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The Duct Problem: America's Building Epidemic

The staggering harm ducts in attics are causing your home, wallet, comfort, and America's energy resources.



In our collective quest to save energy, save money, and live comfortably, a formidable enemy is lurking in our homes.


Ducts. Especially ducts in attics.


Houses provide us with conditioned environments that keep us dry and comfortable.  The mechanical system in homes that keeps us comfortable is the HVAC system (Heating, Ventilation, and Air Conditioning).  The components that make up a home's HVAC system can vary, but they all have a source of "hot" or "cold" and a way to distribute that "hot" or "cold" to the rooms where it's desired.  This distribution system often consists of ducts, especially if the home has air conditioning.  Over 90% of homes in the US have air conditioning, and about 70% of those have central AC.  There are 144 million homes in the US, meaning about 90 million homes use ducts as the HVAC distribution system. Most of these ducts are located in the attic.


The rise of air conditioning systems has also influenced the type of home space heating systems found in newer construction. Central air conditioning and central space heating equipment often work in tandem or as a unified system. About 91% of homes built since 2000 have a main space heating system that includes central ducts. - US Energy Information Administration

Basic Terminology


A duct is a tube that air flows through to bring hot or cold air from a home's source of heating or cooling to the rooms.  The air is pushed through the ducts by an air handler, which contains a fan and the source of heating or cooling, which can take the form of a furnace, a hot water coil from a boiler (hydronic), or a refrigeration coil from a heat pump/air conditioner.  The only signs of the ducts visible from the living space are grilles located in the floors or ceilings called registers.  The ducts are connected to the other side of those registers.  Ducts located upstream of the air handler that pull air from the rooms are called return ducts.  Ducts located downstream of the air handler that push heated or cooled air into the rooms are called supply ducts.




The Problem with Ducts


Ducts in unconditioned spaces, especially attics, are the largest source of "wasted energy," wasted money, and discomfort in most homes.  Let's explore why.


Fourier's Law


Thermodynamics tells us that whenever there is a temperature difference between two things, heat will flow from the hotter thing to the colder thing.  Fourier's Law states that the amount of heat that is conducted through a material over time is equal to the material's thermal conductivity multiplied by the cross-sectional area of the material multiplied by the temperature difference across the material divided by the thickness of the material.  Or in equation form:


The term R-value is commonly used to describe the performance of insulation materials.  The R-value is thermal resistance and is equal to the reciprocal thermal conductance.  I'll spare you the math, but this means that the amount of heat transferred through a material is proportional to the cross-sectional area of the material and the temperature difference across the material and inversely proportional to the material's total R-value.


Keep this in mind.


Let's say it's summer, and it's 93°F outside (these are actual measurements I took at my house).  Attics get much hotter than the outside air in the summer because of the sun's energy heating up the roof, which heats up the attic.  On that 93°F day, the attic is 116°F.  My house is 76°F inside, and the air coming out of the air handler with the air conditioning running is 59°F.  The temperature differences, ∆T, across different parts of the home building assembly are:


Walls:  ∆T = 93°F - 76°F = 17°F

Ceiling:  ∆T = 116°F - 76°F = 40°F

Ducts:  ∆T = 116°F - 59°F = 57°F


We want to minimize the amount of heat flowing into the house through the walls, ceiling, and ducts.  The greater the temperature difference, the more heat will flow.  We see that the greatest temperature difference in any part of the house's boundary between conditioned and unconditioned space is between the attic air and the air in the ducts.  Therefore, we should have the most insulation, the highest total R-value, of anywhere in the home on the ducts.  That's logical, right?


Building Codes for Insulation


Houses are usually insulated to building code standards.  Building codes vary based on states and regions, but they follow a similar pattern in most areas of the US.  The current building code requirements for insulation in homes where I live, for example, are R-20 in walls and R-38 in ceilings, but only R-8 on ducts in the attic!  The very place we have the greatest need for insulation in our homes is the place with the least amount of it – and that's what the building codes specify.



The Effect on Heating and Cooling


How much of a problem is this?  I did a detailed analysis in my house and measured the air temperatures going into and coming out of every register.  The temperature changes combined with the mass flow rates of the air can be used to calculate the amount of heat that is leaking into the ducts from the hot attic.  Another way to think of this is how much cooling load is being lost.  We will simplify the results here and ignore mass flow rates for brevity.


The temperature of the air going into the air handler is 76°F, and the temperature of the air coming out of the air handler is 59°F, for a 17°F temperature drop across the cooling coil.  The average air temperature coming out of the supply ducts is 69.1°F.  The air conditioning system gives us 17°F of cooling, but only 6.9°F of cooling is being delivered to the rooms.  We are losing 59% of our cooling capacity through the ducts!  That means the air conditioning needs to run more than twice as much to cool the house because of the heat gain through the ducts.  And this is a house built in 2022 according to code.  There are tens of millions of homes in the US with much greater duct losses, and older homes tend to be worse.


