The First UK Cavity Wall Passivhaus - Denby Dale - 45
The Denby Dale Passivhaus - first cavity wall Passivhaus in the UK. Over the last 10 months, the construction arm of Green Building Store - Green Building Company - has been building what has come to be known as 'the Denby Dale Passivhaus' - the first certified Passivhaus in the UK to be built using traditional cavity wall construction. Bill Butcher and Chris Herring report.
Built for private clients in Denby Dale, West Yorkshire, the project is all 8m2, 3 bedroom detached house, built to a tight budget of £141 k. It received its Passivhaus certification at the end of April, one of the first 3 projects to go through the Passivhaus certification process with Pete Warm of WARM: Low Energy Building Practice.
The Passivhaus diaries
The project has already attracted much attention throughout its build, in part due to our Passivhaus Diaries blog about the project. Following every stage of the build, the blog attempts to provide an accessible, technical guide to Passivhaus construction techniques, suitable for the layperson and professional alike.
Cavity wall construction
Another reason the project has attracted such interest is its application of Passivhaus methodology to UK cavity wall construction techniques. European construction is commonly solid masonry with external insulation and render or timber frame, and so Passivhaus buildings often use one of these two methods. For a number of reasons, and perhaps controversially, we decided to stick with familiar cavity wall building methods and materials for this project. This was for the following reasons:
• Masonry wall construction, including cavity wall, contributes to a high thermal mass within the insulated building envelope - allowing a more even living environment, in terms of both acoustics and temperature. Through use of dense concrete blocks and concrete groundfloor slab within the thermal envelope, the house will have a greater thermal mass, stabilizing temperatures and optimizing passive solar gains.
• Cavity wall construction is the method that we, in the UK, are most familiar with. We wanted to build a Passivhaus using traditional construction techniques with, as far as possible, materials you could find in any local builder's yard.
• West Yorkshire planning rules require natural stone facing on the exterior of new buildings and so ruled out the option of block-built construction with rendered finishes.
(Photo above) the completed building.
However, by deciding to use cavity wall construction at Denby Dale, we undoubtedly made things harder for ourselves. We were going into uncharted territory. Although we have heard of some cavity wall Passivhaus projects in northern Germany, there is little technical documentation readily available about them. So, in effect, the team had to start from scratch. To meet the airtightness requirements of a Passivhaus, cavity wall construction relies on wet plastering as the airtightness barrier. It might have been more straightforward to go down the timber frame route, which can achieve airtightness by lining with vapour barriers and airtightness tapes etc. By using cavity walls we needed to work out new detailing, particularly around the junctions of elements such as floors and roof to walls.
The experimental nature of our project inevitably did lead to a certain amount of anxiety through the build. Fortunately for us, it was a gamble which paid off, with final airtightness results at the Denby Dale Passivhaus coming in at O.33ach@50Pa. In the process of the build we have also developed some really excellent detailing which can be applied to cavity wall Passivhaus projects in the future, such as how to fit the windows into the cavity wall. The bespoke plywood boxes and aluminium cavity closers, we developed for this purpose, offer a robust and unique solution which make cavity wall a viable way of getting to Passivhaus standard. We had already been contacted by other Passivhaus project designers keen to go down the cavity wall route. We very much see cavity wall as offering an alternative option for Passivhaus construction in the UK, with timber frame and block and render offering equally valid solutions for achieving the Passivhaus standard - dependent on the specific needs of the project. There are many ways of crossing the river.
Professor Wolfgang Feist, originator of the Passivhaus methodology and founder of the Passivhaus Institut in Germany, has been supportive of the project and of the company's attempts to adapt Passivhaus design to cavity wall construction. He said,
“The Green Building Store team is to be commended for adapting the Passivhaus methodology to British building techniques and for constructing the first Passivhaus using cavity wall construction in Britain. The Denby Dale project has proven that Passivhaus design can be easily adapted to British building techniques. Airtightness is not a myth - the rules of physics are the same everywhere - and wet plaster offers an excellent airtightness barrier for cavity walls, if applied to all surfaces of external walls and connected properly at the joints. The Green Building Store team has done a great job at Denby Dale and also in helping to spread the word about Passivhaus design in the UK. Congratulations!”
Lots of careful detailing was needed to get a cavity wall design up to the exacting standards required of Faaslvhaue buildings.
Lessons from Denby Dale
We were pleasantly surprised at how relatively easy it has been to meet the Passivhaus requirements. We easily beat the airtightness requirement (0.6 air changes/hour @50Pa) by about 45%. We also managed to come in well under the space heating requirement of 15kWh/m2/annum, meaning that space heating costs are anticipated to be less than £75 per year.
Partnership working
Passivhaus and low carbon buildings require high levels of airtightness and close attention to thermal bridging. To achieve that level of attention to detail requires the design team, on-site workforce and client to all be educated in Passivhaus theory and practice. You have to create a team ethos where everybody understands their part in the process, where there is continuity of personnel, and ideally subsequent monitoring of the building's energy use is included.
