The Marchmont flat woke up at nine degrees on a January morning before we started. It now wakes up at nineteen, with the same boiler. The difference is 100 mm of wood-fibre board on the inside of the sandstone, and a set of details I have now used on six flats. Here they are, because I would rather the city copied them than got them wrong.
Why not the outside
You cannot insulate a listed tenement externally, and you should not want to: the ashlar front is the point of the building. Internal insulation is the only route, and the danger is well known: warm damp air from the flat reaches the cold stone behind the insulation and condenses there, and the stone never dries.
The rules I use
- Vapour-open everything. Wood-fibre board, lime plaster, clay or lime paint. No plastic anywhere in the build-up.
- Leave the stone a way to dry. The board is bedded in a lime adhesive with no air gap, so moisture that reaches the stone can wick back into the room in summer.
- Insulate the reveals. Thinner board, 40 mm, wrapped into every window and door reveal. The cold bridge at a reveal is where the mould starts.
- Fix the roof and gutters first. Internal insulation on a wall that is being rained on from a broken rhone is a wet wall with a warm coat.
The numbers
The wall went from a U-value of roughly 1.3 to about 0.35. The flat's gas use fell by a little under half in the first year. The owners kept a thermometer diary for me, which is the kind of client I like.
The measure of a retrofit is not the U-value. It is whether the stone is still dry in ten years, and whether the shutters still shut.
I am writing a longer version of this for the practice notes, with the drawings. If you own one of these flats and want the details, write to me and I will send them.
