Timber frame wall build up

[southcoastseagull]

Hello everyone, I'm very much enjoying the knowledge on this forum and I've perused it a far bit over the years. Just to introduce myself, I'm an English architect who studied in the UK, however pretty much all of my practical project experience has been abroad in tropical climates abroad so I'm really not familiar with best practice over here when it comes to technical details.

I'm currently working on an extension at my own home in south east England, which will be constructed in timber frame and vertical clad in narrow GFRC panels (perhaps Equitone or Reider, not sure yet).

Due to site constraints the aim is have the walls as thin as practically possible.

I've drawn up the following wall sections (in plan) with my first attempts at detailing the external walls and would greatly appreciate any feedback.

For wall build up U Values I've been estimating using this tool: .... link removed .....

Which option would be best to go for, or if there would be a better build up I'd love your knowledge and suggesttions.

I should add that, according to my rudimentary calculations the U-Value of all the options is below 0.18.

Another though - for the timber battens and counter battens to the exterior side, would I be best off using a composite ..link removed .... batten rather than wood? They're much more expensive and presumably more durable, however people have been using timber for centuries...

Many thanks!


The thin option (224.5mm)
For this option, air-tight electrical boxes would be required as the VCL would be created by the foil back plasterboard, and they would sit behind it. I'm also not sure if this is a definite no-no due to cold bridging through the timber studs directly to the plasterboard. I've seen details online like this though, and would appreciate everyone's thoughts as it allows for the thinnest wall.

The medium option (249.5mm)
With this option the VCL moves back to the studline and a 25mm service void is created using timber battens.

The thick option A (274.5mm)
I've added 25mm PIR between the studs and the service void.

The thick option B (274.5mm)
I've added the 25mm PIR between the studs OSB/3 and the UV membrane

 

[23vc]

Insulation wise, I’d have thought the only one that would meet building regs would be thick A, and even that may need 50mm rather than 25mm across the studs.

 

[^woody^]

My frame, designed to be as thin as possible, is 38x89 CLS with 90mm Celotex between and 50mm Celotex across the inner face - this could have been 20mm.
18mm plasterboad and skim internal
11mm OSB sheathing.
25mm battens and drained/vented air cavity
19mm cladding.

So 163mm in all, could have been 133mm total.

U- value regs changed last June, I suspect the 90/50 insulation would comply, but you'd need to check.

You don't need an internal air void.

 

[southcoastseagull]

Insulation wise, I’d have thought the only one that would meet building regs would be thick A, and even that may need 50mm rather than 25mm across the studs.

Hi thanks for taking the time to reply. When you refer to building regs are you talking about the U Value of the wall? I've been calculating this for all the options I posted using an online tool (I can't seem to post links, but see attached screen shot). Anyway, using this tool the U Value of the thin wall is 0.17 so within regulations. Perhaps I am using the tool wrong though, or missing something.

This would be the U value for the 224.5 Thin Wall

 

[southcoastseagull]

My frame, designed to be as thin as possible, is 38x89 CLS with 90mm Celotex between and 50mm Celotex across the inner face - this could have been 20mm.
18mm plasterboad and skim internal
11mm OSB sheathing.
25mm battens and drained/vented air cavity
19mm cladding.

So 163mm in all, could have been 133mm total.

U- value regs changed last June, I suspect the 90/50 insulation would comply, but you'd need to check.

You don't need an internal air void.

Thanks, I suppose I will need to speak to a structural engineer to see if I can get the timbers down to 89mm with my design. Definitely something worth looking at!

Also, your bulid-up doesn't seem to include a service void, how did you deal with these?

What are you referring to by not needing an internal air void? None of the wall drawings contain an air void other than the service void and the build up for mounting the open rainscreen cladding. Thanks!

 

[^woody^]

Your service void appears to be an air void.

My services were recessed into the insulation layer, that's why I increased the thickness of it. It's not as if there are loads of services that require a service void across the whole face, and the condensation and fire risk of any air void means that they are best designed out.

 

[southcoastseagull]

Your service void appears to be an air void.

My services were recessed into the insulation layer, that's why I increased the thickness of it. It's not as if there are loads of services that require a service void across the whole face, and the condensation and fire risk of any air void means that they are best designed out.

Cheers, were your studs at 400 centres being so small or were you able to keep them at 600?

 

[southcoastseagull]

Your service void appears to be an air void.

My services were recessed into the insulation layer, that's why I increased the thickness of it. It's not as if there are loads of services that require a service void across the whole face, and the condensation and fire risk of any air void means that they are best designed out.

Something like this?