So how do you get a leaky Edwardian building to be so airtight that it can be heated with a single towel rail? Robert Prewett, the architect behind the retrofit, takes us through the project’s first steps …

Making period homes highly energy efficient is one of the most challenging jobs facing the industry. Robert Prewett, partner in Prewett Bizley architects, has bravely taken on the job of refurbishing a terraced home in Balham, south-east London, to near Passivhaus standards. To get this into perspective, the leaky old 1910 three-bedroom solid walled property will be transformed so it can be heated with a single towel rail. This means the house has to be highly insulated and airtight: the team are targeting 0.6m3/hr/m2 which is 17 times better than the Building Regulations demand. Prewett is posting a fortnightly diary on progress at Here are excerpts from the first month on site.

Preparation and installing the thermal and airtight lining

Dave Manby, a builder we worked with on another low-energy refurbishment at Culford Road in north London, has stripped the building back to its bones, removing loose plaster, plumbing and wiring. This allows problems to be identified and repairs made. It also gives the team a better chance to create a continuously insulated and airtight lining.

For the relining work we carefully balanced material and labour costs against performance and buildability. The resulting form of construction is, in our view, a good balance between economics and robustness. For the first floor ceiling 70mm of phenolic insulation was tacked to the underside of the existing timber joists and all joints taped for air and vapour tightness. Below this, 12mm of oriented strand board (OSB) has been fixed through the insulation into the same joists. Again the OSB has been taped as it will form the primary airtightness layer. At wall junctions similar taped joints will ensure continuity of the airtightness layer.

Below the OSB, battens will form a servicing zone for cables and below that 32mm of insulated plasterboard will supplement the overall thermal performance. Eventually the joists above will be filled in and overlaid with glass wool insulation, and building paper will be laid between layers to reduce “wind washing”. All of this will provide a U-value of just below 0.10W/K/m2.

The following had to be borne in mind on the design of this particular build-up:

  • Avoid cutting rigid insulation into strips to go between timber structure - it is labour intensive and quality of fit can be hard to control
  • Use glass wool in the timber structural zone as it fits snugly and any moisture that enters can evaporate later
  • Use OSB and tape for airtightness at ceiling (previously successful on other projects)
  • Form air-tightness layer in such a way that all services can be installed before airtightness testing but still allows finishing linings that can be fitted quickly and safely after testing
  • Avoid penetrating airtightness layer with cables or other services
  • Avoid thermal bridges at edges - having some insulation below the joists eliminates a cold bridge that the joists can create.

Making the house airtight

Our energy calculations indicated that the loss of energy through uncontrolled air leakage represented about a third of the total heat loss. Of course the insulation linings are likely to help control air leakage to some extent but experience shows that even the modest gaps and cracks left during “normal” building works mean that this energy path is reduced by only about half. In order to reach our target of reducing heat loss by a factor of 10, we need to do vastly better than that.

Previous experience shows very good results can be achieved if the following considerations are carried through on site:

  • Have a clear strategy for the whole house, and make it simple to understand and build. Any confusion or requirement for origami-type skills will lead to failure
  • Name the airtightness layer explicitly. One bucket without any holes is better than one leaky bucket inside another leaky bucket
  • Maintaining absolute continuity of the airtightness layer. This is essential. Any compromise or design fudge in this is likely to cause big problems
  • Every time there is a geometric or a material junction, there is a potential weak spot. Although special tapes can solve most of these problems, designers should give special care to how each junction will be made and the sequence that should be followed
  • All penetrations though the airtightness layer must be managed carefully. Special grommets and sleeves are available
  • Everybody who comes on site should be made aware of the importance of the airtightness strategy and how it will relate to their work.

The air-tightness layer in our project consists of two commonly available materials: either a wet plaster layer or a dry OSB layer and two types of tape. Almost all conditions can be dealt with in this manner. Consistency of detailing lessens the burden on the site team and allows them to refine their technique.

Ceilings, the front wall, party walls and suspended ground floor are sheathed with OSB (or particle board) as an airtightness layer over the insulation layers. Every joint between the OSB sheathing is taped.

Corner junctions can be done fairly easily with this dry form of construction. We are using a tape that is very sticky and can accommodate a little movement. All window frames will sit within plywood boxes that can be taped to the internal sheathing and thereby provide continuity to the window frame.

The walls to the back of the building, which will be insulated externally, have an internal airtightness layer formed with two coats of sand and cement with a skim finish. This finish will extend through the joist zone at first floor and down to the ground-floor slab.

We have kept the number of penetrations of the envelope to the minimum as services run through dedicated zones in front of the airtight OSB or render layer. Special grommets or sleeves will be used for the 12 unavoidable penetrations to reduce air flow as much as possible.