A study of heat movement, home
insulation
and
radiant barriers in homes
Horizon Energy Systems
Copyright 1997/2009
By: Brad Lindsay
Copyright 1997
ARE WE LIVING IN OVENS?
Homes in hot climates are unknowingly designed and built to
act as Dutch ovens, baking the people living in them. Homes built in hot
climates using today's building standards are collecting, storing, and,
re-emitting heat energy long after the sun goes down. Not only is this
phenomenon of heat re-emission expensive for the home owner because of the costs
for cooling a home, it's also uncomfortable.
The re-emission of heat energy can be easily experienced by entering an attic at 10:00 p.m. after a hot day. Even though the sun has been down for several hours and the ambient temperature is under 100f, the attic temperature is still very warm. What is the source of this mystery heat? Hot air trapped in the attic? There is not enough volume or mass in air alone to store this many BTU's for so long. The roof? Tracking roof temperature with an infrared camera shows the roof matching outdoor ambient temperature 40 minutes after the sun goes down. Placing a thermometer into the insulation will reveal the source…stored heat in the insulation. Since the purpose of insulation is to slow heat movement, it takes hours for heat to escape once the insulation gets hot. It is very important to keep the insulation as cool as possible during the hot summer months in a hot environment. Cooler insulation means cooler ceiling. Cooler ceiling means less to cool and more comfort.
WHY ARE HOMES BUILT THIS WAY?
Two reasons: First, insulation levels used in today's homes
began in cold climates where heat moves upward and is lost mostly through
convective and conductive losses through the ceiling. Building codes soon
followed. However, since they are not climate specific, there is no
difference in building codes between a home built in North Dakota or the desert
of Phoenix AZ. A layer of insulation in
the attic resisted these heat losses (in cold climates) and saved energy.
The term "R-factor" was then created by insulation manufacturers as a tool to
gauge their new product. "The higher the R factor, the more you save" is
what we hear.
Second, when fossil fuels were cheap and seemingly endless, generating electricity was inexpensive. This fostered monthly electric bills under $50 dollars and minimal interest in energy conservation. Homes were built accordingly. Recent energy audits performed on homes here in Phoenix, Arizona illustrate the chronological history of conserving energy or lack thereof. Many homes built in the 40's had little or no insulation in the walls or ceiling.
The “R” stands for resistance to heat flow. Driven by rising energy costs year after year, we began to look for ways to conserve. A higher R-factor seemed to be the answer since it worked in cold climates for heat loss. The standard in the 80's was R-19 in the attic and R-11 in the walls. It was then recommended to increase the walls to R-19 by using a 6” wall stud and R-30 in the attics. The “more insulation is better” train of thought continues today as some home builders are now offering R-60 in the attic. This theory does not work in hot climates were heat becomes trapped inside insulation.
INSULATION IN HOT CLIMATES
Does fibrous insulation work for radiant heat? To
some extent. However, during the hottest part of the day, it can be
confirmed that it is 10 to 30 degrees hotter 1" below
the surface than the hottest air in the attic! What does this mean?
Several inches above your ceiling is a heat source hotter than your
attic!
INSULATION IN COLD CLIMATES
Insulation was originally designed to minimize energy
wasting conductive and convective heat losses through walls and ceilings.
But what about radiant heat? Again, placing a reflective surface in the
path of heat in ANY direction will reduce the need for heating OR
cooling. Our Thermal Control Membrane or RBS Chips installed
over conventional insulation will change the direction of the heat leaving the
building, warm the ceilings, increase comfort and lower energy costs.
"Insulation for your insulation" is a good way to describe this.
