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I made “rafter vents” from 1″ polyiso foil-faced foam board. I have a 1″ air gap and then the foam cut to fit between the rafters. Yesterday I measured the roof decking (underside) at 122F and the underside of the foam board at 90F for a 32 degree drop across 2″ (1″ airspace, 1″ foam). It didn’t matter where on the board I measured (center, or right at the edge) I got the same temperature. There was no detectable air movement (i.e., “convection). The board is stamped R-7.2 per inch, and a 3/4” air gap is supposed to be R-2.6 — is there a way to tell what the delta temperature should be?
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Crusty, Sorry for the delay in replying
I can quickly tell you math that will get you close. The equation is for a wall, but there are only a couple tweaks that would make it apply to the cathedral ceiling style installation of the foam. These coefficients are different based on the thermal resistance of the air film. On a sloped ceiling the air film convects (whether or not you detect it). Oh by the way, did you use a smoker to "detect" the air movement?
The equation is a legitimate ASHRAE equation with the values plugged in and the math done uch that we can plug and play with our numbers.
How did you measure the temperatures?
See the attached picture now....
I just looked up the coefficient for cathedral ceilings. REMrate uses .61 for a cathedral ceiling.
I would think it is safe to plug in 122 degrees for the outside air temperature and 90 for the inside surface temperature. The interior air temperature would be less than 90, I'll say 88 just for fun.
I get the effective R-value of the foam at 9.76
note that the interior air temperature is very important. If we assume 86 dgree interior air, the effective R-value is 4.88. so the interior air temperature is important.
I wouldn't count on the air space for R-value becuase it has free convection thru it. Next time you are up there, get me temperatures of the air going into the soffit and out of the ridge vent. I am looking for even more facts on the effectiveness of roof venting at removing heat. Since the sun can impart 1000 btu per square foot, and we could say half the roof is 400 square feet (to say that the hottest sun is from the west in the afternoon, so only shining on one side) This means that the venting would need to move enough air to remove 400,000 btu per hour to keep the roof at constant temperature. We know that it doesn't remove all the heat because as you pointed out, the underside was 122 degrees, so roof venting is not 100% effective at heat removal. What I am trying to determine is the actual percent effective.
I want to do this because I am almost surprised at some of the IR images I see on roofs that have vented and unvented areas. I am beginning to think that my assumptions on the amount of heat a vent can remove have been too high all along!
Hope this wasn't too confusing.
-Rob
*Rob, I use a small handheld IR thermometer made by Exergen. It has automatic emissivity compensation and is pretty accurate from what I can tell. But it won't measure AIR temperature since it has to have a surface that is radiant. So I didn't measure that.I don't think most venting is all that effective. Convection is heavily dependent on air velocity, and you just won't get high velocity with a typical "open" rafter bay. I'm thinking that since mine are basically enclosed, I will get higher velocities due to thermal currents. Any reduction in the temperature of the underside of the roof deck is going to reduce the radiation of heat towards the living space.
*I agree with your statement about the reducion in temperature of the roof surface = reducion in heat radiated. I am just starting to think that the amount of radiant reduction is not worth the effort.-Rob
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I made "rafter vents" from 1" polyiso foil-faced foam board. I have a 1" air gap and then the foam cut to fit between the rafters. Yesterday I measured the roof decking (underside) at 122F and the underside of the foam board at 90F for a 32 degree drop across 2" (1" airspace, 1" foam). It didn't matter where on the board I measured (center, or right at the edge) I got the same temperature. There was no detectable air movement (i.e., "convection). The board is stamped R-7.2 per inch, and a 3/4" air gap is supposed to be R-2.6 -- is there a way to tell what the delta temperature should be?