How to Screen Foundation and Gable Vents

Why this matters

Foundation vents and gable vents are holes the house is supposed to have. They let a vented crawlspace and a vented attic dry out, so the one thing you cannot do is close them. What you can do is cover them with a mesh that stops rodents and keeps the air moving, and that is a job tradespeople get wrong in both directions: a torn insect screen left in place that mice walk through, or a well-meaning seal-up with sheet metal and foam that rots a crawlspace in two seasons. This card owns screening a vent so it is rodent-tight and still a vent. The material and gap rules come from 'Rodent Exclusion Materials and Opening Sizes': a mouse passes about 1/4 inch and a rat about 1/2 inch, so the screen is 1/4 inch galvanized hardware cloth or a fitted metal vent screen with openings no larger.

First question: is this a vent that has to stay open?

Answer it before you measure anything.

Vented crawlspace. Foundation vents are the crawl's ventilation. The International Residential Code, in section R408 as adopted by your jurisdiction, sets a minimum net vent area for a vented crawl and requires the openings to be covered with a corrosion-resistant material whose least opening dimension does not exceed 1/4 inch, listing hardware cloth of a minimum wire size among the acceptable coverings (check the current edition for the full list). A 1/4 inch hardware cloth screen therefore satisfies the code covering and the mouse threshold at once. Your job is to keep every vent opening at its full size; whether the crawl has enough vents is the building's question, not yours, and you record it rather than fix it.

Sealed or conditioned crawlspace. A crawl with a ground liner sealed up the walls, insulated foundation walls and no working vents may have been built or converted to be closed. Those vents are often already blocked on purpose. Do not open them, and do not decide on your own to close vents on a vented crawl to "encapsulate" it; that is a moisture and building-science decision for the owner and a contractor. Record what you found.

Seasonal vent covers. Some owners block foundation vents for winter with foam or covers. The screen goes behind or within the vent opening so the owner's seasonal cover still fits over the outside.

Gable vents. An attic gable vent is part of the attic ventilation path, and the IRC's attic ventilation section, R806 as adopted, similarly caps the least dimension of the covering on vent openings at 1/4 inch. Before you cover one, look behind it from the attic: a powered gable fan, a bat roost, a squirrel nest or a wasp nest changes the job (see below).

Roof penetrations route out. Ridge vents, box vents, turbines and plumbing stacks are roof work: flashing and shingles are the roofer's, and a fastener through the wrong place is a leak you now own. Record them and refer.

Hazards at the vent, step by step

• Silica when you drill masonry. Fastening into a block or poured foundation means drilling. Drilling releases crystalline silica dust, which is an inhalation hazard: use a drill with a dust-extracting shroud or a wet method, stand upwind, wear safety glasses, and follow the silica standard that covers the work (29 CFR 1926.1153 Table 1 where it is construction, 29 CFR 1910.1053 in general industry). A respirator, if the controls call for one, is worn under a written program per 29 CFR 1910.134.
• Cut edges. Snipping hardware cloth leaves needle-sharp wire ends. Cut-resistant gloves and safety glasses for every cut, and fold the cut edges under so nothing faces a hand or a pet.
• Ladders at a gable. The ladder goes on firm level ground at the right angle, extends past the work point where you need to step off, and you keep three points of contact (29 CFR 1910.23 for general industry work, 29 CFR 1926.1053 for construction). A second-story gable you cannot reach that way is a lift or a scheduled two-person job, not a stretch.
• Inside the attic. Step only on framing, watch for roofing nails through the deck at head height, and limit time in a hot attic. Do not disturb loose gray-brown pebble-like fill insulation that may be vermiculite, which can contain asbestos; work around it, record it, and leave it for a licensed abatement contractor.
• Powered gable fan. Turn it off at its disconnect or breaker, verify with a tester proven on a known live source before and after that the fan circuit is dead (the live-dead-live check in NFPA 70E, current edition), and keep hands out of the blades until it is. Turn it back on and watch it start when you are done.
• Animals behind the vent. Wasps first: look before you reach, and treat or remove the nest per its label before working. Bats in a gable are a wildlife job: many states protect bats and restrict exclusion during the maternity season, so check your state wildlife agency and route it to a wildlife control operator; never screen a vent with bats or squirrels still inside.

Steps

Start state. You need 1/4 inch galvanized hardware cloth (the heavier wire your supplier stocks for exterior exclusion) or factory metal vent screens, snips, a tape, corrosion-resistant screws with washers (stainless or galvanized; concrete screws for masonry), a drill with a dust shroud, the 1/4 and 1/2 inch drill bits used as go gauges, and the camera.

1. Walk every vent and record each one. Location, type, size of the opening, the condition of what is there now, and whether the 1/4 inch shank passes the existing screen or the frame. Reason: screening four vents of eight leaves four open, and the crawl does not care which four you did. Skip it and the record cannot tell the next tech what is done.

2. Measure the opening and cut the screen larger than the hole. Measure the clear opening, then cut hardware cloth about 1 inch larger on every side so the fasteners land in solid material around the opening, not in the opening. Fold the cut edges under where they will be exposed. Reason: screen cut to the hole has nowhere to fasten except the vent frame itself, which is often thin, cracked or plastic. If the vent is a stock size, a fitted metal vent screen made for it is neater and faster than cut cloth.

