Q&A – Vents on a 60-year-old ranch

When adding an addition to a 60-year-old ranch home, this homeowner wanted to make sure that they had the right ventilation in place. 

Joe, a homeowner from Conneticut, recently reached out to the AskARoofer experts for some advice about ventilating an addition they made on their 60-year-old ranch home. This is what Joe asked: 

I have a 60 year old ranch, it was built with gable vents. We put an addition on that makes the ranch an L shape in the late 90s. The addition had soffit and ridge vents. When I had the house sided the sider said I shouldn't have gable vents because I had ridge vents. At this point I have put soffit vents for 20 feet on one side and maybe 30 feet on the other.

I'm wondering if that is enough and if I should just not have ridge vents on the main part of the house and have the gable vents put back in. The house will be roofed within the year given a nasty hail event we had so I could just dump the ridge vents on the main part of the house at that point. I wouldn't mind paying someone that knows what they're doing to come and tell me what is best, just don't know who I'd try. The roofer that did the roof 5 years ago didn't mention is, just put in the ridge vents.

One of our experts, John Kenney from Cotney Consulting, responded to Joe, saying this:

Joe, I would not remove the ridge vents and go back to gable vents until the entire ventilation system is evaluated. The first question is whether you currently have enough low intake ventilation to support the ridge vents properly.

In a conventional vented attic, the preferred airflow path is low intake at the soffits and high exhaust near the ridge. Mixing gable vents and ridge vents can interfere with that pattern because air may enter through the gable and exit at the ridge instead of being drawn from the lower portions of the attic.

The amount of soffit ventilation you have cannot be determined by length alone. Twenty feet on one side and 30 feet on the other may or may not be enough depending on the specific soffit product and its published net free vent area.

A quick preliminary calculation is:

Attic floor area ÷ 300 = total net free ventilation area, when the 1/300 provisions apply.

For example, a 1,500-square-foot attic would require about 5 square feet, or 720 square inches, of total net free vent area. The system should then be balanced between low intake and high exhaust according to the applicable code and vent manufacturer requirements.

Because your house is now L-shaped, I would also make sure the original attic and the addition are truly connected in a way that allows unrestricted airflow. If they function as separate attic sections, each area may need to be evaluated independently.

Since you plan to reroof within the year, this is the right time to have the system checked. I would look for a roofing contractor experienced with attic ventilation, an independent roofing consultant or a qualified building-envelope or home-performance professional. Ask them to measure the attic area, calculate the actual net free intake and exhaust, verify that insulation is not blocking the soffits and confirm whether the ridge vent is properly sized and installed.

My recommendation is to calculate the system first. The ridge vent may not be the problem. The issue could simply be insufficient soffit intake and if that can be corrected, keeping the soffit-to-ridge ventilation system may be the better path than returning to gable ventilation.

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