exterior covering

[deuce22]

Hi.

 

I am planning to construct a stick build timber frame on site, but I'm not 100% sure what to do for the exterior. I have done a timber frame with block and a traditional double block construction in the past, but I have been told that I do not need an outer leaf of block/brick.

It will be a twin wall stud at around 300mm thick. I have spoken with warm cell and have been told this is enough to achieve the thermal value. I was then told that I could render straight onto the OSB and there is no need for a cavity. I've never seen this done in the UK, but supposedly it is starting to happen on a lot of builds.

My concerns doing it this way are, will it be 100% watertight and will there be any issues for mortgage purposes?

 

Thanks.

[jack]

7 minutes ago, deuce22 said:

I was then told that I could render straight onto the OSB and there is no need for a cavity. I've never seen this done in the UK, but supposedly it is starting to happen on a lot of builds.

My concerns doing it this way are, will it be 100% watertight and will there be any issues for mortgage purposes?

 

You're right to be concerned, and I'm surprised anyone is suggesting rendering directly onto an outer layer of OSB. 

 

First, OSB isn't ideal as an outer cladding. You want to have a think about vapour openness from interior to exterior. For example, we have a twin stud construction, which (from inside to out) is airtightness membrane > OSB > warmcell-filled twin stud > panelvent (I think that's the brand - vapour permeable board, in any event) > breather membrane > vertical battens > cement board > brick slips.

 

You definitely don't want to be rendering directly onto whatever is used to close the twin stud wall. It needs a ventilated cavity.

[nod]

32 minutes ago, jack said:

 

You're right to be concerned, and I'm surprised anyone is suggesting rendering directly onto an outer layer of OSB. 

 

First, OSB isn't ideal as an outer cladding. You want to have a think about vapour openness from interior to exterior. For example, we have a twin stud construction, which (from inside to out) is airtightness membrane > OSB > warmcell-filled twin stud > panelvent (I think that's the brand - vapour permeable board, in any event) > breather membrane > vertical battens > cement board > brick slips.

 

You definitely don't want to be rendering directly onto whatever is used to close the twin stud wall. It needs a ventilated cavity.

I agree with the above 

I wouldn’t render directly onto any timber surface Even with a mesh 

A vapour barrier with batons on top then a cement board is normal

Or insulation Like this one I’m trying to do this morning

If it ever stops raining in sunny Garstang  

[A_L]

I would not want to (for various reasons) render directly onto timber based sheets. However render direct onto a substrate (without cavity) is possible. @ProDave has proprietary render onto wood fibre insulation board. In the attached link there is an exposure map, direct render in zones 1 to 3 is perfectly possible with render onto something like cement bonded particle board or proprietary systems.

 

https://www.parex.co.uk/GetFile.ashx?file_id=1453

[Mr Punter]

Timber frame needs a drained and ventilated cavity. to do otherwise is a risk.

[jack]

1 hour ago, A_L said:

... direct render in zones 1 to 3 is perfectly possible with render onto something like cement bonded particle board or proprietary systems.

 

Possible, yes. Recommended...?

[K78]

Even done properly most of the big mortgage companies won’t like it. 

 

Silly but that that is the uk for you.  

[deuce22]

Thanks all for the advice.

 

So batten, cement board and then render is the correct way. If this is correct, are mortgage companies ok with this?

[Russell griffiths]

Batten, cement board and render is certainly a way to do it, but I wouldn’t want it. 

Cavity, concrete blocks, render 

 

has anybody done a price comparison??

those cement boards are not cheap, then the fixings are not cheap, then the bagged render is not cheap

 

concrete blocks are not that expensive to have laid and is a far more robust detail IMHO. 

 

 

[deuce22]

I've helped on 2 self builds in the past and a few extensions and there's always been an outer block or brick used. I agree it's more robust and know that I won't have a problem getting a mortgage.

The reason i'm looking into this other way is that, I have been told that when using a twin wall stud the insulation value is achieved and so there is no need to fit insulation in the cavity. A block/brick is then just an unnecessary wall covering.

I think I'll calculate the prices between the two and see how different they are.

Thanks.

[deuce22]

Would this be the correct product to use?

 

http://www.builderdepot.co.uk/cemboard-general-purpose-cement-board-2400mm-x-1200mm-x-12mm.html

 

 

[Temp]

Here...

https://www.buildingboards.co.uk/products/rcm-cemboard/

It says...

Can RCM Cemboard be used as a render carrier board?

• No, RCM Cemboard is not suitable for use as a render carrier board. Renderflex however is.

Also has advice on sealing joints.

[davejura]

We were looking into all the options including rendered block and rendered cement board. We have now decided to go with Cedral cement render cladding, as putting a decent coloured render system onto either of the other options was going to be expensive. As my engineer friend keeps telling me, the strength is in the kit. Block work is just a skin!