Diathonite Insulating Lime Plaster – Part 2, Detailing and Preparation

In Part 1 of this topic I described the process of evaluating the options for internally insulating the front wall. The process led to the decision to install 50mm of Diathonite Thermactive. The Diathonite serves several functions on the front wall; it forms the thermal envelope, the airtightness layer and it manages any moisture. It also serves as a wall finish ready for painting. For these reasons, we chose to also apply a 20mm layer of the Diathonite Evolution to all the ground floor walls except for the rear wall, which will remain as exposed stone. Several of these walls had previously had a “dot and dab” plasterboard finish which was showing signs of mould growth (and in some places it had moss!), had uncertain airtightness, and would be a thermal bridge to the uninsulated floor slab; the application of Diathonite would remedy these problems. 

There were several important preparation jobs necessary for the installation of the Diathonite and the mitigation of thermal bridges. These included structural adjustments, relocating pipes and wiring, airtight treatment of gaps, replacing cement pointing for lime, and masking up surfaces for protection during the spraying. This post explains the preparation details.

Thermal Bridges and Airtightness

The finished U-value of the front wall will be an estimated 0.58 W/m²·K, representing a fairly significant improvement on the untreated 2.7 W/m²·K. However, the thermal envelope is not entirely continuous on this project owing to a combination of cavity and internal wall insulation (CWI and IWI), and a few other thermal bridges. Thermal bridges represent an increasingly significant source of heat loss where the rest of the building fabric is improved, and so attention to these details is important.

At the junctions between the front IWI and gable walls’ CWI, the Diathonite would be returned along the gable by approximately 1m for the full height of the wall to overlap with the cavity wall insulation (see sketch plan in gallery above- I’m working on quantifying the effect of this in energy terms and I’ll post my findings when I get there). The same treatment was applied to the internal solid partition wall. However, in each of these locations, existing floor joists were bearing onto timbers supported on these walls, and which would have presented thermal bridges in themselves. We decided to take the time consuming step of peeling away the ground floor ceiling and the first floor boards to allow us to reconfigure the floor structure (see pictures below). The floors in these positions will now be supported by an internal timber structure, within the thermal envelope, and these can be boxed in or decorated to match the rest of the room. Whilst this probably represented a few days of work, it will allow continuity of the insulation, thus minimising the heat loss, and will also protect the timber structure of the first floor from condensation and possible decay over the long term. Importantly, it revealed that the stone work behind these timbers had not be re-pointed with the rest of the walls, and this allowed us to address these potential air leakage pathways.

Additional airtightness preparation included the use of tapes and other airtightness products at the window reveals. On some of the windows we used Contega Solido SL tape with the Tescon Sprimer (primer spray), both by Pro Clima. The tape has a fuzzy finish, which means that a wet plaster coat can be applied directly over it. The backing strip is in two pieces, one is about 1cm wide, which can be peeled off first and adhered to the face of the window frame; the remaining width of backing is peeled off after, and adhered to the wall. I found the tape pretty easy to use, and it stuck well onto the primed brick and rough stone surfaces.

The other approach we used was an airtight expanding foam, Illbruck FM 330. This is designed to be more flexible than a conventional expanding foam, and so will still be airtight even if there is settlement or slight shrinkage of the substrate. We used this where there were deep gaps to fill around the window frame.

Cement vs Lime pointing

Once the thermal bridges and the structural considerations had been addressed, we also chose to chisel out the cement pointing which had been applied to the stone walls at some point historically. Because cement is less porous than lime, it can block the transit of moisture, and therefore trap it within the wall structure. The disruption of the moisture balance can lead to accumulation over a number of years. Problems can manifest as damp areas of the wall (which can damage internal finishes), poorer thermal performance of the stone, and in turn, a harder to heat building. Of course the Diathonite is specifically chosen to handle these moisture problems, but given the cost of the product it seemed prudent to prepare as well as we could and mitigate any foreseeable issues in advance. Fortunately the cement pointing came away from the wall easily with no damage to the stone, and we then repointed with a lime mortar to limit the amount of expensive Diathonite being lost into deep gaps between the stones.

Services

The remaining preparation required the removal of the central heating system because the pipework was routed largely along the front wall. The central heating will be replaced later with underfloor heating.

We ensured that the wiring for the light switches, sockets, burglar and fire alarm systems, and thermostats was as required for the completed layout; this included mapping where different light switches would be positioned after the interior reconfigurations and then running wiring, where necessary, behind the Diathonite. To allow the treatment of the wall behind the electricity meter, consumer unit, burglar alarm panel and wifi router, we mounted these onto a free standing board which allowed access to the wall behind. This board will later be fixed to the wall and will have a cupboard built around it.

Inconveniently, we also had to un-plumb the toilet and bath, as well as remove the stud-walls which made up the bathroom! This was necessary to allow a continuous application of Diathonite behind these on the perimeter walls. Thanks to Dawn for letting us stop at her place whilst our toilet was out of action!

Masking

Finally, it was necessary to mask off any surfaces which needed protection from the Diathonite sprayer: This meant all window frames, flooring, other structures (like roof trusses, underside of staircase), and also wires and pipes which couldn’t be moved out of the way. Our furniture was stacked up in the centre of the room and sheeted over. Thanks to Alex and Adam for helping us out with the last minute furniture removal!

Check out Diathonite Part 3, with photos and videos of it being sprayed.