Replies

1. 
   m2ew | Nov 19, 2012 06:08pm | #1

   Here is an attachment with the approximate location of the vents. The roof vent is located in the same spot as my neighbors house, with a similar roof build

   
2. GBA Editor
   Martin Holladay | Nov 20, 2012 06:15am | #2

   Mike,
   Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. For 128 square feet of attic, you need at least 0.43 square feet of ventilation area (61 square inches).

   According to Joe Lstiburek, "Put more vents down low than up high. ... You want more entry points at the perimeter than exit points at the top." So if you have 60 square inches of ventilation up high -- that's what you have chosen -- then you need more than 60 square inches down low.

   However, note that the square roof vent you have chosen for exit ventilation is twice as big as you need.

   You can use an online ventilation area calculator if you want. Here is one from GAF:
   http://www.gaf.com/Roofing/Residential/Products/Roof_Vents/Ventilation_Calculator

   To get 60 square inches of soffit ventilation with 4-inch round soffit vents, you need 12 round soffit vents, not 5. That still doesn't meet the Lsitubek requirement -- that's just balanced ventilation. To follow Lstiburek's guideline, you would need more than 12 soffit vents.

   Another option: choose a smaller unit for the exit ventilation.

   Here's another perspective: attic ventilation is overrated. Most of these calculations are made to satisfy a building inspector rather than to solve a problem. If you are worried about ice damming, the most important rules are these: build an airtight ceiling and include lots of attic insulation. Don't worry so much about attic ventilation.

3. 
   homedesign | Nov 20, 2012 06:51am | #3

   Speaking of Ice Dams
   I stumbled onto this excellent Lstiburek Video
   https://www.greenbuildingadvisor.com/green-basics/video-class-ice-dams-insulation-and-roof-venting
   the reason I say "stumbled".... as far as I know a link to the Video was never announced here at GBA
   or if the Link was published... I just happened to miss it

4. GBA Editor
   Martin Holladay | Nov 20, 2012 07:57am | #4

   John,
   Thanks for providing the link. GBA has many pages of articles and videos, and we add to our library every week -- so it's easy to miss some of our content.

   I provided a link to that Lstiburek video a couple of years ago in the "Related Articles" box on my blog about ice dams ("Prevent Ice Dams With Air Sealing and Insulation").

5. 
   m2ew | Nov 20, 2012 08:58am | #5

   Hi Martin,
   Thanks for the info. According to the calculator and 1:300 ratio (which is code) I need 30.72" inches of venting for soffit & roof. For the soffit vents I'm squaring the radius * Pi (3.14) which gets me 12.56" of venting per unit. That would require 3 vents total, correct? If so, I would need to find a roof vent small enough to cover 30.72".

   As far as air tight, I don't know if that is easily done given that there is exposed beams in the ceiling. Plus my guess is that the insulation is not sufficient. It was likely done 20 years ago.

6. GBA Editor
   Martin Holladay | Nov 20, 2012 09:18am | #6

   Mike,
   You're right, of course, that the area of a circle is π r². However, the net free vent area of a 4-inch-diameter round soffit vent is significantly less than the area of a circle with a radius of 2 inches. The reason is that the louvers and the screening restrict air flow.

   Frustratingly, the link you provided to a Home Depot web page doesn't include the net free vent area for this product (unless I'm missing something). However, the online calculator from Gaf.com (see my link in the earlier answer) uses data that show that the net free vent area for a 4-inch-round soffit vent is about 5 square inches -- not 12.5 square inches as you optimistically calculated.

   However, if you have a ceiling that isn't airtight, and if you suspect that your insulation level is insufficient, you are definitely barking up the wrong tree. I can't imagine why you're worrying about your attic ventilation level. It's time to come up with an air-sealing plan and a plan to improve your attic insulation.

   If your ceiling is leaky, adding attic ventilation can increase the rate of air leakage through your ceiling -- increasing your energy bills and making a bad situation worse.

7. 
   m2ew | Nov 20, 2012 10:46am | #7

   Thanks Martin. I didn't even think about the air restriction with the louvers.
   It seems to be a better investment to work on the insulation and ceiling.

   Thanks for the advice.

   Cheers

8. Expert Member
   MALCOLM TAYLOR | Nov 20, 2012 11:21am | #8

   You don't need to buy a roof vent with exactly the size of venting you require. Just cut the hole in the roof sheathing underneath the right size.

9. GBA Editor
   Martin Holladay | Nov 20, 2012 11:33am | #9

   Malcolm,
   Your suggestion is intriguing, but the calculation is more complicated than you imply.

   Rectangular roof vents (mushroom vents) usually include louvers and screening. All of them restrict air flow to some extent, so the area of the opening in the roof sheathing will differ from the net free ventilation area provided by the opening once the vent unit is installed.