Attic ventilation requirement

[Joe Funderburk, CMI]

Here's an example of how I word it:


"The attic space is considered to have inadequate ventilation. Only two gable vents provide ventilation for the structure. For venting to be effective the air has to be able to move. It is recommended that the structure be ventilated with both: A) An intake preferably located as close to the eaves as possible. (Most often this is done through soffit vents. Ensure the soffit vents are not obstructed by insulation.) B) An exhaust preferably located as close to the ridge as possible.
“The total net free ventilating area should not be less than 1 to 150 of the area of the space ventilated except that the total area is permitted to be reduced to 1 to 300...” provided mechanical ventilators are used. When only gable vents are used, effectiveness is more determined by wind movement. Area calculations were not conducted; it is recommended that a state licensed general contractor evaluate the adequacy of attic ventilation and add ventilation as deemed necessary. Reference: International Residential Code R806.1/R806.2"

[Russel Ray]

Very nice, Joe. Very, very nice.

[Gary Reecher]

Any addition of attic exhaust fans should be followed up by recommending that the draft of all natural vent equipment be tested with a draft meter. Addition of any exhaust motors can cause back drafting.

[David A. Andersen]

Also note that the vent area is NET AREA. That is the opening of the vent, less the mesh or screen restriction. The vent Mfg lists this info.

[Dale Duffy]

I'm inspecting 24 new Townhouses, they all have a flat roof system with no ventilation at all, and excepted by the AHJ.

The building plans do not call for any ventilation of the rafter system.....

[Russel Ray]

Originally Posted by dandersen

Also note that the vent area is NET AREA. That is the opening of the vent, less the mesh or screen restriction. The vent Mfg lists this info.

I've never seen a manufactured soffit vent, gable vent, or ridge vent. I've always constructed them myself as a necessary part of the structure.

[David A. Andersen]

If you make it yourself, do the math.

[ccarrington]

Free is exactly that, Unencumbered by nothing, not so much a 1 single louver. Louvers and coverings reduce the free flow of air. I use a 50% reduction of free flow for louvers vents, 75% reduction for turbines and I follow the MFG instruction for other systems. Wood louvers are very inefficient at a 75% reduction to free flow… Hope that helps

[Mark A. Timpani, CMI]

I did a new house on Monday. 1600 sq ft floor plan. No turbine, ohagen(sp) or soffit wents. Only one 2ft gable vent on one end of the attic area. I recommended more ventilation and insulation. The insulation was only about R-20 and was supposed to be R-32. My client brought this up to the super who asked the builder and they all ended up with the prints at the county building.

I guess here in AZ., for ventilation,they do not use the IRC. In 2001 or 2 due to thermal imaging, they came up with thier own formula and can almost have zero ventilation to create a dead air space. They said the lack of moisture here does not create problems that are found elsewhere.

Today (Wed.) I got a call from our client saying that although the builder stated that the current vent is ok according to the code, he is going to install 8 Ohagen(sp) vents. My client also said that today they went and blew in more insulation.

Sometimes things work out the way they should...

[Dale Duffy]

Originally Posted by mtimpani

I guess here in AZ., for ventilation,they do not use the IRC. In 2001 or 2 due to thermal imaging, they came up with thier own formula and can almost have zero ventilation to create a dead air space. They said the lack of moisture here does not create problems that are found elsewhere.

Mark, I have been finding less ventilation, more insulation on the ceilings, and a thermal barrier on the OSB lately (new construction).

[Russel Ray]

Originally Posted by mtimpani

ohagen(sp) . . . vents.

More information, please.
Pretty please.
Oh please oh please.
Darn margaritas.

[Mark A. Timpani, CMI]

Sorry R2, so far I can't find a pic. They are called O'hagen roof vents or invisivents. They look just like the tiles when you are looking at a tile roof. Same colors, same profile, they just stick up about 1/2 inch or inch higher then the tiles. So from the road you can not even tell they are on the house.

[Russel Ray]

Ah-ha.

I know exactly what you're talking about. Thanks.

[Jay Moge]

not to hijack another thread, but i've been looking for some info on metal roofs installed over shingles. it seems to be common practice up here, my question is, does this effect the venting at all, they do have ridge vents, kind of, and there's about 1'' of air space between metal and shingle. the ridge and drip edges aren't "air tight" but they do deflect water well.

[Mark A. Timpani, CMI]

If any one has pics of O'hagen vent could you please post them?