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Old 10-04-2011, 08:33 PM   #1
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Combining Gable and Soffit vents


Hello,

I just bought my first house and have been busy making improvements/repairs. My current project involves redoing the insulation in the attic as it is in terrible shape right now. The home does have problems with ice damming as the winter before we bought the house a chunk of siding fell off the front of the house. My plan to deal with this is to seal up the attic (with caulk, can of spray insulation etc) and then roll out some R30 fiberglass insulation on top of the existing insulation. My question is that since my house has gable style vent system is it worth while to install a bunch of soffit vents? Just from intuition it seems to me that this would work although not quite as well as a ridge/vent system. I appreciate any advice.

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Old 10-04-2011, 09:15 PM   #2
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Combining Gable and Soffit vents


For your system to work efficiently you need to have an equal amount of ventilation at the soffit level as you have at the highest point of your roof.

For ever 150 sq.ft. of attic space you will need 1 sq.ft of ventilation divided equally between the soffit and the highest point of your roof.

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Old 10-04-2011, 09:17 PM   #3
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Combining Gable and Soffit vents


Above is well said. The soffit vent and ridge vent should be related to each other.
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Old 10-04-2011, 11:04 PM   #4
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Combining Gable and Soffit vents


So I measured my gable and it's about 1 square foot on each end for an attic that is 700 square foot. I guess I need to install a larger gable... Is this something I should try to take on or hire a contractor?
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Old 10-05-2011, 09:31 AM   #5
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Combining Gable and Soffit vents


Use this example;

If your attic floor measures 24'x30'= 720 sq.ft. divided by 150 sq.ft = 4.8 sq.ft of ventilation needed, so approx. 2.5sq.ft at the soffit and 2.5 sq.ft at the peak.

2.5sq.ft x 144= 360 sqin lets assume your current gable vents provide 75sq.in of NFA ( Net Free Area) each. 360-150=210 sqin of venting needed at the peak.

So you would need to install at the peak;

4- roof vents with an NFA rating of 60 sq.in. or....

12' of ridge vent with an NFA rating of 18 s.in. per lineal ft.

and at the soffit;

6- 8" x 16" Soffit vents with an NFA rating of 60 sq.in.


Only you can decide if you are capable of doing this project. Make sure you install air chutes above each soffit vent to prevent insulation from covering the vent and blocking air flow. IMHO I think you will get more bang for your buck if you blow in, cheaper, faster and easier. Most places let you use the blower for free. I would bring it up to an R-50 while you are at it.
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Old 10-05-2011, 10:04 PM   #6
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Combining Gable and Soffit vents


Welcome to the forum!

Air seal first (as you know), to stop the ice dams: http://www.buildingscience.com/docum...d-135-ice-dams

Here is help on finding them: http://www.finehomebuilding.com/PDF/Free/021105092.pdf

More important than venting to control ice dams: http://www.greenbuildingadvisor.com/...stop-air-leaks

Seal the crawl space/basement first, before venting: http://www.wag-aic.org/1999/WAG_99_baker.pdf

Where are you located for your required insulation R-value?

Gary
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Old 10-06-2011, 10:40 PM   #7
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Combining Gable and Soffit vents


Thanks for all the great links. I live in Ohio. I think I've decided I am going to blow in some of the insulation till it is even with the joists and then roll fiberglass over that. The reason I feel I have to do it this way is I have knob and tube wiring and have to avoid insulating over it. I had my electrician/firefighter friend take a look at it and he said it was in good shape and did not need to be replaced. I am using chicken wire to keep the blown in away from the recessed lighting. There was maybe 500 square feet of old carpet scraps that someone got a great idea to try to use as insulation. I pulled that down and am throwing it over the garage as there is no insulation over that area right now.

Any further thoughts on the knob and tube? Also for the soffit vents I don't need to put a spacer in between each joist just where the soffit vents are, right? But I shouldn't stack insulation next to where the roof meets the floor of the attic regardless of a vent.
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Old 10-07-2011, 09:31 AM   #8
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Combining Gable and Soffit vents


Quote:
Originally Posted by Shabby View Post
...The reason I feel I have to do it this way is I have knob and tube wiring and have to avoid insulating over it...

...I am using chicken wire to keep the blown in away from the recessed lighting...

Don't let the K&T wire get covered by insulation. In fact, if you can it would be best to replace it.

Are your recessed lights "IC" (insulation contact) rated? If not, I don't think the chicken wire will be enough to keep blown cellulose out. You might want to consider building a box out of 1x boards to keep a safe distance around the recessed cans.
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Old 10-07-2011, 11:06 AM   #9
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Combining Gable and Soffit vents


You should install a plastic insulation baffle in each vented rafter bay for optimum ventilation, even if the soffit is communal. Insulation touching the colder roof sheathing will act as a heat sink wicking away room heat from below as well as block venting.

Just block along the top of wall, should have been done at framing anyway. Don't extend them any.

Sister rafters with 2-3' longer than split.

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Old 10-07-2011, 03:19 PM   #10
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Combining Gable and Soffit vents


The problem is....that when you add large holes to your home, your expensive heat escapes!
Adding holes to a roof may seem a good idea but, when the wind blows it creates an area of low pressure above and to the lee of the building, this low pressure pulls the warm air from your home, along with the air from the attic.

What you need to do, is to stop the wind pulling the heat from your home into the attic, warming the underneath of the roof and melting the snow, creating an ice dam and a leaking roof.

Start by checking the upstairs ceilings for holes, block every one, check that the door into the attic is an air tight fit, then paint every ceiling with either a gloss or latex paint to stop the water vapor in the air getting into the attic and forming condensation. Then pile in the insulation.

Go round the rest of your home finding and blocking every hole, keep in mind that warm air rises, as it does it pulls cold air in from outside to replace it, every time the air changes up goes your heating cost.

When winter comes open a window for a few minutes each morning to let the warm wet air escape and some fresh dry air in, if its really cold use a dehumidifier to remove the water vapor.
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Old 10-08-2011, 05:57 PM   #11
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Combining Gable and Soffit vents


Air Seal, Air Seal, Air Seal, Air Seal....then insulate while maintaining proper air flow.

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