Physics of Heat
Flow
www.concertinafoilbatts.com www.afia.com.au Aluminium Foil Insulation Assoc. August 2005 Page 1/6
There are three methods by which heat flows –
i) radiation - the emission of heat waves passing through air or a vacuum
ii) conduction - contact from one body to another
iii) convection - upward movement of warm air
Radiation is the first and predominant form of heat transfer. In winter, heat is lost through ceilings, walls and windows by radiation followed by convection, whereas framed timber floors lose heat only by radiation downwards – there is no convected heat loss. Heat gain in summer through ceilings, walls and windows is entirely by radiation, not convection.
Federal and State building energy efficiency regulations exist to
reduce energy used for artificial heating and cooling, in order to reduce
national greenhouse gas emissions.
Heat flow in and out of buildings in Australia is influenced by many factors.
·
Identifying the climate and the dominant direction of heat flow – in
or out or mix of both.
·
Temperature difference between indoor and out. Outdoor ambient shade
temperature versus direct radiation entering buildings through ceilings, walls
and glazing. How to stop radiation penetration.
·
Amount, type and cost of heating and cooling needed to maintain desired
human comfort levels.
·
Building design and orientation to the sun, mass versus lightweight
construction.
·
Advantages of Open versus Closed building design in hot
climates.
Open
design
homes are energy efficient and healthy using cool air by natural ventilation, assisted by ceiling fans or
evaporative appliances. Most effective cooling time is in the evening.
Closed
design
requires cold refrigerative air to be
produced in order to maintain an artificial internal temperature. It is the
most expensive form of cooling and is unhealthy.
·
Thermal performance of all building materials (including insulation)
against heat flow in or out. Expressed as Rvalues – the higher the R, the greater the
resistance. The ‘material’ Rvalue is different to the installed or ‘total’
Rvalue – eg R2.5 fibre batts in ceilings is not continuous R2.5 because of
bridging effects and produces Total R2.2 for timber joists and lower again for
steel.
·
Insulation levels and the Law of Diminishing Returns. Initial
insulation makes the most significant effect and increasing levels beyond R2.0
in ceilings has rapidly diminishing economic and comfort benefits – twice the
Rvalue does not give a doubling in energy savings for heating or cooling. It is
better to consider all of the heat paths in a particular building rather than
to insulate one of them heavily – this is called the overall principle of insulating.
·
Determining the most appropriate and effective insulation for varying
Australian climates.
Insulation materials are considered the single biggest factor affecting
the resistance capabilities of the building envelope and there are generally
two types of insulation: bulk and reflective.
i) Bulk insulation works by means
of small air pockets which retard or restrict the flow of heat - and have
“material Rvalues”. Resistance is always measured under steady-state conditions, commonly in a Heat Flow Meter, where the
material is placed between two metal plates set at fixed unchanging
temperatures of 33 and 13degC, where the mean or average temperature is 23deg
and the testing duration is 4 hours.
Bulk insulations originate from the northern hemisphere (about 60 years
ago) to keep heat inside buildings and are not tested against high temperature
radiation entering buildings through roofs/ceilings and walls, as what occurs
across typically across Australia, where the underside of metal roofs can have
been recorded with downward radiating temperatures of approximately 80-100degC.
p2/6
ii) Reflective insulation reduces the flow
of heat by reflecting radiation away on
the warm side and giving off very little on the cool side. Aluminum foil
insulation, with airspaces, will provide a continuous
and permanent resistance to the flow of radiation – by 97% reflectivity on
the side of the heat source and 3% emissivity, or re-radiation, from the
opposite surface.
Aluminium foil
insulations have been used in Australia since 1954 to thermally insulate
buildings in roofs, ceilings, walls and floors. In 1998, Wren Industries
(Concertina FOIL BATTS) and two other foil companies created the very first
foil association in Australia, the Aluminium Foil Insulation Association Inc.
(Vic) – AFIA - which has representation
on Standards Committees as well as the Australian Building Codes Board (ABCB)
Industry Technical Advisory Committee.
