Retrofit for the Future – Passive House Retrofit (Enerphit)

Anne Thorne Architects LLP won two of the Retrofit for the Future bids; both of them are mid terrace houses. One is in North London and one in Stoke on Trent. The Edwardian terrace house near Alexandra Palace, London, (Hawthorn Rd) was built as two flats, and is being combined into one 3 bedroom house, and the client is Metropolitan Housing Partnership. The Stoke-on-Trent house (St Lukes St) is part of a small terrace on a steeply sloping site, which was saved from demolition under the late Government Pathfinder scheme through local opposition. The client is Sanctuary HA. Fran Bradshaw and Jennie Swain report...

Both houses are 225mm solid brickwork walls with timber floors and tiled/slated roofs, although the St Luke Street front elevation is rendered, with 102mm thick party walls. Both have back extensions; Hawthorn Rd is two storeys while St Luke St is one and needed to be completely rebuilt.

The architects have been concerned at the divergence in views between those for whom the priority is to retrofit or renew all buildings to as high a standard as possible using the latest and most effective technologies and products, and those for whom conservation of existing buildings and building elements takes precedence. Our own view is that reducing energy use will be achieved most effectively through a combination of conservation, remodelling and rebuilding, and that working with natural materials is the most effective approach, as they deal with moisture in the fabric better, and have lower embodied energy.

These 2 houses are part of well liked street terraces, with economical footprint and shared party walls. The brickwork street elevations form part of their popularity, we think we should try to retain them. At the rear, where the kitchen is often located, and where people spend a great deal of time, because of the extension there is a much greater surface area. Our strategy was to insulate the rear of the building externally, while the front elevation (largely window) is insulated internally. We also wanted to test out using natural insulation materials internally. This strategy poses some challenges, and we wanted to take the opportunity the Retrofit for the Future competition offered to test it out, and get some really detailed monitored results.

Energy targets

PHPP modeling of the existing house reflects the energy required to keep the two houses at an internal temperature of 21°C (set by the Technology Strategy Board competition standards) See Tables 1 and 2.

Detailing issues

Accurate site information

Our initial funding application modelled the two terraced houses in PHPP, assuming U-values for walls, floors and roofs of Passivhaus standard, i.e. 0.15 W /m2K, and was based on existing plans given to us by both clients.

Once we had accurately drawn plans and sections of the house, we could more accurately model the heat loss of the thermal envelope, and tailor details and junctions to each of the existing houses. This was particularly important in terms of addressing and minimising thermal bridges. Key junctions, where we were unable to remove thermal bridges, such as internal solid walls bearing on the existing slab, were modelled in THERM, their impact minimised cost-effectively where possible, and their heat loss added to our PHPP model.

Wall construction

We were aiming for low U-values using breathing/natural building materials for issues of carbon sequestering, low embodied energy, and as a means to deal with moisture within the construction fabric and internal atmosphere of the houses.

Located in a conservation area, although not itself the main focus of the designation, Hawthorn Road originally had timber double hung sash windows. These had been replaced with UPVC windows, some of which were fire damaged. A sympathetic conservation officer allowed the new triple glazed timber replacement windows, while internal insulation to the front wall means brickwork remains the material of the front elevation. We promised we would find a copy of the Edwardian triple panelled front door to replace the damaged modern one, so that the elevational composition, with its 2 front doors, is retained (one door now leads to an extremely useful external store).

St Luke Street terrace could not be insulated externally because the front elevation opens directly onto the street, (front line of brickwork is site boundary). The windows for both houses are Green Building Store Passivhaus certified, Ecopassiv triple glazed. We sought internal insulation solutions, and researched possible insulation materials, including wood-fibre, sheepswool, hemp and cellulose insulation (recycled newspaper). As yet, none of these products come in a rigid panel form that could achieve the U-values we were targeting, so we adopted a timber frame approach to hold the insulation. We initially considered using I-beams as our internal lining frame to contain the insulation to minimise the amount of timber bridging the insulation. However, throughout the project we were working closely with a contractor who was costing the project, and to keep within budget and the cost of the wall lining down, we 'down-specified' to using timber battens and counter-battens, with insulation full-filled between. The advantage of this is that the contractor finds it very quick and easy to install on site, using standard sections. To overcome the thermal bridging of the timber battens as they cross, we then internally lined the insulated framing with a continuous woodfibre insulation board.

