We plan to insulate the loft at rafter level with Celotex between the
rafters (allowing an air gap). Then a layer of ply screwed to the
rafters (mostly for sound insulation but also to strengthen the roof
and give us something to screw fittings to). Then another layer of
25mm or 50mm Celotex and a layer of plasterboard on top of that.
I can't decide if we should fix the ply to the rafters directly or on
top of the second layer of Celotex. Any advantages to doing it one way
over the other that anyone can think of?
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.
If you do the insulation right ie air tight, then the ply shouldn't add
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
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 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
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
Thanks to all.
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
2 sheets of normal would be better than one. Of course, the Celotex
itself provides no sound insulation.
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