Lets start a discussion about rooftop A/C units and exposed ductwork.


The worst place to put a metal A/C unit is where?  Yeah, on the roof.  We are trying to keep the air inside cold and we are pushing that air through a 190 degree metal tunnel.  That metal tunnel gets so hot because of what?  The radiant heat from the sun.  On a 95 degree day those metals can reach over 190 degrees F.

The air is one temperature when it leaves the A/C unit.   The cold air absorbs that heat energy and it is  warmer before it even enters the building.  Homeowners pay part of their cooling costs because of those hot metals.  Keep that tunnel cooler and the air enters the building at a colder temperature.  The thermostat is satisfied sooner and that saves the building owner cooling energy costs.

Those are simple facts, simply explained and they make sense.. 

The avatar for this group is a photo of a residential rooftop A/C unit that has been coated with a radiant control coating. .  This small application took about 3 pints of the coating and about 1 hours labor.  The result;  the air temperature coming into the home was 5 degrees colder than before.

Is this not a substantial improvement?  Is this not a good bang for the homeowners buck?  Yes it is!


Here are the problems with these small jobs that have a good return for the investment..

1.  Most contractors are not willing to do such a small job.  Not enough profit for the contractor.

2.  Most contractors do not believe that such a small application of an energy saving product an possibly bring a noticeable improvement.

3.  Most homeowners are under the same opinion as in #2..

4.  Most A/C manufacturers give a 5-year warrany on their units.  If you apply a radiant control coating to an A/C unit that is still under warranty, you just voided that warranty.

As a Home Energy professional, do you ever consider reducing the load on a rooftop A/C unit as an additional option you could offer your homeowners?

I am curious what others think about this OTHER energy saving use of a radiant barrier coating.


 This application dropped the air temperature coming into the house12 degrees according to the homeowners AC contractor..  Is this not a substantial difference?  This dropped their total electric bill by 25%, a $125.oo savings per month.  Would your customers be impressed with you?


 U&nit below, 5 degree drop on air temp

Unit below ic a hspital rooftop air handler, dropped air temp 10 degrees coming into building




 Unit below dropped air temp 5 degrees on 85 degree afternoon

Coating the small AC duct and evap cooler ducts dropped the air temp in the building around 12 to 15 degrees.  Sent his employees calling home to get sweaters brought to them.  First time that had ever happened on a 90 degree day in the 35 years the manager (Previous owner) had opened the restaurant.

Unit below, coated the top and duct of this evap cooler on smasll store in the country.  Also coated the AC unit and duct on hios 2 story home across the street,  Made big difference in the comfort levels of both.


Units below, dropped air temp coming into building by 10 degrees.  Satellite dyalisis center.

Unit below, very large evap cooler or poultry processing plant.  Dropped air coming in by 7 degrees

Unit below dropped air temp coming in by 10 degrees+, manager not certain exactly but said huge difference.  Contractors did not want to come in during afternoon hours.  Always uncomfortable before, very nice now.

Unit below, spoke with thne wife a year later.  She said it resulted in a 25$ drop in their total electric bill, a $125.00 per month sasvings

CBS Newsletter
Winter 1998
pg. 6

Efficiency of Exterior Exposed Ductwork

Most of California's commercial buildings have thermal distribution systems, the majority (63%) of which are air-based and distribute air through ductworks. Thermal distribution ductwork systems in small commercial buildings are similar to those in residential construction [Winter 1995, p.8] and have the same leakage and conduction-loss problems. The extent of these duct-related thermal losses depends on the location of the ductwork--the largest thermal losses occur when the ducts are entirely outside the building envelope.

Leakage, conduction losses, direct solar radiation effects and solar reflection all affect the magnitude of thermal loss. Differences in the lengths of exterior ducts also affect a distribution system's energy efficiency, as well as the temperature of air delivered to interior spaces at the registers. When long duct runs are exposed to sunlight and high outdoor temperatures on roofs, the supply air can experience a significant temperature rise before reaching the registers during periods of demand for interior cooling. This configuration has a direct impact on interior thermal comfort conditions and can cause uneven temperature distribution within the building.

To examine the thermal energy issues of exposed exterior ductwork, we conducted a case study at a building on the campus of a community college in Sacramento, California. Most of the building's ductwork was located on the roof, providing an opportunity to evaluate the effects of duct leakage, conduction losses, and other issues on the energy performance and efficiency of the duct system.