This layer of insulation had 2 advantages: firstly, it brought our U-value to target; secondly, it protected the air-tight membrane within the build-up from being uninten­tionally punctured.

At Hawthorn Road we used the woodfibre board as a carrier for internal lime plaster, whereas at St Luke Street, for comparison, we lined the walls with plasterboard and skim finish.

Thermal bridges

The mixed construction creates a second 'thermal bridge' where internal and external insulation meet, and this has been carefully detailed with an overlap. Other bridging points have been designed with an insulation overlap, where the internal brick walls are taken up through the floor for instance. Because the external insulation is such a deep layer, the rafter ends of the roof have had to be extended to overhang the insulation, in this case not a problem as the roof covering was being replaced. At the ground level the insulation has to be extended well below ground level, to overlap the floor insulation; it is not possible here to form a continuous layer of insulation.

On site - Build tight

Both contractors and site teams have embraced the air-tightness issue on site and have really committed to the principle of building air-tight. One carpenter said he'd been having paranoia dreams about whether he'd taped and sealed his framing properly.... However, keeping in mind which layer is acting as the air-tight layer, and the importance of a continuous 'line' is a challenge. Any gap was sealed, plugged and taped, even if it was not related to the air-tight layer, as a 'belt and braces' approach ­e.g. a service penetration through an externally insulated wall was air-tight taped to the external face of the EPS insulation, even though the internal plaster is the air-tight layer. However, we took it as evidence of the real pride and enthusiasm with which both contractors have taken on the retrofit challenge and the energy targets.

Mid-construction air-tests were vital to push the build to achieve our air-tight targets. At St Luke Street, we hadn't made it explicit to plaster behind electric back-boxes in the party walls, so these, along with the patched plaster to the party walls were a major source of air leakage. This was remedied by an all over skim coat and re-plastering and sealing behind the back-boxes.

Surprising air-leakage paths

At St Luke Street during the mid-construction air-test, air flow was evident through the (so-called) air-tight cable grommets, and the cables had to be secondarily sealed with sealant. We think this was due to electric cables being rectangular, whereas the hole cutter that comes with the grommet, cuts circular holes, so a case of a squarish cable in a round hole! A small amount of air flow was felt through the staples that fixed the airtight membrane to the timber framing to the walls. As a belt and braces approach, we taped over the line of staples to all walls and ceilings.

Airtightness and drying out

For the internal woodfibre insulation used at Hawthorn Road, we used a lime plaster finish straight onto the walls. However, to save on costs, rather than the proprietary thin-coat finish system, we used standard lime-hemp plaster mix. This gave real problems on site because of drying out times. As the house retrofit progressed, and the house became more air-tight, drying times were elongated, and the plaster stayed soft for several weeks, which led to the the plaster being bumped and damaged during the remainder of the construction. At St Luke Street we drylined the walls with plasterboard, so no issues in this respect as yet, fortunately.

Air test results

Throughout both retrofit projects PHPP modelling, we were aiming for Passivhaus retrofit (Enerphit) air-tightness standard of a maximum 1.0 air changes an hour, and set out from the beginning of the design with this target. The existing houses were air tested to give approx. 15 and 10.9 air changes for the London and Stoke houses.

The London house final result of 2.2 air changes an hour was at first disappointing, but we knew we had high aspirations - the contractor carrying out the works regularly gets results of down to 1.5 for new-builds, so our target was ambitious.

Interestingly, when the door to the boiler cupboard was sealed, containing the solar thermal store, boiler and MVHR unit & intake, the result came down to 1.4 air changes an hour, so definite leakages around service penetrations that we need to concentrate on in the future, and hope to remedy with some taping, although hard to retrofit the retrofit of the ducts now installed due to space to work.

A further air-leakage path was the existing stair against the party wall, which we hadn't anticipated, so an area to address on future retrofits.

