I have had to switch my timber frame wall build-up on a large extension, due to my Building Inspector refusing us our original spec of external woodfibre with lime render, on the grounds of fire safety. Now I’m having to switch the woodfibre EWI + lime render to woodfibre IWI, and fibre cement rainscreen cladding. I’ll add below the story of this switch!

Build-up now is:

skim
plasterboard
25mm battens
woodfibre board
18mm osb
timber frame 140mm
infill Knauf frametherm 32 140mm
11mm osb (optional - will this hinder or enhance frame protection from wet weather ?)
vcl - klober permo frame (?)
38mm battens
fibre cement cladding (cedral click or hardieplank)

So far the frame is up, warm roof is on & complete, the internal 18mm osb anti-racking is up, and the battens are on for the service void - I’ll be removing these, installing the woodfibre IWI & putting the service void battens back on first thing tomorrow, so the internal works can continue (electrician due Tuesday to start first fix). The frame is exposed externally still, so that can wait until the services are through, and the build-up is finalised, then I’ll crack on with that.

My question is about the VCL(s) - if I put Klober permo frame or similar on the outside of the frame, do I also need a vcl internally across the front of the woodfibre board, behind the sevice void? Is Klober Permo Frame my optimum option for vcl here?

Also, I’d be interested to know if the external osb is required / advisable / unnecessary? Does it add to protecting the frame from external moisture, or would we have better moisture management with just the vcl across the front of the studs?

All advice welcome on this one, as it’s only just been settled with BC and I need to fit the IWI this week, because the electrician starts first fixing on Tuesday!

slow

Back story:
I initially just spec’d it myself as I’ve been involved in these builds on more than one occasion before, with Steico Protect & Lime Green’s 3-coat lime. The BI immediately had a panic about fire, because one wall is within 1m of the boundary. So I went back to him with NBT’s Pavatex build-up which has a BBA cert, and is a thin render system on woodfibre, but again avoids the ventilation cavity & rainscreen cladding approach. As the extension is my parents’ bedroom, bathroom & kitchen, I was keen to avoid the cavity & cladding, and give them a nice warm space. I much prefer the detailing of the fully enclosed build-up. But my BI still wasn’t prepared to say yes, despite the clear information of the BBA cert showing compliance, even specifically mentioning the build-up when within 1m of the boundary. As there are so many things he’s still picking over and my parents need to get moved in, I agreed to switch to IWI and give him the vented cavity & cement board rainscreen cladding. We met in his office, and as soon as I said I would switch to this build-up he became extremely cooperative - came up with an alternative calcs method for maximum openings; cleared all my proposed doors / openings etc. and essentially just stopped being an obstacle, so this is where I am, for better or worse.

Posted By: slowMy question is about the VCL(s) - if I put Klober permo frame or similar on the outside of the frame, do I also need a vcl internally across the front of the woodfibre board, behind the sevice void? Is Klober Permo Frame my optimum option for vcl here?

Permo Frame is not a VCL!!! "Permo Frame is a single-layer, vapour permeable membrane for use on timber framed walls and behind vertical cladding." http://klober.co.uk/shop/product/permo-frame

Yes, you do want a VCL inside.

But my BI still wasn’t prepared to say yes, despite the clear information of the BBA cert showing compliance, even specifically mentioning the build-up when within 1m of the boundary.

Appeal against the building inspector. Or rather, first step, phone his boss and say you are going to appeal unless he changes his mind PDQ. What's the point of BBA certs? http://www.warmshellinsulation.co.uk/document/Products/BBA%20Certificate%205173ps1i1.pdf

I don't understand what the BI is objecting to. Nor why you've switched from insulation outside the timber frame (good) to insulation inside the timber frame (not so good).

OK, so I think I’m still confused between breather membranes & vapour barriers. I thought a breathable membrane = vapour control later (keeps water from getting in, but allows vapour to leave), and that this is what I want on the outside of the build up. And that I might use another breathable membrane (vcl?) on the room (warm)side of the IWI, but that I would avoid a vapour barrier altogether and keep the entire wall ‘breathable’.

I think you are telling me this is wrong, but I’m not clear where you are telling me to put which type of membrane. I thought the Klober was specifcally for use how I want it - on the external face of a timber frame, when installing a cavity void & rainscreen cladding?

I would appreciate a bit more information to correct my errors, if you can.

As for the BI, James of Lime Green told me they only have a BBA Cert for a masonry system for Warmshell, not timber frame, so the BI threw that out. When I then used NBT’s Pavatex BBA Cert to try again, he just refused to take it on, getting hung up on the woodfibre being combustible and the BBA results only relating to ‘surface spread of flame’ and not an incombustible material. I went in & talked through the BBA Cert results, including when within 1m of boundary (which only requires as an extra that you upgrade the fire times of the internal wall) but he just kept saying I was confused between ‘surface spread of flame’ and ‘non-combustible’. We just went round and round, and he could not take on the information at all. I had to give up & agree to the ventilated cavity & cement board cavity. I simply could not get him to hear or believe the evidence.

Exactly, re more osb, this will depend on your cladding on the outside.

I reckon you could be OK without osb, it will be more to do with vermin/insect control and those considerations could swing the decision.