ARCHITECTURE & DESIGN
manipulation of a number of key design parameters: n Reference to local climate data. n The use of efficient building forms with enhanced insulation levels.
n The meticulous specification and location of high-performance windows and doors to take advantage of free solar gains.
n The detailed consideration of air leakage reduction and thermal bridge-free construction.
n The installation of the quietest, most energy-saving heat recovery ventilation systems.
n Avoidance of overheating risk through shading and manual venting.
n Inspection during the construction phase to confirm that all the above measures have been installed to the acceptable standard.
Specialist software
All of these conditions are checked for compliance in advance through the use of the specialist software design tools, which include designPH and the Passivhaus Planning Package (PHPP). Trained Passivhaus designers use this software to input and record data on thermal performance for wall, floor, and roof constructions, ventilation design efficiency, fuel sources, and internal heat gains. A 3D model of the building can also be used to test the impact of form, shading, and window opening sizes and positions. Just as with any architectural design, Passivhaus buildings are shaped by the local context or streetscape, and by local views but, in addition, they are influenced by an awareness that building form also has a significant impact on energy use and construction cost. The target for all Passivhaus buildings is to reach a level of efficiency whereby the space heating demand is reduced to just 10 watts per square metre (10 W/m2
the course of a year, the adjusted target is just 15 kilowatt hours per square metre per
year (15 kWh/m2 /yr – 1 kWh being
equivalent to one unit of gas or electricity). To put this into a context, it equates to a large three-bedroom house being heated on the very coldest of winter days solely by a small fan heater. Insulation measures are particularly worthwhile in hospitals and other care environments – on account of their high temperature requirements and almost continuous operation.
User comfort the cornerstone However, far from being just a number- crunching exercise, the consideration of user comfort is the cornerstone behind Passivhaus. If building users are more comfortable in their working or resting environment, they are less likely to resort to the application of more heat or cooling to improve the situation. Studies have shown that we generally equate comfortable conditions with even temperatures and stable relative humidity, which means external walls and floors need to be sufficiently well insulated to avoid colder surface temperatures whatever the external conditions – this equates to a minimum thermal performance U-value of 0.15 W/m
2K for
external walls, floors, and roofs (although it is typically lower for overall energy conservation reasons), and 0.8 W/m
2K for
glazing and doors. This insulation needs to be contiguous, avoiding the all too common thermal bridges formed by structural elements in conventional construction. Windows also need to be triple-glazed, with insulated frames and glass spacers. Since external wall surfaces do not fall below 17/18˚, mould and condensation cannot form, and cold downdraughts are not induced, even adjacent to the glazing.
). Over
‘Build tight, ventilate right’ The level of airtightness required from the building fabric is perhaps the most significant difference between a Passivhaus building and conventional
Ventilation’s importance However, improvements in building insulation and airtightness standards must go hand in hand with greater
Retrofit cladding. Mark Elton said: “A new approach, involving prefabricated retrofit, is gaining interest, where a new external skin is fabricated in its entirety under factory conditions.”
48 Health Estate Journal April 2017
consideration being given to ventilation. In such cases, it is no longer healthy to rely on manual ventilation and draughts alone to remove poor quality air; nor will it be acceptable in energy terms to simply exhaust that warm air to atmosphere. By limiting the level of air leakage through the building envelope, it becomes viable to recover the heat energy from the exhaust air – a system known as heat recovery ventilation. The more airtight the walls, floors, and ceilings, the more efficient the ventilation system’s performance for the minimum of fan energy use. The best heat exchangers can achieve efficiencies of between 75% and 90%, with filtered, fresh air being supplied, slowly and quietly, to all occupied rooms to balance against the same volume of stale or moist air extracted from bathrooms or utility rooms. The energy used to run the fans is more than compensated for by the recovered energy from the heat exchanger. Dust and pollen filters, together with acoustic silencers and low air speeds, ensure that accommodation is always fresher, quieter, and cleaner, even in the harshest of urban environments, and all without any noticeable loss in temperature, even on the coldest of days. This is a huge positive for any healthcare environment. Ignoring the associated heat loss for one moment, you can always open the windows if you want to, as all Passivhaus buildings have windows that open, not least for easy cleaning, and this may be of particular importance in the summer as
construction. Poor airtightness is one of the main reasons for the performance gap in other so-called low energy buildings, whereby the warm air within the property escapes through gaps in the construction under buoyancy and pressure differentials. UK Building Regulations permit a relatively high level of air leakage – Passivhaus compliance requires a standard some 16 times better (at least 0.6 air changes per hour, when tested at 50 pascals pressure). Typically, this is achieved through a detailed design and specification strategy from the outset, followed by careful and comprehensive installation. Components developed that are Passivhaus-compliant in terms of air leakage include window assemblies, membranes, tapes, and grommets. Testing of the assembly throughout the construction phase ensures that the quality and fabric integrity is maintained right through the fit-out and during the building occupation. This enhanced airtightness also helps with durability, as moist air is unable to penetrate the barrier and lead to condensation with the wall build-up.
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