The study building is a single-story brick structure with a 2,000 m2 (21,500 ft2) floor area containing classrooms, laboratories and office space. There is no shading from the south and east and some tree shading on the west side. The building is one of two served by a centralized chiller plant. Its systems were completely renovated in the 1980s with the installation of 15 roof-mounted, constant-volume air-handling units with chilled water coils and air-side economizers.

In 1995, the Sacramento Municipal Utility District conducted a "cool-roof" retrofit of the building, which involved improving the roof-deck insulation and increasing the surface reflectivity of the building's roof. (This strategy was developed as part of a joint research project between SMUD and Center researchers in the Heat Islands Project [Spring 1994, p.6]). A contractor sprayed a closed-cell polyurethane coating that added a 1.2- to 1.7-cm-thick coating to exposed ductwork and 10 to 15 cm to the roof. After the insulation, a highly reflective coating--reflecting up to 85% of incident solar radiation, according to the manufacturer--was added to only the top and sides of the ductwork.

For this experiment, we selected a building air-handler system serving a lecture hall with a floor area of 147 m2 (1,580 ft2). Diagnostic measurements included system duct leakage, system air flows, outside air flows and duct insulation and conduction efficiency. The study included short-term monitoring of temperature and solar radiation over two three-week periods in the summer of 1995.

The analysis of these measurements focuses on quantifying the magnitude of conduction losses and the effect of direct and reflected solar radiation on the ducts, the delivery effectiveness and efficiency, and the effect of the "cool-roof" retrofit on system performance and thermal-comfort issues. We developed and verified a simplified computer model to evaluate the effectiveness and efficiency of the delivery system.

The Table below summarizes conduction losses, expressed as capacity losses, measured at each air supply register studied. Conduction losses in the ducts, when in cooling mode, raise the supply air temperature. The capacity loss is the energy lost as a fraction of the capacity before cooled air reaches a room.

Despite the fact that the ducts started off with a conduction efficiency of 97%, the delivery efficiency was, on average, only 73%. (Conduction efficiency is a measure of how ducts behave as a heat exchanger; the higher the conduction efficiency number, the better. Delivery efficiency is defined as the ratio of energy delivered to the space divided by the energy put into the duct system.) This is because the ducts were located on the roof, where they gained heat from the ambient environment. The retrofit increased the delivery efficiency to an average of 89%, reducing the average energy use for conditioning by 22%. The model predicted these results, on average, within 10% or better of the measured results.

Table: Summary of average register conduction losses. Capacity loss is energy loss as a fraction of capacity before reaching the room.


Uniformly Weighted Average


Capacity Weighted Average




Percent Change
  (post-retrofit in
  relationship to




Percent Change
  (post-retrofit in
  relationship to

































--Woody Delp, Nance Matson, Mark Modera

Woody Delp
Indoor Environment Program
(510) 486-5864; (510) 486-6658 fax

An LBNL report, "Exposed Exterior Ductwork: Delivery Effectiveness and Efficiency," LBNL-39083, describes the methods and results of this study in detail.

This work was supported by the California Institute for Energy Efficiency and the Department of Energy's Office of Building Technology, State and Community Programs.




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Replies to This Discussion

I'm interested in these coatings, have seen something in trade mags and know it's $$$ off the bill to apply them.

My experience was Phoenix, AZ, where the attic was 160F every day for months. The other part of moving cold with air is humidity, in deserts people have swamp coolers feeding the AC to help move more therms of cold to rooms.

It's not logical for AC+cooler to be in the attic where delta-T is huge, it should be on the shady side of the home and perhaps draw from under the homes, distributed from the basement.

For the question of other uses, thermal collection & storage is ignored in architecture passive solar or not, can these coatings be used to help preserve heat from a solar collector in piping to a tank, and the tank itself?

Seems it should ... another reaction was in the crawl space a lot of heat is lost to radiation from the floor to the ground, batting has a foil side as a radiant barrier but could be improved. 

In my buildings I'm using water in pipes below the floor for thermal-mass and tear out the batting to put them in. Then closing it off with foam board, so, if you coated the pipes and/or board how much less loss would there be?

Then, if that's valid for reducing losses what if you spray it on the sheathing under the sealing layer to help reduce wall radiant losses?

Hi Tom.

It looks like I will have to start off most of my answers to questions with this statement;    I am a 30 year engineering rep for the one coating I work with.  I am an expert on that coating.  I can tell and show you what I and a few others have proven with it.  I cant speak for any of the other coatings.


