Soffit vent install question

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Old 05-12-10, 08:15 AM
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Soffit vent install question

My soffits on the garage are made of 5/8" OSB. The garage is 24'x24'. I need to install soffit vents, but I am unsure of two things:

1) How many vents/what size do I need? (There is a ridge vent and I believe the pitch is 4" for every foot)

2) To cut the holes for the vent, should I use a dremal (i have) or rotozip? If you know what cutting tool/blade I need to get the job done, I could really use that info as well!

I am currently planning to install 4 soffit vents on each side of the garage for a total of 8 vents. I have the white 16"x8" vents ready to go, but want to be sure I am doing this with the right materials and going about it the right way before I begin.

Thanks!
 
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Old 05-12-10, 10:35 AM
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Hi mrkris82,
Is the garage finished and insulated on the inside or just open studs and rafters?

In a typical house with an attic, you need one sq. ft. of vent space for every 150 to 300 sq. ft. of attic floor. If there is a vapor barrier and the ceiling is well sealed, you use the 300. And that's actual vent rating, as the screens and louvers will block some air flow. I'll let you do the numbers, but off the top, It looks like you are in the ball park.

However, if this is an unfinished garage, I have no idea, I usually don't vent them.

Finished or unfinished? Heated? Moisture issues, like snow melt?

Bud
 
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Old 05-12-10, 10:58 AM
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Thanks for the information!

Right now, the garage is unfinished and uninsulated, but I have the drywall ready to roll once the soffits are completed and will be insulating it as well.

It is heated and the roof sheathing and shingles are brand spanking new because of issues that I discovered over the winter caused by one of th eprevious owners.

The garage is 576 sq ft. I need 3.84 16x8 vents to meet your criteria. Doubling that (since vents will be under the soffits one ach side of the garage) I will have 7.68 sq ft of soffit ventialtion, which is either just about the right amount of way too much (if that is even possible)
 
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Old 05-12-10, 01:30 PM
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The ridge vent counts as well. The 16 x 8 vents you have are 128 sq. in. but may only allow 70% air flow. There may be a mention of the effective ventilation area on the package. In any case, area wise, you probably meet the minimum.

Having a continous vent the full length of the soffit works better at distributing the air flow to each bay, but as long as you don't block any bays you should be ok.

Where ceiling insulation comes up to the soffits, it improves the efficiency to cover the end of the insulation. I see another post under insulation, so will continue there.

Bud
 
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Old 05-29-10, 11:52 PM
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ventilation calculation

I've found this thread looking for some basic information how to calculate ventilation. There is a lot of info on the whole net, but all together it is not clear. This is what I found so far:
-rule 1:300, 1 sf of ventilation per 300 sf of attic floor
-rule 1:150, 1 sq inch per 1 sf of floor, or 2 sf per 300 sf of attic floor
-resulting from these rules ventilation net area should be divided 50/50 between soffit intake and exhaust, like pot vents
-another sources say, this calculated area is for intake only, and the same amount should be used for exhaust. This approach gives us 4 sf per 300 sf of floor.

You have explained the difference btw 150 and 300. Can you clear up the rest of that?

I have another problem with vents rating. You estimated the net areaa of soffit vent 16x8 as 70% of (nominal) 128 sq inch. I've measured one of my soffit vents. There are 48 slots, approx 3/16" by 4-1/8", which gives 37sq inch. This should be reduced for mesh, maybe 66-70%, if the gap btw wires is about 2 x wire size. What is left is about 25 sq inch, less than 20% of nominal vent size. Is this calculation ok?
How to estimate (or where to find) the net area of pot vents without getting up to hot attic.
Thanks for your help.
 
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