Milton Keynes' Passivhaus: the most airtight house ever?
EcoDesign Consultants complete ‘The Passive House’ in Howe Wood Park
A cold damp day, although thoroughly unwelcome in July, was the perfect test environment for Footprint’s visit to eco Design Consultants’ recently completed Passive House near Milton Keynes.
Replacing a derelict burnt-out house that had been troublesome for the community, client The Parks Trust wanted to build a new home that could be a positive icon for the future. Opting to construct the first Passivhaus standard building in the area meant footing the bill for a construction cost of £2,000 per square metre.
On approach from the drive, there is a pleasing dialogue between the house and the elevated home-office folly opposite, which tricks the eye into reading the considerable height of the house as slightly lower.
An oversailing roof to the south is designed to prevent excessive solar gain, however funds were available to install back up roller shutters – just in case. Compartmentalisation of the top floor for fire regulations allows an open plan ground floor despite the house’s three storeys, which are necessary to accomodate the five quite generous bedrooms.
Although the scheme largely conforms to typical Passivhaus north-south window arrangements, interior daylight is improved by vertical slit windows to the east and west, generally a Passivhaus faux-pas. This addition of dynamic changing light to north-facing rooms goes some way to improve the rather dark upper floors – the diminished natural light here is a side-effect of satisfying other more stringent requirements.
Of course, the open plan living area makes up for this by being super bright. Practice director Alan Budden noted it was a shame to lose a double height space over the lounge in order to increase the lettable floorspace to 182 square metres - there was an estate agent on the client board - but he admitted it has helped with the Passivhaus Planning Package calculations.
A timber frame made for a quick eight-month construction period and facilitated airtightness. The walls contain 300mm of Warmcel insulation against an inside face of taped up OSB3, a material whose airtightness was hotly debated at this year’s AECB Conference according to Budden. Yet with an airtightness value of 0.065 air changes per hour, it seems to have done the trick. And it felt like that on the day; the cool, muggy outdoor air was not apparent inside the house.
The MVHR system extracts and recycles air from the toilet and kitchen to help warm the home; care has been taken to ensure the ducts are easily accessed from the bathroom and utility room on their respective floors.
Other green features include solar water heating and 4kW peak photovoltaics, again because the money was there.
Attentive cold bridge detailing has helped alleviate U-values at thermal weak points; for example using inward opening windows has reduced frame width and allowed for an extra external layer of phenolic foam around windows. It’s worth noting that many still query the use of environmentally un-friendly materials such as this in order to attain the high performance required for Passivhaus.
The Parks Trust are keen to follow through on their sustainable aims for the project by collecting and analysing energy data. Building performance monitors are at the ready,and the house has now been taken over by its first tenants, who have been fully briefed on how Passivhaus works.
With the Trust keeping its eye on another house opposite that is currently up for auction, The Passive House at Howe Wood Park might even be the beginnings of a Milton Keynes Passivhaus community.
Energy Standard (PHPP)
Heat requirement / year: 14 kWh/m²a
Heat load (Thames valley data): 12 W/m²
Primary energy required: 95 kWh/m²a
Air-tightness (n50-value): 0.065 h-1 @ 50 Pa
Floor slab: 0.122 W/m2K
Exterior walls: 0.137W/m2K
Roof sloping: 0.116 W/m2K
Windows glass: 0.6 & 0.7 W/m²K
Windows frame: 0.97 W/m²K
Client: Parks Trust, Milton Keynes
Architect: eco Design Consultants
Passivhaus Consultants: eco Design Consultants
Passivhaus Certifier: WARM
Project manager: Jackson Coles
CDM coordinator: Jackson Coles
Main contractor: Parkway Construction
Timber frame specialist: Touchwood homes
Structural engineer: Allan Consult