Pitched Roof Thermal vs Acoustic insulation trade off

[Nye]

I think I'm going insane trying to understand how to layer up my refurbished vaulted ceiling. The issue I guess is I am trading off between these 3 things:

 

Acoustic performance (I have a train line at the back of the garden in a city)
Thermal performance (I will be using a heat pump so want to get the U value down below 0.18)
Ceiling height (Roof pitch is around 30 degrees and ive already lost some overhead height with wet underfloor heating)

I'm currently thinking along these lines (from outside to in):
 

1. Roof tile
2. 25 mm Battens

3. Breathable membrane draped 10mm

4. 15mm air gap in 175mm rafters at 400mm centres

5. 150mm rockwool slab insulation between rafters

6. Tecsound 10 mass loaded vinyl 5mm

7. Vapour barrier

8. 50mm PIR in an unbroken layer

9. 50mm battens, space between filled with 50mm rockwool slabs

10. 17mm resilient bars to create service layer, non ventilated air gap, and decoupled ceiling

11. 2 layers of acoustic plasterboard (25mm total)
12. Skim coat

That should give me a build up of 150mm and allow for recessed ceiling lights that won't interfere with the PIR layer. The tecsound further back will also counter act noise leaking through the ceiling lights I hope.

But All the U value calculators I have found are very limited, even within the manufacturers range of options. And I can't begin to calculate the acoustic performance. It feels like I'm throwing grass into the wind and hoping for the best. I spoke to a couple of architects who seem to have their preferred methods (usually double plasterboard and as much PIR as can fit).

Does anyone know a better way to do this ?

Thanks

[Russell griffiths]

In my opinion the tec sound and vcl are in the wrong place, I would have them on the room side of the pir. 

 

Have tec sound said this this is where it should go. 

 

I would also also look at the weight you are adding, have you had it calculated with two layers of 12.5 plasterboard. 

That is a huge weight. 

Edited January 20 by Russell griffiths

[Iceverge]

As I see it you can stop noise by:

 

1. Blocking airpaths for sound to travel. 

2. Decoupling 

3. Adding an absorbent layer to stop resonance.

4. Adding mass. 

 

There's no secret sauce to sound block plasterboard or mass loaded vinyl. Its just a trade off of mass Vs thickness Vs cost.  More standard plasterboard is probably more economical. 

 

What U value do you need? 

 

 

[Mike]

In terms of tools, I've used the BRE U-value calculator for many years, though it's not free. For sound transmission, AFMG SoundFlow may be helpful and is available on a free trial, though I've not used it.

 

Of course if you have any windows facing the noise, they are likely to be the weak point.

 

 

[Mike]

4 hours ago, Nye said:

That should give me a build up of 150mm and allow for recessed ceiling lights that won't interfere with the PIR layer. The tecsound further back will also counter act noise leaking through the ceiling lights I hope.

The best way of preventing sound leaking through recessed ceiling lights is... not to have recessed ceiling lights. Do something more creative instead. The second best way is to create a sealed box from one or two plasterboard / plywood for them to sit in, or to use a substantial commercial hood.

[Iceverge]

17 hours ago, Mike said:

course if you have any windows facing the noise, they are likely to be the weak point.

 

Stayed in an airport hotel recently. 

They had 2 windows. One at the a building external face and another at the room side. Dead silent. 

 

It wouldn't be an too expensive an option for domestic houses to put 2x double glazed windows in. It should be thermally magnificent too. 

[Nye]

Thanks all! Having read everyone's feed back I think I'm going to avoid recessed ceiling lights and move the vapour barrier closer to the interior.

So it might look more like this:

 

1. Roof tile
2. 25 mm Battens

3. Breathable membrane draped 10mm

4. 15mm air gap in 175mm rafters at 400mm centres

5. 150mm rockwool slab insulation between rafters

6. Tecsound 10 mass loaded vinyl 5mm

7. 50mm battens (perpendicular to rafters), space between filled with 50mm rockwool slabs

8. Vapour barrier

9. 50mm battens (perpendicular to last layer), space between filled with 50mm rockwool slabs

10. 17mm resilient bars (perpendicular to last layer) to create service layer, non ventilated air gap, and decoupled ceiling

11. 2 layers of acoustic plasterboard (25mm total)
12. Skim coat

One more question though - rafter specific insulation is incredibly expensive compared to normal rolls of insulation. Knauf have told me normal insulation is not suitable for rafters however I would have thought the counter battening on the underside will be enough to support any insulation above it? I understand why it might be needed on a roof where it is resting on the plasterboard but it shouldn't be needed here - am I correct?

[Iceverge]

I would be cautious of the mass loaded vinyl. It's a material of low permeability and as such you risk trapping moisture in your roof as you already have a vapour barrier in there.  At £25/m2 it's not cheap either. 

 

I would prefer to put a layer of 18mm OSB on top of the rafters. It's about the same mass/m2 but costs about £6/m2. It'd allow you to do a really top job here sealing any airpaths to prevent airborne noise entering in the first place. You'd need battens and counter battens to prevent pooling of any water that blows between the tiles. 

 

If you can get cellulose then it's got an installed density of 55kg/m3. Much better than mineral wool with a density of 10kg/m3 but more importantly it fills every single corner and really helps with airtightness too. 

 

Here's an idea at today's prices. As you can see insulation is also a very dear way to add mass. Plasterboard, Soundbloc and OSB are all about the same. 

 

 

How about the following buildup. 

 

Roof Tiles, 

Tile battens

Counter battens

Breather membrane.

18mm OSB sealed with flexible sealant at all joints. care taken at edges to do the same with appropriate noggins to block every airpath.

220mm Rafters with dense packed cellulose. 

A/T membrane. 

50mm battens at 600cc  with rockwool rwa45 infill at 90 deg to rafters. 

17mm Resilient bars at 90deg to battens. 

15mm plasterboard sealed with acoustic sealant 

15mm plasterboard sealed with acoustic sealant. No penetrations other than a wire for pendant lights sealed with acoustic sealant.  

Skim

 

 

 

 

It's 44.5kg/m2 excluding timber and skim and membranes and costs about £55.75/m2. 

 

With the same caveats your previous suggestion comes out at 36.6kg/m2 for a cost of £78.87/m2 for a U value of 0.15 W/mK.