extrapolating temp/energy use difference from small boxes to my condo
November 19, 2013 12:02 PM Subscribe
Physics geeks! Help me save energy! How do I estimate how much less energy will be used by light siding vs gray siding, based on an experiment with small boxes? I made some identical boxes, put them in the sun, measured temperatures, and am trying to figure out how to scale up. The net has info about different roof colors, but not siding colors.
My condo association is installing vinyl siding. They want to put up gray siding, but I think lighter-colored siding would be a better choice.
My condo is in direct western sun with little shade and gets very hot in the summer. The association seems mystified that I would prefer a lighter color, so I want to set up some kind of credible evidence that this would make a difference to energy use, my utility bills, etc.
Anecdotal: in summer, the outside front walls are very warm to touch, even though there's enough overhang that they aren't exposed to the sun for very many hours. We changed from a dark gray front door to a very pale door, and that allows us to not burn our hands when grasping the doorknob.
I made two boxes, one with a gray color that matches the proposed siding, and one with a lighter color, color matched to another siding color from the same brand.
Each box has a thermometer mounted inside behind a transparent window, so that the thermometer can be read without opening the box. Readings were taken after about an hour in the sun.
Here's a photo of the "experiment" in process.
This is obviously _very_ crude, but I did:
- assure that, before the boxes were painted, they registered the same temperature in the sun and inside the house. In fact, I built three such boxes and discarded one with an inconsistent temperature reading.
- assure that, when all boxes are indoors (after painting), the thermometers consistently read the same temperature, to within ~.25 degrees.
- temperature readings were taken after about 1 hour outside in the sun, with a third thermometer taking ambient temperature readings in the shade (also tested for consistency with the other two thermometers).
- boxes were placed with the largest painted side oriented similarly to the condo wall that receives the most light.
- There are four condos in my building, in a 2x2 grid; my condo is the northwest quadrant of the building. The west and north walls of the condo get sun and air, but the back and south side are insulated by abutting other condos. This means that heat absorbed is difficult to dissipate.
- There are no trees shading my unit from the setting sun. Some shade comes from the soffit/roof overhang, but it still gets very hot.
I could:
- take additional temperature readings in ~65F sunny weather
- measure/calculate the box surface area in direct sunlight
- measure/calculate the air volume inside each box
- measure/calculate the surface area that would receive the vinyl siding
Obviously, this isn't going to be very exact, but I think the differences are significant enough that I could get some crude figure and good logic to present. Besides, it's kind of an interesting problem, no?
Here are a few measurements. All temperatures are in degrees Fahrenheit:
Bg = gray box;
Bw = "white" box (actual color close to almond);
Sh = ambient temperature in shade
Sh: 92; Bw: 100; Bg: 107
Sh: 95; Bw: 107; Bg: >123
Sh: 43; Bw: 56; Bg: 68
(the latter was taken with all boxes in _shade_; even in shade, the color made a difference: Sh: 92; Bw: 97; Bg:102)
This is all in Chapel Hill, North Carolina, United States.
My condo association is installing vinyl siding. They want to put up gray siding, but I think lighter-colored siding would be a better choice.
My condo is in direct western sun with little shade and gets very hot in the summer. The association seems mystified that I would prefer a lighter color, so I want to set up some kind of credible evidence that this would make a difference to energy use, my utility bills, etc.
Anecdotal: in summer, the outside front walls are very warm to touch, even though there's enough overhang that they aren't exposed to the sun for very many hours. We changed from a dark gray front door to a very pale door, and that allows us to not burn our hands when grasping the doorknob.
I made two boxes, one with a gray color that matches the proposed siding, and one with a lighter color, color matched to another siding color from the same brand.
Each box has a thermometer mounted inside behind a transparent window, so that the thermometer can be read without opening the box. Readings were taken after about an hour in the sun.
Here's a photo of the "experiment" in process.
This is obviously _very_ crude, but I did:
- assure that, before the boxes were painted, they registered the same temperature in the sun and inside the house. In fact, I built three such boxes and discarded one with an inconsistent temperature reading.
- assure that, when all boxes are indoors (after painting), the thermometers consistently read the same temperature, to within ~.25 degrees.
- temperature readings were taken after about 1 hour outside in the sun, with a third thermometer taking ambient temperature readings in the shade (also tested for consistency with the other two thermometers).
- boxes were placed with the largest painted side oriented similarly to the condo wall that receives the most light.
- There are four condos in my building, in a 2x2 grid; my condo is the northwest quadrant of the building. The west and north walls of the condo get sun and air, but the back and south side are insulated by abutting other condos. This means that heat absorbed is difficult to dissipate.