3. Clean out the opening. Remove torn insect screen, nest material, leaves and old foam from the vent and the opening behind it. Gloves on, and do not sweep droppings dry. Reason: a screen fastened over a nest is a screen with a mouse behind it, and debris left against the screen blocks the airflow you are supposed to keep.

4. Fasten the screen over or behind the opening. On a foundation vent, set the screen behind the vent grille where you can reach it, or over the outside face, and fasten every 3 to 4 inches around the perimeter with screws and washers into the foundation or the framing around it, not into the vent's own louvers. On a gable vent, fasten the screen to the framing behind the louvers from inside the attic, or over the face from outside. Staples alone pull out as wood moves; use screws with washers or metal strapping. Acceptance: the screen is taut, covers the whole opening, and the 1/4 inch shank does not enter anywhere around the perimeter. Skip the close spacing and a rodent pushes a corner of the screen in and walks around it.

Cut through the wall, a foundation vent screened from behind looks like this:

  outside           foundation      crawlspace side
                    wall
                |############|
                |############|o  screw + washer
  owner's       |            |*  into the block
  seasonal      | / / / / /  |*
  cover fits    |   grille   |*  1/4 in hardware
  over this --> |   louvers  |*  cloth, taut, behind
  face          | / / / / /  |*  the grille
                |############|o  1 in past the opening
                |############|

1. Check you have not reduced the airflow. Look through the vent from outside. The screen should cover the opening without anything else narrowing it: no framing, flashing, sealant or folded cloth intruding into the clear area, no foam stuffed behind it. Where the vent has an operable damper or louver, it still moves. Reason: hardware cloth leaves most of the opening clear by design; a strip of wood or a wad of foam added to fill a gap is what actually chokes a vent. If a gap between the vent frame and the foundation needs closing, close it with mesh and sealant or mortar in the gap itself, per 'How to Seal Pipe and Utility Penetrations With Mesh', not with something across the opening.

2. Restore anything you switched off and photograph the result. Gable fan back on and running; seasonal covers back where the owner keeps them; a before-and-after photo of each vent on the ticket.

Finish test

At every vent on the record: the screen is taut and covers the full opening, the 1/4 inch shank enters nowhere at the frame or the screen edge, no fastener is in the louvers, and nothing but the screen sits in the clear opening. On the next visit, no fresh gnawing at the screen edges and no droppings inside the crawl entry.

Worked example: a 1960s crawlspace house with a rat history

The account: Norway rat activity in the crawl, runways along the sill, a vented crawlspace with no ground liner. The walk records eight foundation vents and two gable vents.

Counting it out. Ten vents recorded. Seven get screened (F1 to F5, G1, G2). One gets screened plus a mortar repair at the frame (F6). One gets screened after the foam comes out (F7). One needs nothing (F8). That is nine vents worked and one passed as found.

Why F7 mattered. The foam the owner stuffed in for winter had been tunneled; it was the one vent blocked to air and open to rats at the same time. The tech removes it, screens the opening, and explains that the foam both stopped the crawl drying and failed as a barrier. If the owner wants winter covers, they fit over the outside of the vent and come off in spring.

F6, drilled masonry. Concrete screws with washers every 3 to 4 inches into the block around the screen, drilled with the dust shroud on, tech upwind. The mortar, mixed from a small bag outdoors with gloves and glasses on (wet mortar is alkaline enough to burn skin, and the dry mix is dusty), fills the cracked frame bottom and is the barrier for that gap; the screen alone would have left the rats a route under the frame.

G2. Fan breaker off, tester proven on a live outlet, fan circuit read dead, tester re-proven live. Cloth fastened to the framing behind the louvers, clear of the blade path. Breaker on, fan starts and runs without striking anything.

What went on the record beyond the screening. No ground liner in a crawl with damp soil, recorded as a conducive condition for the customer, per 'Moisture and Conducive Condition Report'. No roof vents were touched.

Follow-up. Rat sign in the crawl falls off over the next two services as the interior trapping continues; no gnawing at any screen edge.

What changes the method

• A mouse-only account. Same screen, same 1/4 inch openings and the same 1/4 inch shank test at every edge. Only the survey reading changes: a frame gap the 1/2 inch shank does not enter is still a mouse route, so survey with the 1/4 inch shank too.
• Brick veneer weep holes at the foundation line. These are not vents. They drain the wall and take inserts made for weeps, never hardware cloth fastened over them with sealant, per the materials card.
• Plastic foundation vents gone brittle. Replace the vent with a metal one or fasten the screen to the foundation behind it; screws into brittle plastic crack it and the screen falls off.

References

• See related: Rodent Exclusion Materials and Opening Sizes
• See related: How to Seal Pipe and Utility Penetrations With Mesh
• See related: Moisture and Conducive Condition Report
• International Residential Code sections R408 (under-floor ventilation) and R806 (roof ventilation), as adopted by your jurisdiction; check the current edition
• 29 CFR 1926.1153 and 29 CFR 1910.1053 (respirable crystalline silica); 29 CFR 1910.23 and 1926.1053 (ladders)