Download The fundamental principles of radiant heat barrier

The fundamental principles of
radiant heat barrier / reflective foil
Reflective insulation materials work
on a different concept than conventional
bulk insulation like rigid foam boards
or fibrous blankets. Unlike conventional
bulk insulation, reflective insulation has
very low emittance values “e-values”
(typically 0.03, compared to 0.90 for most
insulation) and that significantly reduces
heat transfer by radiation.
In order to understand reflective
insulation capabilities, one must be aware
that the radiant heat rays of the sun
do not become heat until they strike an
object such as a home or everything that
is in it. Thus, radiant heat rays must be
kept out in warm weather; while in the
cold weather, warmth must be kept in.
ion
S
SUN
t
Hea e
c
r
sou
at
t He
ian
Rad
t
Hea
‘e’ Exitance (Emitted) Heat to the atmosphere
See definition also Sol Air
diat
Ra
olar
e
Refl
ndu
t co
ea
H
‘c’
cted

’
d ‘a
orbe
Abs
at
He
cted


Heat not reflected is absorbed by
the structure once temperature
exceeds that of surrounding air,
structure will emit excess heat to
the cooler surrounding atmoshpere
‘e’ Exitance (Emitted) Heat to the interior of the building
Cold side
Thermal Resistance of Attic Spaces
Ventilated and Non Ventilated
Thermal Resistance ‘R’
(Heat Resistance Units) (m².K)/W
Heat Flow
Downward
SUMMER
Heat Flow Upward
WINTER
No Ventilation
0.282
0.176
Natural Ventilation of approximately 0.005m³/s for each m² of ceiling area.
0.458
0.112
Natural Ventilation of approximately 0.005m³/s for each m² of ceiling area.
1.092
0.564
Natural Ventilation of approximately 0.005m³/s for each m² of ceiling area
1.356
0.335
Non Reflective Attic Space: -
Reflective Attic Space:-
Total ‘R’-resistance values as shown in columns, is for foil facing the air space with ‘e’ - emissive values of 0.05.
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Absorptivity for Solar
Radiation ‘a’
Emissivity ‘e’
at 10 - 38˚C
Black non-metallic surfaces
0.85 - 0.98
0.90 - 0.98
Red brick, concrete and stone, dark paints
0.65 - 0.80
0.85 - 0.95
Yellow brick and stone
0.50 - 0.70
0.85 - 0.95
White brick, tile, paint whitewash
0.30 - 0.50
0.85 - 0.95
Window Glass
Transparent
0.90 - 0.95
Bright aluminium paint, gold, or bronze paint
0.30 - 0.50
0.40 - 0.60
Highly polished aluminium
0.10 - 0.40
0.02 - 0.05
Surface
NOTES:
1. A Reflective attic space is defined as one in which either the upward facing surface or the downward facing surface (or both) comprises
2.
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5.
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7.
bright aluminium foil, having an emissivity of 0.05.
It is virtually impossible to provide a completely non - ventilated attic space in a roof construction where the roof cover is metal, cement
tile or slate.
Expansion and contraction of a metal roof takes place as well as air leakage at the flashings at the parapet wall and at the eaves (soffit
area).
Roof tiles do not nestle so as to be air tight; the same applies to slate and flat profile tiles as a result of expansion and contraction.
A ventilated attic space is an advantage where Reflective Foil is applied as a tile underlay and radiant heat barrier - the movement of
natural air flow will enhance the performance against downward heat flow.
For the retention of heat, reflective foils are less efficient.
Ventilated attic spaces are a distinct disadvantage where open cellulose boards or fibrous materials are installed as a ceiling insulation
layer.
0860 527 725
www.sisalation.co.za
Don’t build without it!