- There are no trees shading my unit from the setting sun. Some shade comes from the soffit/roof overhang, but it still gets very hot.
I could:
- take additional temperature readings in ~65F sunny weather
- measure/calculate the box surface area in direct sunlight
- measure/calculate the air volume inside each box
- measure/calculate the surface area that would receive the vinyl siding
Obviously, this isn't going to be very exact, but I think the differences are significant enough that I could get some crude figure and good logic to present. Besides, it's kind of an interesting problem, no?
Here are a few measurements. All temperatures are in degrees Fahrenheit:
Bg = gray box;
Bw = "white" box (actual color close to almond);
Sh = ambient temperature in shade
Sh: 92; Bw: 100; Bg: 107
Sh: 95; Bw: 107; Bg: >123
Sh: 43; Bw: 56; Bg: 68
(the latter was taken with all boxes in _shade_; even in shade, the color made a difference: Sh: 92; Bw: 97; Bg:102)
This is all in Chapel Hill, North Carolina, United States.
This is quite difficult to estimate. Sorry, but while your box experiment certainly shows the difference in reflectivity / absortivity between the two colors, it won't help much in calculating your energy savings.
The energy use depends on many factors, but the main ones are the difference between the indoor and outdoor temperatures and the insulative properties of your wall. Vinyl siding is somewhat vented, so you have an air space behind the siding that may be at a warmer temperature than the outside air temperature. That system would be quite difficult to model mathematically though and would be best tested by building an actual model of your wall.
I think this has been done for roofing and you may be able to make some estimates based on those experiments. Solar reflectivity or reflectance will be useful search terms.
Bear in mind that while vinyl siding color will have some small effect, it would be much, much more effective to install a couple inches of insulation underneath the new siding.
posted by ssg at 12:28 PM on November 19, 2013
The energy use depends on many factors, but the main ones are the difference between the indoor and outdoor temperatures and the insulative properties of your wall. Vinyl siding is somewhat vented, so you have an air space behind the siding that may be at a warmer temperature than the outside air temperature. That system would be quite difficult to model mathematically though and would be best tested by building an actual model of your wall.
I think this has been done for roofing and you may be able to make some estimates based on those experiments. Solar reflectivity or reflectance will be useful search terms.
Bear in mind that while vinyl siding color will have some small effect, it would be much, much more effective to install a couple inches of insulation underneath the new siding.
posted by ssg at 12:28 PM on November 19, 2013
You would indeed need to build a scale model of your actual wall to have valid data. How much of a difference the siding color will make will depend on the whole wall assembly, since the heat will need to transfer to the layers between the vinyl and the interior before reaching the inside.
The sunlight getting directly in your house through the windows, especially on the West side, is likely to make a much larger difference than the siding. Adding shade, a window film or special storm windows would enable you to reduce your solar cooling load.
posted by Monday, stony Monday at 12:35 PM on November 19, 2013
The sunlight getting directly in your house through the windows, especially on the West side, is likely to make a much larger difference than the siding. Adding shade, a window film or special storm windows would enable you to reduce your solar cooling load.
posted by Monday, stony Monday at 12:35 PM on November 19, 2013
Response by poster: What if I could just estimate the insulating effect of the current walls (from the starting inside/outside temperature difference, and the change over time with no added energy from the HVAC), and calculate the insulating effect of the boxes (same thing)? Maybe call those R1 and R2, and understand that they won't be that precise.
Could I then apply those to a model to relate the box to the condo?
posted by amtho at 12:49 PM on November 19, 2013
Could I then apply those to a model to relate the box to the condo?
posted by amtho at 12:49 PM on November 19, 2013
Response by poster: Also, I could potentially measure insulating effect with some siding that has already been installed on another building. Or get an idea of the difference that the layer of air makes from previous studies of siding.
posted by amtho at 1:18 PM on November 19, 2013
posted by amtho at 1:18 PM on November 19, 2013
Best answer: I think you'll probably find what you need in the ASHRAE Book of Fundamentals and in the references it cites. Look at the chapters on cooling load calculation. The 1997 edition does have table that account for the color of the walls.
posted by Monday, stony Monday at 5:39 PM on November 19, 2013 [1 favorite]
posted by Monday, stony Monday at 5:39 PM on November 19, 2013 [1 favorite]
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I know that this reads like the lamest science fair project ever, but I'm working with what I've got, and trying not to spend _too_ much time on it. Besides, maybe it will help someone else.
(Also - keeping the place warm in the winter is less of a concern.)
posted by amtho at 12:06 PM on November 19, 2013