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Name: Roberto
Status: other
Grade: other
Location: TX 
Country: N/A
Date: 9/25/2005


Question:
I was wondering if an engineer could resolve a debate I'm having with a friend. The debate is this: Is it more energy efficient (in terms of cooling cost) to have a high-ceilinged house than a low-ceilinged one? One view is that a high ceiling will allow the heat to accumulate at the top of the house, be more comfortable at the floor level, and require less cooling (lower cost for air-conditioning). The other view is that a high ceiling (with the same floor space) would result in a larger volume of air that needs to be cooled - that the air will naturally circulate - and so drive up the cooling cost.

We have also argued over whether a flow through or vent at the top of the house is energy efficient since the removed air would have to be replaced and ultimately would have to be cooled.


Replies:
The parameters you specified above are not complete enough to answer yes or no. How well the ceiling and walls are insulated is going to be a major factor. Whether by "cooling" you mean the entire room, regardless of shape, is at the same temperature, or whether you are going to be satisfied with a temperature gradient between the floor and ceiling (and if so how large a temperature gradient). What is the temperature difference between the outside of the house and the inside. What is the air "turn over" between ceiling/floor and inside/outside, and what is the heat transfer between the floor and the ground (that is, does the room have a basement below or is it sitting on the ground) and in that case what is the ground temperature. You have not specified the relative humidity. A significant factor in heating/air conditioning is controlling the humidity in the room. This is very energy intensive. So there are a lot of undefined parameters that could change the heat balance in room.

Vince Calder


This from a structural engineer, a mechanical engineer, or an architect would be your better source.

There are a few issues involved.

First, air changes, the air needs to be recycled, or recirculated. The older buildings leaked, so outside air got in, but the new buildings with their insulation, the issue of air changes has to be resolved. Without recycling, the carbon dioxide builds up as well as the other contaminates. You heard reference to sick buildings; I would say the cause is the air circulation.

Regarding the high vs. low ceilings, hot air rises, recycling the warm air is also important from the energy angle. In the desert, ceiling fans more effectively use the air conditioning. Las Vegas in the past 10 - 15 years have gone to two story buildings. Personally, I could not see the utility in the second floor, they mainly were bedrooms. To me, the cost of cooling a second floor prohibitive, their only uses for sleeping in the cooler nights, but the developers placed more cost on land, and designed the buildings closer together. There are zero lot lines, but the people bought the change. In the cold climates, fans can help the heating.

A lower ceiling would be advisable, but the living space also is involved. The higher ceilings give benefit toward the feeling of openness, enhancing livability.

Implied in your question is a second floor, that floor being heated from beneath, that is true, but what percents of time is spent on the first, or second floors? Watching this Old House, they put radiant heating in the first floor slab , for the very reason of having heat from below. So, the design of the house, the use of the rooms, the health and energy costs all have to be factored into a solution.

James Przewoznik


I think there are usually about two paths for everything thermal, and you will have to have a working estimate for each to understand the trade-offs for yourself.

In my southern California house a mile or so from the ocean, heat usually enters as sunlight on the roof, which gets much hotter than the outside air. Then attic vents are essential, and ceiling vents are helpful, because replacement air coming in the windows usually feels cool. The heat gets down from the attic how? perhaps an air circulation in the room, but perhaps as thermal radiation from the warm ceiling. The interior walls accumulate a lot of heat; I wonder if there is downwards air flow in the walls, insufficiently blocked by my wood frame.

In Miami the entering air would _not_ feel cool. That house would need a different design. Texas would be somewhere in between.

I grew up in a two-story house with a large, tall entry hall where air stratification could be felt. The ground floor remained cool, but climbing the stairs might put you into uncomfortably warmer air. It was quite effective at keeping our ground floor cool, except for a few weeks each year.

I tend to feel you need to be able to do a moderately broad set of calculations or computer simulations before you can design from a learned philosophy.

Air does not necessarily "naturally" circulate. Thermal stratification can largely stop circulation, if nothing strong is pushing it. On the other hand, ceiling fans are a popular way to "cool" a room, but if one uses them, the air is definitely circulated. One might not wish to design a house in which the net effect ceiling fans is adverse or poor for at least the first hour of operation. Air-conditioning usually circulates the air too, unless one makes sure the cool-air outlets are at a low altitude.

Thinking clearly about layers of "blocking" and "shunting" heat flow helps too.

Insulation partly blocks heat flow, slows it down. Shunting, such as vents or air-conditioners, throws heat off to one side, also with partial effectiveness. If one had a tall ceiling the air stratification would be insulation slowing heat flow from ceiling into the floor-space, then the AC would shunt away the heat that made it through. The AC airflow loop both starts in the cool space and ends in the cool space. To do this without circulating to the ceiling, both intake and outlets would need to be near ground level, and not pointed vertically. A gentle cross-flow without much turbulence would need to be set up. I am not sure the right duct-grills for this are common. I think they would be rather large or numerous, or use a little-used side-room as a turbulence-subsidence volume. I guess 7-foot high wall outlets vane-directed downwards towards the floor are usually used. I wonder if pointing downwards into little hallway would work nicely. That air might emerge evenly from the whole aperture as it meets the large room.

If you deliberately allow stratification, the ceiling will be some degrees warmer than the floor, and will be radiating heat downwards by infra-red radiation. If the ceiling was 20 degrees C warmer than the ground-air, In still air I think I could feel it glowing down on me as I walked into the room.

But I'm unusually aware of such things, and a little airflow would mask that sensation. I think you need to get a number for this heat-flow path, in watts/meter2/degreeC, or BTU/sqft/degreeF, and multiply it by the number of degrees difference you expect to develop across this insulation layer you are designing, and compare with your air-conditioning horsepower.

In comparison, a low ceiling with an attic having an electric-power attic fan might be considered a custom-built blocking and shunting structure protecting the rooms from sunlight-heating. The cheap fan-flow greatly reduces the burden on the AC, if the ceiling has its own insulation, and the separate ceiling works a little more predictably than stagnant air because it is a solid structure..

High ceilings definitely have some cooling effect, but they could also be rationalizations for other preferences, and they do fail in poor situations. Then you are fashionable but impractical and uncomfortable. Personally, it bothers me when I encounter that kind of mistake.

If your AC loop goes from living-space to living space, it is not your fresh-air intake. Allowing the occupants to have a separate fresh-air intake path which they can turn on and off their discretion is valuable to them, and is pretty near the best affordable cooling economy. Especially if the airflow is captured in ducts well enough to insert a heat-exchanger into the path.

I clearly know less about minimizing heat entry through side-walls in 100-degree locales. near as I know all you can do is make one good layer of insulation, and get the AC inside right. keeping sunlight off the wall exterior and making walls massive might help.

my 2 cents...

Jim Swenson


Dear Roberto,

This is a reasonably subtle question and I suspect the answer depends on the details. If you can avoid mixing the air, there might be an advantage to higher ceilings. However, if you are running an air conditioner, I should think it would be almost impossible to avoid mixing the air thoroughly and often. In that case, you just have more wall space which you have to insulate and would nevertheless allow heat to enter the house.

If, on the other hand, the air is calm enough and the ceiling is high enough, much of the heat entering the room could rise to the top by convection and stay there until night when it might be cool enough to cool all the air in the room in preparation for the next hot day.

Nonetheless, I would opt for smaller rooms in my house.

Best, Dick Plano,
Professor of Physics emeritus, Rutgers University



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