For Good Attic Ventilation, Use Ridge And Eave Vents

HOME CLINIC: A house was built next to ours. The contractor installed something called a ridge vent when he put on the roof. What is this for? Could I or should I install one on my house, which was built in the '40s? And if so, which type is best?

R.C., Woodinville

DEAR R.C.: Ridge vents are an excellent way to achieve attic ventilation, but they are only half the system. You also need eave vents. Since your home was built in the 1940s, it may or may not have adequate attic ventilation. The building codes and standard practices in your area may not have called for it at the time.

The primary purposes of attic ventilation are to keep the attic cool during warm periods and to vent moisture that could accumulate in the attic. In both cases, the objective is to protect and extend the life of the structural components of your roof.

This is done by placing screened vent holes through and around the roof structure. Optimum ventilation is achieved when half of the vents are placed high (on or near the ridge) and half are placed low (in or near the eaves). The total vent area should be one square foot of net free area for every 150 square feet of ceiling.

For new construction, additions and remodels (where the roof is being redone), I would highly recommend ridge vents as an attic ventilation component. But in your case, you should ask yourself first: Do I already have attic ventilation? Is the current attic ventilation system working?

Inadequate attic ventilation is evident if you notice any heat or moisture-damage problems in the roofing material or structure (i.e., curling shingles, dry rot, mold or moisture stains). Sometimes by design or accident, the attic ventilation in older homes is just fine.

If you decide that you want to add ridge vents to your attic ventilation system, two types are typically available. One is a metal piece that is placed on top of the ridge, replacing the roofing material cap. This vent can run the full or partial length of the ridge and has a screened vent slot running the length of the vent.

The other type is a plastic material that is placed over the ridge, and the roofing material cap is placed on top and nailed through to the roof. The plastic ridge vent should run the full length of the ridge. The plastic material is a honeycomb type of material with about 1/16-inch-diameter holes. Both types do the job.

The metal ridge vent has more free vent area per linear foot (more ventilation), and the plastic ridge vent is less noticeable, fitting in with roofing materials and colors.

It is also important to check the eave vents and be sure they are clear. Eave vents should be baffled to prevent blockage by attic insulation. Another worthwhile precaution is to seal all attic penetrations (around light and fan fixtures, attic hatches). Warm air from the house is the prime carrier of moisture into the attic.

For the final choice of which type of ventilation system will best suit your home, consult a roofing contractor and see what is available in your area, and what experience the roofer has with different vents. For a free Energy Office factsheet on Attic and Crawlspace Ventilation, call the Energy Hotline, 1-800-962-9731.

Written by John Doyle of the Education and Information Network of the Washington State Energy Office. This information can be made available to accommodate people with disabilities; please call 1-800-962-9731 (Voice and TDD).