Water coolers feeding A/C units.  Defeats the purpose.  One of the functions of an AC unit is to remove excess humidity in the air.  I will give a worst case scenario type of incident.  Many years ago I was called by the maintenanc manager for a large and exclusive golf course / country club.  They had a problem with the AC unit above the mens shower room.  Got there, the shower room was EXTREMELY humid AND they had 2 6-man hot jacuzzis in there.  Got on the roof and the A/C was one big square block of ice!  Never saw anything that came close to tyhat.  I told him to buy a de-humidifier and our coating would not help there.

Heat energy transfers by 3 modes;  Conduction, convection and Radiation.  In your 160-degree attics, which of those 3 modes are affecting the attic AC unit?  Answer; All 3. 

1.  Radiant heat from the underside of the roof deck.  Radiates down and is absorbed by the A/c ub, plenum and the flex or metal ducts and the insulation on the attic floor.

2.  Conduction - the attic;s R- insulation is almost always touching (sometimes piled up against or on top of the unit and ducts.  THAT hot insulation is now transferring that heat energy to the cooling system.

3.  Convection - the attic air passes over the hot surfaces, absorbs heat energy and transfers it to the AC equipment by convection.

I have to explain how our coating works.

Our coating is a water and ceramic based latex.  It contains microscopic ceramic platelets.  In the first 72 hours of curing, as the binder chemicals dissipate out, that process pulls the ceramic plateletys to the outside.  This forms a thin, flexible ceramic tile.  This tile is formed in the first 72 hours of curing.  That ceramic tile reflects heat energy back  in the direction it came from.

Now here is the hard part to believe;  Just as a floor4 tile has an upper and lower side when on a floor (inner and outer side when on a wall)  our coating's ceramic tile also has 2 sides.  Both sides will send heat energy back in the direction it came from.

Now to the different applications that will keep the air cooler in your attic A/C systems.  If an electrical outlet has no power, the electrician will star at that point ad his / her way back.  I will try and do that here.


I guess I got too mlong winded ther, it cut me off.

1.  Spray the coating on the ducts, plenum and A/C unit itself in the attic.  They are now insulated against radiant, conductive and convective heat gain.

2.  Spray thne underdecking of the roof.  That same ceramic tile is going to send that heat back in the direction it came from.  Cooler attic but the roof will be hotter.


When a new roof is installed, the coating can be applied to the sheathing, the black paper OR directly to the shingles  *Not wood shingles.'




sorry for having to break this up but my responses I guess are too long.

You talking about the coating retaining heat inside a pipe, tank or any structure made me sit back in my chair.   The answer is yes.  If the inside of the pipe is hotter than the cold outsider the pipe, then yes it is going to reduce the heat loss.  If the temperature outside is hotter than the temp inside the pipe, the coating will reduce the heat gain.

Using our coating;

We have coated boilers and steam pipes in a hospital and rfeduced the surface temnperature of those metals from 267F down to 155F.  A 35% reduction in heat loss per the hospital engineering chief.  I have his statement if you want it.


Employees of the East Bay Regional Park District coated the unbderside of a 33 foot mobile home in one of their parks.  They also coated the the metal heater duct than ran the full length of the unit.  They documented the results.  An 84% reduction in heater run time.  I have that statement also.


One of the simple sales tools I came up with for our sales people was so simple and believeable and worked eery time.  I took stitchbond polyester roofing cloth and coated it with our coating.  I cut it into 8 inch by 2 inch strips.  Our coating is an elastomeric.  I would turn on the hot water water in the kitchen and let it run very slowly.  I would let the faucet get good and warm,  wrap the coating strip around and pinch it tiht.  Gave it about 20 and asked them to touch the the strip.  Then I had them touch the the bare faucet.  V ery noticeable difference.  Then we went to the water heater and I uncovered a little of the hot pipe.  Wrapped the strip around that pipe and repeated that test.  They touched  the strip, cool.  Then they touched that very hot pipe, normally yelled or cursed.  Had one one little old lady yell at me while shaking her hand  "I CANT BELIEVE YOU LET ME DO THAT!"  

You let them burn their hands and they will NEVER forget you.

Used as eithar an interior or exterior paint or roof coating, it retains heat in the cold months and keeps it out in the hot months.




Sounds like the perfect, thin insulator adding enough thermal inertia to separate two masses and/or mediums a lot better than most insulation.

