Insulate Magazine Issue 17

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Exclusive Insulate Column
The Language of
Insulation
Paul Forrester Technical Editor, Insulate Magazine
The ‘language of insulation’ is not what you
would call romantic. It’s unlikely to inspire
a book of sonnets any time soon; budding
Blakes, Chaucers and Shakespeares tend
to have weightier things on their minds! by Paul
Forrester
No: insulation is a technical, logical language, born
of the laws of physics. Much of it is black and white,
but there remains room for nuance. Used well, it is a
powerful tool for communicating ideas about building
design and performance. The words themselves may
not be beautiful, but they can help create great spaces
that inspire great work from the people who use them.
Thermal Properties
A fundamental measure of a thermal insulation material
is its thermal conductivity (lambda value, units: W/
mK), the amount of heat energy it conducts. Generally,
thermal conductivity doesn’t change with thickness,
so thermal resistance (R-value, units: m2K/W) is a
more accurate measure of a material’s ability, at a
specific thickness, to resist heat transfer.
The resistances of the materials in a wall or roof are
combined with internal and external surface resistances,
then the reciprocal taken, to give the thermal
transmittance (U-value, units: W/m2K) of the element.
Additional calculation procedures apply to floors,
basements and steel framed structures.
Conduction through materials is one mechanism for
heat transfer; another, via the movement of air, is
convection. The third principle mechanism occurs
due to bodies emitting and absorbing electromagnetic
radiation at different rates, depending on temperature
and emissivity.
Installation
Thermal insulation is most effective when installed as a
continuous layer, forming a thermal envelope. Interruptions
to the insulation layer are thermal bridges (or cold
bridges), and should be avoided as far as possible.
They can be the result of structural requirements, poor
design, or poor installation.
Thermal bridges can be repeating, occurring at consistent
intervals (like timber rafters, a structural frame,
or mechanical fixings), or linear, where the geometry
of a construction element changes or at the junction
between elements (the floor/wall junction, the wall/roof
junction, or around door and window openings).
Gaps in insulation layers can result in warm and cold
air mixing, and circulating around the insulation layer.
This is one type of thermal bypass, a collective term
for different types of air movement (also including air
infiltration and wind washing) that result in significantly
increased heat loss.
Taking a fabric first approach to construction centres
on reducing heating demand and energy use through
efficient building fabric (i.e. insulation and well-detailed
thermal bridges) and passive heating techniques.
Maximising solar gains through good design maintains
comfort in winter, with solar shading to limit the risk of
summer overheating.
Condensation Risk
A fabric first approach also relies on airtightness, to
stop warm air leaking from the building, and controlled
ventilation, to make sure occupants have fresh air to
breathe. Design for low energy construction typically
features a continuous airtightness line within the
thermal envelope; the level of airtightness dictates the
ventilation measures that should be installed.
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