Tim Carter, known building expert and nationally syndicated columnist, has
RBS Chips in his home over a R-60 bed of blown fiberglass. He is now a
believer and wrote about the benefits which was printed in every newspaper
across the country: http://www.askthebuilder.com/cgi-bin/column?447
COMMON BELIEFS
The sun heats the roof, the roof heats the air in the
attic, which, left unchecked, will move into the home. Insulation
having an R-factor or resistance to heat flow sounds like a pretty good idea
when placed just above the area being cooled…or is it? What about the
radiant heat being emitted from the plywood roof deck? Does insulation
slow radiant heat? To some degree. But not as well as a reflective
surface as we will see.
REALITY
If
insulation absorbs radiant heat and is hotter than the attic air, what then is
an appropriate method for reducing this commonly overlooked form of heat
gain? A reflective surface with a low emissivity placed between the source
of radiant heat and the area to be cooled seems like a good idea. This
reflective building product is now recognized as a Radiant Barrier System
(RBS). RBS placed correctly in a home can
significantly reduce heat movement and increase the overall efficiency and
comfort. But ten years of research has proven that RBS placed incorrectly
can increase the energy consumption. Refer to fig 1 below.
Figure 1
RADIANT BARRIER TESTING
Figure 1 illustrates the results from testing Radiant
Barrier Systems (RBS), on four identical, unoccupied homes. The
black line is the control home without a RBS. The other lines track three
different types and placements of RBS in a residential home. The graph is
based on energy consumption across a 24 hour period on four identical,
unoccupied homes. Further, all homes were tested to have equal duct losses
and infiltration factors by the Arizona Department of Commerce Energy Office by
using a blower door and energy audits.
RAFTER RBS
This home has the RBS stapled between the roof rafters, up
against the bottom of the plywood roofing material or the roof deck. This
seems like a logical placement for the RBS as the roof is the source the
incoming radiant heat. An obvious drawback to this design is the
difficulty in trying to install it. Cramped quarters, wasted, ripped
material and the potential over time for gravity to pull it down are a few
problems. More importantly is the effect the rafter RBS had on energy
consumption. Not only is this configuration difficult to install, it
caused the home to consume more energy than the control home without a
RBS.
PLYWOOD LAMINATED WITH RBS
This unique RBS (Was called "Koolply" but now sold as
"Techshield") is applied (laminated) to the roof decking material prior to the
construction of the home. No additional labor is required for installation
as the RBS is in place as the roof is being nailed down. This is an
obvious benefit from an installation point of view. However, like
the rafter RBS, laminated plywood RBS caused this home to consume more energy
than the control house without a RBS. How is this possible? The
bottom surface of the wood roof deck has an emissivty of .95 which is lowered to
closer to .17 after having the foil laminated to the underside thus reducing the
emission of infrared heat. I will explain this in greater detail further
down this page. Note: Using a laminted OSB product for roofing or
stapling RBS up under a flat roof deck is not a problem, only in homes with a
pitched roof. A flat roof won't drive convection like a pitched roof
will.
UNDERSTANDING EMISSIVITY
In order to understand the chart above, an understanding of
emissivity is necessary. Emissivity is the ability for an object to
release radiant heat. The lower the emissivity, the more difficult
it is for heat to leave its’ surface. This why a chrome auto bumper is
hotter than one painted black left to sit several hours in the sun.
Most paints emit in the .90 range which is very
high. (See Fig 1.2) Chrome has an "E" value of .05. It
will take the chrome bumper longer to get hot due the high reflectivity value,
but the low emissivity of chrome traps the heat making it much hotter than the
black one. Another example is leaving your toolbox open to the sun while
doing car repairs. Ever try to pick up a chrome socket or ratchet
handle? How about a chrome car door handle or chrome ignition starter on
the steering column. The low E value of chrome prevents the absorbed heat
from escaping making them very hot. This is why black chrome solar panels
provide hotter water than panels painted flat black. Black chrome will
take a little longer to get hot, but once it does, the low E selective surface
traps heat in the absorber which in turn transfers it through conduction into
the water passages. Fig. 1.2 lists the emissivity of various substrates
and building materials.
