Your services do not end when the inspection does
Your services do not end when the inspection does
As wildfires become more common, protecting a house involves several layers of defense. The first is maintaining defensible space around the property. The second is hardening the house itself to keep embers out of attics and crawlspaces. Most older houses don't have ember-resistant mesh on their vent screens, and some have no screening at all.
1. Risks from Older Venting Systems
Crawlspace vents have typically used 1/4-inch steel mesh, and attic vents 1/8-inch steel mesh. Home inspectors frequently find damaged attic and crawlspace screens. The damage may be from animals seeking entry or contractors cutting screens to run pipes and conduit. The large openings, either from damage or 1/4-inch mesh, will not stop embers from passing through and collecting on the other side of the screen. If enough embers accumulate against combustible material, such as stored boxes or kraft paper vapor barrier on insulation batts, they can generate enough heat to start a fire. The goal is to reduce or prevent embers from reaching the crawlspace or attic, which in turn reduces the chance of igniting anything on the other side.
In older houses, it is also not unusual to find vent openings with no screening at all. Large wood lattice covers over gable vents or crawlspace vents with wood lattice and nothing behind them are common in some older houses from the 1920s and 30s. Missing screening easily lets pests and water reach the interior, and embers along with them.
The vent hardware itself can be a weak point even when undamaged. On houses with retrofit vinyl or aluminum siding, you may find vents with plastic or aluminum louvers. Neither material handles high heat well. Plastic will melt, and aluminum may deform or melt if the fire gets close enough. Plastic dryer exhaust hoods deserve a mention here too. Many use thin plastic louvers that swing open when the dryer runs, and they will melt or deform readily during a fire. Replacing the plastic exhaust hoods with metal will reduce the risk of ember entry.
2. New Vents and Ember-Resistant Mesh
New vents and screening need to be tested to ASTM E2886 and approved by the State Fire Marshal for use in WUI areas. Under current building codes, these screens must have openings between 1/16 inch and 1/8 inch. Anything larger than 1/8 inch allows embers to pass through. Below 1/16 inch, air flow drops off enough that the vent stops functioning as a vent. Even at 1/16 inch, the reduced open area often means more or larger vents are needed to meet ventilation requirements. The mesh must also be made of a noncombustible, corrosion-resistant metal like stainless steel or galvanized steel to withstand high heat. Mesh size alone does not make a vent compliant. The assembly has to be tested and listed, which is why an older 1/8-inch attic screen is not the same as a WUI-rated vent.
Many of the newer vents do more than tighten the mesh. Some use baffles or a honeycomb matrix to keep wind from carrying embers straight through from the exterior to the interior. Others use an intumescent coating that swells when heated and closes off the vent opening entirely. The coating only reacts to fire-level heat, so it has no effect on how the vent breathes day to day.
3. Replacement Vent Considerations
The tighter mesh in these newer vents affects air flow in two ways. First, the smaller openings reduce net free area (NFA); a 1/16-inch mesh may drop air flow by half compared to a standard 1/4-inch mesh. This often means a homeowner cannot simply swap mesh sizes; they may need to install more vents, add mechanical ventilation, or enlarge the existing ones to satisfy standard attic ventilation building codes (such as the 1/150 rule). If your attic or crawlspace gets too hot or stays damp, you may need to improve your ventilation.
Second, finer mesh clogs faster. Dust, dirt, and other airborne debris collect in the smaller openings, so these vents may need to be brushed off periodically to maintain adequate air flow.
4. Where to Start
Homeowners can begin by looking at their own vents. Walk the perimeter and check the crawlspace and eave vents for holes, gaps, and screens that have pulled loose. Gable vents are harder to reach, so if a vent cannot be seen safely from the ground, that is the time to contact a contractor or home inspector.
A limited budget may determine the order of work. Damaged and missing screens come first, since an open vent offers no protection at all. After that, prioritize the downhill and windward sides of the house, since that is where embers are most likely to arrive. Most importantly when installing new screens, seal around the vent frame. A compliant vent screen is useless if there is a large gap in the surrounding caulk or framing.
5. Conclusion
The focus here has been on improving the fire hardness of houses in WUI areas by addressing damaged or missing screens. We recommend obtaining area-specific advice from the local fire marshal or WUI inspector.
Upgrading attic and crawlspace screens has value in dense neighborhoods too, particularly ones with older housing stock. As noted above, many older houses have large gable vent openings that were never screened. If a neighbor's house catches fire and releases a shower of embers, those embers can be drawn into an unscreened attic and start the next house burning. Screening those openings may be one of the simpler ways to keep a fire from jumping house to house.
If you're buying or selling in the Bay Area and want conditions like this checked in person, schedule an ASHI-certified inspection with Tarkington Home Inspections.
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