Advance framing - double stud wall

[jen and mark]

I wonder if someone familiar with stick build construction could help me out with a bit of design. Originally, we were going to build a twin concrete block wall to two extensions on a steel barn conversion. Due to lack of brickies, I have decided to stick build my two single story extensions. The build up is 450mm x 1m deep trench fill and two courses of block with 100mm cavity. I am now building a double stud wall (each wall off each block) in 95mm x 45mm C24 timber with 600 centres. The outside of the inner stud (weight bearing) is sheathed 11mm OSB. The outside of the outer stud wall will also have OSB and then Tyvek house wrap, batten and steel cladding. The inner stud wall (warm side) will have services running through, vapour control layer, plaster board.

 

My questions are:

1. Do I need to add Tyvek House wrap to the cold side of the inner stud wall or just leave the OSB as it is?

2. Insulation - my thinking is something like Kingspan 80mm K112 between the studs and also full fill the cavity between the studs with something like Rockwool so there would be no thermal break.

 

Any advice would be appreciated

 

 

[tonyshouse]

Stagger the studs, mitigate bridging effect of soleplate/bottom rail of stud walls.

 

why two different insulation types

 

likely insurance will be tricky as full fill double stud is innovative. 
 

 

 

 

[jen and mark]

As we have an empty cavity (should have been double block wall) we thought we should fill it with something cheaper than PIR and there would be no thermal bridging at all as the insulation would go below the sole plate of both stud walls and all around the middle of the 2 stud walls.

 

It would only be the outer stud wall that would have PIR between the studs - limited to 95mm max. Do you think filling the cavity with PIR would be better? We are trying to keep costs down. Although it feels like we are building 2 walls and probably should have gone for a single 6 x 2 single stud instead!

 

The inner stud will carry the services and vapour barrier.

 

Insurance will always be at a premium for anything other than the norm! We are a double steel agricultural barn conversion. We have a real mish mash of a build due to the limitations of class Q barn conversion. Our main walls are 4ft high block with timber studs above, 25mm cavity and then a 2x4 inner stud wall with PIR between studs ( no noggins  to minimise thermal breaks) faced in a further layer of insulation with no thermal breaks. Outside Clad in house wrap, batten and steel sheeting.

 

We had to enclose all the steel stanchions with insulation to prevent condensation - a new build seems a dream compared to a steel barn conversion ?

 

[SimonD]

54 minutes ago, tonyshouse said:

as full fill double stud is innovative.

 Only in the UK, or maybe it's just England? Elsewhere they've been doing it since the early 70s and maybe even earlier ? But I do know what you mean. I had to deal with probing questions from building control re why I was wrapping the outside of my tf with additional insulation.

 

31 minutes ago, jen and mark said:

Although it feels like we are building 2 walls and probably should have gone for a single 6 x 2 single stud instead!

 

From an energy perspective the double wall is better. a 6 x 2 does gives you a lot of repeating cold bridges.

 

I reckon you've got 2 choices.

 

1 is to continue with what you're planning, so basically two timber frame walls decoupled and with this you don't need Tyvek on the outside of the inner wall. I would suggest adding a service cavity to the inside of the inner wall as it simply protects the vcl and you then don't have to worry about your services, wiring and boxes etc. having to be sealed up with your vcl.

 

2 is to build a full fill stud wall which could use a wider piece of ply or carcassing timber as sole and top plate and then fill the whole thing with either pumped cellulose, mineral wool or something else of choice. This would be like building a very thick single wall. This is similar to what you've suggested as an option but with a slightly different build approach.

 

Like @tonyshouse I'm wondering why the different types of insulation, especially giving the total thickness of the wall you've got to play with.

[SteamyTea]

Can you use cellulose insulation?

[Iceverge]

What stage of construction are you at?

Some photos would help. 

 

I really like double stud walls but like you say it's like building two houses. Concrete blocks are very cheap in comparison to timber. You could make a jig for setting the blocks if you are a competent chippie. Eps bead insulation and a layer of sand cement plaster inside. You'll have a cheap wall. 

 

PIR has no place in a timberframe wall. I maintain board insulation should only be installed on a flat surface like floor slabs, ewi or warm roofs. 

 

However send a pic or a drawing and it'll paint a thousand words . ☺️

 

 

[SteamyTea]

7 hours ago, Iceverge said:

PIR has no place in a timberframe wall.

For what reasons?

[Iceverge]

It's PITA to fit 100%. More often than not it's not done perfectly leading to thermal bypass. 