Rvalue is the
term used in Australia for expressing the thermal resistance of any insulating
material. Foil insulations do not have "material Rvalues", as do bulk
insulations, but rather expressed as "Total
Rvalues", being the sum of the entire wall, ceiling/roof or floor
structure, including the resistance values of foil airspaces.
Publications of the AIRAH(Aust) Handbook 2000 and ASHRAE(USA) Fundamentals, are the historic base references for selecting Rvalues of foil airspaces. These Rvalues are derived from physical testing undertaken in the USA - Report HR32 (Robinson&Powlitch 1954), of variable width parallel foil airspaces in varying orientations – ie in horizontal, vertical and sloping planes. CSIRO Melbourne physically tested the winter thermal performance of aluminium foil in brick veneer walls in 1982-84, the results of which validated the original 1954 USA testing.
AFIA has developed and utilises the computer program REFLECT 3 (Aynsley-USA: 2003) which provides a modified methodology for the calculation of foil airspace Rvalues better suited to Australian climatic conditions, rather than USA or Europe.
As of 1 July
2003, the BCA (Building Code of Australia) requires all insulation materials to
conform to the new Australian Standard AS/NZS4859.1:2002 "Materials for
the thermal insulation of buildings". 4859.1 requires all factors
known to affect thermal performance of any insulation material to be
accounted for, including high temperature radiation effects. Bulk insulations
require a routine Rvalue testing procedure, whereas foil systems do not and have thermal performance undertaken by
calculation only. Also, 4859.1 requires that the thermal effects from any
framing (ie floor joists, wall studs, ceiling timbers), is excluded from the
calculation method, and the Total Rvalue of any insulation is for the insulation path alone. Sometimes unusual
foil insulation configurations are so complex that expert opinion is the only
realistic practical option for presenting an indication of performance.
Thermal
performance of Concertina FOIL BATTS
conform to a deemed to satisfy requirement
under BCA ClauseA2.2 with evidence of suitability recognised via a
Certificate from a professional engineer
or other appropriately qualified person. Calculations for Concertina FOIL BATTS are undertaken by James Fricker (MIE Aust)
in accordance with “AFIA Guidelines and Assumptions” Nov 2004 – refer www.afia.com.au.
These Assumptions are endorsed by RD Services (USA) – www.rdservices.com
As from 23 June
2005, Wren Industries products are marked with the Benchmark Certification
trademark. Refer www.concertinafoilbatts.com
AFIA believes
that there is a lack of thermal testing facilities which are needed in
Australia to better demonstrate in-situ/in-service performance of insulation
materials, in particular, the need to test bulk fibrous insulations for the
effects of summer radiant heat. For the past 50years, the fibreglass industry
have consistently refused to subject their insulation products to the effects
of high temperature radiation entering buildings.
Further examples are when foil is wrapped around a cooked chicken or
with an aluminium coffee pot and you press your hand onto the surface -100%
heat transfer by conduction occurs, but once you draw your hand away, virtually
no heat is felt. Why? Because the properties of
aluminium foil are such that only 3% of the internal heat will be emitted from
the opposite surface, but there must be airspace.
Foil insulation is commonly manufactured in roll form and is used in
houses mainly to be fitted under roofs and on external walls for waterproofing
and thermal insulation. It can be single or double-sided aluminium and is
coloured on one side to indicate that this is the side which faces the sun for
safety “anti-glare” reasons. In both walls and roofs, the dominant insulating
surface of roll foil is the visible aluminium side which always faces into the building and, in conjunction with an
efficient airspace, has two functions:
i) 3% low emitting surface
- summer
heat flow in
ii) 97% high reflecting surface - winter heat flow out
Aluminium foil airspaces produce maximum (or optimum) thermal resistance with:
i) Vertical airspaces - walls: minimum 20mm to 40mm wide
ii) Horizontal airspaces - ceilings and under floors: minimum 50mm to100mm deep,
ie less than 50mm or more than 100mm - the resistance decreases.
