Flat roof insulation on ground floor apartment
Apologies for another flat roof question but I’ve read them all and circumstances for this built greatly limit what I can do….
I’ll be helping a friend refurbish a ground floor space of a three storey terraced building dating from 1890’s. The location is in London, UK, probably climate zone 4 by US standards. The ground floor is empty with two occupied apartments above it. This empty space will be refurbished into a two bedroom apartment. A portion of the ground floor ceiling sits below existing conditioned spaces (ie other apartments) but also directly onto the exterior where the current buildup is 2x12s, sloped furring strips, 3/4″ ply, then some form of felt/asphalt roofing.
I’ve done a number of flat roof projects all doing a “hot/warm roof” principle. However, the exterior is not owned by the owner of the flat, so it cannot be altered. The insulation must be on the interior. Spray foam here is hard to come by and expensive.
The existing soffits have been fitted with vent strips so I could in theory vent below the existing plywood, however I am aware of the limitations on flat roof venting, especially with air only coming from one direction (the rear of the building) in this case.
What are your expert thoughts on the best build up considering this is a low budget build not targeting high insulation (this old Victorian property has very little insulation anyway)?
My U Value code minimum is 0.18 (we don’t use R Value much here) which appears achievable with 4-5″ of xps from online calculators I’ve found.
My initial thoughts exterior to interior :
– 4″ layer of XPS fitted tight between joists and canned foamed around the perimeter. Do I leave an air gap below sheathing and attempt to vent this space as much as possible or not worth it?
– Fill remaining space in joist bay with Rockwool type insulation.
– Worth adding an air control OR an air and vapour control membrane (Pro Clima for example readily available here) under the joists?
– 1″ layer of XPS foam seams taped then drywall to act as thermal break
I’ve already suggested to the client to avoid recessed ceiling lights in the ceiling areas that back onto the exterior.
Much appreciated as always.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part