How to Replace a Vent with an Ember-Resistant One

Why this matters

Swapping a vent looks like a twenty-minute job, and the physical swap is. What takes judgment is everything around it: whether the opening is really a ventilation vent or also feeds a gas appliance, how much airflow the space legally needs, whether the new vent's smaller open area still meets it, and whether the gap around the frame lets embers walk past the mesh you just paid for. Get those wrong and the house either loses the ember protection the customer asked for or loses ventilation it needs, and neither shows up until a fire season or a moldy crawlspace makes it obvious. Why embers matter and what the rules require is the sibling card on ember-resistant vents; this one is the procedure.

Before you start: the hazards at a foundation vent

The swap itself is low risk, but three things at a foundation or attic vent are not.

• Rodent droppings and nests. Crawlspace vents are a common rodent entry. Dried droppings disturbed by brushing or vacuuming can release hantavirus into the air. Follow CDC guidance: do not sweep or vacuum; ventilate, wet the material thoroughly with a disinfectant or a 1 part bleach to 10 parts water solution, wait 5 minutes, then wipe it up with gloves on and bag it. Heavy contamination is a respirator job, and for employees that means a respirator under a written program per 29 CFR 1910.134; without one, the heavy cleanup goes to a crew that has one.
• Old paint around the frame. On a house built before 1978, assume painted trim may contain lead. Prying and scraping it makes lead dust. Disturbing more than 20 square feet of exterior painted surface on a target house triggers the EPA Renovation, Repair and Painting rule (40 CFR 745) and a certified renovator; below that, still work wet, avoid dry scraping, and protect your own breathing per the OSHA construction lead standard (29 CFR 1926.62).
• Cutting masonry. Enlarging an opening in block or brick is a silica exposure. Under 29 CFR 1926.1153, use the Table 1 controls for the tool (water delivery or a shroud with a dust collector), and respiratory protection where the table requires it.

Step 1: Confirm what the opening actually does

Look inside before you order anything. Use a flashlight through the vent or from the crawlspace hatch, without entering.

Why first. A foundation vent is usually just underfloor ventilation. But if a gas furnace or water heater sits in the crawlspace and takes its combustion air from it, those vents are also combustion air openings, and the fuel gas code, as adopted (IFGC 304.10), sets screens on them no smaller than 1/4 inch mesh. Fitting 1/8 inch ember mesh there can starve a burner and make carbon monoxide.

What breaks if skipped. You install a compliant ember vent that creates a combustion problem. If the crawlspace holds a fuel-burning appliance, stop and route the combustion air question to whoever services the appliance before any vent is changed.

Step 2: Work out the airflow the space needs

Measure the underfloor area and calculate the required net free area (NFA), the open area air actually passes through.

The rule. The IRC, as adopted (R408.2 in the 2021 edition), requires 1 square foot of NFA for each 150 square feet of underfloor area, with one opening within 3 feet of each corner. Where the exposed earth is covered with a continuous Class I vapor retarder with joints lapped at least 6 inches, it may be reduced to 1 square foot per 1,500 square feet. That is a tenfold difference, and it usually decides whether an ember upgrade is possible at all.

What breaks if skipped. Fine mesh and ember baffles cut NFA. Without a target number you cannot tell whether the new vents still ventilate the space, and an under-ventilated crawlspace grows mold and rots sills on a timescale of years.

Step 3: Inventory the existing vents

Count them, measure each rough opening, find the NFA of what is there (from a label, or from the manufacturer's data if you can identify the product), and note the corner locations.

Why. You need to know whether the existing system was compliant to begin with. Many older houses were built to an earlier, looser rule. If the existing vents already fall short, the replacement is not making it worse, but it may make an existing shortfall unfixable without other work, and the customer needs to hear that before the vents are bought.

Step 4: Choose a vent that is actually ember-resistant

Pick either a vent listed to ASTM E2886, confirmed in the testing laboratory's online certification directory, or, where the local rule permits it (California's Chapter 7A and R337 do), noncombustible, corrosion-resistant mesh with openings at least 1/16 inch and no larger than 1/8 inch. Match the frame to the rough opening, and read the NFA printed on the label.

Why. "Ember-resistant" on packaging is a marketing claim. The listing is the test. Fine mesh of the right size but made of plastic or uncoated steel fails the noncombustible and corrosion-resistant conditions.

