Timber Frame Wall build up for extension

[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: https://www.changeplan.co.uk/u_value_calculator.php

 

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 (like this) 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

 

 

Edited December 19, 2023 by southcoastseagull

[PhilC]

Not a pro but the last one puts a non-breathable layer (external PIR) outside the timber frame so I would avoid that, water won't escape the frame even with a breather sheet on. I am looking to put thick option A in, albeit with differing stud sizes and PIR depths between and internally.

[john0wingnut]

I’m currently going through the same process for my rear extension / new porch.

 

My current plan is your thick option A, then tape the 25mm PIR layer to act as a VCL. 150mm studs, 120mm PIR inside and 30mm clear cavity.

 

As above, I was advised to use rigid wood fibre insulation if I wanted to go the external route as it’s breathable.

 

John

 

 

 

 

[DTM]

hi mate,

 

Im looking at similar calcs for my roof extension. I've been planning to approach it similar to your thick option B. but biasing most of my insulation to outside to insulate the studs better from cold bridging.

 

And @john0wingnut and @PhilC is having PIR in a cavity any different to running it externally on a timber frame? I would expect with proper humidity control for it not to be a problem? Im planning to be adding a MVHR system to the house as a part of my extension.

 

@southcoastseagull i like that you have presented multiple options but they differ in thickness and U value? makes it a bit odd to compare them. 

[mike2016]

Try this site - the main reason is to show the moisture layer. https://www.ubakus.de/en/r-value-calculator/index.php?

You don't want to see that moisture line inside your house! 

I was going to use Koolterm in mine but it's not breathable so going with 220mm Moy Metac in the timber frame & 140mm Gutex wood fibre behind this which is all breathable, my preference to PIR etc but it adds cost. 

[john0wingnut]

On 15/02/2024 at 20:13, DTM said:

hi mate,

 

Im looking at similar calcs for my roof extension. I've been planning to approach it similar to your thick option B. but biasing most of my insulation to outside to insulate the studs better from cold bridging.

 

And @john0wingnut and @PhilC is having PIR in a cavity any different to running it externally on a timber frame? I would expect with proper humidity control for it not to be a problem? Im planning to be adding a MVHR system to the house as a part of my extension.

 

@southcoastseagull i like that you have presented multiple options but they differ in thickness and U value? makes it a bit odd to compare them. 

Hey

 

Ive now gone down the route of wood fibre insulation and sheathing boards as it’s all breathable.

 

I used that ubakus tool mentioned above to get my u values sorted etc and it’s also useful to demonstrate phase shifting of heat through the walls and humidity control etc.

John 

[PhilC]

On 15/02/2024 at 20:13, DTM said:

hi mate,

 

Im looking at similar calcs for my roof extension. I've been planning to approach it similar to your thick option B. but biasing most of my insulation to outside to insulate the studs better from cold bridging.

 

And @john0wingnut and @PhilC is having PIR in a cavity any different to running it externally on a timber frame? I would expect with proper humidity control for it not to be a problem? Im planning to be adding a MVHR system to the house as a part of my extension.

 

@southcoastseagull i like that you have presented multiple options but they differ in thickness and U value? makes it a bit odd to compare them. 

Not a pro as I said but PIR is not breathable so if moisture gets into the frame and you have another non-porous layer VLC/PIR internally then the moisture can't escape anywhere.