Flexible Rockwool Roof Insulation

I'm currently having a job done on my roof. The inside of the attic has been converted to a bedroom/en-suite and has a plasterboard ceiling. The rafters are 4 inch and there isn't any insulation behind the ceiling.

The builder is taking off the roof tiles to replace the felt. He has suggested that we put in 4 inch flexible rockwool insulation and use breathable felt over it.

Is this the best way to go? He said that it would be difficult to cut rigid insulation properly.

not the best way to go no... as it may expand and blaock your 4 inchs and cause damp issues.

make sure you leav a minimum 25mm gap that gives you 75mm to play with try somthing like Rafter lock it bends in place rather than cutting.. cutting insulation is hard and needs doing properly or you are wasting your time....

after that you must put a minimum of 25mm insulatated plasterboard across the rafters this will give you great insulation and stop the cold going through the wood... bridge it.

Thanks, Bluemac. I was aware of leaving a space for air circulation/ventilation to prevent damp but the builder reckons that the breathable felt would take care of that.

Have you any idea how 4 inches of rockwool would compare to rigid insulation (phenolic, etc.) when it comes to insulating properties?

+ 1 AFAIK with breathable felt you do not need an air gap, but I would use a polyurethane insulation board for its much higher U value : [broken link removed]

It would be much easer for the builder to use rockwool than do a good job with a rigid board, so that is what he wants to use. I would keep a good eye on the job, because if he leaves any gaps it is a waste of time.

Thanks, Towger. Excellent link.

rockwool

have been advised to use rockwool as insulation in my new build as its cheaper than greener alternatives and has good u-values. was also advised to use something called rocksil externally - seems to be a solid but flexible form of rockwool. anyone know anything about this, please?

feileacan said:

have been advised to use rockwool as insulation in my new build as its cheaper than greener alternatives and has good u-values. was also advised to use something called rocksil externally - seems to be a solid but flexible form of rockwool. anyone know anything about this, please?

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Hi Feileacan. When you say you've been advised to use it externally, do you mean as a form of external wall insulation for your house?

rockwool insulation

yes, meb, advised to use it as a sort of cladding under the galvanised mesh which will then be plastered. not sure of spelling - rocksill, rocksil, or rocksilk? - but it comes in panels which are thin enough to be flexible .

In my opinion rockwool flexi slab would be a better job than using a rigid board insualtion in between your rafters as it is nearly impossible to cut the rigid boards into the space between the rafters without leaving gaps and once there is a gap there goes your u-value out the window. The rockwool flexi slab will give you a snug fit with no gaps and is easier to fit and quicker.

On site application must be taken into consideration when appying a product no u-values. Another good product is the Xtratherm rafterloc. this is a rigid insulation but will allow compression in between rafters.

All these products can be viewed on the manufacturers websites our at build4less where you will also see prices as well.

As for Rocksilk on the outside of a building be very carefull it is not something I would do but again that is myn opinion. I stick to tried and tested methods. This is not tried and tested and could give problems in 10 years time who knows?