The Passivhaus standard requires an airtightness of â‰¤ 0.6 ach (air changes per hour) @ 50Pa (current Building Regulations require 10.0 ach @ 50Pa). This high standard ensures draught-free comfort, protects the building fabric from condensation due to leakage of humid air, and ensures that the efficiency of the heat recovery ventilation is not bypassed by leakage ventilation. The completion air test by Paul Jennings of Aldas confirmed an Airtightness result (at 50 Pascal’s): 0.58 ACH or 0.52m3/m2/hr
As a bonus to meeting the Passive House standard criteria the house achieved an ‘as Built’ EPC A rating and a numerical SAP figure of 104 making it eligible for a level six Code for Sustainable Homes rating. Tim & Sarah have now moved in, but are still finishing off certain superficial completion tasks and will monitor actual performance against design criteria over the next twelve months. We will report back in due course to let you know how it performs in reality.
Building Fabric: Working with Viking & MBC Timber Frame we developed the low U value, super-insulated, timber building fabric build ups as set out below:
Ground floor: 20mm reclaimed maple flooring, on 50mm battens with Knauf glass wool insulation between, on 100mm thick reinforced concrete floor slab, on 300 mm Aerofloor EPS insulation, on Viking Passive Slab (PHI certified). U-value: 0.105 W/m2K.
Ground floor walls: 10mm thin coat silicone K render, on 100mm Masterblock recycled aggregate concrete block, on 50mm wide drained cavity, on Pro Clima Solitex WA wind-tight membrane, on 12mm thick Panelvent sheathing board, on MBC 300mm stud twin wall with full-fill Warmcel 500 cellulose insulation, on 12mm OSB with taped joints. The walls are finished internally with 50 x 50 battens to form service void insulated with Knauf glass wool insulation, on 12.5mm Gyproc plasterboard internal lining with plaster skim. Dulux Ecosure emulsion paint. (PHI certified). U-value: 0.112 W/m2K.
First floor walls: Eternit Cedral weather boarding on, 50 x 50 mm vertical battens to form ventilated cavity, on Pro Clima Solitex WA wind-tight membrane, on 12mm Panelvent, on MBC 300mm twin stud timber wall with full fill cellulose insulation, on 12mm OSB with taped joints, The walls are finished internally with 50 x 50 battens to form service void insulated with Knauf glass wool insulation, on 12.5 mm Fermacell board with Dulux Ecosure emulsion paint finish. (PHI certified). U-value: 0.110 W/m2K.
Unfortunately the sub-contractors on site omitted to install the specified Pro Clima Intello Plus vapour control membrane assuming the 12mm OSB with taped joints would suffice. This goes some way to explain the marginal pass rate at the final airtest, when compared to other subsequent timber frame projects, where we have come to expect a result of 0.2 to 0.3 ARC (at 50 Pascal’s) as standard.
Roof: Bob tail fink truss rafters at 600 c/c with 620mm full fill Warmcel insulation, followed underneath by Pro Clima Intello vapour control layer / air tightness barrier, 25 x 50 battens to form a services void, 12.5mm drylining Gyproc board, Dulux Ecosure white emulsion paint finish. (PHI certified). U Value: 0.065 W/m2 K.
Unfortunately the sub-contractors on site failed to install the specified OSB ceiling board to support the loft insulation, so we had significant sagging of the air tightness membrane between the service battens which made taping difficult.
Windows: Ecohaus Internorm KF410 triple-glazed aluminium clad windows and doors with, ISO glazing spacers (Overall U-value: 0.72W/m2K) were supplied & installed with air tight tape seals by Ecohaus to a very good standard. We certainly recommend a similar supply and install package on all our other PH projects to retain a single point of responsibility and high levels of quality assurance.
Passive and Active Solar Design: A large percentage of the high performance solar glazing is orientated due south to the private courtyard back garden to obtain the benefit of passive solar gain to the living spaces. Shading prevents summertime overheating but permits low level winter sun to penetrate to the heart of the house. Windows to the north, east and west elevations that have less passive solar gain potential and are in comparison deliberately kept more traditional in scale and modest in size to reduce heat loss.
Site Generated Solar Hot Water: One the largest consumers of energy within any house is the heating of domestic hot water to service the kitchen and bathrooms. As part of the sustainable development, 7.8 square metres of 3No Consolar Plano 27H flat plate integrated solar Panels are located on the south-facing roof of the house and are connected to an Akvatherm 500litre solar plus thermal store located in the centre of the house off the landing.
Site Generated Solar Electricity: A 4kWp Solar photovoltaic integrated array, consisting of 16No Hyundai 250W modules are mounted on the south-facing roof of the house to convert sunlight to domestic use electricity and hot water via a Immersun controller unit to transfer excess electric to the Akvaterm solar thermal store.
Low Energy Appliances: All appliances have been carefully considered to eliminate unnecessary electrical demand and to optimize the efficiency of the essential items (cooker, fridge, low energy LED lighting etc).
