Roof insulation question

[Welderpaul]

I'm doing a shopping list for materials.. As per Engineer's details, the existing rafters are sistered with 38x125 C16s with 38mm insulated platerboard across the face. This is giving me a 125mm or so gap behind (Sarking felt and pantile roof covering)
An air gap to the underside of the felt will be a good idea, so any issue with just sticking 100mm rigid Knauf/Celotex/Kingspan board in there?

 

[angellonewolf]

not sure but from memory yearsago i thought there should be a 50mm gap for air flow

im sure some one @roofman or @gaz1

 

[gaz1]

this up to you paul i dont like the stuff but for speed

insulation board is faster to install however its not recycleable

fibreglass insulation is recycleable just harder to install

its not known yet but i suspect many will find an issue with there joists rotting as insulation board dosnt breathe any water gets through and it will run down the boards

the insulation board is just flamable but can still set afire if it does it will burn and smoke like a git

fibreglass insulation dosnt set afire and will protect really good

post 9 if you want to take a look which roof survives within a fire take a good look at that black acrid smoke from the insulation board it does set afire once past flashover point

Thread 'Insulation'

Nov 30, 2022

Hi chaps ,
I have loft insulation , also cavity wall insulation , the tops of the walls above windows and corners of outside walls at top are cold enough to cause condensation on wet days esp when humidity is high , Q could I use cavity wall batts to bulk up the insulation under the eaves ? Iwas thinking of 100mm rockwool , the semi ridgid nature I'm hoping would aid installation ,doing it myself , it is a 70s built detached bungalow in Cornwall so damp climate .
Cheers

• rosinante
• Replies: 50
• forum: The Brew Room

 

[gaz1]

new build same thing

check your insulation info as you might be under the regs

this build was installed with 4" insulation board and then another 2" overlapping the roof rafters as well for a total of 6" insulation

 

[Bobtail4x4]

if its a new build you need more insulation to meet the regs,
can you got for 75mm between rafters and 75mm ins plbd
make sure you use a decent breathable felt,the cheap stuff fails in cold weather,

 

[addjunkie]

if its a new build you need more insulation to meet the regs,
can you got for 75mm between rafters and 75mm ins plbd
make sure you use a decent breathable felt,the cheap stuff fails in cold weather,

Id agree here, you can get 80 or 90mm kingspan or increase the insulated board.

 

[Welderpaul]

This is a retro fit so roof is existing. The thicker the insulated plasterboard, the more headroom I loose. Having said that the job needs to be right!

 

[gaz1]

if its an existing rafters insulation fit out, then fit the best insulation that you can paul

so if your happy with 4" then fit that if you want 5" insulation and plasterboard your only losing 38mm off the sides

 

[addjunkie]

An inch air gap in my roof…but it was new and membrane was tight and not sagging