Home Hardening Basics: Vents, Screens and Eaves

What is home hardening, and why does it matter?

Home hardening means changing the building so it resists ignition: the materials, the openings, and how they are put together. The University of Nevada, Reno Extension's Wildfire Home Retrofit Guide says homes are threatened in a wildfire by wind-blown embers, radiant heat and direct flame contact, and that 60 to 90 percent of home loss is due to embers. It calls for a coupled approach: the construction and the surrounding vegetation and combustibles have to be addressed together.

The Insurance Institute for Business & Home Safety (IBHS) says wind-blown embers are the principal cause of building ignitions during wildfires. Montana DNRC's landowner guide for west-central Montana makes the same point in plain terms: embers can easily enter a home through vents, and open windows and gaps under garage doors let embers in.

How do embers get into a house through vents?

Attics and crawl spaces have to breathe, so they have vents, and vents are openings. The UNR guide describes attic and crawl space vents as an entry point for embers that can ignite materials inside, such as stored boxes. IBHS tested vents on a rotating test building with wind from different directions and found that vent type, configuration and orientation significantly affect how many embers get in.

Common vent types include soffit or eave vents, gable-end vents, ridge vents and other roof vents, and foundation or crawl space vents. Each sits in a different place relative to wind and debris, which is why the fixes are similar but the priorities differ house to house.

What mesh size should vents have?

DNRC's guide says all vent openings should be covered with 1/8-inch corrosion-resistant metal mesh. The UNR guide gives the reasoning behind the sizes. Quarter-inch mesh lets many larger embers into the attic and crawl space and should be replaced or covered. One-eighth-inch mesh is the common choice because it balances maintenance with effectiveness. One-sixteenth-inch mesh keeps out more and smaller embers, which tend to burn out quickly once inside, but it clogs more easily, reduces airflow and needs more cleaning.

The guide adds two cautions: screening will not stop every ember, only reduce their size, and debris clogging a vent screen can itself become a source of embers if it is not cleaned. A finer screen you never clean is not an upgrade.

What are ember-resistant vents?

They are manufactured vents tested to resist both flames and embers rather than relying on mesh alone. The UNR guide notes that vents meeting the flame- and ember-resistant standard are listed on the California Office of the State Fire Marshal's Building Materials Listing Program website, and suggests them where defensible space is hard to maintain, where combustible materials are closer to the home, or where the siding is combustible.

Whether such vents are required in your situation depends on local building rules, which apply mainly to new construction and additions. We cover that in building in the WUI in Montana. For an existing home, they are an upgrade to discuss with a contractor, not an obligation.

Why are open eaves a weak point?

The UNR guide explains that there are two basic eave designs: open eaves, with exposed rafters, and soffited or boxed eaves. Open eaves are more vulnerable to flames, because heat builds up between the rafters and spreads sideways, increasing the chance fire finds a way into the attic. Vents in the blocking between rafters on open eaves are also more vulnerable to embers than soffit vents.

The guide's fixes: inspect open eaves for gaps where embers could lodge or pass through, screen all vents, fill other gaps with durable caulk, and consider enclosing the eaves to create a soffit. Caulking and screening are reachable for many owners. Enclosing eaves is carpentry.

What about windows, skylights and the garage door?

The UNR guide says the most vulnerable part of a window is the glass, which can crack and fall out under heat, and that tempered glass is three to four times more resistant to heat than ordinary annealed glass. Its recommendations: choose multi-pane windows with tempered glass when replacing, keep screens on all operable windows, and close windows when fire threatens. It notes domed plastic skylights are riskier than flat glass ones and that skylights should be closed before evacuating.

Garages store fuel, solvents and cardboard, and embers can get in under or around a poorly sealed door. The UNR guide recommends sealing the gap between the garage door and its frame, treating garage vents and windows like the house's, and adding a battery backup to the door opener so it can be opened or closed in a power outage. DNRC's guide also flags gaps under garage doors.

Does siding near the ground matter?

It does when there is fuel next to it. The UNR guide explains that combustible siding can ignite from flames or radiant heat, and from embers when they ignite something close to the house such as bark mulch or a woodpile, especially if the siding runs all the way to the ground. It describes a detail with about six inches of concrete foundation exposed and a gravel ground cover against it. It also says fire-retardant coatings and paints are not recommended for siding, because their performance degrades with weather.

Replacing siding is a construction project. Keeping the ground against it clear is not: that is the job of zone 0, the first five feet, where IBHS calls removing all combustibles the most effective defensible space action an owner can take.

Which of these can a homeowner do?

Several, with care. Replacing torn screens or adding 1/8-inch metal mesh behind accessible vents, caulking gaps in open eaves, checking the garage door seal, closing the flue in fire season and keeping cushions and boxes away from attic and crawl space vents are all within reach of a handy owner. The UNR guide specifically advises against storing combustibles like cardboard and newspapers near attic or crawl space vents.

Enclosing eaves, replacing vents with listed ember-resistant models, replacing windows or siding and reworking deck framing are contractor jobs. Anything on a roof or tall ladder is a fall risk. If you are not sure where to start, the options in free wildfire risk assessments can help you set priorities.

Can you cover vents when a fire is coming?

Only if there is plenty of time. The UNR guide says vent covers made ahead of time can be installed when wildfire threatens, and that this can be effective, but it takes time and should only be attempted if there is ample time to evacuate. Preparation becomes dangerous if it delays leaving. If you want this option, cut and label the covers now and store them where you can reach them, and treat them as a backup rather than the plan.

Closing up is simpler and faster. The guide says to close windows, skylights and garage doors when fire threatens. Our Ready, Set, Go guide has the household checklist, including when to stop preparing and go.

What does HJ do, and what do we not do?

We do not build or modify houses: no vents, siding, eaves, windows or roofing. For those, hire a licensed general contractor or the relevant trade. What we do is the landscape side of the same plan: clearing roofs and gutters of needles, cleaning out the first five feet, removing limbs over the roof, thinning and limbing trees, and replacing wood mulch near the house with rock or hardscape.

The two halves depend on each other. The UNR guide's own advice for siding, decks and fences comes back to keeping the ember-resistant zone clean. Fences and decks have their own guides: see wood fences attached to a house and decks and the space under them.

Where should you start?

Start with the openings and the ground next to them. Walk around the house with this guide and look at every vent, the eaves, the garage door seal and what is touching the siding. Fix screens and gaps first, clear the five feet next to the house, then plan the bigger upgrades over time. Our guide to how homes ignite explains why that order works, and the rest of the topics are in our Montana Wildfire Guide.

For the yard side of hardening, our fire mitigation service handles the near-structure cleanup and tree work. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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