This is a complicated subject and decisions shouldn’t be based on price alone.

Upgrading the thermal envelope needs to take a holistic approach which accounts for the materials and breathability of the existing structure, cold bridges, architectural details, the disruption to the homeowner, the skill level and professionalism of the contractor, etc.

There are plenty of examples where a failure to account for any one of these has caused issues, irrespective of the insulation materials.

It is true to say that retrofit is less risky with a vapour-open approach using natural materials, but much of this has to do with the ability of the materials to absorb and release any moisture that builds up. Man-made insulations either can’t do that or can’t do it as well as a natural fibre so, for instance, a build-up of interstitial moisture will increase faster than with a breathable system. A ‘bad’ installation with natural materials will still cause issues – it will just take longer for those issues to become apparent.

It is much harder to get a vapour-closed approach right – the SD value and detailing of the VCL needs to be perfect for the situation. You also do not want any gaps between the insulation and the wall surface as this increases the risk of condensation forming.

In my opinion, this is impossible to achieve with the insulation pre-bonded to plasterboards, and should always be built up in individual layers instead. This takes time, expertise, and attention to detail, which a lot of tradesmen unfortunately do not have.

We often get calls from people who have had a poorly installed vapour closed upgrade and looking to replace the insulation with our woodfibre boards to ameliorate the symptoms. Often this is not due to the materials, or the vapour closed approach, but lack of attention to other areas. The holistic approach I was talking about earlier.

Internal wall insulation will move the dew point within the construction, and this may cause issues with the existing structure. Standard U-value calculations will not correctly account for the sorption properties of natural fibres, nor their ability to pass on liquid water through capillary action.

The external brickwork and pointing will need to be assessed and repaired if in poor order to reduce the passage of moisture to what will become a colder wall. I’m constantly amazed at how often this isn’t done. People try to treat the symptoms of dampness and mould, not the cause.

For IWI a WUFI calculation is a useful check - this purely considers moisture issues and how the various elements of the building fabric will deal with the moisture volumes based on site-specific conditions.

It is important to note that there is a balancing act between achieving the best U-value, the increased risks of moisture in the wall, the loss of space, and the cost. The cost should always be the last one on that list.

In basic terms, a natural insulation will cost more than a plastic or glass one. However, this is often offset by not requiring additional VCLs and additional labour time for detailing which speeds up installation. Overall costs can be the same, or at least not vastly more.

It is more important to worry about the competency, (and honesty) of the installation than the bottom line cost – a poor install might end up costing more in the future to put right.

The ASBP has a webinar on IWI which covers most of these topics:
https://asbp.org.uk/webinar-recording/rethinking-iwi-with-natural-fibre-insulation

Another question ... are there any vapour open systems that have a BBA certificate?

BBA certification is voluntary rather than mandatory.

We do not hold BBA certification on our own woodfibre insulation boards; the boards are manufactured to the relevant EN standards and CE marked accordingly.

In addition, all of our products have installation detailing available and third-party certified EPDs which is not the case for all manufacturers yet.

Regulation 7 (1) states CE marking and the appropriate Declaration of Performance can be used as a way of establishing the suitability of a product for its intended use.

Additional national or international technical specifications are referenced in Section 1.14 and Independent certification schemes in Section 1.15, but these are not mandatory - it is explicitly stated that they ‘may be in addition to, but not conflict with, CE marking’.

Given the issues surrounding UKCA, I don't see that changing soon. But given the economic kamikaze tactics of our elected representatives, I'd never say never.

Posted By: sgt_wouldsour own woodfibre insulation boards;

Which ones are these?

As I see it the questions I'm asking in this thread are totally related to the "real world".

Maybe I should clarify that in this case I'm looking at things from the perspective of specifier. I have done a fairly major IWI project on a DIY basis and the decision making process in that scenario can be rather different.

To explain why a BBA certificate is useful to a specifier - part of it is about covering your tracks in demonstrating you've taken reasonable steps when making choices on someone else's behalf. BBA certs don't just tell you that something has been manufactured according to relevant standards, they also tell you what are judged to be appropriate uses, installation methods and so on. Often this is subtly but significantly different from what the manufacturer says or implies.

To me the existence of a BBA certificate is also some signal that the product is not obscure in the UK context. This can be rather important if you are worried about scaring off contractors with things they have never heard of or will find hard to get hold of. This is a very "real world" concern.

Of course a BBA is not the be all and end all when looking at options. But certainly a nice-to-have.

https://www.insidehousing.co.uk/news/bba-published-materially-wrong-certificate-on-grenfell-cladding-after-manufacturer-stonewalled-data-requests-69997 for example

People do get hung up on BBA and most BCO's, Architects, Quantity Surveyors, etc., don't really know what a BBA entails or that it is not mandatory.

My earlier explanation was cut and pasted from a standard e-mail response that I send out on a nearly daily basis.

All natural insulations are obscure in the UK context. That won't change until the DIY boxes start selling it off the shelf and promoting it. Not to mention education and policy change to promote its use over 'traditional' materials. This has been done successfully in France and to some extent Italy which are the world's biggest markets for woodfibre right now. Can you see our current rulers taking note of successes in foreign lands?

The sad truth is that the UK market for insulation in general, and natural insulations in particular, is absolutely tiny in the grand scheme of things. Paying for BBA does not make economic sense at the moment. Especially when it achieves nothing above what EU technical approval does - other than add a typically British layer of red tape, delay and cost.

I do understand the 'warm tummy' feeling a BBA gives to the end users, and I also understand that using CE products requires a bit more effort in explanation from specifiers, but it really shouldn't be a stumbling block.

Posted By: sgt_wouldsmost BCO's, Architects, Quantity Surveyors, etc., don't really know what a BBA entails or that it is not mandatory.

I'm sure a lot of small builders don't have a clue, but I'd be very surprised if more than a handful of qualified BCOs, Architects & Quantity Surveyors don't.

If you're looking at a building product or system though, and wondering - is this fit for the purpose I want it for, how does one actually answer that question?

There's no easy route to a definitive answer, so you have to attack it from as many angles as are available. You still won't usually get a definitive answer, but you'll have done the best you reasonably can.

Asking if there's a BBA certificate is just one of the things you can do as part of that process. If an architect or surveyor asks whether there's a BBA certificate, that doesn't necessarily mean they think it's mandatory, or that it's the only check that they are doing.

Posted By: sgt_wouldsIt's true to say that once we point out the regulations, the architects are normally satisfied, but they still have to convince their customers and BCO who have been gaslit into thinking a certificate is required. It is a shame - and a surprise - that we have to point this out to them in the first place. Given the cost and length of their education and the amount they charge for their expertise, this is one of many areas where I - as a layman - am amazed by their lack of knowledge.

In a perfect world, checking the product certification would be just one step in a chain of due diligence that would result in compliance with building regs and product installation specifications. Unfortunately - all too often - having the certificate on file seems to be where accountability stops.

I'm not really clear about what exactly it is that you think that architects in general should be doing, compared to what you observe them doing.

The fact is, there is no real failsafe "chain of due diligence" that guarantees compliance with building regulations. There is no set procedure that an architect (or anyone else) can go through that tells them absolutely definitively that what they are designing and specifying will be OK. Much of it relies on subjective judgement and obtaining opinion from people with practical experience.

The architect's life would be much easier and less stressful if there were some check-box procedure that led with certainty to the right decision each time.

When you say the equivalent EU version - do you mean the CE mark and associated DoP?