, leaving a 50mm void that can vent into loft space, then replace the plasterboard?

Should I use wool-based insulation?

Anything else i should do?

 

[Bonni]

The hemp panels, probs about 50mm thick, was the old way of doing a warm roof. Now, the insulation has to be 120mm on more Kingspan insulated boards under current regs, that gives you much bigger facia boards. So you could make it a cold roof but then ventilation needs added and I'm not sure if the hemp boards should be left in place, possibly though. You don't have insulation above a cold roof so I'm not sure about condensation. I tend to steer clear of flat roofs. If you want peace of mind, just get local buildings for it and quiz the inspector for guidance.

 

[Alastairreid]

Full fill the void with rigid insulation and tape the joins with foil tape..use foil backed plasterboard.
Steer clear of using downlights to prevent piercing the ceiling.
Your guy should have created a warm roof whilst replacing the felt.

 

[stuart.macintyre@hot]

Thanks for the replies. An insulation supply company has advised that this is a cold roof due to the insulation being under the flat roof. The current insulation that is at the top of the void, under the deck is 1-2cm thick, its a hemp roll, not a board. The advice I got was to remove the current insulation then fit foil-backed PIR above the ceiling leaving a 50mm gap/void to the deck and to add ventilation to the external.

If I remove the old insulation that would leave a void of 130mm. So 80 mm PIR board would leave the required 50mm. Then I could fit a PIR-backed plasterboard to replace the ceiling.

The above is kinda in line with what Bonni advised. or could I do what Alastairreid suggested (this was what I was originally thinking) and fill the new void with 130mm foil-backed PIR board?

Just not convinced that insulating from below makes this a warm-roof design as most of what I read says that warm roof insulation needs to be above the deck, not below.