Using hardware cloth in heating vents

Since moving into my house almost two years ago, I have had a continuous problem with mice.

We kill them, plug up the holes, everything is fine for a while, and then a new bunch move in.

From what I have been reading, I will probably never get rid of them totally because I live in an old house with a stone foundation and it is close to impossible to plug up all the holes.

Now they have gotten into the duct work. Most of the ducts on the first floor are in the floor.

So far there have been no signs of mice upstairs.

What I want to do is remove the grates, cover the opening with hardware cloth or copper mesh, and then replace the grates. This includes the air intake, which is about 18 inches by 3 feet.

Can I use mesh with a quarter inch squares? I read somewhere that holes that small will restrict air flow. However, since mice can fit through a half inch hole, I don’t know if it’s worth it to use mesh with half inch squares.
Can I use galvanized metal? I know galvanized metal gives off poisonous fumes when burning, but would the vents get hot enough for that to be a problem?

I’m no professional, but I had a little experience with this kind of thing while managing apartments. I don’t think it’s the mesh itself that restricts airflow - it’s more the likelihood that it traps bits of dust, fibers, etc that accumulate over time and restrict airflow. If you committed yourself to cleaning off the mesh regularly, you should be fine.

You might be better off with something that uses slats instead of a mesh. This item is the wrong size for your intake, but might be exactly what you want for the floor vents. (Either way, it illustrates what I’m talking about.) A mouse can’t get through the small spaces, and without the vertical obstructions of a mesh, you’re less likely to restrict air flow.

We vacuum almost everyday, so cleaning the mesh won’t be a problem.

Every floor vent in this house is a different size, I don’t know if what you linked to would work.

I hate floor vents, about as much as I hate outlets in the floor.

How are mice getting into the ducts? They should be sealed.

Screens cut down airflow a lot. It’s more than just the net opening size, each wire has turbulence around it that further reduces the effective opening. But with a careful selection of wire size and mesh per inch, you will be able to minimize it. I use McMaster-Carr for most of my industrial purchases, they have more wire then you can imagine. Here is a link, look through the tables under the type of screen you want (steel, stainless, etc), then pick an opening size you can live with and check the various screens for “open area”. You don’t need a very heavy wire for what you are doing. Dennis

If you own the house, you should start filling in holes in the foundation. If mice or rats get into a structure, they will keep trying to get into your living areas, anyplace they can find food. You might not get all the holes right away, but a can of spray foam will help a lot.

I don’t know if they are getting in that way but I’ve seen them run into the vents and come back out. I thought they would fall in and die, but after looking online it seems like this not an unusual problem.

The experts suggest taking of the grates, setting traps to catch the mice there, and then using hardware clothe so the mice can’t get back in. Also, the check the ducts for any cracks or holes where the mice could be getting in and seal them.

Thank you for the link.

The size is what I worry about because I know I read somewhere that quarter inch holes can restrict air flow, but if mice can fit through half inch holes, I’m not sure there’s a point to doing this.

I should be buying stock in Great Stuff and steel wool. I found one place where they pushed the steel wool out of the way and came right on in.

It’s been very frustrating, I’ll get rid of them for a few months and then they come back.

What about floor vent filters? They are made to go over/under floor vents so I suspect the airflow wouldn’t be too affected.

Would the mice just eat them though?

For the holes in the foundation, you can use mortar or concrete. You can buy small quantities at a hardware or home improvement store.

My house was originally all-electric, and the guys who added the gas line knocked a hole in the wall for the pipe. I noticed mouse traffic, so I patched it with a small batch.

Having a stone foundation means you must patch up the chinks in the wall once or twice a year. Tradition!

I wouldn’t worry about hardware cloth as long as it’s open enough. You already have a furnace filter (right?) that’s going to be far more restrictive than pretty much anything else you could cover a vent with.
But, off the top of my head, I can’t think of anywhere that a mouse could enter my HVAC system from. That’s where you need to start, don’t trap them in there, keep them out.
If I were you, I’d pick up some sheet metal and a pop rivet gun so if you do figure it out you can patch the hole right away. If you want, you could set some snap traps outside the hole to kill them instead of letting them die trapped inside, that’s up to you. But you need to seal up the system.
One place you might check is near the filter, if it just slides into a slot between the return duct and the furnace, they could be getting in there.

As for the stone foundation, I don’t know much about those. I probably wouldn’t bother with steel wool, it’s just going to rust away as soon as it rains (and what is that rust going to drip on?). I’ve heard of people using copper wool (brillo pads) for that reason. You could use Great Stuff as mentioned above or if you find a few holes, mix up a small batch of mortar and patch them up.
If there’s a lot of holes, it might be worth seeing if you can hire a mason/handyman to come and spend a few hours patching up the inside and outside.

You should also make sure you don’t have any rotted wood, sills, soffits etc. No open gable vents. I assume you don’t want to hire Orkin, but walk around the house like they would. Don’t just focus on the foundation. Take a really good look at the entire house for any place an animal can get in.

If you get along with cats, they can be your friend. Plus they are entertaining.

Keep food in mouse proof containers. The more the mice have to eat, the bigger litters they produce.

Mice eat the foam.

Buy the coarsest steel wool available. Using gloves ( cause it’s awfully sharp ), tear off hunks and jam them into any holes you find.

The coarse steel cuts into the jaws of the mice. They bleed, it hurts. They stop chewing. Worked for me.
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You should see if the furnace fan could be turned up to a higher speed if you install the screens. Other than that maybe one of these could help:

May I suggest a different approach - trap the mice and interrupt their breeding cycle. I have rid my outdoor shed of mice for years.

I used a plan like this (I put a mouse sized hole in the side of the bucket and leave the lid on) and used antifreeze in the bucket (make sure animals you care about can’t get to it). Apart from never freezing, it also delays the decomposition of the occupants and prevents horrendous stinking. Water works to kill the mice, but doesn’t stop the stench - which can be really, really bad.

In the first week I used the trap, I caught over 40 mice. I now get one or two a year, and they never build nests in my lawnmower anymore. I would swear they file in within hours of setting it up.

Absolutely seal the holes on the house’s exterior, but the trap makes a great second line of defense.

What you do with the dead mice once they’re in the bucket of antifreeze? You can’t just dump it outside or you’ll poison whatever dog, cat, or omnivorous/carnivorous wildlife are attracted to the anti-freeze.

I fish them out with a basket for frying food - I don’t know what its exact name is - with a wooden handle and wire tray. Put the dead mice in a plastic bag and toss out with the trash. If/when I clean the bucket out, I’ll usually do it the night before our trash is picked up so it’s not sitting in the barrel for any substantive amount of time.

I think they would eat through it.

My son went down and checked. He didn’t find any holes or cracks in the duct work, so they may not be coming in that way, but they are going down in the vents. I don’t want then getting in there at all, and I especially don’t want them getting in there an dying.
I have a service man coming out in a few weeks to do a service check and I’ll mention the problem to him.
I do have a filter, but I think there is a door. Horrible that I don’t know but my son does all that kind of stuff around here.