Insulation detailing advice for new extension

[pilgrim]

Hello,

 

Im planning a single story extension, with a pitched roof that will be built of a steel portal frame with steel ridge leading back to the existing house. so I can have open vaulted ceilings in the extension.

The walls and roof will be timber frame stick built onsite. 

 

I would like to have a warm roof if I can as this sounds like the best option but maybe I am making it more complex for my self than it needs to be for the benefits. I will be doing nearly all of the construction myself.

 

I have had the structural engineers drawings done, but I would like some advice on the best construction details to minimise cold bridging and making my extension as warm as I can. 

I have been reading through various forum posts and watching youtube videos but think I might have confused myself a little bit.

 

I have attached the plans from the engineer and where I have got to so far with my model. At the moment I am not sure the best way to clad the steel pillars and wether the roof details are correct for a warm roof with standing seam roof. Any advice on this would be really appreciated.

Thanks

 

[Redbeard]

Off-topic, but out of interest, is the v shallow roof pitch as a result of using PD rules?

 

... or is there a window in the main elev  just above ext'n ridge height?

Edited Tuesday at 10:28 by Redbeard
re ? window

[pilgrim]

1 hour ago, Redbeard said:

Off-topic, but out of interest, is the v shallow roof pitch as a result of using PD rules?

 

... or is there a window in the main elev  just above ext'n ridge height?

Yes you are right, there are windows above the ridge height, in reality I can probably safely raise the ridge height up once I have worked out the correct makeup for the roof.

[Redbeard]

My ext'n was post-and-beam, EWI'd with 160mm rigid wood-fibre, and a further 100 (or maybe 80 - can't remember) between posts. Thus the majority of the insulation outboard of the structure. Could you do that?

[pilgrim]

2 hours ago, Redbeard said:

My ext'n was post-and-beam, EWI'd with 160mm rigid wood-fibre, and a further 100 (or maybe 80 - can't remember) between posts. Thus the majority of the insulation outboard of the structure. Could you do that?

Yes I could put some insulation on the outside of the structure. 

[Iceverge]

What's the proposed wall cladding going to be? 

 

I would encourage ensuring the steels are completely insulated. If they are outside the insulation and get they will be a magnet for condensation and the timber will rot where it was attached. 

 

 

[Iceverge]

Also why WBP and not OSB?

[pilgrim]

15 hours ago, Iceverge said:

What's the proposed wall cladding going to be? 

 

I would encourage ensuring the steels are completely insulated. If they are outside the insulation and get they will be a magnet for condensation and the timber will rot where it was attached. 

 

 

Im planning to use vertical larch cladding, but a hardy plank style synthetic cladding for the wall that will run along the boundary so that it shouldnt need maintenance 

 

15 hours ago, Iceverge said:

Also why WBP and not OSB?

Im not sure that is what the structural engineer specified to use throughout.

[WoodpeckerDentMan]

On 09/09/2025 at 17:40, Iceverge said:

Also why WBP and not OSB?

I'd never heard the term WBP, so had to google it! I don't know much about OSB, but I'd have expected a quality plywood to be more structurally sound than OSB? Is that not the case?