Concrete Today May 2010 - the Irish Concrete Federation

concrete today -zero carbon emissions
Concrete Moves towards Zero Carbon Emissions
By Liam Smyth FIEI, Sustainability and Marketing Manager, ICF
In a previous article for Concrete Today, the initial design and early stages of
construction of Ireland’s first Zero Carbon Emissions Concrete House were discussed.
Now, as the project nears completion, key aspects of achieving the design targets as well as
lessons learned are reviewed.
Design Overview
Originally designed to Passive House
standards (and an A3 BER Rating in
this case), the Irish Concrete Federation
became involved in the project prior to
construction, having agreed with the
owner/builder the much higher targets of
Zero Carbon Operational Emissions (from
primary energy needs) and an A1 BER
Rating (0-25 kWh/m² per annum for
primary energy).
Target U values of 0.1 W/m²K were
adopted for the ground floor, external
walls and roof, while an overall U value
of 0.8 W/m²K was specified for windows.
Outstanding thermal bridging performance
was required, ever harder to achieve in a
super insulated house, by the specification
of a Y value of 0.04 W/m²K.
General Construction Details
Achieving these targets required very
close attention to detail by both designer
and builder as energy performance and
constructability do not always go hand in
hand.
The foundation detail chosen was a raft
foundation which is completely underlaid
with EPS, with 200kPa non-compressive
EPS under the beam sections and 100kPa
specified under the floor areas. This
minimises heat loss through the floor
Window fixed with external brackets
Zero Carbon Concrete House
and substantially eliminates the thermal
bridging heat loss problems associated
with traditional strip and raft foundations
directly in contact with the ground.
Wall construction is a standard
heavyweight block laid on the flat (215mm
thick), plastered internally with EPS on
the external face with a suitable render.
While these details are similar to type 2
wall construction as per the Acceptable
Construction Details, published by
DEHLG in late 2008, thermal bridging
performance is achieved by windows
and doors being cantilevered into the
external insulation on simple stainless steel
L brackets, in addition to the enhanced
foundation details.
At roof level, a warm roof detail whereby
EPS is placed above, between and below
the rafters was chosen. At eaves and
gables, the insulation meets the external
wall insulation to complete the insulation
envelope.
So, while overall a fairly simple build
in principle, the devil is in the detail.
Substantially, a direct build by the owner/
builder, technical expertise was provided
by Aerobord Ltd., the chosen insulation
supplier, and CPI Ltd., whose Baumit
system was the chosen external render.
Getting Foundations Right
Working from a levelled base of Cl804
aggregate, blinded with sand and covered
with a radon barrier which also acts as a
damp proof membrane, overlaid with the
appropriate EPS grade for the location. The
outside of the raft was timber formwork
with all internal support for beam sections
formed with EPS panels in place to provide
the floor insulation. While this led to
increased use of insulation, it saved on
time and added certainty to the process,
ensuring exact coverage and minimal
thermal bridging. As built, the floor U
value achieved was 0.09 W/m²K, even
better than the design target.
Of substantial overall importance
is to bring all service pipes, e.g. sewers,
water, electricity, air ducts, through the
foundation to avoid breaking the insulation
envelope above ground. All such piping
was wrapped in insulation throughout
the foundation to minimise thermal
bridging. This complicated the foundations
somewhat and is a plumber’s nightmare
(given a lack of internal walls for guidance)
but serves the project targets well. The raft,
when poured, can then be power floated to
leave the finished floor, thereby saving time
later in the project.
CEMEX (Ireland) Ltd.supplied the
readymix concrete for the pour, using a low
carbon 30N mix. The same company later
supplied the blocks and first floor structural
screed.
Walls and Intermediate Concrete
Floor
In building a 215mm block wall, it is
important to ensure that it is well jointed
with mortar, to minimise air permeability,
and kept smooth externally, to ensure the
external insulation can be fitted correctly.
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