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Proper attic ventilation...


Haubeil
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I know this has been discussed, but I can't find what I'm looking for...so I apologize in advance for possibly beating a dead horse!!

What is the best attic ventilation methods or design? ...ridge & soffit, gable & soffit, gable, roof & soffit, forced fans, etc? Can an attic be TOO ventilated... and is that just as bad as not enough? Is the 1/300 rule for the attic area or the surface area of the roof?

Do the principles apply for all types of roof coverings (i.e., metal, slate, shaker, asphalt, etc?

Thanks,

Jeff

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1/300 is the floor area of the attic, although I think the standard is 1/150.

I think a combination of ridge and soffit or eave vents is usually considered ideal. IMO, powered vents are unnecessary and a waste of money but may have their place.

You could spend all day measuring attic vents to determine if they meet code. Here is what I say:

"Ventilation appears adequate. "Adequate" ventilation means that vents are installed and no evidence of past or present moisture buildup was observed. Total attic ventilation area is not measured to determine if the amount of ventilation conforms to any certain code or standard."

One of the few times I use "appears" in my reports.

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Originally posted by Haubeil

I know this has been discussed, but I can't find what I'm looking for...so I apologize in advance for possibly beating a dead horse!!

What is the best attic ventilation methods or design? ...ridge & soffit, gable & soffit, gable, roof & soffit, forced fans, etc?

Whenever someone asks me "what is the best method" the first thing I do is reach for my JLC Field Guide*. I like to cross-check my opinion with a published source, whenever possible. In this case my opinion and the JLC Field Guide are the same. The ventilation should be split 50/50 between low and high vents -- soffit & ridge vents are considered to be the best combination.
Can an attic be TOO ventilated... and is that just as bad as not enough?
The primary purpose of the attic ventilation is to rid the attic space of excess moisture that enters it from the house below. Secondary purposes are to lower the temperature in the attic space (and of the roof covering) and to (in my climate) "keep the roof cold" to prevent ice dams from forming. With a passive ventilation system, I can't think of any problems related to having too much -- so I agree with AHI that continuous ridge and soffit vents are the best.

One thing to be careful about, if you have a continuous ridge vent, is to be sure that you have at least the same amount of unobstructed NFVA (net free vent area) in the soffits (and more is better). If the soffit vents are undersized or blocked, under windy conditions the ridge vent can act like a vacuum and draw more moisture laden air into the attic space from the home below.

Is the 1/300 rule for the attic area or the surface area of the roof?

It's the area of the attic floor. Use 1/300 if there is a well performing vapor retarder (not exceeding 1 perm) present between the attic space and the heated portion of the home, otherwise go with 1/150.

Do the principles apply for all types of roof coverings (i.e., metal, slate, shaker, asphalt, etc?

Thanks,

Jeff

The purpose of the attic ventilation does not change based upon which roof covering is applied, so I'd say the answer to your question is yes. I cross-checked my opinion with the IRC and the codes to not take into account type of roof covering when they specify minimum attic ventilation requirements.

*"JLC Field Guide to Residential Construction -- A Manual of Best Practice"

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According to William B. Rose "Water in Buildings - An Architect's Guide To Moisture and Mold", the 1/300 rule is a wild ass guess with no basis in fact. It first showed up in the 1942 in FHA's Property Standards and Minimum Construction Requirements for Dwellings. All of the subsequents tests to confirm the 1/300 rule have been inconclusive.

So far as I can tell there is no one method guaranteed to solve ventilation issues in all vented attics.

Chris, Oregon

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Originally posted by Bryan

What would be the proper way or recommendation for eve vents in a home with no overhangs?

Bryan

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Air Vent makes a "vented drip edge". I think I remember that it provides just under 10" of net free area per linear foot.

Have your local chapter ask Air Vent to come out and speak at a meeting. They have a presentation for inspectors that includes principles of attic ventilation.

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Yes, proper ridge and soffit venting should provide a uniform flow of air up underneath the roof sheathing. The 1/150 rule of thumb is if half the ventilation is at the soffits and the rest at ridge.

If ridge venting is added (during a re-roof, for example) and gable vents or power vents pre-exist, some shingle manufacturers (such as CertainTeed) require the gable or power units be covered or removed to prevent hot spots. Other major manufacturers (like Tamko and GAF) highly recommend this. Not doing so can void the CertainTeed shingle's warranty. I find this often and it appears that most home inspectors overlook this issue. I report it as Mixed Ventilation Types.

Most homes have insufficient attic ventilation.

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