Emissivity explained in layman's terms
Main Entry:
black·body
Pronunciation: 'blak-'bä-dE
Function: noun
Date: 1710
: an ideal body or surface that completely absorbs all radiant
energy
falling upon it with no reflection and that radiates at all frequencies
with
a spectral energy distribution dependent on its absolute
temperature
Ok ok...in layman's terms. Emissivity is the ability for radiant heat to leave the surface of an object. It matters not what the density, mass or thickness of the object, only the surface. As can be seen below, emissivity (E factor), plays a significant role in how heat moves into or out of our homes, our cars, our bodies.
Fig 1.2 EMISSIVITY OF VARIOUS COMMON MATERIALS
Material Emissivity value
Gold, polished .02
TCM Radiant Barrier .03 (second layer .02 )
Silver, polished .04
Chrome .05
Aluminum,
polished .04
oxidized .78
Brass, polished .04
oxidized .61
Iron, polished .21
oxidized .69
Copper, polished .05
oxidized .78
Human skin .98
EMISSIVITY OF BUILDING MATERIALS
Wood .95
Glass .94
Paint,
average of 16 colors .94
Brick, common red
.93
Concrete .92
Plaster, rough coat .91
SUMMARY OF PERFORMANCE
Using Fig 1.2 as a reference, lets get back to the
different RBS applications and see how the emissivity affects energy
consumption.
News Flash! Click here to view recent university testing of our TCM product.
RAFTERS
The
RBS placed at the rafter reflects the incoming infrared (IR) back to the surface
of the roof. This in turn heats the roof hotter than it would have
been without the RBS. The hot roof heats the air in the attic which then
increases the temperature of the insulation which in turn increases heat flow
into the home. As the sun moves towards the horizon, it becomes apparent
in Fig.1 that the heat is trapped inside attic, raising the demand for
electricity. The RBS is reflecting this heat back into the home instead of
allowing it to escape through the roof.
This is not a recommended placement for a RBS.
Increased roof temperature over time may also lead to premature
degradation of roofing components such as shingles and laminated wood
products.
LAMINATED PLYWOOD RBS (Koolply)
Since the emissivity of the plywood has been reduced by the
RBS laminate, the heat is trapped in the plywood roof deck much like the
chrome bumper discussed earlier. This increased roof temperature has
the same effect as the rafter RBS in that it increases the temperature of the
air in the attic. Similar to the rafter RBS, this application also traps
the heat in the attic much like a thermos bottle keeps coffee hot: by
reflecting the IR back to the source, which at the end of a day in the desert,
is the attic insulation. And the higher the R-factor of the insulation,
the greater potential to retain it.
RADIANT BARRIER CHIPS
The RBS Chip product is installed directly over the attic
insulation offering an effective shield from radiant heat. Since the
emissivity of the roof has not been lowered, heat in the attic can move back
through the roof at the end of the day thereby minimizing the thermos bottle
effect seen in Fig 1. Placing an RBS directly over the insulation was
the
original application years ago when
Radiant Barriers gained attention as a viable energy source. It soon
became apparent that airborne particulates such as dust would settle on the RBS
thereby reducing the reflectivity and subsequently losing thermal
performance. The RBS Chip product overcomes this performance degradation
problem by having many layers of RBS stacked upon each other. Dust will
settle on the top layers which protect the layers below. Testing by the
Florida Solar Energy Center (FSEC) in 1989 showed a 42% reduction in heat flux
over a test cell without a RBS and an R-factor of 19. Since then, Horizon
Energy Systems, manufacturer of the RBS Chips, has done field testing in
homes in Nevada, California, Michigan, Arizona and Mexico. Recently, the
RBS Chip has been redesigned (a new shape and size) which is even more
efficient, installs easier and offers better coverage .
DESERT TESTING
Our first full size test home was built without insulation
in the attic, only two layers of RBS, one stapled up to the rafters and
one layer on the attic floor where the insulation. The RBS was a highly
metalized film product with a tested emissivity of .05. An identical home
was built next door as a control house for comparative analysis.
