View as PDF - CIBSE Journal

mild climate, annual solar gains tend to
be lower than other parts of the UK, so
high levels of insulation and large areas of
glazing were incorporated into the design to
compensate for this and to enhance natural
daylight levels,’ says Williamson.
Construction
The building uses a hybrid approach to
construction. Its structural core of internal
partition walls is constructed from concrete
blocks mounted on a cast in-situ concrete
ground floor slab, while the external walls
are prefabricated timber frame and cassette
construction. The wood-faced cassettes
incorporate recycled newspaper insulation.
The first floor is constructed from hollowcore
concrete slabs. This solution adds
thermal mass to the interior while allowing
a highly insulated facade system to be used.
‘We took a fabric-first approach to the low
energy design because the fabric will be
in place for at least 60 years,’ Williamson
explains.
An asymmetrical pitched roof caps the
building. Its south-facing, slate-clad aspect
is pitched at 35 degrees to
ensure its 7 kWp covering
of photovoltaic panels
performs effectively. The
north-facing slope is much
gentler to enable the living
sedum roof to flourish. A
green roof also blankets the
building’s large entrance
lobby to help enhance the
site ecology.
Fabric airtightness
is fundamental to the
building’s low energy
performance. An interior
breathable membrane is used to seal
the facades. ‘The biggest challenge was
detailing around the windows, floor plate
and wall junctions and where M&E services
enter the building,’ says Williamson. ‘The
junctions had to be airtight without creating
a thermal bridge and the designs had to be
approved by the Passivhaus Institut. ‘It was
important the timber cassette manufacturer
and structural engineer bought into the
detailing because they have to stand by their
warranty,’ explains Williamson.
C Sneade, a family-owned local contractor,
was selected to construct the low-energy
building. The contractor worked alongside
the designers to ensure the building was
airtight. Construction joints were pressuretested
before being covered with internal
linings. This enabled any reworking to be
done with the minimum of disturbance.
However, it also required a change to the
conventional construction programme to
ensure the building’s fabric was complete
and windows installed earlier than usual in
the programme, which could be an issue
on larger schemes if the works cannot be
phased accordingly.
JPW challenged the contractor to make
the building more airtight than their
German counterparts were currently
achieving. The applied psychology clearly
worked; when the scheme was fully
pressure tested it registered 0.37cu m/h/
sq m at 50 Pa, which is one of the lowest air
leakage rates ever achieved in the UK.
Ventilation
With so little air infiltration, the ventilation
design is critical. The designers considered
natural ventilation for the scheme, but
at £500 per actuator per window it was
expensive and potentially ineffective on
days when there was no wind. Instead JPW
Associates opted for mechanical ventilation
with heat recovery (MVHR). The problem
with this option was
that the Passivhaus
fresh air requirement
was only eight litres per
second, but Part F of the
Building Regulations
have a greater fresh air
requirement of 10 l/s
per person. Williamson
says the designers
compromised by
designing a system with
step control capable of
delivering fresh air at
quantities up to 10 l/s.
Five Passivhaus-approved Drexel and
Weiss ventilation units have been installed,
one for each computer room, with the
remaining two units serving the offices
and public areas. The units operate on
three speeds: high, medium and low, with
medium being the default for public areas.
In summer the MVHR system will supply
fresh air during the day. At night the system
will run to purge heat from the heavy
construction elements if the outside air is
greater than 4C cooler than the internal
temperature. To save fan energy and prevent
any heat transfer, the ventilation units’ heat
exchangers are removed in summer to
enable them to run at 100% bypass.
Williamson did consider using
mechanical cooling for the scheme as
an alternative to running power-hungry
It proved difficult
to satisfy both the
BREEAM and
Passivhaus Institut
rating systems.
Creating two sets
of data was time
consuming and
added to design costs
OffiCe Case study cibse AwArd winner
KeyfaCts
POwys cOuncil’s
cAnOlfAn hyddgen in
mAchynlleth, wAles
Structure and fabric:
A central masonry core from
reclaimed and recycled masonry
in conjunction with in situ
ground-floor slab (60% ground
granulated blast furnace slag),
with an external solid timber
frame and I-beam Cassette roof;
7kw PV array.
airtightneSS: n50 = 0.249-1
@ 50Pa (Passivhaus Institute
requirement is 0.6); Q50=0.37cu
m/(h.sq m)@50 Pa (UK
measurement).
venilation/heat recovery:
Five Drexel & Weiss Aero
business units operate across
five different zones. Three
ventilation levels are varied
automatically according to PIR
and timer controls.
Space heating: 9-24 kW gas
boiler; point-of-use electric water
heaters in toilets, kitchen and
cleaners cupboards.
lighting: PIR and daylight
balancing is used throughout the
building which complements the
hybrid decentralised/centralised
building ventilation strategy:
rooms and equipment only
power up when they are in use.
energy management
SyStem: System control board
is linked to County Hall, allowing
remote monitoring and enabling
local key holders and caretakers
to manage the building under
instruction from centralised
energy managers.
monitoring: JPW has applied
for funding to expand monitoring
and analysis of the building and
hopes to use CIBSE TM22 and
BUS questionnaires to gather
more information and compare
actual efficiencies with building
physics methodologies and
prediction tools. They will do this
using: sub-meters to monitor
individually tenanted office spaces;
focused mobile monitoring of
selected workspaces for actual
and perceived comfort levels
based on CO2, relative humidity,
temperature and daylight levels;
and the actual use of operational
energy in IT and similar
equipment.
www.cibsejournal.com April 2011 CIBSE Journal 37