A Personal Case Study


I used closed-cell spray foam to insulate my ducts.  Admittedly a miserable job in the heat of the summer, I sprayed the foam onto the flex ducts and trunk lines I could access in the attic.  I only had a 600 board-foot foam kit, so I couldn't cover all the ducts, and I couldn't cover them as thick as I would have liked.  I ended up adding about 1-2" of foam to about 80% of my ducts before I ran out.



I then used the exact same measurement procedure, measuring the temperatures of the air going into and coming out of the registers, at the same time of day at the same outdoor and attic temperatures.


The air conditioning system is still giving us 17°F of cooling, but now the average temperature of the air being delivered to the rooms is 65.9°F, still with a 76°F return air temperature.  Now 10.1°F of the original 17°F of cooling from the air handler is being delivered to the rooms.  We are now losing 41% of our cooling capacity through the ducts.  This is still bad, but way less bad than before.


Essentially, we have increased the cooling capacity of the air conditioning system by 46%!  That means the air conditioning needs to run 32% less in order to cool the house.  The opposite is also true for the winter when we need to heat the house.  Just by doing this one thing, improving the duct insulation, we have reduced our heating and cooling costs by 32%.


The Scale of the Problem


On average, homes in the US use 54% of their energy on heating and cooling.  As we saw earlier, about 90 million homes have ducts.  The average US household uses 23,000 kWh (78 million BTUs) of energy per year.  If we assume that duct losses are wasting 59% of a home's heating and cooling load, as in my new house (most houses are likely losing more), then the average US house is losing 32% of its total energy (0.54 x 0.59) through duct losses alone.  That's 7,360 kWh per year per house wasted.  If we assume all electric heating and cooling at the average US electricity cost of $0.23/kWh, each household is wasting $1,693 per year through duct losses alone.


Nationally, duct losses in homes are wasting 662,400,000 kWh worth of energy in the US!  That's 662 gigawatt hours.  Just by adding 1-2 inches of foam to those ducts, we can save 32% of that, or 212 GWh. That's insane!


Other Problems with Ducts in the Attic


The problems with ducts in the attic go beyond just heating and cooling losses.  Duct systems are composed of a main trunkline coming off of the air handler, often made of duct board or sheet metal with an inch of insulation on the inside, with branch ducts attached to it, often made of flex ducts that are made of a flexible wire structure with plastic surrounded by a thin layer of fiberglass insulation and a vinyl or foil layer.  These connection points often have gaps and holes in them.  The amount of air that flows through a hole is proportional to the size of the hole and the pressure difference across the hole.


In the case of the supply ducts, there is very high pressure inside of the ducts and lower pressure outside.  So when the heating or cooling system is running, conditioned air is being forced out of the gaps in the duct system.  That's air we paid to heat and cool.  When air leaks out of the ducts into the attic, now there is less air inside of the house, causing a negative pressure, which causes unconditioned outside air to be pulled inside the house from other leaks.  Those leaks can be located in the living space, or worse, they can be located on the return side of the duct system.  We then have to heat and cool the new unconditioned air that leaked in, adding to the problem.


But it's worse.  When the air leaks out of the ducts in the winter, it generally has humidity.  When it hits the cold surfaces in the attic, that air's relative humidity drops and can drop below the air's dew point, causing condensation.  Now, stuff in the attic is wet, and mold grows on it, especially on the bottom of the roof sheathing. I've seen this in hundreds of homes. Here's a video about this.



The new cold, dry air that comes into the house in the winter gets heated, and its relative humidity drops even further, causing extremely dry air in the house that most people don't like. To compensate, people might use a humidifier, which causes mold city in the attic.


In any season, when the unconditioned air comes into the house, it's often full of pollutants such as pollen, mold spores, vehicle exhaust soot, and other fine particulate matter.  This adversely affects the air quality in the home.


Although heating and cooling losses are the biggest problem with ducts in the attic, humidity levels, moisture, mold, and air quality are also significant issues.


How Do We Fix This?


I certainly do not have all the answers, but here are a few ideas.  There are really two different approaches depending on whether it's an existing home or new construction.  Through Fourier's Law, we've seen that to minimize heat transfer through the ducts, we can do one of three things:


  1. Decrease the surface area of the ducts.

  2. Increase the R-value of the ducts.

  3. Decrease the temperature difference across the ducts.


Let's start with existing homes.  We can decrease the surface area of the ducts by eliminating the excess length.  Often when HVAC contractors install ducts, they leave the ducts too long.  If it's 15 feet from the main trunkline to a supply register and the contractor has a standard 25-foot length of duct, rather than taking the time to cut it down, they may leave it long and let it coil up in the attic.  By cutting this 25-foot duct down to 15 feet, we can reduce the surface area that heat can flow into or out of the duct by 40%.



