Fixing Celotex and Plywood. Over or under?

If you want the ply to add strength, then it will probably do it best screwed to the rafters directly. Having said that, I am not sure you really need it at all, since it will not make much difference to the sound transmission in addition to the insulation and the plasterboard... (two layers of plasterboard might make more difference).

As for adding strength - depends a bit on what sort of strength you want to add, and why.

Actually, strength was a bit of an afterthought. I don't really need it for that. The main reason is soundproofing. The property is on the slipway of a major road and I want something a bit solid to reduce the noise of the thundering traffic, something more than just another layer of plasterboard.

In article , Rednadnerb writes

If you do the insulation right ie air tight, then the ply shouldn't add anything.

Doing it right means making sure there are no air gaps round the PIR foam boards between the rafters and sealing the smallest gap with gun foam.

Adding the extra layer under the rafters as you propose (no gaps and foil taped) should further help the isolation with 12mm PB finish being the icing on the cake.

Make sure there is no way for air to pass between the 2 layers of insulation as that is a way for air transmitted noise to pass around the first barrier (and negate heat insulation). You can do this by adding a fine bead of low expansion foam[1] to the face of the inner PIR boards at outer sheet boundaries ie where a sheet meets the eaves, gables or apex. Let the foam expand then squish it flat when fitting the inner PIR.

I don't think a layer of ply will give you much extra - unless you are using 18mm, and then its going to be expensive and probably no more effective than a second layer of PB.

The foam and a good air tight fit will knock out most of the high to mid frequencies. You then need mass to attenuate the LF sounds. PB is not so bad at that when supported across its face as yours will be on one side.

Hmm. Even though it was going to be 18mm I think I'll forget the idea of using ply and save myself a bit of money. If it still seems too noisy after 2 thicknesses of plasterboard then I might put a third one up.

Thanks to all.

Brendan.

Keep in mind how much weight you are adding to the rafters though - three sheets would be adding over 30kg/m^2

You can buy acoustic plasterboard, which is supposed to attenuate more sound. The sloping walls of the bedrooms in my dormer bungalow (on the front of the house, where the dormers aren't) are the rafters, which I battened out to six inches, insulated with Celotex, then boarded with one layer of acoustic 15mm plasterboard, and another of standard 12mm plasterboard, then had it skimmed. The road gets traffic at certain times of day, and you don't notice it at all in one of the bedrooms. The other has Velux windows and I think I have sound getting in via the window reveals. It's still not noisy. Obviously I don't know what difference it made using acoustic plasterboard rather than normal, but

Regards Richard