is insulation between rafters essential?

[phil24w]

Hi, I am currently building an extension approx 40m2. Coming to plaster boarding the upstairs ceiling and it has been suggested I roll the blanket insulation in two directions over the top of the rafters rather than between, then apply plaster board, easier and less hassle.
There will obviously be a well insulated cavity between rafters. Does anyone see a problem with this or is it the norm with new builds?

 

[noseall]

Rafters? I think you mean joists.

The insulation is best fitted snug against the foil backed plasterboard, i.e. between the joists. Also, by fitting it between joists allows you to push the ceiling insulation tight up against the cavity wall insulation.

The cavity wall insulation should have been left protruding past the wall plate. If it isn't then you need to tuck some down prior to fitting the ceilings.

Also, don't forget to make provision for airflow, i.e. via eaves ventilators.

What do you mean by "a well insulated cavity between rafters"?

 

[phil24w]

Thanks noseall I meant ceiling joists not rafters.

Re: well insulated cavity in between the joists, means between the plaster board and insulation there will be a cavity/void the same depth as the joists and unfilled if I lay all of the insulation above the joists as mentioned. As trapped air is the mechanism of loft insulation then what will be the advantage of wedging it between the joist if I am still laying 250mm + above this void and up to the internal wall face.

Re: wall cavity, the partial fill insulation in the walls has been capped off and tucking blanket insulation over the wall plate would surely prevent the air flowing from the eaves.

Cheers Phil.

 

[eddiespaghetti]

I suspect the reason this has been suggested is to prevent conductive 'thermal bridging' between the outside, through the joists to the plasterboard.

To cut a very long explanation short: Essentially heat will always take the path of least resistance. In conductive terms, this is going to be the wooden joists, hence putting a layer of insulation between your heated space and the outside.

It is considered 'best practice' and in future will become more common. Currently, probably not that common.