Northern Thermal Comfort
2379 Rux Rd.
Arbor Vitae, WI 54568

The Insulation Lab

Ducts in the Attic – A Very Bad Idea!

Jeff Aderholdt - Monday, June 08, 2015

Not long ago, I arrived at a new house to start insulating. When I surveyed the house, I was surprised to see that some of the duct work for the heating system was in the unheated attic. Surprise quickly moved to irritation. When the heating system installer arrived, I pointed out the situation, reminding him that it's not the 1960's and this is not acceptable anymore. The rest of the day was a bit tense.



 There are still a large number of HVAC contractors that think that installing duct work in unconditioned attic space is an acceptable option. Some even think it is OK to, (I shudder to say this), install the furnace in an unheated attic! This leaves me confused. Out of all of the construction trades, these should be my energy conservation allies. In doing this, not only are they not my ally, they have gone so far as to join the other side!

 Why do it?

So, why is the attic so appealing to HVAC contractors. There are a number of reasons.

1. Space.

In any house, space is limited. Duct work is large and bulky, and a lot of it is required for a forced air system. Many times there is not a route using interior walls and floors to get where you need to go. But here is the wide open attic. It has all the space I need!

2. Design.

Most house designs focus on living spaces. Where to run the mechanical equipment, including the heating system is, well, an after-thought. Many times it seems that the only option is through the attic.

3. Education.

Many HVAC are simply not aware that running ducts in an unconditioned space is bad. They may be unaware of other heating techniques, yes, there are other methods besides forced air.

4. Cost.

Construction is a competitive market. A lot of focus is on the bottom line. Forced air systems are usually the least expensive. Plus, if the HVAC contractor can use less labor and materials, shortening the lines by using the closest route through the attic, he can keep the cost lower while still making a profit. Trying to push a more expensive system may price him out of the job.

Is It Really That Bad?

Is this really a problem? Or am I blowing this way out of proportion? What does the science show?

Over the years there have been several studies dealing with the effects of duct work in an unconditioned attic. A conservative estimate of added energy usage is about 20%. (See This Study ) This is if everything is done exceptionally well. The real world effect is usually much more. Recently I was told that by moving 75-80% of the duct work from the attic into conditioned space resulted in about a 60% reduction in heating and air-conditioning costs. This is an extreme example. I would estimate an average reduction of between 25-30%. This is a lot of heat going right into your attic.

There are several reasons for the increase. I will mention four of them.

1. Penetrations.

Every place the duct work transitions from heated to unheated space creates a hole in the thermal envelope. This includes where the main trunk lines from the furnace enter the attic and every register head that comes through the ceiling.

2. Leaks in Duct Work.

Every joint, every seam, in the ducting is an air leak. Small leaks can cause a lot of problems in static situations due to stack or chimney effect. Have those leaks in a pressurized duct and the issue increases greatly. A small hole acts like a big hole.

3. Increased Surface Area.

Every 1 foot of length of 8” round ducting adds 2.094 square feet of surface area (This is basic geometry [ (diameter)(length)(π) ]). The bigger the duct, the more surface area. This is area added to the most vulnerable area, our ceiling. This adds up quickly! It's like you have added another room to the house.

4. Minimal Insulation.

Not only has surface area been added to the ceiling, it is barely insulated (usually R-4 or R-5). HVAC contractors install insulation, not for energy saving, but to prevent condensation.


The effect is a lot of heat being put in the attic! In an area like northern Wisconsin, this results in the snow on the roof melting, forming ice on the eaves, creating ice damming. There is also an increase in moisture and mold issues. This can happen from leaks caused by the ice damming. It can also be a result of the air leakage carrying humid air into the attic. The resulting moisture can accelerate the deterioration of plywood and OSB board. Shingle life will be significantly shortened. With the heat constantly melting the snow, the shingles are always wet in the winter, washing out the oils. You can tell roofs that have melting issues by the way the shingles cup and curl over time.


Just don't do it! This is not the 1960s. No matter what anyone tells you, there are ALWAYS options. If you want a forced air system, design and build it to accommodate for the duct work. Don't treat it as an after thought. This may mean building a soffit, or a thicker wall to provide the space needed. It may mean loosing a little interior space. It may mean costing a little more. It may mean using other methods than forced air.

At Northern Thermal Comfort, we can help you through your entire project to ensure that you have the most energy efficient home. Ask how we can put our expertise and experience to work for you with our consultation services.

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