Insulation, radiant barrier, energy
conservation, infrared cameras, IR, non destructive testing, Arizona,
A study of heat movement, home
and radiant barriers in homes
Horizon Energy Systems
By: Brad Lindsay
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, unfortunately, 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 11:00 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 above 125f. 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, 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.
Second, 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. A layer of insulation in the attic resisted these heat losses and saved energy. The term "R-factor" was then created by insulation manufacturers as a tool to guage their new product. "The higher the R factor, the more you save" is what we hear.
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 homebuilders are now offering R-42 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 20 degrees hotter 1" below the surface than the hottest air in the 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. Hence, radiant barriers properly installed should be in the building envelope of every home.
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.
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.
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.
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 (Koolply) 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.
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.
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.
Fig 1.2 EMISSIVITY OF VARIOUS COMMON MATERIALS
Material Emissivity value
Gold, polished .03
Metalized Film Radiant Barrier .04
Silver, polished .04
Aluminum, polished .04
Brass, polished .04
Iron, polished .21
Copper, polished .05
Human skin .98
EMISSIVITY OF BUILDING MATERIALS
Paint, average of 16 colors .94
Brick, common red .93
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.
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 .
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.
Ten years of research, intensive thermographic scanning and exhaustive documentation have led to some surprising results: The attics of our desert homes convert our homes into 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 as well as increasing interior comfort. It can be seen that a quality Radiant Barrier System has a place in every home to reduce energy costs and increase interior comfort.
It must be recognized that an alternative form of measuring the performance of insulation in a hot climate is necessary as the "more insulation is better" mind set does not apply to an environment immersed in infrared radiation.
Additional information on RBS and heat movement can be seen on the internet at our website: http://www.savenrg.com
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 for homes, business, farm use, vehicles, hot water tanks and several industrial applications.
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