to insulated plaster board or not

willsrs

I am in the middle of constructing an extension to my home. It is a dormer style extension that matches original building.
It is a cavity block construction with aeroborard insulation in the cavity. I had intended also of putting insulated plasterboard on the internal blocks and skimming them.
Is this a waste of time and money or will I benifit from doing this...

Poor Uncle Tom

What thicknesses? What other insulations are you intending on putting in the floors and roof.

Specifics Bob, we need specifics.

willsrs

25mm insulation on back of board..already 50mm platinum aeroboard in cavity. 100mm platinum aerboard in rafters and 150mm fibre wool going into attic space. also 50mm kingspan under concrete floor...

Poor Uncle Tom

Assuming you have the necessary 50mm air gap on the outer side of the rafters, it is my opinion that you do not have enough insulation in the walls, rafters and attic.

I would double the quilt in the attic by running another layer of 150mm quilt over and at right angles to the first layer. I would also up the cavity width to 150mm and cavity insulation to 140mm and then put the insulated plasterboard on the inside giving a dry finish. Then I would put the thicker insulated plasterboard on the inside of the rafters.

Well, that's what I would do.

willsrs

Thanks for that uncle Tom!!

As the cavity is now not accessible I will continue with the original plan of using the insulated boards.

Any more advice would be welcome!

soldsold

Pump the remaining cavity with bead insulation. No question but I would do this. Then you "might" be able to use a drylining board but it would still be a close call as to whether you would get mould growth inside the drylined boards or not. You could use in theory 50mm polystyrene-backed drylined boards with 100mm polystyrene insulation in the cavity but probably safer to use a 25mm+12mm board (25mm insulation plus 12mm plasterboard).

The current insulation values in your wall/ floor/ roof do seem really low.

dfader

Poor Uncle Tom wrote: »

Assuming you have the necessary 50mm air gap on the outer side of the rafters, it is my opinion that you do not have enough insulation in the walls, rafters and attic.

I would double the quilt in the attic by running another layer of 150mm quilt over and at right angles to the first layer. I would also up the cavity width to 150mm and cavity insulation to 140mm and then put the insulated plasterboard on the inside giving a dry finish. Then I would put the thicker insulated plasterboard on the inside of the rafters.

Well, that's what I would do.

Uncle Tom,
would leaving just a 10mm cavity not endanger the cavity insulation becomming wet and therefore lessen its thermal performance?

Poor Uncle Tom

dfader wrote: »

Uncle Tom,
would leaving just a 10mm cavity not endanger the cavity insulation becomming wet and therefore lessen its thermal performance?

No, the insulation I am referring to is a solid cavity fill, the outer 10mm is fashioned as a water throw off from the insulation.