How do you vent the cavity in a double frame stud wall?

[Waka]

Hi all, I've been working on my garden studio design, which is going well.

Just a question I'm struggling to find an answer to.
How do I vent my cavity in an 2-leaf timber wall room within a room design and maintain the sealed air cavity?

Reading through Rod Gervais' book (Home recording studio build it like the pros) I can't seem to find it mentioned. He speaks about the roof being best just built as a "warm roof" design. That is: put purpose designed rigid insulation on top of roof deck and wheather proof on that, therefore not requiring the roof space itself to be vented. But how does this work when it comes to walls? The standard design here in the UK seems to be weep vents at the base of the wall. These seem incompatible for the external leaf as they break the seal. How do studios avoid this?

Thanks,
Dan

[Waka]

Apparently I've got confused! (Not hard to do these days) Weep vents seem to need to be installed if the facing skin is sealed (such as in a brick outer skin). The wall needs a vented cavity between the internal stud wall and the facing material - you need to include a breather membrane on the outside of the inner timber frame wall.

This image represents this:

external frame design.jpg

The above image achieves ventilation by using battens to make the cladding stand-off from the wall allowing free flow of air between the cladding and the sheathing.
It looks to me though that a decoupled wall will require ventilation between the skins. Is this not the case?

[Soundman2020]

How do I vent my cavity in an 2-leaf timber wall room within a room design and maintain the sealed air cavity?

I'm not even sure why you would want to do that! Why would you want to vent the wall cavity? What reason is there for doing that?

Reading through Rod Gervais' book (Home recording studio build it like the pros) I can't seem to find it mentioned.

Right. Because there's no reason to do it, and doing so would trash your isolation.

He speaks about the roof being best just built as a "warm roof" design. That is: put purpose designed rigid insulation on top of roof deck and wheather proof on that, therefore not requiring the roof space itself to be vented.

Right. That's the roof. And for the roof there are situations where, there is no choice, and the roof MUST be ventilated, in which case you also have no choice but to build a three-leaf roof. But I can't think of any reason why you would want to ventilate a wall.

The standard design here in the UK seems to be weep vents at the base of the wall. These seem incompatible for the external leaf as they break the seal. How do studios avoid this?

Seal the wall so that no water can penetrate. Build a sealed wall, not one that needs drainage holes.

It looks to me though that a decoupled wall will require ventilation between the skins. Is this not the case?

No. I still don't understand why you want to ventilate the cavity. For what purpose? If the wall is built correctly, with the moisture barrier and vapor barrier in the correct locations, and properly sealed, then I can't see any reason to want to ventilate it.

- Stuart -

[Waka]

I'm not even sure why you would want to do that! Why would you want to vent the wall cavity? What reason is there for doing that?
...
No. I still don't understand why you want to ventilate the cavity. For what purpose? If the wall is built correctly, with the moisture barrier and vapor barrier in the correct locations, and properly sealed, then I can't see any reason to want to ventilate it.

- Stuart -

Hi thanks for responding Stuart. I was coming to that conclusion myself. It is a fact that building regulations in the UK require brick vents to be installed when facing a timber frame construction with brick. I wrongly assumed this might apply also when putting another skin in front of an already insulated timber skin. Logically a two leaf decoupled wall is really no different to a wall with the sheathing attached too the same stud. The breather membrane should allow any condensation to escape and the vcl should prevent inside vapour entering the studs.

As a side point. Is cladding as the picture above likely to cause issues being a third leaf? I'm interested in other materials for facing that do not require airflow behind them if you have any suggestions?

Thanks,
Daniel

[Soundman2020]

Logically a two leaf decoupled wall is really no different to a wall with the sheathing attached too the same stud.

Logically it is no different, but acoustically it is...

The breather membrane should allow any condensation to escape and the vcl should prevent inside vapour entering the studs.

As long as you keep those in the correct relationship for YOUR climate, then yes. The water-proof but breathable barrier goes on the outside, to allow the wall to breathe and lose moisture to the outside without allowing liquid water to get in, and the vapor barrier controls the movement of humidity between the wall cavity and room interior. Provided that those are arranged correctly for your climate, you should be OK.

As a side point. Is cladding as the picture above likely to cause issues being a third leaf?

Possibly, if it is sufficiently massive. But it also might vibrate, rattle, ring, etc.

I'm interested in other materials for facing that do not require airflow behind them if you have any suggestions?

I have used fiber-cement panels directly over the Tyvek. Provided that you seal it well, and tape the joints well, and finish it well, it can do a pretty good job. Of course, that assumes that you have a good solid layer on the studs behind that. I've used both OSB and plywood for that. In other words: studs, then thick OSB (19mm), then Tyvek, then sealed fiber-cement panels (thick: 8mm) screwed firmly into the OSB, with counter-sunk screws, with the joints sealed and taped, then sealant, then stucco, then paint. Massive, solid, rigid, water-tight, looks good, not too expensive.... But you should check to make sure your local code allows that. Don't do anything that is not allowed by local building codes.

That's for the outer-leaf, of course. There's nothing on the other side of those studs. Then comes a gap, then another stud frame, then the vapor barrier, then the inner drywall. The entire air cavity is filled with suitable insulation.


- Stuart -