View Full Version : roof vent question

Clarence Martin
12-19-2015, 12:49 PM
Found some specs on roof vents.

Roof exhaust ventilator blows out 500 CFM. The square footage on the roof that needs venting is 630 SQ FT. There are roof vents on the other part of the roof, but not this part. How many would I need ? I am assuming Intake as well as exhaust vents.

Clarence Martin
12-19-2015, 12:52 PM
Also, would it be a good idea to locate the intake or exhaust vent directly over the spot on the roof where the heat ductwork is closest to the roof ? That is where all the ice build up is the problem.

Brian Elfert
12-19-2015, 3:06 PM
The better solution for heat getting into the attic is to seal the air leaks that are allowing heat into the attic. If you have your heating ducts in the attic then I don't know how to help. I had the floor of my attic sealed with spray foam to stop air leakage. I did not have a real vapor barrier so I decided to have the spray foam done.

Jerome Stanek
12-19-2015, 3:11 PM
I can't answer that as you need to know the cubic feet of the roof. If it has a shallow pitch or a steep pitch will make a difference.

Clarence Martin
12-20-2015, 9:40 AM
I can't answer that as you need to know the cubic feet of the roof. If it has a shallow pitch or a steep pitch will make a difference.

Low sloop rubber roof. About 7 feet attic ceiling height on the high end, and not sure on the low end. I know the duct work is less than a foot from the ceiling on the low end, where the roof meets the wall. It is possibly a 1/12 2/12 or 3/12 . No more than 3/12.

Rob Nusbaum
12-21-2015, 9:35 PM
If you're getting ice on the roof above the heat ducts, then perhaps the first thing to do would be to seal and insulate the ductwork. Sounds like you're losing heat from your warm ducts to the cold attic. You could vent that heat out, but don't you want it to heat your shop?
The Owens/Corning website has a good calculator for figuring attic ventilation. www.owenscorning.com (http://www.owenscorning.com)

Lee Schierer
12-21-2015, 10:21 PM
Installing a powered vent when there are not powered vents in other parts of the roof may lead to problems. With regard to air inlets you want them as low to the roof as possible, preferably under the soffits. More is better than too few. You need at least as many square feet of inlet as you have in the outlet. However, since your outlet is powered, you don't want high velocity air coming into the attic. Divide 500 cfm by the square feet of the ventilator outlet and you can determine the speed of the air flow in feet per minute. For example if the outlet of the ventilator is 2 square feet, the air is moving at a rate of 250 feet per minute. Probably 3-4 times (or more) the outlet opening would reduce the velocity of the air coming in to a reasonable speed.

Sealing and insulating your air ducts in the attic is a better way to go than trying to remove leaked heat that you paid for.

roger wiegand
12-22-2015, 9:31 AM
We transitioned to a sealed attic with spray foam at the roofline (bringing our ductwork inside the heated envelope of the house) from traditional attic floor insulation and had not a single icicle, much less ice dam, last winter in the worst winter on record for ice damming in our area. Many similar houses around us, also including relatively new construction, had $100-300K in damage from ice dams-- a couple houses were teardowns because the damage was so extensive. One of the best investments I ever made I think, dwarfing the actual energy savings.

Clarence Martin
12-22-2015, 5:49 PM
14 inches of blown in insulation in the attic. Duct work is all taped and wrapped. Problem is ducts are too close to the roof AND lack of roof vents on the EPDM Rubber Roof. Found vents that would work on flat rubber roofs. The vents under the eves are not providing enough ventilation