Energy efficiency: building a home to the Passivhaus standard
18 August 2016
Tony Godwin describes the process of building his home to the Passivhaus standard
It is now 15 months since we moved into our Passivhaus. Humidity and warmth are consistent, to the extent that you cannot believe how cold it is when you go outside. The mechanical ventilation heat recovery (MVHR) system quietly rids the house of lingering smells and excess water vapour, meaning that our laundry dries overnight, if not sooner.
Figure 1: Rutland Passivhaus south-west elevation
The designed temperature of 20°C does not feel cold because the limiting values – including a requirement to be 12 times more airtight than UK Building Regulations Part L – ensure that thermal comfort is maintained, and you can sit right by the windows without feeling cold.
Passivhaus is a methodology for achieving very low-energy buildings in all climates. There is also a standard for retrofitting with less stringent heating demand and airtightness targets, and two for nearly zero-energy buildings that factor in renewable energy generation (see Hitting the target).
Passivhaus is not a prescriptive construction method, but the heating demand of 15kWh/m2/year and the limiting values make airtightness and thermal bridging far more critical than they are in UK Building Regulations. Although they should be well within the capability of construction workers to achieve, the standard still involves some unfamiliar techniques and materials that require planning, care and teamwork.
Designing for the Passivhaus basics from the outset is essential, and buildings should be orientated and glazed to enable solar heat gain. Architects ought to be able to use the Passivhaus Planning Package (PHPP) with a sketchUP plug-in for 3D modelling to assess early concepts; but I would still recommend using an experienced Passivhaus designer because data input is critical for certification, and experts can help with detailing. The Passivhaus Trust, the UK affiliate of the International Passivhaus Association, is a helpful resource, and its annual conference provides a great opportunity to engage with other practitioners.
Surprisingly, the UK has 22 different climate datasets, among which the East Midlands – where our home is built – is one of the coldest. Our first PHPP iteration predicted a heating demand of 22kWh/m2/year because its ‘T’ shape with single-storey sections and a garage under the first floor resulted in a high ratio of external surfaces to floor area, and there was also a greater proportion of glazing to the north. The redesign, featuring improved U-values and envelope thicknesses up to 600mm, required a new planning permission.
With the lower floor of the house built into the slope of the site, the external envelope had to have detailing for above- and below-ground conditions, maintaining continuity of waterproofing, insulation and airtightness.
The solution comprises a 300mm double-stud structural timber frame filled with Warmcel – blown recycled newsprint that fills the voids around the studs – set on a ground bearing slab in front of the reinforced concrete retaining wall.
Figure 2: Timber frame in progress: living, dining, kitchen space with bedroom wing beyond
The slab sits on a minimum of 200mm high-compression expanded polystyrene (EPS) over a damp-proof membrane, joined to a self-adhesive tanking membrane applied to the retaining wall. The wall is insulated externally with EPS, protected by a composite plastic drainage membrane and geotextile.
This arrangement exposes the concrete slab internally, with its mass providing thermal storage that delays the effect of external temperature fluctuations and also reduces the resultant energy requirement, keeping the insulation dry for thermal performance as well.
The timber frame is lined internally with a Durelis vapour-proof oriented-strand board that acts as the airtight layer, and the plasterboard is battened off this to provide a 37mm zone for services. A mineral wool ‘overcoat’ is then fixed externally in the cavity behind the various cladding types, providing continuity with the EPS below ground.
Energy, heating and ventilation
The high level of airtightness makes an MVHR system an integral part of a Passivhaus. The ductwork has to be designed and integrated into the building at an early stage. The air extracted from the bathroom, the kitchen and the utility room, where the plant is located, warms the 100% fresh-air input to all other rooms via the heat exchanger, changing the air around every three hours. A gap under the internal doors provides a route for returning air.
