Closing Cavity/Ventilation Timber Frame with Stone Cladding

[James1234]

Hello all. Was keen to get other's expertise on the detailing of the cavity of my timber frame extension.

 

The construction from outside to inside is

 

-125mm thick natural stone wall

-50mm cavity

-breather membrane

-OSB

-150 stud with PIR insulation

-AVCL

-50mm PIR

-Plasterboard

 

The wall is built and next week the builder is putting in the soffit boards/closing the cavity at the top.

 

TOP OF CAVITY

I am getting mixed opinions on closing the cavity at the top. Some say keep it open to provide ventilation, others say close to prevent insects getting in, other say close for fire reasons.

 

The builder says he normally leave them open and/or lays a board over the cavity as per the below sketch. As it will be a board laid over the stone work it won't be a tight fit.

 

What would others recommend?

 

 

 

BASE OF CAVITY

This is all built. A cavity tray has been installed with weep holes above it. These are the tapered type of weep holes so I assume provide limited ventilation. There are no vents below the cavity tray.

 

 

 

 

I am concerned that the current weep holes above the cavity tray don't provide effective ventilation and the lack of them below the cavity tray will create a humid environment that the sole plates are exposed to.

 

The relevant section of the NHBC standards say full perp end vents above and below the cavity tray so I am mined to now install those.

 

https://nhbc-standards.co.uk/6-superstructure-excluding-roofs/6-2-external-timber-framed-walls/6-2-10-protection-from-moisture/

 

 

[Eric]

I would fit the soffit to the timber frame, closing off. Stone then gets built up to the soffit.

I would have the vents below the dpc.

 

Where is your service void? I think the AVCL should be on the outside of your insulation.

[James1234]

Thanks for the reply Eric.

 

My builder has said he doesn't see the need for a service void and he will just chase the electrics out of the PIR. Any downsides to that? I guess it makes any future changes more difficult (although after this build I am hoping not to make any changes for at least 20 years).

[Eric]

I would suggest to fit a service void.

 

How about your 150 timber frame with pir, 25mm pir over the studs, AVCL, 38*45 battens creating the service void then plasterboard?
 

 

[G and J]

Given what I think is built already I guess you are limited on your options. As your VCL is under your inner PIR then channeling into that is messy and a pain but won’t affect the VCL, so I guess that sounds ok to my limited understanding as long as you never puncture the VCL and don’t go too deep too often to undermine the insulation. 
 

I wonder how much ventilation is ideal for the void between stone and frame?  Given that you have a breather membrane between that void and frame I assume the frame dries with moisture vapour travelling into the void.   The stone itself may not be vapour permeable but the mortar may be plus cold stone will at times be cool enough for condensation to form on the inside face to either soak into the mortar or run down to the weep holes.

 

Perhaps whoever designed the build up made an assumption based on the above rather than ventilation - can you check?

 

If you do need that cavity to be ventilated I would always look to close with metal mesh but how you get air in at the bottom defeats me.   I’d be interested to hear the resolution. 

[James1234]

On 26/07/2024 at 08:46, G and J said:

 

I wonder how much ventilation is ideal for the void between stone and frame?  Given that you have a breather membrane between that void and frame I assume the frame dries with moisture vapour travelling into the void.   The stone itself may not be vapour permeable but the mortar may be plus cold stone will at times be cool enough for condensation to form on the inside face to either soak into the mortar or run down to the weep holes.

This makes sense and gives me some confidence that the timber frame should stay dry even if the head of the cavity is not ventilated. 

 

On 26/07/2024 at 08:46, G and J said:

If you do need that cavity to be ventilated I would always look to close with metal mesh but how you get air in at the bottom defeats me.   I’d be interested to hear the resolution. 

I will achieve this with perpend vents

 

 

Regarding the closing of the cavity at the top of the wall I have found the below in the LABC, i.e. close the cavity at the top and provide ventilation through perpend vents if you are in Scotland of Northern Ireland. As I am in Wiltshire I don't need to do this.

[G and J]

2 minutes ago, James1234 said:

This makes sense and gives me some confidence that the timber frame should stay dry even if the head of the cavity is not ventilated. 

Hang on, I was asking a question, not stating what I know to be true.   I think it would be great if the person who designed and/or signed off your design shared their view with you.  
 

Remember, finding an idiot like me on the net that just happens to have a convenient opinion doesn’t make that opinion true.  And if the guys in those areas have put in a rule it will be fir a reason, i.e. some bad experiences, so that rule indicates to me that it’s highly likely that at some point in the past someone in the know believed that such cavities need active ventilation.   What did said info source say about venting at the bottom?