View Full Version : OT: Aluminum Radiant Barriers
07-12-2009, 12:11 AM
Anyone reading have experience with using aluminum radiant barriers over their attic insulation???
I'm looking in that direction and trying to separate fact from fiction.
07-12-2009, 12:41 AM
Please share whatever you find. I've often wonderd how much a reflective surface would impact heat migration in a dark space like an attic. Radiant heat can be infrared, but I don't know if aluminum is any better than a simple plastic.
A sheet of aluminum is good for about R-3. The effectiveness isn't in doubt as it is basic physics. The emissivity of a surface is the same as the absorption. The absorption is the reciprocal of the reflectiveness. Aluminum foil reflects about 80% on the dull side and about 90% on the shiny side. That means it absorbs 20% and 10% respectively. Of that 10% or 20% it emits 10 or 20% respectively. It is because of the low emissivity that aluminum gets hot in direct sun even if it is very reflective. When you add the good reflectivity and the low emissivity a sheet of polished aluminum is an effective insulator.
A very simple way to prove this is to place a sheet of aluminum foil in front of a radiant electric heater. You will feel absolutely none of the heat from the heater against your hand held near the foil. It is all being reflected or is heating the aluminum which is a very poor radiator.
07-12-2009, 03:09 AM
Is there a lot of radiant heat in an attic? And if it's reflected off the aluminum, where does the energy go? Back into the roof?
The amount of radiant heat present anywhere is a direct function of the temperature of the objects in the environment times thier mass. The amount radiated by a specific object is a function of it's emissivity while it's maximum temperature is a function of the temperature of incident radiation.
The two values that represent energy are temperature (quality) and heat (quantity). The temperature of something is not a measure of how much heat it contains while the amount of heat is the product of temperature and mass.
A burning match is at a far higher temperature than a bath tub of warm water but the tub of water contains far more heat. The measure of the heat contained in the match flame is how much it can raise the temperature of the tub of water. Since the amount of surrounding material in an attic is significant even a small change in temperature represents a lot of heat.
If you have a roof with an area of 2000 sq feet and if we assume it absorbs half of the solar energy that falls on it on a summer day the amount of energy it collects is an astounding 90 thousand watts per hour. If your roof is dark in colour then raise that to around 140 thousand watts per hour.
All of that energy shows up as heat in the material of the roof. The majority of it must be re-radiated into the environment while the remainder is carried away by the movement of air through the attic and by the conduction of heat through the materials of the roof to other cooler parts of the structure.
That majority which is reradiated radiates in all directions including down within the attic space upon the ceiling structure below. If the first thing it meets is a highly reflective surface then that bounces that energy back to where it came from according to the reflectivity of the material. A sheet of aluminum foil is highly reflective and will bounce back 90% of that heat. That causes the roof temperature to increase whiile keeping the mass below cooler. The hotter roof then radiates more efficiently into the outside surroundings and sky since the quantity of heat radiated by anything increases with temperature.
07-12-2009, 11:30 AM
Most new construction in this area now uses the foil backed OSB decking.
07-12-2009, 01:06 PM
When we built a new back porch on our house I sprayed the Sherwin Williams radiant barriar (looks like a thick silver paint) on the bottom of the roof decking and it made a big difference. If I remember correctly, the bottom side of the coated deck was around 6 degrees warmer than ambient and the uncoated area was about 20 or so degrees over ambient. It worked so well that I sprayed the bottom of the roof deck in the rest of our house, and the attic is noticeably cooler. Now, before you get too excited about spending about 40 dollars a gallon for the SW stuff, when I built the woodshop (used the same style framing and same shingles) I sprayed it with a bucket of cheap white paint that I had and the attic there is at least as cool as the one in the main house.
07-12-2009, 02:14 PM
When we had our roof done in 1999, I asked the guy doing the survey and quote for the company. His response at that time was "maybe, maybe not - now would be the time since we're going down to the rafters but I can't tell you you'll see any kind of ROI on the cost". So we didn't. White paint is a good trick and cheap too.
Painting the underside of the roof deck is a heck of a good idea. It will help in summer and winter since it works just like a thermos. Keeps hot things hot and cold things cold. Your attic is a lot like a thermos bottle and the better it is insulated in all ways the better it works. I am sure everyone has noticed that a glass thermos bottle is coated with mirror finish inside. Tacking up aluminum foil to the underside of the roof deck would probably work even better than paint but it would be a pain. I bought an industrial sized roll of 24" aluminum foil and put a layer of that between the old insulation and the extra layer of insulation I added when I stepped up the insulation in the attic. On top of that we installed a new roof last year and I put on a nearly white fiberglass shingle roof to replace the dark red roof. The difference in heat gain right now is significant, especially since we don't have AC.
You can see by the spectral plot of the reflected light in the photos taken under identical conditions at the same time how much more light the white roof reflects.