 

It's expensive. 

 

Poor decrement delay. 

 

 

 

 

Edited October 22, 2021 by Iceverge

[jen and mark]

[jen and mark]

How do I upload more photos? 

[markc]

Click to choose files at bottom of box then select pics from your library

[jen and mark]

Tried that - it says 'you are only allowed to upload 5.86MB'?

[markc]

Just now, jen and mark said:

Tried that - it says 'you are only allowed to upload 5.86MB'?

Ahh, I never use hi res for pics so never have a file size problem.

[Iceverge]

Thanks for the pic, 

 

How wide is the cavity shown and I'll try to determine a cheap solution. 

 

 

[jen and mark]

I'll try one at a time!

[Iceverge]

150mm? 

[jen and mark]

Cavity ranges from 100mm - 120mm and outer stud would be 2x4

[jen and mark]

sorry, 100 - 110mm

 

[jen and mark]

Inside stud wall for services

[jen and mark]

Inside main barn showing outer block and stud wall and 25mm cavity then inner stud wall just being built. 100mm Ravatherm insulation below 1st sole plate and 2nd sole plate on top

[Iceverge]

On 21/10/2021 at 21:31, jen and mark said:

We are trying to keep costs down

 

How important is this? 

 

What level of thermal performance do you need?

 

Do you have a list of materials you have already bought and are committed to using? 

 

You're clearly able to do and are doing a lot of work on site yourselves. Is this coming at a cost or are you fortunate enough to be in a position that your time isn't an issue? 

 

Sorry for all the questions. I'm just trying to come up with a balance of what I think is best. Cost vs performance vs time. 

 

 

[Iceverge]

Oh also are you using a nail gun? 

[jen and mark]

You are so right in that we are doing all of the work ourselves and cost is more of an issue now than ever. When we costed the work (May 2020) we hoped for more of a downward trend on material prices post Covid - and how wrong where we!

 

 Also, to top it all off, our designer was diagnosed with cancer (really sad as lovely bloke) so we are now building through a building notice rather than full building plans. We need to meet building regs. We have 0.15 for roof (using Kingspan composite steel panels) and would like to achieve a minimum of 0.21 for walls.

 

We are a class Q steel barn conversion with no nail gun ?.

 

We have moved into a caravan on site and don't really have a time limit. The total build is approx. 366m2 (internal 354m2). 

 

Thanks for you advice - greatly appreciated ?

[jen and mark]

Sorry forgot to add - the only thing we are committed to is the roof. Everything else is whatever we can pick up at a good price! 

[Iceverge]

Thanks for getting back to me. I've had a think. 

 

Firstly winter is coming. You need to get this closed in as quickly as possible. 

 

If cost is a priority I'd forget double framing. Timber and sheet materials are gone far too expensive. It was pioneered in the States where they get extremely cold and prolonged winters and lumber is (was?) cheap. Neither are true at the moment in our fair isles. 

 

It might seem like a kick in the teeth but I think you are better off knocking down the 2 courses of external blockwork and getting on with your external cladding. I would also take down the internal studwall being built. 

 

My proposed build up is as follows. 

 

15mm Soundblock plasterboard + Skim

45x45mm battened service cavity @600cc insulated with 50mm Frametherm 35 or similar.

Airtightness membrane. 

50mm PIR. 

95x45mm structural stud with 100mm Frametherm 35 insulation. 

11mm OSB sheathing,

Tyvek breather membrane. 

75x25mm treated vertical batten.

75x25mm treated counter batten. 

 

 

 

You could increase the PIR layer if you're really chasing U values but 0.16 is more than adequate I wager. If PIR is too dear you could omit this but it will really help with the 4ft section of block wall to bring that up to a good level of insulation.  by forming a continuous layer of it you would really minimise any thermal bridges with the existing structure. 

 

 

Nothing to stop you using seconds for the PIR. However I would be slow to go above 50mm+45mm service cavity as screws get expensive above 150mm. 

 

https://www.secondsandco.co.uk/product-page/50-55mm-kingspan-non-foil-1-2-x-2-4

 

 

I reckon that it'll be £18/m2 less and a lot less labour. 

 

I'll have a look at the roof if I get a chance tomorrow however a quick google suggests it is a very expensive way of purchasing insulation. Are you absolutely totally committed to this thickness?  

 

 

 

 

 

 

 

 

 

Edited October 29, 2021 by Iceverge