10 November 2001
This is a critical time in Australia’s efforts to achieve
energy efficient buildings and reduce its greenhouse gas emissions. Australia needs verifiable scientific
methods for determining realistic R-values for external envelopes of
buildings. Current test methods fail to
adequately account for radiant heat transfer or the high temperatures commonly
experienced in roofs in Australia. One reason for the inadequate accounting for
radiant heat transfer in roof spaces is the difficulty posed by the geometry
(view factors) of roof spaces. The
Australian home owner deserves reliable R-values for insulation products
derived using a common method for all products. Now is the time to deal with these issues with some good science.
United States encountered similar problems some years ago,
including R-value feuding among insulation manufacturers. To resolve the issue
of radiant heat transfer in roofs, USA built the large-scale climate simulator at
the Oak Ridge National Laboratories in Tennessee. A full-scale residential test
module can be accommodated inside the climate simulator chamber. I had the opportunity to inspect this ORNL
facility on March 14-15, 2001, and to discuss many issues with Dr. Desjarlais
and Dr. Wilkes. ORNL is about two and a
half hours drive north of where I work in Marietta, GA.
Probably the most valuable feature of this large-scale
climate simulator facility is its ability to simulate dynamic climatic
conditions, including radiation, which in the US follow the Typical
Meteorological Year (TMY). This allows
direct comparison of heat fluxes from dynamic physical simulation with output
from the computer simulation software ATICSIM written by Dr. Ken Wilkes. It uses TMY climate data (Petrie, Wilkes,
Childs, and Christian, 1998). This
software was validated against the large-scale climate simulator data and is
now the ASTM C 1340-96 method for determining R-values in roofs.
It is extremely difficult to validate dynamic computer
simulation software such as NatHERS etc. from field study data. The beauty of
the large-scale climate simulator is that the facility can determine the
dynamic performance of all types of construction with any type of insulation
under a very wide ranges of climatic conditions. A large-scale climate
simulator offers the Australian insulation industry a level playing field; a
single method for determining R-values for any form of insulation or envelope
construction under Australian climatic conditions.
Impediments to
progress:
· Feuding
within the thermal insulation industry
· Inadequate
standards for establishing R-values and testing thermal insulation products
· Difficulties
in accommodating radiant heat transfer through roof spaces in estimating R-values
· Difficulties
in accommodating effects of air change rate through roof spaces in estimating
R-values
· Lack
of a common method for testing thermal resistance of bulk and reflective
insulation products, or combinations of these materials.
· Australia
does not have a large-scale climate simulator such as that at ORNL in the USA
which was established to address the issues listed above.
Benefits of
full scale thermal simulation approach:
· Full
scale simulation avoids scale effect issues associated with radiant heat
transfer
· Physical
full scale simulation reproduces roof space geometry influences on radiant heat transfer
· The
same Australian month-by-month, hourly climate data used in computer models
such as NatHERS can be simulated physically in the large-scale climate
simulator, including radiation and roof space ventilation rates. This avoids difficulties associated with
trying to compare output from computer models with data from field studies.
· Provides
opportunity to validate computer modelling against physical simulation (not
just other computer simulations).
· Provides
a common independent means for establishing overall R values for both reflective and bulk insulation products
(or combinations) and envelope construction.
· Validated
ORNL computer heat transfer software which runs under DOS is available now in
Btu/°F
units. ORNL is interested in having the
software rewritten by a third party to run under Windows using dual British/SI
units.
Reference
Petrie, T.W., Wilkes, K.E., Childs, P.W., and Christian, J.
E. (1998) Effects of radiant barriers and Attic ventilation on residential
attics and attic duct systems: New tools for measuring and modeling. ASHRAE Transactions., Vol.104, Part 2,
Paper number TO-98-20-1, pp. 1175-1192.
(Note: this paper includes a detailed description, and
diagrams, of the ORNL large-scale climate simulator. Under severe summer conditions simulated in the tests, the
average cooling benefit from the radiant barrier was 34% with ducts in the
attic and 29% with no ducts in the attic.
The computer model ATICSIM uses attic geometry and thermal properties of
the attic together with hourly climatic data [Typical Meteorological Year] to
calculate the heat flux through the ceiling).
Richard Aynsley B.Arch(Hons I), MS (Arch.Eng), Ph.D.
Dean, School of Engineering Technology & Management
Southern Polytechnic State University,
1100 South Marietta Parkway, Marietta, GA. USA
Tel. 1 (770) 528 7205, Fax 1 (770) 528 7134
email <raynsley@spsu.edu>
&
Adjunct Professor of Tropical Architecture
James Cook University, Townsville, QLD.
Australia