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37
ZEB
annual report 2015
Heat transfer through cavities can be reduced
by good air tightness, by filling the cavity with
thermal insulation material or by reflective
layers to reduce heat transfer by radiation. Air
leakages can be reduced and even eliminated
by air tight material layers like vapour- and
wind barriers.
The traditional solution, to fill some type
of thermal insulation in the entire cavity,
will reduce or eliminate both radiation and
convection. Still air has higher thermal
resistance than mineral wool. For air filled
cavities the thermal properties can be
improved by reflective surfaces to reduce
radiation. Convection is kept low by reducing
the thickness of the cavity. Conduction cannot
be reduced in air filled cavities since the
thermal conductivity of air sets a lower limit for
heat transfer:
F
min
= λ
air
· DT / d [W/m²]
λ
air
thermal conductivity of air, 0.025 W/(mK)
DT temperature difference across the cavity, K
d thickness of the cavity, m
ZEB focuses on reducing the carbon footprint
from construction materials. The idea behind
utilizing air cavities in building envelope
components is to reduce the amount of
thermal insulation materials while keeping the
thermal resistance of the envelope.
By using reflective foils in floors cavities (such
as crawl space), one can achieve a significant
reduction of heat loss. The air in the cavity will
be relatively stable as long as the temperature
in the cavity is higher than the temperature in
the ground under the building. This is because
the heat transfer by convection becomes
small. The heat transfer by conduction in the
stagnant air will also be small, because the
thermal conductivity of the air is low, about
0.025 W/(mK). Heat transfer will be dominated
by radiation from the underside of the floor
structure to the ground, but the radiation can
be reduced using one or more reflective foils
mounted horizontally in the cavity, parallel to
the floor area.
The maximum theoretical heat resistance that
can be obtained in crawl space are about 3.5
m²K/W and about 4.1 m²K/W, respectively.
This corresponds to a continuous layer of
normal insulation (thermal conductivity 0.035
W/(mK)) with a thickness of around 120 mm
and around 140 mm. In theory, this will reduce
the heat loss through the floor with about 20%.
In addition to the reduction in heat loss,
the temperature on the underside of the
floor joists is raised a few degrees during
THERMAL INSULATION PERFORMANCE OF REFLECTIVE MATERIAL
LAYERS IN WALL AND FLOOR CONSTRUCTIONS
VARMEISOLERENDE EFFEKT AV REFLEKTERENDE BELEGG I VEGG- OG
GULVKONSTRUKSJONER
Silje Kathrin Asphaug and Sivert Uvsløkk (SINTEF)