R806.2 exceptions, balance high/low vents, ambiguous language

[hepp527]

1:150 rule does not require a 50:50 balance upper vs lower. 1:300 requires approx 50:50 balance.

So a 3000sf attic requires 10sf of venting IF 5 to 6sf are lower, and 4 to 5sf are upper (min 40% max 50% in upper third). So 8sf upper and 2sf lower breaks this rule. That much is clear. Also 2sf upper and 8sf lower breaks this rule. What about 4sf upper and 12sf lower?

In this example we break the min 40% requirement for the 1:300 rule, but use of the words "...not less than 40% or more than 50% of the REQUIRED ventilating area must be in the top 1/3..." muddies the water as follows:

using the 1:300 rule, 10 square feet of ventilators are REQUIRED. So if i exceed the minimum by providing 16 square feet of venting, but do not reach an amount that satisfies the 1:150 rule, do i need 40% to 50% of the "REQUIRED" 10 square feet to be in the upper third (which would make my "4sf upper/12sf lower" example perfectly acceptable), or do I need 40% to 50% of WHAT I ACTUALLY PROVIDE to be in the upper third (meaning i would need min 6.4sf upper, and therefore my 4:12 example is a violation)?

My suspicion is that the near 50/50 balance is required for speed of air movement (i.e. air flow relies on the "stack effect" at this ratio). At low venting ratios air speed is more critical since it is the only force present that can move water vapor out of the attic (I'm speculating here), so balance has to be maintained at all ratios below 1:150 to preserve the stack effect. Air moves upward and out the vent as it warms against yhe roof sheathing, pulling cold air in at the lowest opening to replace it. The stack effect is foiled if upper and lower are not in balance (same reason barely opening the door of a wood stove causes rapid air movement into the fire, but fully opening it slows it down. More isn't always more).

At 1:150, the stack effect is no longer required because there is enough vent area to allow the combined pressures of "diffusion" and the "ideal gas law" to move water vapor out of the attic and into the open air outside.

Thoughts?

 

[steveray]

For those of you attempting to follow along from home:

R806.2​

The minimum net free ventilating area shall be 1/150 of the area of the vented space.

Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided both of the following conditions are met:

1. 1.In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
2. 2.Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

 

[ICE]

Your example of 16 sq,ft. ventilation for a 3000sq,ft. attic space equals 1/187.5 That is less than the minimum 1/150 and greater than the exception of 1/300; There is no guidance provided for this scenario. Given the near balance between upper and lower vents required by the exception which allows 1/300 it seems logical to apply the same formula to any amount of vent opening below the 1/150 minimum.

 

[wwhitney]

The 16 sq ft scenario complies with the language of R806.2 Exception 2. It is easy to write language that requires the upper and lower ventilation areas to be balanced, but that was not done. If there is a technical reason the ventilation areas should be balanced, then the language requires revision.

Cheers, Wayne

 

[hepp527]

Your example of 16 sq,ft. ventilation for a 3000sq,ft. attic space equals 1/187.5 That is less than the minimum 1/150 and greater than the exception of 1/300; There is no guidance provided for this scenario. Given the near balance between upper and lower vents required by the exception which allows 1/300 it seems logical to apply the same formula to any amount of vent opening below the 1/150 minimum.

Agreed, logical. But after several round table discussions when I worked as a construction inspector and plans examiner, we could not find justification to "act as if" this were the intent. Therefore we allowed the mismatch as long as top and bottom met the requirements of whatever the "minimum" would have been. Then the plane were approved and stamped. When inspected during the build process, builders just rolled the dice and let the framers fight with the inspectors site by site. On each house something that should have been agreement was reached, but was more like a cease fire and the battle would continue on the next house.

I always felt this was an imperfect solution and wished the commentary addressed it.

 

[hepp527]

The 16 sq ft scenario complies with the language of R806.2 Exception 2. It is easy to write language that requires the upper and lower ventilation areas to be balanced, but that was not done. If there is a technical reason the ventilation areas should be balanced, then the language requires revision.

Cheers, Wayne

Agreed and all counts. But that doesn't answer the question, it only shifts it. After crawling through many moldy attics in the pacific northwest (attics that comply with the language but have molded regardless)...

Should this be revised? And if so, is the revision a "change" or is it a "clarification of what was there all along"?

 

[wwhitney]

Should this be revised?

I have no opinion, it is not clear to me that taking a compliant installation and adding additional low ventilation would negatively impact performance. Figuring out the answer to that would require a better understanding of the relevant physics than I have.

And if so, is the revision a "change" or is it a "clarification of what was there all along"?

Definitely the former, that is my point. The current language is not ambiguous as to your original question. The phrase "required ventilating area" refers to the result of the computation (area/300) and does not reflect the amount of ventilating area actually installed. Likewise for "required ventilation."

Cheers, Wayne

 

[steveray]

Agreed and all counts. But that doesn't answer the question, it only shifts it. After crawling through many moldy attics in the pacific northwest (attics that comply with the language but have molded regardless)...

Should this be revised? And if so, is the revision a "change" or is it a "clarification of what was there all along"?

You have until January 10th to submit your code change to ICC to get it to say what you want it to say...Looking forward to your public input.

 

[Joe.B]

After crawling through many moldy attics in the pacific northwest (attics that comply with the language but have molded regardless)...

I also live in the Pacific northwest and have seen lots of attic crawlspaces that were in really bad shape even though they technically meet code. I've also seen plenty that are just fine.

All the roofers up here add extra ventilation, way above code minimum. It's the way it is. The local jurisdiction could make amendments based on the specific climactic/geographic conditions, but I haven't seen one do that yet.

