Multiple layers of insulation to insulate flat roof?

[Tim B]

I have some left over insulation from some other work and a ceiling below a flat roof which has been removed to take out the asbestos.

 

I have enough insulation to put a layer of 25mm kingspan and then a layer of 50mm kingspan. Am I right in thinking that the best way to do this is to have air gaps either side of each layer of insulation, rather than, say pushing it all up to the top in one 75mm layer? So that the reflective foil can do its job.

 

Also, if I were to put the 50mm in between the joists and then have the 25mm under the joists would the plasterer I get round to put the ceiling back think I was mad, or (assuming I mark where the joists are) would he be fine to put the plasterboard on and use 25mm longer screws than if it wasn't there?

 

Thanks,

Tim

[Iceverge]

What is your total roof build up. Primary consideration is not to rot the roof.  You don't want to trap moisture in there with the PIR as it is impermeable. 

 

As far as i know PIR under the rafters is a common detail but the roof must be able to dry upwards and outwards. 

[Temp]

Make sure you know if its a warm or cold roof construction. Details here..

 

https://www.tendringdc.gov.uk/sites/default/files/documents/planning/building control/Flat roof guide.pdf

 

 

 

[Tim B]

It is a warm roof, so as I understand it condensation on the under side should not be a problem?

[Temp]

5 hours ago, Tim B said:

It is a warm roof, so as I understand it condensation on the under side should not be a problem?

 

If its a warm roof (insulation above the rafters) then you shouldn't add too much more insulation on the underside. The thinking is that there will be a thermal gradient from hot inside to cold outside and that If you put a lot of insulation on the underside as well you effectively move the structural parts of the roof towards the outside temperature increasing the risk of condensation forming. 

 

I believe the general rule is no more than 1/3rd of the insulation should be below or between the rafters with 2/3rds above. So if you had 100mm of insulation above the rafters then don't put more than another 50mm below. 

 

Also worth adding a vapour barrier before plaster boarding to reduce risk further but this may have an implication for downlights (ideally they shouldn't penetrate it).

[Mr Punter]

8 hours ago, Tim B said:

It is a warm roof, so as I understand it condensation on the under side should not be a problem?

 

I doubt if it is.  A warm roof has all the insulation on top of the joists.  Warm roof construction would not coincide with asbestos use.

 

On 24/08/2021 at 13:24, Tim B said:

Also, if I were to put the 50mm in between the joists and then have the 25mm under the joists would the plasterer I get round to put the ceiling back think I was mad, or (assuming I mark where the joists are) would he be fine to put the plasterboard on and use 25mm longer screws than if it wasn't there?

 

This is fine.  Not as much insulation as ideal, but still better than nothing.  You need to make sure there is good ventilation under the roof deck.

[Dutch]

Can you put all the insulation externally say 300mm, 2 sheets of 150mm cellotex ? So inside you just run the power cables under the plasterboard, plaster the ceiling.