In walls, vertical foil airspaces have basically the same Rvalue heat
flow sideways, in or out,
ie summer and winter Rvalues are the same. In ceilings, horizontal foil
airspaces have about 2-3 times greater Rvalue heat flow down than up, ie the
summer value is much greater than for winter.
For winter heat flow out of
buildings, the highest Rvalue for foil is achieved with the existence of still airspaces, ie no air movement. However,
with summer radiant heat flow in, moving
air against the surface of foil actually increases thermal resistance, ie foil
airspaces in roofs, ceilings and walls gain a benefit in summer with high
ventilation rates.
Concertina FOIL BATTS are segmented pieces of double-sided aluminium foil laminate which are innovative and unique in walls and floors because they are recessed and stapled between timbers to form uniformly deep foil airspaces producing constant, optimum and uniform thermal resistance. Roll foil, on the other hand, dished behind weatherboard walls or draped over floor joists has varying airspace depth and consequently changing or inconsistent thermal resistance.
A timber frame house wrapped with aluminium foil insulation plus a Concertina FOIL BATT in the stud
cavities will provide high summer and winter thermal performance.
FOIL
BATTS in walls totally take the place of fibre batts, are self-supporting, rigid,
cannot slump and split the one stud airspace cavity into two
uniformly deep thermally efficient foil airspaces producing consistent
unchanging Rvalue over the entire wall surface area – unlike dished roll foil
if fitted behind plasterboard. Still airspaces on either side of the FOIL
BATTS provide greater thermal resistance in winter than ventilated
airspaces. In brick veneer walls, external house wrap foil makes a valuable
contribution – three multiple foil airspaces are created, two are still and the
cavity space is ventilated. Trapped still air is a good insulator, just like
wearing multiple layers of clothing will keep you warmer than wearing one
sweater. And every aluminium foil airspace has the
capacity to be a low emitter or high reflector of radiant heat.
p4/6
R1.5-2.0 fibre batts in walls must remain dry and need to be restrained from touching outer brick or clad walls because they can absorb moisture (increasing the risk of slumping) and transfer this moisture onto internal walls causing damage and loss of thermal performance. Outer foil wraps are commonly used as a waterproof and restraint device but the fibre batts are pushed into stud cavities making direct contact to the inner aluminium foil surface, which then transmits conducted summer heat directly into the fibre batts, which is stored and then transferred onto internal walls. In other words, the thermal contribution of the foil insulation (inner foil surface: summer 3% emissivity, winter 97% reflectivity) is practically the same as brown building paper.
Western and eastern walls in southern latitudes such as Melbourne experience long periods of daily solar radiation in summer, as the sun sets very slowly compared to more northerly States. With cooler evening time temperatures, fibre batts in the walls continue to radiate heat into the house. Replacing the fibre batts with FOIL BATTS will create the vital non-conduction airspace to the foil wrap and it will now be impossible for any daytime summer radiation to penetrate inwards through the wall structure, achieving a Total R2.6 in BV walls (NB:Total R2.0 approx for foil wrap plus R1.5 fibre batts).
Clad walls – fibre cement, blue board or timber such
as unpainted weatherboard or western red cedar – must also be permitted to
“breathe” by allowing the free movement of moisture vapour through walls,
inwards or outwards. Tyvek breather
paper or pin-pricked “breather” external foil wraps, dished slightly inwards
between studs, assists the passage of moisture vapour to pass through into the
stud cavity and dissipate or vapourise. Fibre batts pressed up against breather
foil push the foil hard up against the cladding which restricts the free
movement of air and it is more likely for a moisture buildup to occur between
the foil and the cladding as well as the ever present possibility of moisture
forming in the matrix of the batt fibres increasing the slumping risk.