Next, we can take the remaining ducts and increase their R-value with more or better insulation.  We can use closed-cell spray foam to coat the outside of the ducts, use rigid foam board on the outside of rectangular ducts, and/or bury the ducts in loose-fill cellulose insulation.  Although spray foam is a fantastic performing insulation material, it can be hazardous to install and is not the most environmentally friendly material.  If you have any ideas for alternative ways to insulate existing ducts better, please let us know.


The third thing we can do is decrease the temperature difference across the ducts.  Since we can't change the temperature inside of the duct (nor do we want to), we can change the temperature of the attic that the ducts are in.  That can be accomplished by creating a conditioned attic and insulating and air-sealing the bottom of the roof deck rather than the ceiling.  This is not always possible or practical though, and can be expensive.


The best time to address the issue is when the house is being built. We have way more options for solving the ducts in the attic problem with new construction.  Less lossy air distribution systems can be built into the design of the house.  Spaces inside the thermal boundary of the house can be used to carry the ducts.  Chases in walls and ceilings can be used.  Many homes have multilevel ceilings for decorative purposes anyway, and these can be used to run the ducts through.  This would be an ideal solution since any heat that leaks into or out of the ducts would still be inside the living space - effectively reducing duct losses to almost nothing.


Another alternative would be not to use ducts.  Mini split heat pumps that have smaller decentralized air handlers (heads) in rooms don't require ducting.  Using a hydronic heating and cooling system where a heat pump is used to heat or cool water, which gets pumped through insulated pipes to either radiators or fan coil units in rooms, could also virtually eliminate distribution losses if designed correctly.


Why don't builders do these things to fix these energy losses and building problems that will last the life of the home?  Because the builders don't care how much energy the homes use.  They don't pay for it.  It's not that they're evil; they simply bear the upfront installation costs.  Business as usual is to do things how they've always been done most cheaply, regardless of how much it costs the homeowners or the environmental impacts in the long run.


Initial System Cost


Of course, the long-term savings of fixing attic duct losses are obvious in terms of heating and cooling costs.  However, upfront costs can also be lower.  If distribution losses of the HVAC system can be reduced from 59%, where only 41% of the heating and cooling capacity gets to the rooms, to just 5%, where 95% of the heating and cooling capacity gets to the rooms, a smaller system can be used.  How much smaller?


95% ÷ 41% = 2.317. That means 2.317 times the heating and cooling capacity would get to the rooms.  So if a home would normally require 7 tons of air conditioning capacity, now a 3-ton system can be used (7 tons ÷ 2.317 = 3 tons).  Furthermore, residential air conditioning systems typically don't go over 5 tons, so to get 7 tons of cooling, a 5-ton system and a 2-ton system would have to be used.  My HVAC installer friends say this system would cost $28,000.  By cutting down duct losses, a single 3-ton system can be used, which costs only $12,000.  The initial system cost would be $16,000 less!



Broader Implications


If duct losses are significantly reduced or eliminated, not only can smaller HVAC systems be used, but different kinds of HVAC systems can be used.  As homes become increasingly electrified (which is the direction we need to go in for a sustainable future), heating can be accomplished with a heat pump rather than a propane, natural gas, or oil-fired system.  A heat pump is nearly identical to the air conditioning system, which in most cases is installed anyway, with the addition of a small part called a reversing valve.  Reducing duct distribution losses makes it possible to use a heat pump for both heating and cooling needs, even in very cold climates.  The cost of installing a hydrocarbon-based furnace or boiler can be eliminated, and using electricity for all the home's energy needs becomes possible.


If a house is fully electric, rooftop solar panels can now be used to power the entire home.  A home can be independent and immune to fluctuating hydrocarbon prices because it simply doesn't need them.  If the rooftop solar energy is stored with a battery or by other means, that home can be fully self-reliant for all energy needs.  If homes are energy-independent, the country's electricity grid, pipelines, and heating oil and propane delivery infrastructure would be significantly reduced.


Conclusion


To summarize, if duct losses can be reduced to almost zero:

  • The initial home HVAC system cost could be $16,000 less

  • Annual savings per household could be $1,693

  • Annual energy savings per household could be 7,360 kWh

  • Annual energy savings in the US could be 662 GWh

  • Annual CO2 emissions in the US could be reduced by 250,000 metric tons

  • Homes can be fully electrified and even energy-independent


It all starts with addressing America's building epidemic: ducts in the attic.


Questions for you:
  • What ways of reducing duct losses do you find the most promising?

  • What are you going to do in your home to fix this problem?

  • How can builders, HVAC contractors, and homeowners work together to align their interests in solving this problem?


* Some math, details, and assumptions have been omitted from this article for brevity, but they have been considered and accounted for.

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