ECOBuilder-Specifiers Journal spring2018

Building Better Thermal
Performance into Public
Sector Projects
Andy Mitchell from NBT, UK distributor of Pavatex woodfibre insulation,
discusses the reasons why so many buildings fail to meet their designed
thermal performance.
Pressure on public sector budgets
has prompted a more creative and
strategic approach to planning and
designing public sector property
assets. Local authorities have not only
reviewed their property portfolio and
how it can deliver best value; many
have also embarked on rationalisation
programmes, often involving the
construction of new buildings fit for
the 21 st century, where services can
be combined to reduce operational
costs.
Critical to that cost management
goal is the sustainability of public
sector buildings, in terms of
thermal performance, maintenance
requirements and service life. The
insulation used and the way in which
this is specified and installed as part of
the building fabric is critical to all three
sustainability parameters because it
will determine heat loss, solar gain
and the risk of issues associated with
moisture build up, such as damp,
mould and condensation.
The same principles apply to
social housing properties, where
longevity of the asset, minimising
maintenance, reducing heating bills
and ensuring tenant comfort are all
important specification and design
considerations.
All of this may seem fairly
fundamental and integral to Part
L compliance. However, there’s a
hitch. Many buildings fail to meet
their designed thermal performance
due to issues with air tightness and
thermal bridging and there is no legal
requirement to check if the energy
performance of the building, once
built, meets its designed energy
performance. Meeting Part L is
essentially a paper exercise and this is
a missed opportunity to work with the
supply chain to simplify design details
and ensure thermal coherence of the
finished build, which would often
result in a faster build programme and
less waste too.
The U-Value Issue
The most commonly quoted thermal
performance criteria is the U value,
which is calculated based on the heat
loss of a building’s principal areas
– such as walls, roof, windows etc. –
and considers every component of
28 ECOBUILDER - THE SPECIFIER SPRING 2018
each. However, to achieve genuine
thermal performance this should
not be considered in isolation but in
combination with, psi values (thermal
bridging) and air tightness.
To achieve the required U-values, the
specifier selects an insulation material
with low thermal conductivity –
the Lambda value. The lower the
conductivity of the material, the higher
performance it gives as an insulator.
In theory, therefore, materials with
very low conductivity, such as PIR
insulation, can be specified in smaller
quantities to achieve high levels of
thermal performance in the finished
structure. But there are a couple of
problems with this assumption.
Firstly, we don’t construct buildings
using a single material in isolation, so
the Lambda value of one element of the
wall or roof build up, i.e. the insulation,
can be seriously compromised by the
additional materials that surround
it, often required for structural or
weatherproofing purposes. Instead,
the thermal conductivity of the
insulation material needs to be
considered in the context of the entire
building envelope and floor structure.
A good illustration of this is woodfibre
insulation, like Pavatex. Woodfibre
has a higher Lambda value than PIR
but it is high density and can be
applied to the external envelope of a
building as a complete thermal wrap
requiring much smaller section fixings
for external weathering surfaces such
as cladding. Its thermal performance
in practice is better, therefore,
despite the fact that its Lambda
value would suggest otherwise on
paper. Consequently, it is vital that
the specifier considers the verified
Lambda value within the context
of all the repeating components of
the principal areas of the building
envelope when designing a project.
Secondly, the effectiveness of the
building envelope also depends on
the psi values (thermal bridging
detailing), collectively known as the
Y-value. Proportionately, thermal
bridging has little impact on poorly
insulated older buildings as there is so
much heat loss through the principal
areas. For a Part L compliant building,
however, as much as one third of a
building’s heat can be lost through
thermal bridging.
Once again, the solution to this is
delivering a thermal wrap around the
principal building fabric, which can
perform even better when solutions
are incorporated to address high heat
loss thermal bridge details, such as
overlapping insulation onto window
frames, for example.
Achieving Improved Airtightness
The other major cause of heat loss is
air escaping from the building and we
measure this as air tightness at m 2 /
m 3 @50pa. The Part L requirement of
5m 2 /m 3 @50pa is the equivalent of a
hole the size of a 20p in every square
metre of the building envelope:
not quite the hermetically sealed
boxes that people fear air tightness
will deliver! Insulation can only trap
the heat within the building if the
envelope provides good levels of
air tightness because warm air will
naturally escape through any gaps,
increasing heat loss.
While designers and specifiers cannot
always control construction integrity,
good airtightness can be aided at
the design stage with simple details
that are more easily executed on
site. In most cases, airtightness is
better delivered from the inside of
the building envelope, enabling a
pressure test at first fix to show up
any issues so that the contractor
can make good. Additionally, careful
consideration should be given to the
sequencing of the construction.
We must be mindful also that
incorporating elements to improve the
thermal performance of our building
changes the physics of the structure.
We must, therefore, avoid creating new
problems, such as trapped moisture or
summer overheating as a byproduct of
the focus on preventing heat loss.
Ultimately, we must stop focusing on
U-values alone; the most costly of
the three ways of reducing heat loss.
By delivering thermal performance
through a combination of U-values,
Y-values and air tightness, we are
more likely to deliver the low energy
public sector built environment we
need to drive performance, whilst
reducing cost.