We carried out an exercise using PHPP when the mid-­construction air test was carried out, where we got 4.4 air changes per hour. We double-checked the size of the heating system (MVHR air heating and top-up wet radiators) to check it could cope if we didn't improve our air-tightness from this result. Capacity of the boiler and radiators was within range, so no concerns that even if we didn't hit our target on air tightness, the tenants wouldn't suffer with an under-sized heating system.

On reflection, considering the challenge of the geometry of the London house and its initial air test results, we still think the contractors have done a great job, and we can see the areas for definite improvement on the next job.

The Stoke-On-Trent house has yet to have its final air test, but as it is a smaller house with a much simpler volume, we are more optimistic that we may hit our 1.0 air tightness targets.

The contractor conducted tests with the 'standard' sealing and with some 'additional' sealing to the ventilation plant area. He did a pressurisation and depressurisation test in each case to produce an average as Passivhaus requires,

The air test results for the Hawthorn Road property can be seen in Table 2.

Contractors; different structures

The contractor for the St Luke Street project, Seddons, is a management contractor, solely employing subcontractors for each trade, so it was of paramount importance that all information was in our drawings and specification, and the contractor was contractually, as well as in practice, tied to delivering the project. The site manager at 23 St Luke Street organised a tool box talk on site for the carpenters regarding the use of lntello tapes and membranes. The architects explained the design and construction principles to all site operatives.

Site solutions

Although you always hope you've thought of everything in tender drawings and specifications a few site issues arise that require site solutions, particularly as unknowns were revealed during the stripping out of both houses.

Snaking ducts

Installing the MVHR unit and the metres and metres of ducts each house needed proved a challenge for the service installers. There is never an ideal duct route when retrofitting ducting to an existing house, and both our houses had rear extensions with changes in level that meant we had awkward junctions and boxing out to negotiate and plan. We'd recommend planning and drawing out each duct and associated silencers to scale when planning an installation, as both installers had trouble fitting the kit in the space allocated, which were considered generous at the design stage.

One fantastic idea of the St Luke Street installer, was to make a mock-up of the MVHR unit out of cardboard to use on site to plan duct routes, rather than have this expensive bit of kit knocking around on site.

Ventilation, heating and hot water

In spring, summer and autumn the windows can be opened for fresh air, but during the winter. This would allow all the carefully retained heat to be lost. Instead a heat exchange on the ventilation system removes the heat from exhaust air (extracted from kitchen and bathroom) and puts it into the cool fresh air - MVHR by Maico. This system is almost 90% efficient, so that only a small amount of heat is needed to keep the building and its occupants warm. Warmth is also provided by the people themselves, their electrical goods, like fridge and television.

A small amount of heat can be delivered with the fresh air (to bedrooms and living room), while small (gas fired) radiators are provided as back up in case they are needed. This is one of the things which will be monitored to see how often, or if, the radiators are needed.

Hot water is provided by the same gas boiler, a Rotex GSU 320, and by solar thermal panels on the south facing rear roof. The kit takes up far more space than conventional systems, and we think this could be a major problem for social housing, and for small houses and flats generally - although in both these houses we found the space required.

Contractor's comments

We asked the contractor at the 10 Hawthorn Road retrofit to give us his feedback on how he felt the project went.

How did this differ from a new build project?

"Areas of the building where we were dealing with integrating new layers of insulation with triple glazed windows we found very successful, it was dealing with existing building elements that was much more difficult. The stairs caused some problems for example. If specifying everything from scratch it would make life simpler, but most people don't live in brand new houses!"

What do you feel were the main factors in achieving high levels of airtightness? ''It's all about the detail. to get the building air tight you need a dedicated team with the time to spend getting each little bit right Taping all joints of the lntello membrane was time consuming but effective. "

What would you change were you to repeat the project?

"We had a great team and generally the project went very smoothly. If we were working on more houses in the terrace it would have helped by reducing cold bridging issues with party walls. Also in terms of sequencing to allow for extended drying times of the lime plaster in cold weather.


For the new residents, living in the two houses will be a very different experience from their previous flats, and we are really pleased that they are very interested in how the retrofitted building will work, and will be partners in helping us to understand this, over the 2 year monitoring period. The control systems have been designed to be easy to operate, and monitoring includes both energy used, and temperatures and humidity experienced, both in the house and within the building fabric.

Jennie Swain and Frances Bradshaw