Attic Ventilation Improvement

Question

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:

Gable vent:

Roof vents:

Attic fan:

Answer 1

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.

Answer 2

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.