firewall construction

[CharlotteJ]

Hi, we are converting a large building into 2 dwellings - we need an internal firewall to separate them.  From various builders advice and the internet, we are thinking a stud wall using 6"x 2" timber, filled with either rockwool or rigid celatex type insulation and boarded both sides with pink plasterboard, then skimmed.  This will continue up into the roof space.    This will all be approved by our very helpful building inspector before installation, but I would like to have something to show her. Any comments, advice or additional information is very welcome. Thank you.

[kandgmitchell]

You'll need sound insulation as well so the celotex wouldn't help with that. I'd refer to specifications in British Gypsum's White Book or Knauf details. Find one that suits the situation and follow it to the letter. Give the BCO the spec and that should put their mind at rest.

[CharlotteJ]

20 hours ago, kandgmitchell said:

You'll need sound insulation as well so the celotex wouldn't help with that. I'd refer to specifications in British Gypsum's White Book or Knauf details. Find one that suits the situation and follow it to the letter. Give the BCO the spec and that should put their mind at rest.

Thank you, I will check out the guides that you mentioned.

[Russell griffiths]

Are these to sell both, or will you be living in one. 
i would want something far more robust if I was living in one of them, a stud wall with the correct boarding will meet the regs, but i would want something better. 
 

as mentioned above sound transfer will be very relevant as well. 

[saveasteading]

21 hours ago, kandgmitchell said:

Find one that suits the situation and follow it to the letter.

For fire this is good advice. For sound, go to the next level, because the tests are done in labs with perfect construction and no flanking effects.

 

[CharlotteJ]

Thanks so much for all your advice. On talking with BC today, the firewall also needs to be 'structural' although not load bearing (single level building). So a 100mm solid concrete block wall is suggested as being appropriate for fire rating - 60 mins required - I need to check if it will give adequate sound insulation but would appreciate your advice and any opinions on this.

[Russell griffiths]

A block wall will give next to no sound insulation 

you will end up building 3 walls, block in the centre and two skins of some sort for sound and insulation properties. 

[Mr Punter]

You could do 100mm blockwork in the middle, resilient bar and 2 x 15mm DB plasterboard either side.  About 200mm total.  You may not need thermal insulation if both sides are normally heated.

[saveasteading]

2 hours ago, CharlotteJ said:

the firewall also needs to be 'structural'

Meaning, it stays in place even if one side of it collapses in fire. It can be in stud as long as it isn't going to be dragged down by a collapsing floor.

That is doable in stud.

[CharlotteJ]

Thanks everyone.  Would stud wall with insulation, ply both sides (this makes it structural according to BC) then boarded each side be suitable? - I could then find out the stud wall thickness and what insulation and plasterboard to use in order to meet the soundproofing requirements. 

[ADLIan]

You may have to build a twin stud wall - basically 2 stud walls back to back. Fully fill all cavities with mineral wool (at least 18kg/m3) to give zero U-value and required acoustic performance. See Appr Doc E of the Building Reg which gives typical details for new build separating walls. The acoustic standard for conversions is slightly relaxed compared to new build but Appr Doc E will give you a starting point. I believe these types of wall need acoustic testing on completion but check with our BCO.

[saveasteading]

A double stud wall is my favourite too. This will be more stable and good for sound reduction. It should not need plywood. 

This will all be published in the manufacturers catalogues. Knauff etc, fire, sound, stability.

But it sounds as if this is yourself and bco as designers, so I think I will leave you to it from now. 

[Nickfromwales]

In flats above shops near me, I noticed the wall build up included a 25mm acoustic plasterboard sandwiched between the dividing stud walls, 150mm studs full filled with acoustic insulation (does heat & sound then) and then 2x 15mm FR plasterboard plus skim & paint. 
 

Seemed a very good solution, studs at 400mm OC iirc. 

[Gordo]

I would recommend 215mm solid common concrete block wall. 8 hours fire resistant and sound testing always works good (better than a cavity wall. Because there is no vavity you will not need to consider insulation to avoid thermal by pass. You will have to provide some returns to buttress the wall and limit flanking sound. otherwise it would be too slender and unstable, particularly in the event of a fire next door when everything starts collapsing. Structural timbers should not be built into separating walls to for sound resistance.

 

If you want a stud wall it will generally have to be double stud construction with mineral wool insulation and two layers of 12.5mm plasterboard to each side. this stud wall will also require buttressing to be stable.

 

The separating wall needs to extend to roof level and be fire stopped over to underside of cladding