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I am closing in an existing screen porch with 2x6 rafters on 16s. I plan to put soffitt ventilation, but I need to know how to figure out the number of mushrooms or turtles I need. I cannot use a ridge vent (at least I do not think I can) b/c the center beam is 4inches wide.

I plan create a common vent space by placing 2x6's across the top of the ceiling (i.e. like a small triangle) to join flatten the ceiling out and create a common space (a mini attic about 8inches high).

thoughts on how to spec the number of mushrooms?
 

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Sure but first you have to tell us the Sq. foot of the celing you are trying to vent. You are on the right track with soffit ventilation which is the intake and now you need exhaust vents. I must say I never heard the term mushroom or turtle vents but I know what you mean.
 

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How did I miss a ventilation question? I salavate at the mouth for these!

RUle of thumbe you divide the floor plan by 300, and that gives you a number of mushroom vents. It's fast math and not always accurate. Tell us more Marker.

Never heard mushroom?! LOL the word "mushroom breather vents" is in all of our contracts... and your from WI. Man something get's lost int he translation from WI and IL. Us FIPS.
 

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Well I have learned something tonight Grumpy. I had to laugh at the terminology. We call them either R-61 vents or just plain roof vents. And I agree with your calculations.
According to building code a Balanced ventilation system means at least 50% of the required vent area must be Intake and 50% must be Exhaust. However many engineers recommend installing additional intake to maximize airflow. Many times I have observed little or no intake or exhaust ventilation on homes. Most codes recommend a minimum of 1 square foot of net free ventilation area for every 300 square foot of attic floor space,
If it's balanced between exhaust vents and intake vents regardless of weather a vapor retarder is used. Otherwise the recommendation is 1/150.
 

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Discussion Starter #5
Ventilation Specs

Gentlemen-
Thanks for your advice on the ventilation, did not know I would cause a stir with the terminology for the vents. Its a turn we use down hear in Gawga, probably b/c no one really knows the right name.

My floor sq. footage is 360 (20w x 18d), rafters are 11-8 feet in total length, 10 feet on the inside of the structue, 14" for the soffitts, So the roof sq ft(inside) would be 20x10x2 or 400?. Soo using 3 or 4 of them should be fine with the either hardi or certainteed cement board vented soffit. Is there a chance that I'll be overdoing it? Is there a certain type you prefer, plastic or alum? My concern with alum is the sound that the rain may make when hitting the shell of the turtle, mushrom or R-61.

Thanks for your help... :Thumbs:
 

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Don't worry aboutt he sound of the mushroom vent's I have never heard anyone complain about it.

Seems to me with quick math, youa re fine. I assume you are referring to 2" continuous soffit ventilation. This is a good form of intake. Just don't put the soffit intake at the rakes/gables.
 

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Gunncon is right. One thing to remember is 1sq foot of vent space per 300 sq ft of attic space on the roof. You need to have the same intake at the soffits as you do on the roof. Otherwise 1 sq ft of vent for 150 sq ft of Attic floor space.
 
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