BenS

SIP with Block and Render or SIP with Cementboard and Render?

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Hi,

I'm planning on using SIPs as the build system for our Self Build (If I manage to find a plot!) but I see that there are a lot of options on what to combine SIPs with. For example, SIP and Brick, SIP and Block & Render, SIP and Cementboard & Render etc.

 

We probably want a render finish, so that gives us the choice of block and render or cementboard and render for the outer walls, but I've seen very little comparing these options.

The obvious points are:

  • Cementboard and render is going to be a lot thinner than block and render
  • Presumably cementboard will be a lot quicker than building a block wall?
  • Presumably cementboard is cheaper (Both in terms of purchase costs and fitting costs)?

 

But what are the downsides? i.e. why would anyone choose block and render over cementboard and render?

Cementboard is obviously a lot thinner than blocks, so does this have any effect on thermal performance? If so then presumably having thinner walls would allow me to have thicker SIPs panels, which would hopefully more than compensate?

 

Thanks in advance

Ben

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If you are going render then consider Durisol or ICF - lots of benefits over SIPs and probably slightly more forgiving when it comes to getting the foundations perfectly level.

 

Other options include wood fibre on timber frame - check what @ProDave has done with his build.

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Thanks Peter but I've done a lot of research into the different methods and I've decided on SIPs as my main build system (I have considered the ICF and Durisol options). I'm sure it's a passionate subject and some people would swear that build system X is best and others would swear that X is flawed and build system Y is better, and so on. 

 

In this case having chosen SIPs as my main build system I'm now considering the options for the outer facade. i.e. block and render vs cementboard and render.

 

Thanks

Ben

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Hi Ben

 

As Peter has mentioned, I have a (almost) conventional timber frame with a 10mm thick wood fibre cladding and render straight onto the wood fibre board.

 

The system is described here http://www.greenspec.co.uk%2fdownloads%2f%3ffilename%3dpavaclad1.pdf&usg=afqjcnhzc6lrtooxtnhnayrqcv6moscmmg/

 

The benefit is that as well as providing a suitable board to render onto, it is also adding more insulation, and particularly important to a SIPS build that insulation has a low decrement delay (in rough terms means it only reacts slowly to external changes in temperature)  Most of the foam type insulation used in SIPS has a fairly high decrement delay.

 

You might need to run it past the SIPS supplier or a structural engineer to ensure the SIPS panel structure can support the additional weight, and check the structure has sufficient racking strength to stand alone with no brick or block external skin (in my case, the structural engineer specified two layers of OSB on the frame to give sufficient racking strength)

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Hi Ben, 

 

I'm in the process of scoping SIPS for a smallish extension project in London. Interestingly, we also looked at some of the alternatives others have mentioned here including Durisol which I was quite keen on but they never returned my emails. One of the key considerations for us was wall thickness and it looks like we can shave up to 90mm off each wall by using SIPS which would be quite considerable on our project.

 

In terms of your questions, a 4th option you could consider is one of the SIPS manufacturers that allow render to be applied straight to the exterior face of the SIPS panel. To achieve this the exterior sheathing board of the panel needs to be either cement board or MGO board - both of which are slightly more expensive than OSB SIPS but have other advantages - ie. they are fireproof, vermin proof and unaffected by moisture rot. I was slightly suspect as to whether this system was building in a future weather-tightness issue but managed to track down a BBA certificate for a provider using this system ("Panablok 100 preformed structural wall") and it was ok'd for 60 year design life. Looks like the certificate has been taken off the BBA website now. If you accept that the risk should be low because both cement board and PUR foam are essentially waterproof then this option would save you cost and wall thickness (suppliers like Mantle and others are still encouraging it).

 

Weighing up the merits of cement board and render vs block and render. In my mind; 

 

Cement board - supported from walls so may reduce foundations and cost; reduced materials and storage; reduced wall width

 

Block - may have be more durable in the (very) long term because its self-supported; better acoustic separation; slight thermal benefit (easily outweighed by adding insulation); may help reduce overheating by adding thermal mass

 

 

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One thing to consider: the architect that did our house used MGO-based SIPS on a project two or three before ours. He said that they'd had serious problems with the panels being curved.

 

I can't say whether that was anything to do with the supplier, storage conditions, or the product more generally, but I know it caused huge issues during assembly. 

 

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Thanks Jack, that's useful - when you look into it on the internet it seems that quality control has been an issue for the MGO board industry and is one of the factors preventing adoption becoming more widespread. On the face of it, its something of a wonder material even having strong sustainable credentials. There are a few suppliers who are quite meticulous about where they source the boards to avoid issues that have been experienced in the past. If we proceed down this route, we would certainly be asking for that information - and also the supplier would be erecting and assembling which should remove some of the risk, at least initially. 

 

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1 hour ago, Archer said:

On the face of it, its something of a wonder material even having strong sustainable credentials.

 

Agreed. At one point we were considering using MGO boards for the internal lining of the house. I can't remember why we didn't - I think it was one of those many ideas that just never quite got important enough to make it into the final plans.

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