This is a typical industry standard way of insulating solid walls internally (IWI). Use a high performance insulation board (or between stud) with a vapour control layer. This appears to give a great new U value (better insulation) for the wall and also deal with damp.
The implication is also that the more insulation that you put on the better the wall performs thermally. This is a quick and easy solution for cutting carbon emissions, surely?!
Well, if you have read my other posts on U value calculators and damp predictors then you should know that there are several alarm bells ringing.
1. The existing wall is probably performing around 70% better than you expect.
2. The accreditated insulant and system has only been assessed for water vapour movement, NOT liquid movement
Work by Dr Caroline Rye and Jo Atkinson (nee Hopper) also highlight some more issues.
Dr Rye’s in-situ testing (i.e. measuring what is really happening) shows that the movement of water in the wall can be seriously affected, so much so that by applying non-breathing insulations with vapour barriers etc can actually pull water into the wall. The ‘dew point’ is also brought further in the wall and the chances of it happening are increased. This means that it is more likely that water will be sitting around delicate areas of the wall like joist ends. This is a major concern.
Jo’s work (and some new work that is currently on-going) also shows that poor workmanship of insulation can lead to thermal bridging. This thermal bridging was mean that levels of insulation beyond a certain level become fairly meaningless. Evidence seems to be suggesting that any more than 60mm of conventional insulation on internal walls is fairly pointless as the heat will just escape through the thermal bridges. So more is not always best!
So IWI has it’s issues. Is there an easy answer then?
Well there certainly appears to be an easier way of maintaining a much healthier wall. This is to use a breathable wall insulation (without a vapour control layer) with a breathable finish over the top (a lime or clay plaster finished with a breathable paint for example). This advice translates into something like using 50-60mm wood fibre boards and ensuring that reveals are also insulated as much as possible (this tends to be a 10-20mm board as a maximum due to space constraints).
By maintaining the flow of water vapour through the walls from inside to out allows the wall to operate as it was designed. It also allows liquid water to show up, so if there is a problem with a leak, rising damp, etc then at least you know about and can fix the problem before it starts to affect the wall adversely.
By having a slightly ‘less efficient’ system for the wall allows a little more heat into the structure and this means that the potential for any dew point is reduced to within very safe levels and also kept closer to the outer elements of the wall and hence away from joist ends etc.
So even though on paper this type of solution appears to be the worse option for carbon savings, actually in the real world it is the best option. It will give you thermal comfort, a healthy internal environment, natural humidity control, a safe structure, visible contact with the wall and also have less embodied energy in the products.