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 September 9 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?

[HughF]

On 11/09/2025 at 16:47, WoodpeckerDentMan said:

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?

OSB is more rigid, more uniform and holds up better if it gets wet. It’s a superior product.

[sgt_woulds]

OSB is more rigid?  It may have slightly more tensile strength, but the difference is marginal, and like every material choice, the characteristics suit different scenarios.

 

OSB3 is cheaper and generally more vapour open, which is useful if you place it inside the structure and use it as a racking board and moisture vapour-variable control layer for a 'breathing wall' system.  This is common in European construction using natural insulation systems.

 

WBP (and make sure it is WBP, as there is a lot of fake crap coming in from China) will better survive getting wet a few times and living in a moist atmosphere.  Especially if you use a hardboard face WBP ply.  It dries quicker when wet, and is less likely to swell if building over winter.

 

OSB is generally fine, and will also survive a short period of being wet, but it will swell and lose its structural strength faster than WBP in the same situation.

 

WBP is also much more expensive, harder to source, heavier, and slower to cut.  

 

If using as external racking in an exposed location in summer OSB, winter WBP.  If you need something to take structural connections, then WBP will hold a screw better.  

 

 

[Iceverge]

How about something like this. 

 

 

 

You could keep the steel inside the external layer of insulation which would stop any condensation and rot issues at the metal timer connection in future.