Anne Thorne Architects’ PassivTerrace retrofit

Last week Footprint visited Anne Thorne Architects‘ recently completed PassivTerrace retrofit in Hawthorn Road, LB Haringey for client Metropolitan Housing Partnership.  This mid-terrace, solid-wall Edwardian house is part of the Technology Strategy Board’s Retrofit for the Future programme which is funding the retrofit of 86 different low rise affordable homes with the aim of 80% reduction in CO²emissions.

Front of house: internal insulation

Rear of house: external insulation

The approach for the retrofit was ‘Fabric First’ so high levels of insulation (250mm) and airtightness were key parameters of the project.  Consistent with what is proving to be the norm with terraced house retrofits, insulation is external at the rear of the house and internal at the front of the house driven largely by external appearance and its location in Campsbourne Cottages Conservation Area.

Photos showing the depth of insulation required

Triple-glazed windows from the Green Building Store

The house is approached as a whole circular system: the heat recovery ventilation system is 90% efficient. Additional heat requirements come from solar and internal gains, although these are unlikely to be as high as with a new build Passivhaus. Although many Passivhaus projects have no central heating system, the project team opted for a small boiler with strategically placed radiators to alleviate extreme weather conditions.  A gas boiler supplies hot water, approximately 60% of which is predicted to be powered by solar thermal panels on the roof.

Small radiators are used for extreme weather conditions

Natural insulation has been used throughout the interior: sheeps wool for the front of house and wood fibre on the party wall, to prevent heat being lost to neighbours.  Chimney stacks were also filled with insulation. Due to financial constraints rendered polystyrene insulated panels were used for the rear wall; it was deemed more important for the internal walls to have natural insulation than the external walls due to breathability.  Moisture and humidity are a concern with internal insulation in retrofits, and alongside the standard post occupancy evaluation that will take place over a two year period, UCL’s Ben Croxford and his students will also undertake additional evaluation to monitor moisture in the internally insulated walls. See UCL student Elisa Prestia’s informative blog which documents the construction process from June 2010 to completion and includes the house plans and interviews with the project team.

A draught-free layerof continuous internal plaster links the building membranes within the walls, ceilings and floors, whilst a new triple-glazed internal lobby allows the existing Edwardian front door to remain.

According to Anne Thorne Architects, PHPP was used to model existing and potential heating needs and energy use, and to understand the effect of small adjustments to the design of insulation, quality of windows, air-tightness , cold bridging, ventilation strategies and heating & hot water systems.

Final Passivhaus testing for certification will take place shortly.  The total budget for the project was £200,000, which included refurbishing the fire-damaged property, converting what was previously two flats into one home, and the energy retrofit which was funded by £80,000 from the Technology Strategy Board.

Primary energy target: 90 kWh/m2/yr
Annual carbon emissions: 15 kg/m2


Front Wall: 0.21 W/m2K
Party Wall: 0.31 W/m2K
Rear Wall: 0.15 W/m2K
Ground Floor: 0.18 W/m2K
Loft and Roof: 0.11 W/m2K
Windows: 0.75 W/m2K

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13 Responses to “Anne Thorne Architects’ PassivTerrace retrofit”

  1. Was it £200k for one house or 86 houses?

  2. £200K for one house!

  3. I’ve been doing something similar but haven’t spent as much money (about £5k) and so haven’t managed the same reduction in energy (about 50% reduction), but it’s probably more realistic for what people can afford to do. I’ve written about it every month or so on The Ecologist’s website - do have a look and compare, next one coming in a week or so on wood burning stove.

  4. not really a sustainable solution to the problem, £200K unfeasable!!

  5. What is the point of doing a refurbishment for £200k?

    In termsof improving the national housing stock, we need to know what can be done which is of benefit for £20k to £40k.

  6. I’m working on an Retrofit for the Future Scheme also. For the £150K project funding we are improving the performance of not one but two properties. This is an all cost for design, and construction whereas most construction budgets exclude design fees.

    We are not going full PassivHaus (or even EnerPHit). We held back on such aspirations for pragmatic reasons:
    1) we wanted to enable tenants to be able to remain in the properties
    2) the homes had recently had new kitchens

    This meant that we did not insulte the floor floor slab (though we did some other things to help improve it’s U-value.) Though we do consider it possible to upgrade the floor in the future. PHPP calcs suggest a reduction in CO2 emissions of just short of 80%.

    We have three such RfTF schemes (6 homes) - each addressing homes built usin different building technologies. So our approach is a highly replicable solution. In essence retrofit construction costs can be brought down to less than £75k. Once there is a fully functioning retofit market (i.e. technology supply and learning curves have been addressed) it is reasonable to assume that costs should come down even further. I’m not suggesting that, say, £55K-65K this is perfect but at about 60% less cost than the example in this article it is certainly at a more replicable end of the scale.

  7. This looks extremely interesting and looks good from outside (I live nearby) but what did it actually cost?

    I am refurbishing and extending an abandoned chapel house in North Wales, using Ecopassiv windows and following details from the Denby Dale project for new external wall construction.

