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Location: Portland, Oregon, USA area

House: 1972 single story, approx 1500 sf attic area.

When we bought the house the prior owners paid for attic mold remediation. I believe the mold was a result of inadequate attic ventilation.

Currently the house has soffit vents consisting of 19 sets of 3 2-inch holes and two gable vents (about 18"x24" each) as shown in the images below. There are currently 8 vents on the east slope of the roof. At some point someone added a powered attic fan venting on the west slope of the roof, but with the limited soffit vents I suspect this actually contributed to the problem instead of correcting it.

One contractor has proposed:

  • Removing the perforated 2x4s completely to open up the soffits (screened, of course)
  • Increasing the number of open segments at the soffit line from 19 to 20
  • Blocking the gable vents
  • Adding 6 more attic vents

From other questions on this site and research on the Internet, I understand I need about 10 sf free area (1:150) for proper ventilation. My concern is that the proposed changes will result in only about 7.8 sf intake free area ( (15" x 3.75" x 20) / 144).

Is the proposed plan sufficient, or should I request the number of soffit openings be increased to 26 (10.15 sf)?

Soffit vents:

Soffit vents (19)

Gable vent:

Gable vents (2)

Roof vents:

Roof vents (8)

Attic fan:

enter image description here

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    I would think the powered attic fan is making things much better. It should be temperature activated and come on at 90+ F if it is running when cold and raining it would make things worse. I installed One of the powered vents in My first Corvallis home and it also helped with my AC $. Since then I have had 4 homes From Corvallis to Dexter Or. Now this is one of the first improvements I add if there is not one already there. – Ed Beal May 17 '16 at 16:55
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The gable vents are short-circuiting the ventilation. The holes appear to be adequate, but as there's a shorter path of less resistance from in the gable vents and out the ridge vents, the holes don't draw very much air.

I'd temporarily cover the gable vents from the inside and see how things change. Maybe take temperature and humidity readings at intervals before and after doing so.

Bottom line: It's rarely a good idea to mix gable and ridge venting.

As to your actual question, you'll need to weigh the cost of achieving 100% of the recommended area vs. 78%. Considering that most standards have some degree of overage built in, 78% is probably adequate, but that's a matter of opinion.

  • You say "the holes appear to be adequate", but the soffit vents amount to only 2.5sf free area which, according to everything I've read, is woefully inadequate. BTW, there's no ridge venting involved here. I agree the gable vents are probably a bad idea. – Jim Garrison May 16 '16 at 20:30
  • Ok, roof vents near the ridge. – isherwood May 16 '16 at 20:31
  • The holes may not meet modern standards for area, but they're substantial. I'd expect that they clear heat and moisture to the point that it's not a major problem. – isherwood May 16 '16 at 20:32
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I don't know why this question popped up at the top of my list after it had been asked 10 months prior, but it's an interesting topic to me.

You're right, in your area the Code requires 1/150 the area of your attic to be in ventilation square feet, if 50% - 80% is in the top half of the attic. Now, I see this as a MINIMUM not a RECOMMENDATION. Also, the Code says "cross ventilation".

When calculating the "free area" of your vents (particularly the round vents) you need to consider the insect screens. Actually, you have significantly less "cross ventilation" when you consider the insect screen.

I'd recommend the following: 1) I suspect attic insulation is blocking your "cross ventilation". I'd recommend you check your attic to see if the insulation has "expanded" along the eaves and are blocking the air pathway. Also common in your area are cardboard insulation blocks along the eaves that keep the airways open. You can add them from the attic...it's difficult but doable. 2) Reduce the area assigned to the round vents by 20% due to the insect screen and recalculate the amount of ventilation provided. 3) Likewise, I'd recalculate the ventilation in the upper one-half to see if it falls within the 50-80% range. If it's more than the 50-80% amount (and I'm sure it is) then add ventilation at the eaves. I would NOT reduce attic ventilation by blocking gable end vents or removing roof top vents, etc. 4) By adding additional eave venting you'll increase the overall amount of ventilation, which will increase your venting to the MINIMUM amount.

  • Oops, (A) the 50-80% rule applies when located a minimum of 3' above the eaves...not when in the upper one-half, and (B) the required area is reduced to 1/300 when the 50-80% rule is used, and (C) the required area can be reduced to 1/300 when a vapor barrier not exceeding 1 perm is installed on warm side of insulation.Sorry about that. – Lee Sam Mar 31 '17 at 19:47

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