 

[ICE]

Some of the better roofing contractors add ventilation when appropriate. There is a theory that well ventilated attics result in a cooler substrate for the asphalt shingles which extends the life of the shingles.

 

[Sifu]

There is a theory that well ventilated attics result in a cooler substrate for the asphalt shingles which extends the life of the shingles.

This has been "confirmed" to me by both asphalt shingle reps and insurance reps. I do not claim to be an expert on why, but I am sure there is some sciency stuff behind it. Maybe hot roof=loss of adhesion? Not sure it holds too much water since you could fry an egg on it most days no matter how much ventilation is under it.

 

[ICE]

This has been "confirmed" to me by both asphalt shingle reps and insurance reps.

Same here. I wonder what effect radiant barrier has on asphalt shingles?

 

[bill1952]

The problem, at least in heating climates, is warm moist interior air condensing on cold roof sheathing. The moldy attics are because of poor air vapor barriers in the ceiling. The benefit of less than half the venting being high (for the 1/300 option) is that more high greats more negative pressure in the attic and exacerbates the problem, pulling more interior air through the ceiling bypasses. You can add venting all day long but if you're blowing warm moist air into attic, expect air problems.

All this made worse because there are lots of venting products to make and sell that are suppose to solve the problem, and much less profit motive in good air vapor barriers.

As far as the shingle life argument, I say BS. The difference between asphalt temperature is many fold greater between light and dark shingles than between be vented and unvented attics.

 

[Genduct]

1:150 rule does not require a 50:50 balance upper vs lower. 1:300 requires approx 50:50 balance.

So a 3000sf attic requires 10sf of venting IF 5 to 6sf are lower, and 4 to 5sf are upper (min 40% max 50% in upper third). So 8sf upper and 2sf lower breaks this rule. That much is clear. Also 2sf upper and 8sf lower breaks this rule. What about 4sf upper and 12sf lower?

In this example we break the min 40% requirement for the 1:300 rule, but use of the words "...not less than 40% or more than 50% of the REQUIRED ventilating area must be in the top 1/3..." muddies the water as follows:

using the 1:300 rule, 10 square feet of ventilators are REQUIRED. So if i exceed the minimum by providing 16 square feet of venting, but do not reach an amount that satisfies the 1:150 rule, do i need 40% to 50% of the "REQUIRED" 10 square feet to be in the upper third (which would make my "4sf upper/12sf lower" example perfectly acceptable), or do I need 40% to 50% of WHAT I ACTUALLY PROVIDE to be in the upper third (meaning i would need min 6.4sf upper, and therefore my 4:12 example is a violation)?

My suspicion is that the near 50/50 balance is required for speed of air movement (i.e. air flow relies on the "stack effect" at this ratio). At low venting ratios air speed is more critical since it is the only force present that can move water vapor out of the attic (I'm speculating here), so balance has to be maintained at all ratios below 1:150 to preserve the stack effect. Air moves upward and out the vent as it warms against yhe roof sheathing, pulling cold air in at the lowest opening to replace it. The stack effect is foiled if upper and lower are not in balance (same reason barely opening the door of a wood stove causes rapid air movement into the fire, but fully opening it slows it down. More isn't always more).

At 1:150, the stack effect is no longer required because there is enough vent area to allow the combined pressures of "diffusion" and the "ideal gas law" to move water vapor out of the attic and into the open air outside.

Thoughts?

You said: "it is the only force present that can move water vapor out of the attic (I'm speculating here)"
I believe you are not Replacing the moisture laden air, You are Diluting the attic air with Additional, drier Air

 

[Genduct]

The problem, at least in heating climates, is warm moist interior air condensing on cold roof sheathing. The moldy attics are because of poor air vapor barriers in the ceiling. The benefit of less than half the venting being high (for the 1/300 option) is that more high greats more negative pressure in the attic and exacerbates the problem, pulling more interior air through the ceiling bypasses. You can add venting all day long but if you're blowing warm moist air into attic, expect air problems.

All this made worse because there are lots of venting products to make and sell that are suppose to solve the problem, and much less profit motive in good air vapor barriers.

As far as the shingle life argument, I say BS. The difference between asphalt temperature is many fold greater between light and dark shingles than between be vented and unvented attics.

This example shows the limits of the code concerning Building Science!

IMHO, Your example is more likely in a wall cavity, not where a vented attic will have plenty of Outside, Cold, DRY Air to dilute air leaking through the imperfect, ceiling air barrier

 

[bill1952]

This example shows the limits of the code concerning Building Science!

IMHO, Your example is more likely in a wall cavity, not where a vented attic will have plenty of Outside, Cold, DRY Air to dilute air leaking through the imperfect, ceiling air barrier

Plenty of posts above where much more venting than required still has damp roofs.

 

[Genduct]

Plenty of posts above where much more venting than required still has damp roofs.

Yes, the situation might be a good example of vented vs sealed attics ( not unlike sealed crawlspaces) where the Outside Air has more moisture than the inside, so instead of diluting the moisture laden air, you are increasing the moisture.

Doesn't seem like that happens all the time and here is where we need to build assemblies that allow the moisture to escape and not get captured.

You in NJ like me are in the Mixed-Humid Climate Zone, the most Bldg Science Challenged Climate Zone

 

[bill1952]

Better air sealing at the thermal barrier would help more than anything else - both the attic moisture and energy issues - but there is not much profit motive in that so manufacturers, vendors, and contractors are not interested in it like they are vents.

BTW in NY, not NJ, on Canadian border. But not a lot different climate wise