For my use I'd spray it on the downward or exterior side of the board, it helps delay changes a lot for so little, this adds a lot of help moving more wall or floor mass into thermal-mass to help maintain comfort zone.

Do you have tables of q or R-factor for it online?

Products like these dont have an R-rating. You still have to have the R-rated insulation to meet codes. Cant substitute this coating for that.

However, tests can be performed to give a material like this an equivalent R-rating.

Several years ago a controlled terst was conducted on our product vs standard white paint.  It was done by the engineering department of UNLV under the supervision of Dr. Semir Moujaes, Associate Professor.   It was done as a Master’s Thesis for (Now Dr.) Arnel Ruffy.  It was a 2 year study. 

Two clone buildings were built on top of one of the campus buildings.  One was coated with standard paint, the other with our coating.

The walls, floor and roof were rated at R-25 with standard insulation.  The results proved the  building with coating used 52.02% less energy to maintain the same temperature inside.

I have one of the executive abstracts of that project.  It is as big as a New York phone book!

We asked Dr. Moujaes how much more R-rated insulation would have had to be added to achieve that same energy saving result.  He did the calcs and said a level of R-32.6 would have te added to achieve the same energy savings.  Now, that is what our ‘equivalent R-rating’ was for that specific application

We have several statements and energy calculations that show 50% reductions in BTU requirements or more.

Right, that reinforces my understanding of how it works, I was after what to plug in for values in this small thermal modeling app MIT did, this is a 400-minute run with the standard wall on top vs adding exterior insulation board on the bottom, has furring strips then replace the siding, cuts conduction by 2/3 ... I'll try to find a ballpark value from the 52.02%.

So, same as under the floor, this coating is added to the exterior of the board under the furring strips & should chop a worthwhile hunk out of losses for the cost of adding that step in a remodel.

For wood-frame homes I prefer hemp-mortar stucco now in place of the foam board used without a vapor barrier over the sheathing for moisture transfer at 1-1/2" thick, this fits here on the wet side of the Cascades, it breathes moisture slowly and doesn't allow fungal growth yet like your coating slows heat-transfer down beyond what pure insulation can do.

This also makes the rooms quieter.

Hey again Tom,

"your coating slows heat-transfer down beyond what pure insulation can do."

That is one way to explain how our coating works.  I prefer this way,  "Our coating allows the R-rated insulation in buildings to perform it's function on a greatly reduced temperature scale." 


If you have furring strips in a wall system, wouldnt that leave an air space in the wall ?  Would that air space be what results in your conductive heat reduction?  Even with that air gap, one side will be warmer than the other and you will still have a radiant heat gain / loss through the air gap.

Radiant heat can travel through a perfect vaccuum.

This discussion has certainly got off the main track here about AC units but it has brought up some interesting points.

I would be remniss in my obligations if I did not point out something about the wall system you described.  Anytime there is an air space in a wall system, it is considered a fire hazard.   It allows a fire to spread much faster through that wall if a fire occurs.

OK Tom, I think I am on the last question now,,,I think.  LOL

Going under the building and applying (Spraying) the coating to the underside of the wood floor.  Yes, it makes a very noticeable difference in the temperature of very cold floor tiles or linoleum in the cold months.

Concrete floor?  Apply it to the concrete, give it a week and then lay your carpets.  Very noticeable improvement.

Here are few examples of residential AC and the results from coating them with our Radiant Control Coating.




Reduced air temperature coming into the house by 5-degrees on a 90-degree day.  Elk Grove, Cal.








  Dropped air temperature 5 degrees on 90-degree day.  Modesto, Cal.


Dropped air temp 12-degrees on 90 degree day, 61 to 49 degrees per the AC contractor.  Porterville, Cal.

All readings taken at the closest and furthest ceiling registers from the unit.  The least I have ever dropped the air temperature on small residential units is 5 degrees.  The most I have ever dropped the air temperature, on a mid sized commercial unit and duct, was 25-degrees.

When making a presentation to a prospective customer; I would quote them a price to paint the exterior of their house. I had absolutely no quallms about saying  "If I coat your AC unit on your roof and that LITTLE job can bring the air temperature down by 5-degrees or more, will you let us paint your whole house?  If our insulating coating can drop the temperature by 5-degrees just by painting a few square feet on your AC unit, how much better will your house be if we painted the entire thing?"


Any time I had to do that free demonstration, we never failed to convince the customer that it was an excellent investment.



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