What's the best wall structure to balance performance and cost in a new stick frame extension, and is continuous insulation worth it?

[Jess Shannon]

We're building an extension on to an old stone cottage. The new foundation will be insulated raft with UFH. The extension will have a pitched roof section and a flat roof section with steel frame connecting to the old building.

A builder friend is helping us design it. He's suggested 220mm studs so we can have a thick layer of insulation. He said twin studs could end up costing more due to added labour over any material cost saving. But will the cold bridging mean it's not worth it in the long run? Should we apply an external layer of continuous insulation too?

I've mocked up his suggestions on ubakus to look at performance (edit it here).



And this is a build up that I've put together based on other research, with an external PIR sheathing (edit this one here). Would this kind of PIR board be able to wrap up and over the roof too?

[Mr Punter]

I think it may work well with either internal pir or insulated plasterboard, so move the 50mm celotex inside.  It follows the rule of thumb of having the most insulating and vapour impermeable materials inboard.

[SimonD]

 

36 minutes ago, Mr Punter said:

It follows the rule of thumb

 

Yes, I wouldn't have the pir on the outside either.

Principles are definitely to have insulation to reduce thermal bridging. In Sweden, for example, it's very common to have a continuous layer each side of the stud walls. You can then use smaller dimensioned stud. I wonder what the cost implications are of this compared to the cost of the 220 studs plus continuous layer?

[Redbeard]

16 minutes ago, SimonD said:

Yes, I wouldn't have the pir on the outside either.

 

... and I was just about to write that too. PIR externally amounts, in my view, to a VCL on the wrong side of the 'sandwich' - and I wince every time I see a TV show depicting it.

How about rigid wood-fibre with a scratch-coat of lime render before the battens and (?hit-and-miss?) larch?

[SimonD]

29 minutes ago, Redbeard said:

How about rigid wood-fibre

 

That's probably what I would go for too.

[sgt_woulds]

Using woodfibre & I-Joists with a direct rendered external sheathing board will give you a quick build, that will give you more decrement delay (probably more important than ultimate u-vale in our ever heating world.  You can also do away with membranes entirely and have no worries about moisture.  

 

 

 

The example build up uses (taped and sealed), 15mm OSB3 as a combined racking and V-VCL layer internally.

 

External, directly rendered woodfibre sheathing board.

 

I've shown 240mm I-joists as these are more common, but 220mm are available

 

I've also shown a service void behind the plasterboard as this makes wiring easier. 

 

 

 

 

Edited 1 hour ago by sgt_woulds

[Crofter]

I designed my place with 50mm PIR on the inside, which allowed me to use 145mm studs whilst eliminating thermal bridging.

 

You want your wall build-up to be progressively more moisture permeable as you move outwards. So mine goes VCL, PIR, mineral wool, breather membrane.

[sgt_woulds]

If you used PIR, that is already acting as a VCL.  If you are careful with taping then an additional internal VCL would not be needed.

 

I have a combination of PIR and Woodfibre in part of my own roof (due to structural loading, woodfbre alone was too heavy) with the PIR Internal and taped as the VCL.