A domestic insulation project. Cast insulation and warm junction details

One of the advantages of doing a deep retrofit and using a cast insulation material is that typically hard to treat areas become accessible to good quality installation (as opposed to an optimistic squirt with a foam gun). What follows is a few photos of some of the tricky bits.

Joist ends

Ever since I owned the house I’ve been aware that there is a huge amount of air movement in the gaps between floors. It’s not been draughty (thanks to wall to wall fitted carpets), but it could be a significant heat loss, as warmth from downstairs rooms is lost in the junction between ceiling and upstairs floor, as opposed to upstairs rooms benefitting from downstairs heat. My assumption has always been that this air movement is due to the way the joist ends are fixed into the wall. When casting insulation onto the existing wall, the easiest option would be to simply start at floor level, but we opted to go for a more complete solution of removing the edge floor boards.

At each point where a joist enters the slate rubble wall, it seems to have been simply placed on a reasonable sized bearing stone, with no mortar holding it in place. Perhaps lime mortar was used, but it has long since turned to dust. As such, these joist ends would have had a lot of air movement around them. Whilst in one sense this is good, as it’s important to not have water vapour condensing on the joists and rotting them, the associated air movement can be a significant heat loss.

Joist ends were pointed with lime mortar. This prevents air movement, but is vapour permeable, in order to limit the risk of water vapour condensing at the junction between slate and timber.

Miscanthus-lime was then cast between the joists and continued up the wall, with floorboards then trimmed and replaced.

Sloped ceiling

The bathroom had a small area of sloped roof, with no insulation in at all; it was just lath and plaster onto the roof joists, with a torched slate roof. The simplest solution would have been to apply a board insulation (e.g. cork) directly onto this area. However, being keen to have a complete insulation layer with minimal junctions between materials of different types, the decision was taken to remove the sloped ceiling to uncover the joists, and then to cast insulation onto the slope.