Timber frame cavity insulation

[tomfe]

Just reading though the kingspan literature for insulating a new timber frame building and it says you need to leave an air gap between the outer sheathing.
Why is this recommend? The osb sheathing will act like a vapor barrier so the back side don't breath will it?
Is there a way to fully fill the cavity, maybe breather membrane, fully filled PIR cavity then osb on the inside?

 

[CambridgeBrummie]

Suggest you have a look at the standard Robust Details

http://www.robustdetails.com/the-handbook/selecting-your-robust-details/timber-walls

 

[^woody^]

The cavity acts as a vent for the timber frame, and it keeps penetrating moisture away from the frame. Membrane also needs air on one side to perform.

So you insulate within the frame.

 

[tomfe]

The cavity acts as a vent for the timber frame

But a vent where? There will be no air flow in that void. The OSB is basically impermeable so is the pir; the tops and bottoms are timber so it's a closed box, there should be no air movement in there.
You might get a bit of moisture if the OSB is saturated but that's what the gap between the rain screen and breather membrane is for.
What am I missing?

 

[tomfe]

But anyway my real question is can I full fill with foil faced board?

 

[^woody^]

OSB is basically impermeable

And yet not many ships are built out of OSB. Strange

The cavity is not a sealed vacumm. It's vented. Either with vents if there is a rainscreen, or via porosity if the cladding is permeable. That's what you are missing.

 

[tomfe]

OSB is basically impermeable

And yet not many ships are built out of OSB. Strange

The cavity is not a sealed vacumm. It's vented. Either with vents if there is a rainscreen, or via porosity if the cladding is permeable. That's what you are missing.

Think I have miss led you, not a cavity between rain screen and breather&OSB but in the frame between OSB and pir

 

[^woody^]

But anyway my real question is can I full fill with foil faced board?

If you like.

It goes against all guidance though.

But why would you want to do that, and not the frame?

 

[^woody^]

but in the frame between OSB and pir

I've got a headache now. There is no cavity within a timber frame between the PIR and OSB

 

[tomfe]

Yes so why have they said you need one?

 

[tomfe]

Never mind, I took a fresh copy off their website and it has that bit removed, I must have had an old copy?

 

[theprinceofdarkness]

Wot no service void, who are these cowboys?
Frank