Both of our businesses are very much in favour of the partnership approach to construction. As we know, individual subcontracting teams for different trades do not naturally 'look out for each other', leading to breakdown of the building fabric and energy use performance. A typical example of the problems that can occur on site would be a subcontracted plumber coming along and puncturing the vapour and airtightness barrier which is then covered up by the subcontracted plasterer.
Trust, honesty, transparency and commitment between all parties, at all levels and stages of the project, will go a long way to answering these questions. Good robust and buildable detailing is essential. An early inclusion of builder and any subcontractors in the design stages for the two-way exchange of. knowledge is preferable. We believe that partnering contractual arrangements - where contractors are chosen for their suitability at an earlier stage of the design process (often used for large complex projects) - could be a useful model for Passivhaus projects.
Above: two stages of the cavity wall build showing the foamed glass for below DPC (top) and the rockwool insulation above DPC.
Minimising thermal bridging
Understanding and modelling thermal bridging is becoming absolutely critical to predicting the heat loss of a building. Measures include:
• Use of 300mm insulation in the cavity, going right down to the strip foundation, so that any heat lost from the concrete floor slab will have a longer thermal transfer path.
• Use of lightweight aerated block below ground level, which does not transfer heat as readily as standard concrete block.
• Use of basalt and resin cavity wall ties (instead of the usual steel ties, see left).
• Positioning of windows and doors at the centre line of the insulation layer.
Maximising airtightness
Passivhaus buildings require very high levels of airtightness (O.6ach@50Pa). The Denby Dale Passivhaus' design and construction pays particular attention to junctions, which are always difficult for airtightness because of the differential movement between different materials. Measures include:
• Wet plaster coating to interior walls.
• Concrete floor slab is carried across the top of the blockwork of the inner leaf of the wall to minimize shrinkage cracking between the wall and the floor
• Attention to airtight detail around window and door openings and junctions between floors, walls and roofs, including use of airtight membranes and tapes.
• To improve airtightness around the window opening, a plywood box was set into the wall. An adhesive-backed airtightness tape was then attached to the plywood with a fleece wrapped into the wet plaster, making the junction between the plywood and plaster airtight. Another airtightness tape was used to seal the gap between the window and the plywood box.
• Various details at first floor junction, to avoid penetration of the inner leaf blockwork including: use of timber wall plate; parging of the blockwork behind the wall plate; use of-anchored stainless steel threaded bar to carry the 302mm timber I-beam structure. Use of l-Bearn for first floor had the added bonus of allowing us to use void for MVHR ducting and all other services, further helping with airtightness (and aesthetics).
Minimising 'thermal bypass'
Thermal bypass refers to air movement through or around insulation, which can reduce its effectiveness. Although not a requirement of Passivhaus design, addressing thermal bypass - by reducing air movement through or around insulation - is becoming good building practice. For the Denby Dale Passivhaus this meant design detailing for windtightness and careful installation of insulation.
Super insulation
Super insulation is fundamental to Passivhaus construction, along with close attention around junctions of these elements. Measures include:
• Walls: 300mm fibreglass batts
• Under groundfloor: 225mm polyfoam insulation
• Roof void: 500mm fibreglass quilt
• Windows and doors: triple glazing with insulated thermal break in frame.
Mechanical ventilation with heat recovery (MVHR)
MVHR is an absolutely integral component for achieving the necessary performance levels needed for Passivhaus buildings. It allows for sufficient and comfortable ventilation to all areas of the house, whilst minimizing the loss of heat gained from the
sun, human activity, cooking, showering, electrical appliances etc. This is achieved by the use of a sophisticated heat exchanger driven by two very efficient fans. The incoming air is blown past the outgoing air and is used to heat the incoming air. This has the effect of saving over 90% of the heat that, due to uncontrolled ventilation and draughts, would be lost in a more 'conventional' house.
A blockwork built Passive House
Key data for the project
• £ 141 k build costs (excluding motorized external sun shading. decorating. garage. external works and incoming services)
• 118m2 three-bed detached house
• Minimal heating - using 90% less energy for space heating than the UK average
• Anticipated heating costs of less than £75 per annum
• Airtightness = 0.33ach@50Pa
• Space heating needs = under 15kWh/m2/ annum
• Peak heating load = 10W /m2 (when outside temperature = -10 degrees C)
• Total heat demand = 1.18kW (equivalent to one bar of an electric fire)
• Condensing boiler heat output = 4.8kW (it was impossible to find a boiler with smaller heat demand)
• Mechanical ventilation heat recovery (MVHR) unit - heat recovery efficiency of 92%.
The future
Having gone through the Denby Dale process, we feel, more than ever, that Passivhaus is the way forward for the UK. It can help create quality, comfortable buildings while also achieving 90% cuts in occupants' heating bills. It offers the UK an easy win solution towards the massive cuts in CO2 emissions we need to make - urgently. We are keen to apply our experiences at Denby Dale to both new build and retrofit projects and are currently researching, using PHPP, a Passivhaus retrofit to a 1930s house. Green Building Store is also expanding and strengthening its Passivhaus products and services including: construction, consultancy, training, and Passivhaus products.