    Any advice would be useful.

    Richard Griffin

  8. Just to fill in on the budget, Metropolitan Housing Trust have a rolling programme of refurbishment to their standard, (more like Decent Homes standard) so the work was originally planned as part of that programme. The building was two flats originally, which had been badly fire damaged, so the work entailed both refurbishment and converting to one house. The extra money from the TSB to get to 80% energy reduction was £80,000 approx, (the rest was £120,000).

  9. I fear that such schemes could almost have been designed to discredit retrofit insulation.

    Let’s see … if the marginal cost really is £80,000 per dwelling, done en masse, and we have to treat 25.5 M dwellings, we’d need just over £2 trillion for them all. That’s more than one year’s GDP - and they still need a (admittedly smaller) heat input.

    But this is not all. Doing the dwellings still leaves all the hospitals, schools, offices, shops and factories to do. At least £3 trillion in round numbers to do the UK building stock.

    My puzzlement is why TSB did not fund a proposal which would have shown how to reduce CO2 emissions by 82% for ca £10,000 per house, and without decanting the tenants. The measures were confined to those which in a mass market would cost less than about £40/tonne CO2 saved, and the bulk much less than this. (Most of the TSB budget would have been spent documenting the cost and performance of different measures and demonstrating a few which are expensive in the UK as they haven’t reached a mass market cost, unlike abroad).

    The UK excels at giving grants which are rigidly technology-specific, even if the technology is a “lemon”. It sometimes seems that the more expensive the techniques, the better their prospect for funding - truly Alice in Wonderland. But the UK can’t afford it. We’d better start looking seriously at energy economics and the cost of different CO2 abatement measures if there is to be a secure future after oil.

  10. At PRP we have completed five of the Retrofit for the Future houses – one of which was a solid wall property. We looked at a range of fabric solutions for the walls, all of which needed to be internal linings due to the house being located in a conservation area, which precluded the use of EWI. The particularly small internal rooms meant that we needed to use very high performing insulation to minimise the impact on internal floor area, whilst achieving reasonable U value improvements.

    This lead us to the use of Proctor’s ‘Spacetherm’ Aerogel insulation, bonded to a foil-backed plasterboard, with a foil-faced phenolic board within the depth of the floor zone. We’ve had lengthy discussions in the office about what impact this vapour impermeable solution would have on the walls and floor structure, but we were unable to find a vapour permeable, natural IWI product, that balanced U value improvements with shallow lining depth.

    We’re aware that this will change the movement of vapour through the structure and have inserted moisture monitoring tags to the ends of the joists (that are embedded in the solid walls) to monitor the moisture content in the wall/ends of the joists, during the 2 year monitoring period. This should give us an insight into the impact of installing the vapour impermeable IWI.

    I’d be interested to know the total build-up/spec of the IWI and any membranes that were used at the PassivTerrace. PRP is keen to explore the use of natural insulation products for other retrofit projects we’re currently involved in, where floor space is less of an issue.”

  11. Hi,
    I have a question to Mark S

    I am currently doing a dissertation for my final year university project, on looking at retrofitting Victorian Houses.I am mainly interested in heating systems and therefore potential cost reduction in terms of bills.I would be very grateful if You could provide me with some additional information on the project You arw working on at the moment .

    Thank you


  12. OK so outcome was impressive in a very narrow and limited way. But the bottom line is - this approach to retrofitting is neither sustainable or affordable - points already made above.!
    The lack of not joined-up thinking in tackling all the environmental issues for buildings is frankly alarming. What about the water, waste and pollutant footrpints across the lifecycle of this building. Please can we start to join-up the dots and that includes the TSB, supporting cast in government and building sector. I hope the National Audit Offcie or someone takes a closer look at this unholistic approach to spending public money!! Shocking!

  13. Three main factors to get right for retrofitting existing housing stock. Insulation ,insulation and insulation.
    Yes we need to ventilate right , however just look at the % savings against cost and it is plainly clear that the most cost effective measures for saving energy and reducing CO2 emissions is simple insulation. Think of it like a thermos flask! simple. It keeps its temperature stored, be it hot or cold, for long periods.
    The easiest way to save energy is not to use it in the first place.
    All other measures are great but very expensive and the pay back time is to extreme.
    There will always be an improvement to the efficiency of yet another boiler ,solar panel, wind generator , window etc in the same way as computers have made dramatic strides.
    Then we see all the old worn out models heading for the landfill/recycling site to make new ones and more expensive super models being fitted in their place.
    Once the fabric of the building has been subject to the simple act of insulating
    (fitted correctly) and draught proofed you have achieved the highest amount of % saving for the lowest pay back term. Educate the occupants on how to live in the newly insulated property such as switching lights off etc and keeping
    the place adequately ventilated and we have the battle won .
    The best part is that it can all be carried out by semi skilled operatives under supervision and so will mean loads of employment opportunities countrywide.
    PS £200,000 would cover a block of 10 terraced houses in an envelope scheme no problem !

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