I’m hoping that most of the blog posts about this project will be more photos than text, but it seems like there are a few elements where explaining the rationale may be of interest, in particular when it comes to the details on insulation choices.
Choice of materials – fireplace
When the old fireplace was being opened up and exposed several years ago, it was clear that the wall immediately behind was extremely thin (one could literally hear voices from outside). Consequently, as part of insulating the room it made sense to improve the insulation behind the woodstove, to increase the amount of heat that would enter the room rather than be lost through the wall.
Regulations designed to prevent fire dictate the minimum distance that combustible materials (timber, insulation etc) must be from a wood stove. If the fireplace had been a more regular shape, a heat shield (essentially a piece of metal to reflect heat) would have been a potential option on the section of wall behind the stove. However, I wanted to keep the original shape of the fireplace, and given that the space was a bit tight, the choice was essentially either to not insulate the fireplace at all, or to use something with the highest possible fire rating (Euroclass A1) in which case the material is deemed non-combustible. Fortunately, a thermally insulating plaster (Diathonite Evolution) is available that has this rating, along with a surprisingly good thermal performance. Whilst I am sceptical that its thermal performance is really so much better than other cork plasters on the market, and it is eye-wateringly expensive (£39/bag), we chose to use it in this application, albeit mostly because there weren’t really any other options. It is certainly a strange material to mix and use; very light and fluffy, with a tendency to float on the water you’re trying to mix it with.
We ordered an extra bag of this plaster, and we will do some in situ U value testing of it on a test wall at work, in order to see how good it’s thermal performance really is.
Choice of materials – window reveals
It wasn’t until the plaster was removed that we were able to decide how to insulate the reveals. The key determining feature in reveals is the thickness of material that it would be possible to apply without it protruding over the edge of the frame over the glazed area. This in turn depends on the size of the window frame in the opening.
Window reveals are particularly vulnerable to dampness. This is due to a combination of relatively still air, heat loss through the frame, the fact that the edge of a glazing unit is less effective than the centre, and the relatively short distance of wall through which heat would need to pass around the outside of the frame. Insulating the window reveal therefore has two main purposes; we are not only trying to decrease heat loss, but we also want to increase the surface temperature of the wall in order to minimise the risk of moisture condensing and causing damp/mould. Since the insulation value of miscan-lime insulation is not as high as for some other vapour open materials, the decision was made to insulate the reveals with cork board in order to maximise the insulation value and the likelihood of having a warm surface temperature. The cork plaster used in the fireplace would also have been an option, but given my scepticism about it’s thermal performance, cork board seemed like the appropriate choice.
The reveals were pointed, and lime plaster used to produce a relatively flat surface. A specific cork adhesive plaster was then applied and the board pressed against it. This board will have a plaster skim applied. The cork protrudes beyond the edge of the reveal to the level that the miscan-lime will be cast. The base of each reveal is built up with miscan-lime, with a cork board on top, and then the window board will be replaced.
How well will these details perform? Next winter I will do some thermal imaging of them and report back on the findings.
Project management and building work undertaken by Steve Cole, Addasu.