However, with Concertina
FOIL BATTS any moisture-laden air
from the cladding is much more likely to freely escape into the two adjoining
foil airspaces. An example of a total vapour seal behind cladding would be the
use of foil styrene wall boards, which is a plastic barrier that cannot breathe unless punched holes are
made. The
FOIL
BATT system does not vapour-seal walls and this is one important reason
why the Timber Advisory Centre (Victoria) give their support to Concertina
FOIL BATTS – refer to TAC letter attached here or on www.concertinafoilbatts.com - see testimonials.
FOIL
BATTS are cut to fit individual cavities and are stapled to meet the top
timber - nogging or top plate. It is not necessary for a tight fit top and bottom in the stud cavities. In winter,
after radiation occurs from internal walls, convected heat starts to form in
the vertical airspaces and, with a sizable gap at both top and bottom of the FOIL
BATT, a rotational airloop could occur between the two foil airspaces.
As stated earlier, moving air is detrimental to foil airspaces but only for
winter heat flow out.
FOIL
BATTS meeting the top (or bottom) timber will break the airloop possibility
and thereby maximise winter Rvalue of these foil cavities. In summer, the walls
of houses are best insulated with FOIL BATTS and foil house
wraps forming up to three multiple airspaces operating which will never
transmit high temperature radiant heat inwards, while also successfully
stopping winter heat escape.
The Rvalue performance of aluminium foil insulations in walls is the same in both winter and summer.
FOIL
BATTS in walls form approximately parallel and uniformly deep airspaces and
have calculated Total Rvalues in brick veneer walls, below: (#house wrap foil is
based on double-sided anti-glare)
·
Uninsulated brick veneer wall = Total R0.5
·
FOIL BATT + Foil house wrap = Total R2.6 (NB: Clad walls=R2.5 *FB+ breather foil)
·
FOIL BATT alone = Total R1.8
·
Two FOIL BATTS + Foil wrap = Total R3.7
·
FOIL BATT + Tyvek breather =
·
Hebel Power Panel + FOIL BATT
=
·
Braceboard + Foil house wrap
= zero thermal value of foil - direct contact to braceboard
·
Braceboard + FOIL BATTS =
Total R2.0 approx
p5/6
Foil wraps fixed directly onto braceboard have a zero foil
airspace, contribute zero thermal performance and serve only one purpose
- waterproofing the framed walls during construction. FOIL BATTS are a much
better insulator with two fully functioning foil airspaces created in the stud
frame cavity.
Up to 93% of the heat passed down from the roof of a building may be
transferred by radiation.
Foil insulations in horizontal or sloping positions have different
thermal resistance in summer and winter – approximately 2-3 times the R-value heat flow down compared to heat flow up. This explains why that for
locations experiencing both hot and cold climates, Concertina FOIL BATTS are recommended to sit
over the top of bulk insulation in standard pitched roofs - they form unique triangular air pockets with low
emitting downward foil surfaces in summer which then switch and become high
reflective surfaces in winter. FOIL BATTS provide maximum summer
and winter benefits at ceiling level because this is the point of final summer
heat gain and immediate winter heat loss, ie winter heat must be stopped at
ceiling level and not be allowed to escape into the roof cavity. Concertina FOIL BATTS are more
effective than a single layer of foil rolled out and laid flat on top of bulk
insulation (or the ceiling itself) because the triangular foil surfaces facing
downward function continuously with a series of formed airspaces with only a
small % surface area of contact, hence very little conduction of heat.
Bulk insulation works better in winter than summer because winter
“convected” heat is gentle at around 24-27degC at ceiling level, whereas summer
heat flow into buildings is very different, with roofs typically radiating
downwards at 80-100degC. Bulk insulation is officially tested at a fixed 33deg
maximum temperature and does not have guaranteed Summer R-values but
rather guaranteed summer performance at only 33deg. This fact ought to be
printed on the labeling of all bulk insulations.
See also Wren website – Thermal Performance & Truth about Rvalues.
