Dunster house framing question

[JackOfNoTrades]

Hi all

Can I pick your brains?

I was looking at alternative cladding options for a timber frame conservatory wall and found a diagram of Dunster House's Addroom wall. It makes no sense to me ... so I'm trying to make it make sense just so I can learn something from the process. 

 

The wall is ...

- 10mm wood polymer composite external cladding 

- 25mm PIR - continuous outside of frame

- 75mm timber frame 

- 25mm PIR - continuous on inside side of frame

- 10mm fermacell fibreboard interior wall 

 

They also say "The walls have a 70mm cavity to increase airflow and ventilation"

 

I know this is a conservatory alternative not a traditional building, but it seems to fly in the face of what I know about timber build. To help me get my head around it, could anyone throw in their two cents on the following?

 

- Would there not be a condensation point in the middle of all this?

- How is it working without an air gap behind the cladding?

- What do they mean by "70mm airgap for ventilation", when it seems from the diagram that the only airgap is in the timber frame itself ... and that's presumably sealed by the PIR?

 

Thanks for your ideas everyone. 

Jack

Edited March 31 by JackOfNoTrades

[Mr Punter]

This currently states "- 25mm PIR - continuous outside of frame" twice.  Is this an error?

[JackOfNoTrades]

Just thought... Here is their diagram in case it helps 

 

[JackOfNoTrades]

  On 31/03/2025 at 12:45, Mr Punter said:

This currently states "- 25mm PIR - continuous outside of frame" twice.  Is this an error?

Expand  

Sorry. That's confusing of me. One of them is on the inside side of the wall, but the reason I was adding that bit was to make it clear it's a continuous sheet and not between the timbers. 

[JohnMo]

Drawing shows number 2 inside and outside of frame, and calls it a sandwich. PIR is the bread, wood is the filling - pretty clear.

 

Rubbish insulation values. Wood trapped between two vapour closed sections - not good either.

 

 

[JackOfNoTrades]

  On 31/03/2025 at 12:52, JohnMo said:

Drawing shows number 2 inside and outside of frame, and calls it a sandwich. PIR is the bread, wood is the filling - pretty clear.

 

Rubbish insulation values. Wood trapped between two vapour closed sections - not good either.

 

 

Expand  

 

Yeah, I understand what they've done from the diagram . It's the *why* they've done it that way that's getting me. And the fact that there's seemingly no ventilation but they claim there is. 

Edited March 31 by JackOfNoTrades

[Russell griffiths]

Duster house sell sheds, it’s not what you want. Avoid. 

[saveasteading]

Their sheds are good as sheds.

This is a shed with some insulation, fixed in and out for their convenience.

The void is a flaw, with some words to try to explain it away.

 

I don't even see it as a basis for any proper building use.

 

" helps maintain room temperature, unlike rockwool". 

What garbage. They might get a solicitor's letter on that.

[JackOfNoTrades]

Thanks everyone. This really helped. I wasn't missing anything and my confusion at their approach was justified. 

 

Thanks all for helping confirm I'm not going mad. 

[ProDave]

"Helps maintain room temperature unlike cheaper alternatives such as rockwool" tells me all I need to know about the designers.

[Alan Ambrose]

>>> Their sheds are good as sheds.

 

Yeah, +1.