Pitfalls and Solutions for Internal Wall Insulation

Nick Lloyd | Urbane Eco

The most important thing to bear in mind when installing IWI is, in my view, to consider the large quantity of water vapour generated by building occupants, and what happens to it. A family of four will produce anything from ten to twenty litres of water in 24 hours; two litres while sleeping, a couple of litres bathing and showering, four litres cooking, a litre washing clothes, and so on (not to mention using an unvented tumble drier, a whopping five litres). Where does this water go? An especially pertinent question if it’s winter time and the windows aren’t opened much and the mechanical ventilation is not used properly or isn’t easy to control, or - even more challenging - where there is no mechanical ventilation at all. The issue of ventilation is a separate topic and is worthy of its own blog, but taking the view that all retrofits should be considered with a ‘whole house approach’, any insulation installation has impacts (e.g. increased airtightness, especially if high performance windows and doors have been installed). External wall insulation does not have the same issues regarding dealing with the water vapour in the house, and in that respect is less of a concern, as well as the fact that thermal bridges are much easier to avoid.

There are quite a few systems on the market for internally insulating walls of domestic properties. The most commonly used are polystyrene or PIR backed plasterboard. People also use timber studwork, infilled between with mineral wool or PIR, covered with a vapour control layer (often polythene) and plasterboard.

I have seen, on more than one occasion, the result of a polythene vapour control layer which has not been sealed around the perimeter of the wall and not taped where it is overlapped. Mould and wet rot doesn’t take long to build up on the internal face of the masonry that the insulation is fixed to (especially as this masonry is now cold due to the insulation). This is hazardous to the health of occupants and the fabric of the building. The thing is, even if the vapour control layer is correctly fixed, there’s nothing to stop an occupant penetrating the layer by deciding to fix something to the wall. Whilst the extent of damage caused by penetrating the vapour control might not have too much of an impact, in my view, using a system that is easy to install incorrectly and vulnerable to future accidental damage isn’t a risk worth taking. Likewise, I am concerned that using plasterboard with a PIR backing could also be problematic in that with a dot and dab installation, water vapour can enter around the perimeter of the wall and condense behind the insulation. The scary thing is that a system can fail over a long period without anyone knowing about the extent of its failure and the attendant damage to the fabric and potential harm to human health.

Something else to consider - now you have internally insulated – is that you are not getting the warmth in the dwelling helping to keep the walls dry. I have seen examples of this where the external surface of the masonry has degraded, as it was not weatherproof, and water ingress has turned to frost and the fabric has spalled. There are some in the industry who say that we should therefore set our target U value considerably less than current building regulations and go for something like 0.6 U value, when retrofitting IWI. Ideally, each property should be assessed according to its own situation. For example, where you have a brick façade which has a northerly orientation, you would be more concerned to keep some heat going into the wall from inside. A WUFI calculation can clarify if this is needed. Not everyone will have access to WUFI software, so the best approach in this case is to be on the conservative side and go for something around a 0.6 U value.

At Urbane Eco we are accredited installers of a range of SWI systems and have done quite a few over the last few years. My favoured IWI solution is to use predominantly a wood fibre product and occasionally a calcium silicate one in more vulnerable situations. Calcium silicate is quite a bit more expensive and gives less thermal performance; however it absorbs and releases water vapour/moisture in record time! Both wood fibre and calcium silicate must be used with lime plaster finishes, and, provided that breathable emulsion is used, will add value to the air quality in the dwelling by helping to buffer the humidity in the room. Gypsum plaster is not suitable as there is compelling evidence that modern gypsum plasters encourage condensation and consequent mould growth if used on walls that are supposed to ‘breathe’. New generation lime plasters are much easier and quicker to use than lime plaster of olden days (complete with horse hair etc). A plasterer who is used to using gypsum plaster can get used to using new generation lime plaster without onerous training. The plasterer will have to get used to a much longer ‘set’ time. Our plasterers invariably prefer working with lime plaster, they find it much more satisfying to use, and the finish they get with the fine top coat is really lovely (some clients have liked it so much they don’t feel the need to paint it).

The cost of a wood fibre and lime plaster IWI system is more expensive than other systems. However I would argue that the robustness of the system and the humidity buffering is a price worth paying, not to mention peace of mind that you are not causing interstitial condensation and damage to both fabric and human health.

Vapour control is still a consideration with wood fibre and I’ve used one product which has a built-in vapour control layer in the board, and another where the vapour control is achieved by using an engineered plaster. The idea is to control the amount of water vapour that’s absorbed into the fabric, especially during the winter, so that it can be released again when the room is ventilated.

So, for me the ideal solution to IWI is using 60 or 80mm wood fibre with a lime plaster finish and a breathable emulsion. When we do these jobs, I always like the ambience in the room when it’s been done. True, we haven’t achieved the holy grail of 0.3 U value, but that’s much easier to do insulating externally and with far less risk. I like that we are using organic and biodegradable products, I like that they are hygroscopic and manage moisture, and I like that they contribute to the internal air quality. (Someone said it’s like natural air conditioning!)

Given the number of period properties we have crying out for IWI, it seems to me that we need a tried and trusted, robust system that we can walk away from after installation and not worry about it failing due to interstitial condensation. I have discussed above a wood fibre and lime plaster option. There are systems on the market that deal with moisture control and as long as they deal with the issues raised above, I would be happy to use them. Building physics, it seems to me, is not well understood in the construction industry. A deeper understanding of the issues raised should be incorporated in IWI training in particular, and the industry in general.

Comments

Many thanks Nick for that helpful contribution to the IWI discussion. I am sure this will take years to resolve as the evidence emerges more widely as to what works more and less well in so many varying situations.
We have just (nearly!) finished a fairly deep retrofit/part rebuild on my own house (where I permitted myself to experiment): we had to partially rebuild external walls which we did in fully insulated dense block cavity walls with 150 extratherm and polymer ties, with reused lias stone tied into the external leaf; massive internal rubble walls remain and we have ground floor heated slabs over insulation so we're well endowed with thermal mass; the external north wall had been well rebuilt in mid 19th c with rubble and ashlar dressings, so this was internally insulated mostly using 75mm tissue-faced phenolic foam board against the masonry (faintly permeable) temporarily glued in place, with a polythene vcl, then 32mm battens fixed through to the masonry, and 25mm foil-faced polyeurethane board between the battens so as to leave a small wiring void, and plasterboard and skim. My theory behind this was to achieve a slight gradient of permeability - but time will tell. For one room's external wall, we applied an alternative - and very costly - treatment of 100mm cork board and lime plaster; for the larder wall we applied none.
We have an MVHR system but have used it seldom so far since our air-tightness was well above target (4 instead of 1) and we're usually 'under-occupied' and the air quality has felt good over the 10 months we've been in occupation.
Perhaps in 5 or 10 years time, we may have something more conclusive to report.

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