Warm Roofs and Sound / Heat trade off...

[paulandfrodo]

Hi,

I've done a warm roof. But, I think I've screwed up a little.

I've done 60mm over the joist and 60mm (the max) between the joist. All well and good.

I've suddenly realised how **** PIR is from an acoustic point of view.

Alas, Building Control are reluctant to allow me to throw some rock wool between the joists as well (I have 85mm void spare), as it would mean the insulation between the joists in greater than that over the joists. Which is a no-no.

As such, I'm going to go with double boarding of plasterboard to help.

Anyone got any other ideas of how to improve the acoustic dampening of the area - without loosing too much height ? Yes, I know... Can't have you cake and eat it !

 

[jeds]

No idea! Is this the same roof as last time?

//www.diynot.com/diy/threads/warm-roof-poor-acoustic-performance.464853/#post-3706371

 

[paulandfrodo]

Yes. But now I'm talking about sound proofing it.... It's all water tight and insulated within an inch of it's life. But I started a different thread, as it's a different conversation.

 

[endecotp]

Anyone got any other ideas of how to improve the acoustic dampening of the area - without loosing too much height ?

Same answers as last time.

 

[paulandfrodo]

Ah ok

At bare min I'm going to double board it. If I can I've used a sound deadening material :-

http://www.customaudiodesigns.co.uk/tecsound-acoustic-membrane.htm

That I've used before and it's stunning !! I'd only use it in one area, as it's expensive. I will also see if the building control officer will allow any leeway...

 

[Notabrickie]

Alas, Building Control are reluctant to allow me to throw some rock wool between the joists as well (I have 85mm void spare), as it would mean the insulation between the joists in greater than that over the joists. Which is a no-no.

I'm missing something. How on earth is that detrimental?

 

[JohnD]

it used to give pattern staining on the ceilings of houses, where dust and dirt stuck to the plaster under the cooler joists, but if you are insulating over the joists as well as between them, it seems most improbable that there will be a perceptible difference in temperature.

It might be entertaining to ask what regulation requires this, so that you can study it to understand the requirement.

 

[Notabrickie]

It might be entertaining to ask what regulation requires this, so that you can study it to understand the requirement.

What regulation requires this, so that I can study it to understand the requirement?

 

[JohnD]

Alas, Building Control are reluctant to allow me to throw some rock wool between the joists as well (I have 85mm void spare), as it would mean the insulation between the joists in greater than that over the joists. Which is a no-no.

I meant, of course, that he should ask whoever fed him the tale.

 

[^woody^]

it would mean the insulation between the joists in greater than that over the joists. Which is a no-no.

Why is it?

If the roof meets the minimum insulation requirement, then any extra is a bonus any it does not matter where it goes.

I tend to specify pir and rockwool where acoustic and thermal insulation is required. Pir to meet the thermal requirement in the minimum thickness, and then rockwool to deal with sound.

You can also use dense sheets (MDF) or fibre sheets to line the roof.

 

[chappers]

The issue with hybrid roofs is whether the roof deck can me maintained at the temperature sufficiently close to that of the air inside the building to prevent condensation. The main issue isn't so much the amount of insulation within the void, but more to do with having, enough over the deck to insulate it.
My personal opinion is that 60mm above the deck may not be enough, if it can't insulate the deck sufficiently from the cold on the outside. any air leaking through to the underside of the deck may not have enough heat driving it from inside of the room to warm the deck, as its is insulated by more insulation on the room side and moisture can condense between the deck and the inner insulation. This is the reason that the PIR manufacturers don't recommend hybrid type roofs. Obviously a VCL on the room side can prevent this but only if it is sufficiently intact with no holes for lights cables etc