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01-16-2009, 10:44 AM   #1
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## Roof venting calculation

Hello,
We live in the northeast in the Washington, DC suburbs. We owned a town house built in 1967. Our house has a flat root. We had all the plywood replaced. The outside is covered with a modified bitumen from certainteed.

This winter we began noticing condensation on the ceiling of our attic. The dimensions of our attic are as follow:

27.5' (L) x 20' (W) x 5' (H).

Intake ventilation is provided by soffits installed in the front and the back of the house (total of 40ft). We have Alside soffit installed (Charter Oak)

http://products.construction.com/Man...ffit-NST2576-P

The net free area of this material is 3.2 square inches of net free area/lineal foot. Now 3.2x40 = 128 sq inches of net free area for total intake.

We are afraid that this intake is not enough for our attic size therefore we are seeing some condensation.

Starting the design from scratch (please correct me if I am wrong, I am an electrical engineer)

If I use the rule of the 1 sq ft of net free area per every 150 sq ft of attic area. See the following document:

http://www.lomanco.com/public.lomanc...lculations.pdf

Given the fact that my attic area is 550 sq ft

= 550 / 150 = 3.666 sq ft of net free area, where 1.83 is for intake and 1.83 is for exhaust (I got this from the document just by replacing values). Is it 50/50 a good assumption?

Now if I work with the intake and put everything into sq in

1.83 sq ft = 144 x 1.83 sq in = 264 sq inc of net free area for intake. You can clearly see that my installed intake is below this number.

Now taking into consideration that my only option for intake is soffit,
I estimate per this design that I will have to reach 256 sq in of net free area using my 40 ft of lineal soffit in my house

Now:

40 ft x (X) = 264
X = 6.6 sq inc of net free area per lineal ft.

The maximum that I can achieve for intake with the soffit materials out there if I were to replace my current soffit to increase the intake is 7 sq in per lineal ft. The product that I am considering is Alliance T4 from Alside:

http://products.construction.com/Man...ffit-NST2579-P

For exhaust, we have a Lomanco attic fan (800CFM that requires 640 sq inc of intake)

Am I right in the assumption that this fine will be OK for exhaust ? What about my assumption for intake/exhaust (50/50) given my current conditions?

Please let me know if my calculations are clear.

I really need help!!

01-16-2009, 11:22 AM   #2
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were the old the old wood soffits removed or a vent slot cut into it?
Vinyl soffit is a very air leaky assembly you couldnt make it air tight if you wanted to,not only does air pass thru the holes,but also around the j channels and fascia.Imo you wont be any better off if you just swap out the vented vinyl soffit for another

01-16-2009, 11:36 AM   #3
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## Old soffits

Hello,
The old soffits were removed just in the front of the house. The old ones were metal I guess. The one in the back of the house is still there and underneath is the vinyl soffit (Charter Oak).

 01-16-2009, 12:18 PM #4 Member     Join Date: Dec 2008 Location: west milford n.j. Posts: 2,786 Rewards Points: 2,002 you have to make sure that there is nothing blocking the air flow in between the rafters.Can you pull some insulation aside and see if you have clear venting from the vinyl soffit?A Lot of siding installers just install the vented soffit without regard as to whether it actually vents or not
01-16-2009, 01:11 PM   #5
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Quote:
 Originally Posted by strublesiding@optonline.n you have to make sure that there is nothing blocking the air flow in between the rafters.Can you pull some insulation aside and see if you have clear venting from the vinyl soffit?A Lot of siding installers just install the vented soffit without regard as to whether it actually vents or not
Soffits are not covered but I you look at the calculations above they are no providing me with enough intake. Please see the previous calculations made and posted on this thread.

 01-29-2009, 12:21 PM #6 Member     Join Date: Dec 2003 Location: Chicago Posts: 614 Rewards Points: 552 Ventilation of low slope roofs is often a different beast all together than ventilation of steep slope roofs. Infact many low slope roofs have absolutely no ventilation what so ever and don't need it either. Is your ceiling insulated, or is your roof deck insulated? If you do choose to put a fan, PLEASE first build up a curb at least 10" high. Most attic fans you can buy at a big-box are not intended for low slope roofs, and you'll need to specifically get a "curb mounted" style fan. __________________ -Grumpy http://www.illinoisroofinspections.com The information found in this post is not to be considered legal advice. The author of this post takes no responsibility for any losses that occur. Use at your own risk.
01-29-2009, 12:48 PM   #7
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## Other suggestions? What about the number?

Quote:
 Originally Posted by Grumpy Ventilation of low slope roofs is often a different beast all together than ventilation of steep slope roofs. Infact many low slope roofs have absolutely no ventilation what so ever and don't need it either. Is your ceiling insulated, or is your roof deck insulated? If you do choose to put a fan, PLEASE first build up a curb at least 10" high. Most attic fans you can buy at a big-box are not intended for low slope roofs, and you'll need to specifically get a "curb mounted" style fan.
Hello,
There is a slight pitch on my roof but I would consider the roof completely flat. There is no insulation on the ceiling of my attic, it that the decking?. There is only R-19 insulation on floor of my attic.

My attic fan is currently installed and is a curb mounted style fan. It is installed on the side of my roof (shingles). My roof is a mansard.

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