Loft conversion - Sound insulation after job 'finished'

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I have just had a loft conversion done, with celotex insulation / single layer plaster board & skim to the walls.

Unfortunately the road noise from outside is making the room unusable as a bedroom. Clearly I need to add some 'bulk' to the walls, but firstly I need to minimise the thickness as the slope of the roof makes any increase in wall really affect the headroom, and secondly i need to work out if i'm better getting the old (still very new!) wall taken down - or build out what's there.

Any suggestions VERY gratefully recieved as I currently feel like i have a 20k white elephant!

Cheers :(
 
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The windows and any air/trickle vents will be the weak spots.
If the noise is still bad with the windows and vents shut, I'd investigate further. Maybe there are soffit or roof vents which are letting sound into the floorspace. Although that should really be insulated as well.

Adding another layer of plasterboard is the only sensible choice if you want to beef up the walls themselves.

Do you know what thickness current plasterboard is? Are you in a bungalow?
 
Thanks for that - no, we're not in a bungalow, but are very close to a busy road.

There are (wooden) soffits holding up the guttering which are fairly old looking. I'm not sure how to improve their sound proofing (it's an old cottage, so plastic soffits might not look so hot...), or if this is worth getting done before the re-do of the walls - from a cost/improvement point of view.

The roof basically has celotex insulation and then the 9mm plasterboard.

We have had secondary glazing put inside the velux, but the velux are those conservation type which rise above the roof line so undoubtedly add more noise.

I think the main culprit is the walls as they feel like a drum skin when lorries go past!
 
Plasterboard should've been 12.5mm at least. I'd add another layer to the existing, using 12.5mm, then get it reskimmed, or taped/jointed might be cheaper.

Should've been specced properly in the first place really.

The window upstands could be beefed up also. Could use thick timber boards or plasterboard. Will need a vapour check probably.

Wooden soffits without vents fitted should be ok. It's the plastic that isn't very soundproof.


From Velux site:

Perhaps the most significant development with the VELUX Conservation Roof Window System is background ventilation. With traditional skylights and rooflights, it just isn't possible to comply with building regulations for background ventilation in habitable rooms. Naturally that places all kinds of restrictions on what you can then achieve with the roof space. But because our Conservation Roof Window System has a ventilation bar which allows for background ventilation when the window is closed, you can create truly habitable rooms in the roof space.

Sound travels through even small airgaps with the greatest of ease.
 
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That's really helpful - thanks. I guess my decision is how far to go down the 'throw money' at the problem. ie: if i'm doing to get another layer of 12mm board / skim added, do i add a rubber acoustic membrane between them?

Great detective work about the velux - i'll hunt those vent holed down!
 
That's really helpful - thanks. I guess my decision is how far to go down the 'throw money' at the problem. ie: if i'm doing to get another layer of 12mm board / skim added, do i add a rubber acoustic membrane between them?

Great detective work about the velux - i'll hunt those vent holed down!

Membrane? Waste of money IMO.

........

Don't forget that vents are there for a reason. If making the room airtight to keep the sound out, don't forget to re-open them/open a window when it's quiet or the room is unoccupied.
 

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