Mechanical Ventilation With Heat Recovery: In winter months when the outside air is cold relative to the required inside temperature, a Passive House Institute certified Paul Focus 200 whole house clean air comfort ventilation system is fitted with a 1kW electric supply duct heater which includes the controller, programmable room thermostat and duct insulation (note: all of the supply ductwork was insulating with 50mm of foil backed fibre glass insulation. (Heat Recovery rate 91%.) All the duct work was installed by the local Cumbrian plumbing sub-contractor, who had little previous experience and the final installation was commissioned by the suppliers, The Green Building Store. Tim and Sarah have the option to not use the mechanical ventilation system in the summer months in favour of natural ventilation.
Water Use: Presently we spend an enormous amount of energy and money collecting and purifying fresh water to a high standard suitable for drinking. We then use this very inefficiently for purposes that do not require this level of purification. In this proposal, household and garden non-potable water requirements are met by collecting rainwater from the main roof via galvanized steel gutters and downpipes and storing it in water butts and a 2000 litre ‘Rainharvester’ underground storage tank. Water from underground storage is filtered and used for flushing toilets and the washing machine. The water butts are used for garden irrigation. Mains top-up water is available from the mains system, but due to the relatively high rainfall in Cumbria this has not been required. Efficient low water use dual flush toilets with a maximum flush of 4 litres and aerated flow restricted taps and shower heads have been installed.
Healthy Internal Environment: Scatter rugs over reclaimed maple timber boarded floors, organic non-volatile solvent paints, avoidance of formaldehyde and other toxic equivalents combined with natural materials along with summer natural ventilation and a winter heat recovery mechanical ventilation system has l lead to a healthy internal air quality which Tim & Sarah are very pleased with.
Household Waste: Kitchen waste and garden debris material is composted until dry and inert and then returned as a valuable non-toxic fertilizer to the food growing areas of the garden and conservatory greenhouse.
Organic Home Produce: The carbon footprint of most households is greatly increased by the reliance on industrial food processing systems and the food miles required to get food to our tables. The lean too conservatory / greenhouse facilitates the propagation of seeds and growing of organic home produce combined with the vegetable garden on the site, which goes some way towards reducing Tim & Sarah’s carbon footprint over an extended time period.
Costs: The overall building spend was £220K which included for a 72m2 one & a half story workshop / utility store and a 15m2 conservatory greenhouse outside the thermal envelop of the 3 bedroom main house. So the actual spend on the passive house thermal envelope was in the region of £1,370m2, which is exceptionally good value for any one off architect designed house never mind a eco house to the passive house standard. Tim & Sarah will have minimal utility bills for life; in fact at the time of writing they have surplus income after paying bills due to the FIT & RHI tariff payments received.
Build Procurement Route: The high quality/good value for money construction was achieved by Tim & Sarah acting as fully engaged active clients, employing two local Cumbrian builders; Sam Nelson & Jim Crawford on a labour only basis (Sam has done a lot of previous work with Eco Arc before; including building our architects office and private home, so he was highly trusted from the outset.) The timber frame element was sourced directly from MBC in Ireland at a good price. Sam opened direct trade accounts in the clients name with several local merchants and most materials were paid for direct to the merchant by Tim & Sarah at the end of each month along with Sam & Jims labor costs based on time sheets. Most other sub-contractor trades submitted competitive fix price quotes which were agreed en-route through the build process. We have used this procurement route many times before, to avoid risk averse over pricing by main contractors who are not familiar with passive house construction and tend to put up tender figures to a super inflated level to cover the un-know risks. Sam had never built a Passive House before, and learnt on the job with some gentle tuition from us, he finished this one on time & on budget without a cross word. Sam has now almost finished his second Passive House in Kendal and has agreed to go on and do a third house in Windermere on the same procurement basis, as it seems to be a winning formula for all parties.
Timber Verses Masonry. We have now finished 45No masonry passive houses across five different sites in North England and are currently working on 18No individual timber frame passive houses across a similar geographical region. Clearly there are pros and cons for both options which vary from site to site and client to client depending upon specific circumstances. Tim & Sarah’s house was a steep learning curve down the timber frame route but it was a success. We have now fine-tuned a timber I beam and Cellulose system with a local Cumbria based timber frame fabricator Trevor Lowis of Eden Insulation and Cumbria based Ecological Building Solutions which has allowed us to erect the whole timber frame thermal envelop consisting of all the pre-insulated walls and roof for a four bedroom house in one day, and follow on with installing the triple glazed windows with air tight seals over two further days and deliver consistent air tightness results in the region of 0.2 to 0.3 ACH at 50 Pascal’s. In comparison our last wide cavity wall masonry passive house project in Cumbria for two affordable passive houses finished 4 months late due to bad weather delays and struggled to achieve the required 0.6 air tightness results with a wet plaster approach. So from a range of experiences in terms of quality control and time certainty we are leaning towards timber frame as a quality assured route to passive house certification delivery. We are currently working on our first single block work leaf with external wall insulation passive house in Chester so that may put the cat amongst the pigeons and knock timber frame off its perch ?