What breaks if skipped. You install a vent that the customer's insurer, or the inspector at sale, rejects, and the job is done twice.

Step 5: Remove the old vent without spreading what is behind it

Wet down droppings or debris first (see the hazard block above), cut sealant with a knife rather than prying painted trim off, and bag the old vent and any nest material at the opening.

Why. This is the moment the airborne hazards are released, not the moment you looked. A monitor or a look does not protect you; wetting and not sweeping does.

Step 6: Install and close every gap around the frame

Set the vent in a bed of sealant or with the manufacturer's gasket, fasten with corrosion-resistant fasteners, flash it into wood siding so water sheds over the flange, and seal the perimeter.

Why. An ember that cannot pass the mesh will pass a 1/4 inch gap between the frame and the block. The vent is only as good as its edges. California's rule and the E2886 listing both assume the vent is installed per its instructions, and a gap defeats both.

What breaks if skipped. Every ember-resistant vent on the house with a visible gap around it is, functionally, a vent with no mesh at all.

Step 7: Verify and record

From inside the crawlspace hatch, look at each new vent with the light outside: daylight through the mesh is correct, daylight around the frame is a gap to seal. Then record the product, its listing, its NFA, the count, the calculation, and photos before and after.

Why. The record is how the next tech, the insurer or a buyer's inspector knows the vents are listed and the space is still ventilated, without pulling one out.

Worked example: nine foundation vents on a crawlspace house

A 1950s house in a mapped California fire hazard zone, crawlspace foundation in concrete block. The owner wants all foundation vents upgraded. The water heater is in the garage and the furnace in the attic, confirmed in step 1, so no vent is a combustion air opening.

VENT WORKSHEET (NFA figures illustrative; use labels)

Underfloor area        30 ft x 45 ft = 1,350 sq ft
Required at 1/150      1,350 / 150 = 9.0 sq ft
                       = 1,296 sq in
Required at 1/1,500    1,350 / 1,500 = 0.9 sq ft
                       = 129.6 sq in
Existing vents         9 x 64 sq in = 576 sq in
New listed vents       9 x 38 sq in = 342 sq in
Ground cover found     6-mil poly, torn, about 60%
Corners within 3 ft    4 of 4

Reading it. The existing 576 square inches was already short of 1,296 under the 1/150 rule; the house was built to an older standard. The new vents, at 342 square inches, are short by 954 square inches under 1/150, and meeting it with this vent would take 1,296 divided by 38, or 35 vents, on a foundation with room for nine.

Under the 1/1,500 allowance, the new vents give 342 against 129.6 required, about 2.6 times the requirement. But the allowance requires a continuous Class I vapor retarder with lapped joints, and the existing 6-mil polyethylene, which is a Class I material, covers only about 60 percent of the ground and is torn. As found, the allowance does not apply.

The call. The upgrade works only paired with a new continuous ground cover. The office quotes both: new 6-mil or heavier polyethylene over the full crawlspace floor with joints lapped at least 6 inches, then the nine listed vents. The ground cover is a crawlspace entry job with its own hazards (rodent material, sharp debris, electrical runs, possible sewage), and it goes on the ticket as separate scope.

What changes the answer. If the owner declines the ground cover, the shop does not install the ember vents as a like-for-like swap and call it done, because that cuts ventilation by 576 minus 342, or 234 square inches, about 41 percent, in a space already short. The options are a vent with a higher listed NFA if one exists for that opening, or leaving the existing vents in place with a written note that the upgrade requires the ground cover. If the house is outside any adopted ember rule, the ventilation requirement still applies, so the same arithmetic governs.

The record this leaves

On the ticket: product, listing standard and directory confirmation, NFA per vent and total, underfloor area, the governing ratio and why (with the ground cover condition), corner count, before and after photos of each vent including the frame edge. And a line for the owner: brush the mesh at least once a year and after any nearby fire, never paint over it, and never block a vent to stop a draft.

References

• International Residential Code R408.2 (under-floor ventilation, 2021 edition), as adopted by your jurisdiction
• International Fuel Gas Code 304.10, combustion air screens, as adopted
• ASTM E2886, and California Building Code Chapter 7A and California Residential Code R337
• CDC guidance on cleaning up after rodents; 29 CFR 1926.62 (lead, construction); 40 CFR 745 (RRP); 29 CFR 1926.1153 (silica, construction)
• See related: Ember-Resistant Vents and Why Embers Burn Houses