Ask a tradesperson


Which is the best roof insulation option for me? cellotex ontop of rafters vs rockwool between?

Hi, hoping someone with experience of this can give me their opinion.

I'm putting up a new bungalow with vaulted ceilings and slate roof. and need to decide on the roof insulation before submitting to building regs.

The architect has said either 100mm PIR/CELLOTEX ontop of the rafters with battons/slates on top of that...

Or 200mm rockwool between the rafters and no PIR ontop.

Alternatively can have a thinner PIR such as 25/50mm plus a correlating amount of rockwool between the rafters to reach the needed u value

My understanding is that the PIR/CELLOTEX will be easier to fit and probably provide better cold bridging between the rafters.

I'm aware the cellotex will be the more expensive option over entirely rockwool

My main concern is I cannot find any info on how long the PIR board will last. Inevitably it will degrade but will that be in 30 years or 100 years? As it will be a structural part of the roof if fitted ontop on the rafters with battons and slates ontop,

Any opinions for pros and cons of either option would be appreciated.
Thanks in advance.

2 Answers from MyBuilder Roofers

Best Answer

no where do you mention any air gap for ventilation to prevent the build up of condensation & stop dampness in your new bungalow. an if this is your architects advice personally i would change company's


Answered 17th Sep 2020

Your architect should advise you on this. He knows the up to date building regs and the cheapest options. We normally cut the board to fit between the rafters then foam and tape all joints. Most suppliers have their own brands so doesn’t need to be a branded name like Velo Rex..At the Apex if you have a horizontal ceiling section use rock wool Cross laid.


Answered 12th Oct 2020

Post your job to find high quality tradespeople and get free quotes

Can’t find an answer? Ask a new question

Question Categories