The R-value performance of Concertina
FOIL BATTS:
·
LAID on top of any existing fibrous insulation + foil sarking under
roof
* Summer Heat Flow Down = Total
4.9 ) **see official calculations
* Winter Heat Flow Up = Total 4.2 )
·
STAPLED between timber ceiling joists or roofing rafters
* Summer Heat Flow Down =
Total R2.0 approx only
* Winter Heat Flow Up = Total R1.0 approx
in ceilings is continuous bulk insulation (no gaps) with a FOIL
BATT, either on top or beneath. At least one foil radiant heat barrier is needed in Australian pitched roof
spaces and two foils are recommended
for sloping or cathedral roofs due to the close proximity of the ceiling to the
high intensity radiation
– eg foil sarking plus FOIL BATT or two FOIL
BATTS. In climates where heat entry dominates over heat loss, then
there is a clear case that foil insulations be used alone, and bulk insulation
totally avoided. Bulk insulations are not radiant heat barriers but rather
radiant heat absorbers.
Insulating ground level timber floors against winter heat loss has
historically been very difficult, impractical or too expensive. Roll foil
dished over joists makes gluing or cramping of floorboards or fixing sheet
particleboard impossible – a gluing bond between timber joists and the timber
floor is meant to occur. To overcome these problems, perforated Concertina FOIL BATTS (for use under
floors) are simply expanded and stapled between timber floor joists.
FOIL
BATTS are regarded by the Timber Promotion Council (Vic) in 1993 as “the most suitable underfloor
insulation system….this product will fit between the floor joists allowing the
floor to breathe, whilst the perforations will prevent any condensation buildup
that may form”. Also, in relation to the introduction of 5 Star house
energy rating in Victoria (July 2004), the Timber Advisory Centre (Vic)
p6/6
have stated that “Wren
Concertina FOIL BATTS under timber floors and in weatherboard walls will
be an important partner to the achievement of energy efficient house design
because of their reflective properties and creation of beneficial airspaces” - see letters on
website. FOIL BATTS under
floors have 50-100mm
deep breathing airspaces. Strip flooring is a living organic material which
breathes by contracting and expanding when absorbing and releasing moisture
vapour and is commonly sealed on the top surface with polyurethane and
therefore can only breathe downwards. FOIL BATTS have unsealed overlaps
and do not reduce or impede the breathing ability of the timber flooring.
According to AS/NZS4200.2:1994, foil insulation in rolls under
floors must not hold moisture and clear drainage holes will do this. Perforated
FOIL BATTS will provide swift escape of any moisture passing
through particleboard platform floors exposed to rain or from possible condensation.
Note also that sheet polystyrene insulating boards are the same as
continuous plastic – the floor cannot drain or breathe, unless there are
defined gap openings. Also, any fibre batt insulations under floors must not be
exposed to any risk of moisture – 1% moisture content can equal 30% reduction
in thermal performance. And any fibre batt insulation requires support
by chicken wire, mesh or strapping. Concertina FOIL
BATTS are
self-supporting and require no additional support devices.
Lastly, to achieve maximum winter thermal performance, the sub-floor space needs to have still air. With clad walls, black close weave shade cloth fitted behind perimeter base boards will act as a wind barrier and deflect air movement from entering and exiting the sub-floor area. Alternatively, for elevated houses on sloping ground with open perimeters, the underside of floor joists can be lined (eg with roll foil with breathing holes) to create immediate stillness of the joist cavity and with Concertina FOIL BATTS stapled halfway down joist sides forming two 50mm foil airspaces within a 100mm joist cavity.
Concertina FOIL BATTS are CERTIFIED and conform
to:
Australian Standard AS/NZS 4859.1(2002) “Materials for the thermal
insulation of buildings”
Further information can be viewed on
www.concertinafoilbatts.com
Wren Industries is a founding member of:
AFIA – the Aluminium Foil Insulation Association Inc. (Vic 1998) www.afia.com.au
AFIA is a participating organisation on:
i) Industry Technical Advisory Committee of the Australian Building Codes
Board (ABCB) in the creation of building energy efficiency measures, introduced
into the Building Code of Australia being made effective State by State from 1
Jan. 2003.
Manufacturers of Concertina FOIL BATTS and RENSHADE
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