Heating and hot water is provided by a small gas boiler serving a wet system circuit with three strategically placed radiators, three heated towel rails and a 110-litre hot-water storage cylinder. The choice of gas as an energy source was entirely pragmatic, providing low capital and running costs with familiar technology. We also have a 1.8kW solar thermal panel and a 3.5kW solar photovoltaic array.
Figure 3: A bunch of services – here pre-insulated solar thermal pipes with sensor cables – passing through the airtight layer are difficult to seal
Construction, procurement and cost
The project team was vital to the project’s success, remaining open-minded and collaborative on unfamiliar territory; but only previous experience could have prepared us for the critical role of airtightness detailing. We air-tested on completion of the airtight layer – an essential process – and carried out two further tests, with many person-hours spent finding and rectifying leaks before achieving the required value of 0.6 air changes per hour (measured at 50Pa pressure).
A schedule of all service penetrations should be prepared and agreed by the mechanical and electrical contractors, and appropriately sized airtight grommets purchased. Proposed substitution of specified insulation material must be reported and agreed; in our case, the mineral wool in the roof was built up in layers of different thicknesses with different lambda values and the contractor varied these, resulting in a loss of U-value.
Figure 4: Expanded polystyrene insulation and tanking at the base of the retaining wall
In selecting contractors and suppliers, we prioritised those in a 30-mile radius of the site for the sake of sustainability, and avoided nominating subcontractors so that the main contractor would remain responsible for all aspects of the project, except for unfamiliar items critical to the Passivhaus standard. We only sourced imported items though UK distributors, which we assessed for reliable support infrastructure.
Prices obtained for the timber frame during the design stage revealed an apparent premium for manufacturer’s systems developed for superinsulation and airtightness, which led us to make this a domestic subcontract. However, the lack of understanding of Passivhaus and the inflexibility of the manufacturer’s system, which was designed to meet UK regulations only, created construction difficulties that probably outweighed the savings we made.
Procuring windows that were triple-glazed from the manufacturer Katzbeck in Austria was a lengthy process due to indirect communication through the UK installation representative. In future, I would always make time to visit the manufacturer.
Note that tilt or lift-and-slide windows are difficult to make totally airtight due to the mechanisms involved; we have two in our living area, and it is noticeably cooler on really windy winter days.
We have calculated the additional construction cost for meeting the Passivhaus standard to be 25.6%, which is based on median BCIS rates for constructing the building on a flat site to current Building Regulations.
A report in 2015 commissioned by the Passivhaus Trust concluded that costs were 10–20% more than for achieving Level 4 of the government’s now abandoned Code for Sustainable Homes. However, these figures do not consider lifetime costs, nor do they value the extra quality and our increased comfort and health.
There is no doubt that the building performs in line with expectations. Visitors always comment on how warm the house is, and our energy usage is well within the limit of the Passivhaus criteria.
Our energy bill for the past year was £900 or £4/m2. To put this into perspective, the 2004 Wimpey-constructed house that we rented during construction – a compact 7.8m x 7.8m on two storeys – cost £18/m2, which is equivalent to £4,185 for the same area as our Passivhaus.
Heat is only required in our new home when the external temperature falls to 7°C, and when we are indoors, it is better to leave the heating on and control it by the remote digital thermostat. This is generally left in the living area, but we do move it with us into the bedroom on colder nights due to the temperature differential. While this was expected, because the bedroom has a higher proportion of external envelope than other parts of the house, the differential is never more than 2°C.
Our studio is heated nicely by ourselves and by our office equipment during the day. Humidity varies little from a level of 60%, and maximum summer temperature indoors is 25°C.
As one of our visitors during an International Passivhaus Open Day exclaimed: “Why would anyone not want to build a Passivhaus?”
Tony Godwin is an architect at FCD Architecture
- Passipedia – The Passive House Resource
- Images © Tony Godwin
- Related competencies include Construction Technology and Environmental services, Legal/Regulatory compliance, Planning and Sustainability
- This feature is taken from the RICS Building control journal (June/July 2016)