Design approach: wall insulation

Retro-fitting insulation to the walls of an existing building is a tricky subject, with much of the theory yet to be substantiated by real-world experience. However, the general consensus seems to be that external wall insulation (EWI) is the preferred option. The key issue is the movement of moisture within the building fabric. For EWI, as long as the insulation material itself is vapour-permeable enough, any moisture within the wall will find its way to the outside. And the original wall will now be on the warm side of the insulation, making condensation less likely (air holds more moisture, the warmer it is). Whilst not without its detailing challenges (window cills, drainpipes, etc), EWI is easier to fit in such a way that it forms a complete thermal envelope around the existing building fabric, with no gaps.

But given the brief of preserving the original appearance of the building, at least as viewed from the street, we opted for internal  insulation on the front elevation, and (mostly) external wall insulation to the rear.

For internal wall insulation (IWI), whilst the jury is still out on the appropriateness of a vapour-open approach in some circumstances, installing a vapour control layer (usually a vapour-impermeable membrane) seems to be the safer option when the thermal performance of the wall is unknown. This article explains the use of vapour control layer in internal wall insulation well.

Then there is the difficulty of completing the thermal envelope within floor voids (necessitating taking up/cutting back floor boards, and possibly cutting out joist ends and re-siting them onto joist hangers, so that the timber is not protruding into the cold side of the insulation, where they are at increased risk of decay). Fortunately for us, almost all the floor joists run left-to-right between the party walls, and cutting back floorboards and re-hanging a few joists is not significant extra work as part of a whole house refurb.

"Super-thin" internal insulation

Three of the rooms we are internally insulating have decorative plaster cornices, which are a key feature of the building and thus need to be retained. The original plan with these was to use a "super thin" insulating material up to the underside of the cornice, leaving the latter in situ, and accepting the cold bridge. But one of the cornices got damaged when we had to replace a rotten lintel, and we found a specialist plasterwork company that offered us a special deal price for renewing all three.

We still need super-thin insulation for two rooms that have bay windows though, otherwise the depth of the reveal created would have a detrimental effect on natural light (and these rooms face north, too). We investigated these two alternatives:

  • Nanopore VIP - Has the advantages of lower cost, increased performance, and no H&S issues. Nanopore (the manufacturer) were helpful, but are fairly new in the UK and couldn't provide a proven fixing method for IWI. We eventually came up with parge-coating the wall with a lime render (downside: 2-3 weeks for it to harden), then sticking the panels on with an adhesive, followed by taping and then plasterboarding. But Nanopore couldn't guarantee that the panels could cope with the weight of the plasterboard and plaster; and any fixing method using battens creates significant cold bridging.

  • Spacetherm aerogel - More expensive, not quite as thin as VIP for regs-compliant U-values, need to use PPE when installing and set up a well-ventilated cutting area. But has been used elsewhere for IWI. Manufacturer's recommended fixing method is using a nail gun directly into the brickwork. Spacetherm product is aerogel glued to a plywood sheet for strength, a VCL membrane, and plasterboard. We will be using Proclima tapes to seal the joints and edges. The overall thickness is 50mm.

After discussing the different approaches with our contractors and weighing up the uncertainty of the Nanopore option, we decided to go for Spacetherm, despite its extra cost.

"Normal" internal insulation

Given the high cost of Spacetherm, we chose an alternative for the rooms that have just a single, flat, wall - a variant of the UdiReco system chosen for the rear wall EWI. The insulation itself is wood-fibre-based panels, which are screw-fixed to the brickwork. The manufacturer supplies a vapour impermeable render to be applied instead of regular plaster, which forms the VCL. 120mm is required for regs-compliant U-value, though we will use 180mm.

External wall insulation

The system chosen for the rear elevation EWI is UdiReco, a wood-fibre product made by German manufacturer Unger-Diffutherm, and sold in the UK by distributor Back to Earth - who have been helpful and patient throughout this project. Although not the cheapest solution, it has good credentials in terms of vapour control, embodied energy, case studies (they've been using this stuff in Germany for years), and recyclability. The reference for our project was Charlie Baker's house in Chorlton.
These are the key elements of the EWI design:

  • Udi Reco 180mm thickness to main building rear elevation and return wall of outrigger.

  • Rafters extended and eaves detailed accordingly.

  • Render colour-matched to original bricks (deep red/terracota).

  • New windows to rear elevation installed flush with the external face of brickwork; this means the reveal depth of 180mm + render approximates to the reveal originally, when the windows were installed on the inner leaf of brickwork.

  • No rainwater or soil pipes on these walls (rear soil stack is internal; rainwater downpipe fixed to outrigger rear wall, which is internally insulated). Unfortunately some penetrations are unavoidable: boiler flue and MVHR terminals (3 of in total).

  • The original stone cills will have been covered over by the UdiReco, so replicas will be fabricated in steel.