Dr. Byard Wood at Arizona State University wired
these homes with a 15 point pyrometer which measured temperatures in the attic,
roof, interior, walls, ducts, ceiling, insulation and ambient. As the
summer pressed on, the RBS home began to take the lead with regard to energy
savings. This despite the large difference in electrical consumption directly
related to the family of seven occupying the RBS home while the control home was
occupied by a couple that both worked during the day and turned the thermostat
up to 85 degrees when they left.
The most
significant data retrieved from this analysis was the observation of the lack of
heat in the RBS attic area, and the length of time heat was “trapped” in the
control home. During the day, the RBS homes’ attic never exceeded 4f above
the ambient. If it was 110f outside, the RBS attic was 114f. The
control house next door with R-30 blown fiberglass exceeded 145f on
several occasions. More important to note is the length of time the
control home had accelerated attic temperatures (above ambient).
The ability for insulation to store heat and
increase attic temperatures became apparent once again as it was decided to add
conventional insulation to the RBS homes’ attic for sound and winter
months. The insulation truck arrived at 9:00am. By noon, a 3” layer
of blown cellulose (R-19) was added above the RBS already laid out on the
attic floor completely covering it. By 2:00 the attic was hotter than it
had ever been. Subsequent testing on other homes illustrated similar data:
the insulation was hotter than the attic air.
Insulation in hot climates when
subjected to infrared heat in the attic and walls has the capacity to store a
tremendous amount of heat for many hours.
CONCLUSION
Ten years of research, intensive thermographic
scanning and exhaustive documentation have led to some surprising results:
The attics of our homes are in reality low-heat Dutch ovens, costing millions in
energy costs and reducing interior comfort. The bottom line? Protect
insulation from the intense radiant energy emitted from the roof deck with a RBS
installed in the correct place. This in turn reduces convective and conductive
heat transfer. Keep the insulation cooler and you reduce the energy required for
cooling increase interior comfort and extend the life of the most expensive
appliances in your home: furnaces and A/C units.
It can be seen that a quality Radiant Barrier System has a place in every home to reduce energy costs and increase interior comfort. Recent installations in cold climates have shown significant reductions in heat loss saving energy and increasing interior comfort. Why wouldn't it? Consider a thermos bottle which works equally well for hot coffee or ice tea.
It must be recognized that an alternative form of
measuring the performance of insulation is necessary as the "more insulation is
better" mind set does not necessarily mean better thermal performance.
Additional information on RBS and heat movement
can be seen on the internet at our web site: http://www.savenrg.com
RBS Chips in a residential attic
Lifetime, transferable performance
warranty
News Update:
Tim Carter, nationally known sydicated
columninst and web
master for www.askthebuilder.com,
has had RBS Chips in
his home for over a year. He
has experienced what RBS
Chips can do during hot summer months
and cold winters of
Cinncinati. Hear what he has to
say about it: click
here
PRODUCTS:
RBS CHIPS
Radiant Barrier Chips are a flexible, metalized film
product which are cut into small, 1" squares. These reflective chips are
then blown attic from a hose, much like fibrous insulation, where they form a
protective shield several layers deep against the incoming infrared heat source
generated by the hot roof deck. The RBS Chip was the only RBS to
illustrate an energy savings over all the other homes tested.
THERMAL CONTROL MEMBRANE (TCM)
TCM is a multi-layered, high performance insulation product
that, based on our performance of RBS Chips, also has a lifetime performance
warranty and is immune to the build up of dust.
About the author:
Brad Lindsay is President of Horizon Energy Systems in
Phoenix, Arizona. Mr. Lindsay has been in the HVAC industry since 1978 and
is now involved in energy efficient home design, infrared scanning
(thermography) solar (thermal and photovoltaic), insulation and currently
manufactures patented Radiant Barrier Systems and solar products for homes,
business, farm use, vehicles, hot water tanks and several industrial
applications.
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