Flat roof insulation question

dwiseman

Hi I'm just looking for some advice on insulating a rear flat roof extension. The flat roof has a ply wood sheeting and 100mls of insulation with a silka trokal membrane covering.

I'm just wondering what to do about insulating internally? I was thinking of using an insulated plasterboard and some insulation between the rafters. So I'm just wondering how thick the insulation should be andwhat to use between the rafters which are 9 by 2.

I don't want to break the bank but I do want to try and insulate my wallet against future heating bills so to speak!

All suggestions gladly received!

sinnerboy

I assume the 100mm insulation is a Kingspan or similar PIR board insulation. If so you have a good U Value already ( about 0.2 w/m2K)

And adding any insulation below the plywood now risks interstitial condensation - by adding insulation below this board you drop the surface temperature of it. Condensation occurs when warm air meets low surface temperatures.

So have your boiler serviced and/or replace all your bulbs to low energy. Your flat roof is fine.

dwiseman

Thanks for the reply, appreciated. It is kingspan board that I used alright, do I take it from your reply that I cant even pit an insulated slab on the ceiling for risk of condensation?

thanks.

sinnerboy

dwiseman wrote: »

Thanks for the reply, appreciated. It is kingspan board that I used alright, do I take it from your reply that I cant even pit an insulated slab on the ceiling for risk of condensation?

thanks.

I wouldn't. Take the better half out for dinner instead .

lennyloulou

hi, dont mean to capture this thread but we are looking into the same - we have a large flat roof- covers 4 rooms ( an extension built onto cottage in the 80s) anyhow the roof is torched on felt, wooden rafters, fibreglass- think the silver stuff painted on the outside is flashing? anyhow it is on a getle slope so rain drains off, no leaks at all- owner put this roof on 1995.
It has never given problems. i know the oqwner always said it was important to let the flat roof breathe- allow some air to circulate.
now here is my question!
we wish to externally insulate the house and flat roof and improve our BER rating.
we dread having to rip up the flat roof and replace it all. it definitely is not leaking and it is not Kingspan insulation by the way in the roof. . ... so can we stick on an outside thermal insulation sheet onto the flat roof????? Is there such a thing and if so is this crazy dear?

sinnerboy

It sounds like you have what is termed a ventilated cold roof - meaning the plywood deck - the part you walk on - is not insulated. It also sounds like there is a ventilated cavity below the plywood and above the insulation.

It is possible but not advisable to place additional insulation over the top of the existing felted flat roof and then apply new felt over that.

But the same risk of interstitial condensation arises as per post no 2 - you have simply came at the issue "in reverse" to the OP.

lennyloulou

thank you for the info. should I just leave it alone so in your opinion?

sinnerboy

Yes.