Are There Mice in Your Attic or HVAC System?
It's 2 am; do you know what that pitter-patter sound is coming from your ceiling? Do you think you have mice in your HVAC system? You've come to the right place.
This article will cover the following:
- three pieces of evidence that signify you have a mice infestation in your attic or HVAC system
- how mice get into your attic in the first place
- the dangers of vacuuming the ducts and why using duct tape is not a good idea
- the lack of rodent-proof insulation
- how my family dealt with our mice infestation
- the risks that come with having an infestation and why it's important to call a professional
- which type of mousetrap you should use in your attic
Evidence of Mice in the HVAC System
I was surprised to hear mice can survive living inside the insulation. After all, fiberglass insulation is hazardous to humans; however, mice actually thrive in this stuff.
In our first home—30 years ago—we had a crawl space where mice came up at night through the heater vent. I followed a trail of mice poop along the floorboards that led me to a nest under the register. It took three weeks and half a jar of peanut butter to outsmart Mighty Mouse and his gang. Mobile or manufactured homes have increased exposure as well (adding a skirt won’t help until you have some sort of defense system in place).
You Might Smell an Odor
How do you know those little critters are up there in the ducts, besides the sounds you might be hearing? Your nose may have also noticed. Notwithstanding the odors you may smell when the air conditioning or heating system is off, the putrid smells may be more pungent when your system is on. Mice urinate in tiny amounts, but it does produce an odor. Rats on the other hand—a whole different stench—drink so much water that if they urinate above the ceiling, it will eventually produce a visible yellowish brown stain. And those nasty rat odors can travel through the ducts as well.
Blowflies Appear Out of Nowhere
A sudden infestation of insects may signal you to mice squatters too. Do you suddenly have blowflies (moths and beetles included) swarming around in the dead of winter? When I saw multiple blowflies coming from the vent, I knew there was something going on.
You Notice a Mouse Nest
Have you ever seen a mouse nest? It consists of a ball of loosely shredded material all bound together, two to six inches in size. But since we are talking about the attic, we are looking for nests probably made with insulation. Mice eat it, pee in it, live in it, and have babies in it. They drag it all around the place and tunnel under it too. Upon inspection, you may not even see the evidence because they tunnel in so far.
How Many Mice Are You Dealing With?
Sprinkle a little talcum powder along the edge of the walls where you think they might scurry in, then check the next morning. How many little footprints do you see? This is will give you an idea of the size of your infestation.
How Do Mice Get Into the Attic?
The older a structure, the more susceptible it becomes to rodent infestation. However, new structures are not immune. Insulation, spray foams, tapes, and other synthetic seals or collars are not absolutely rodent-proof either. Special precautions need to be taken when the heating, ventilation, and air-conditioning ducts become infested with mice.
All kinds of critters, including mice, can enter through crawl spaces under buildings or even foundations on slabs. They also sneak through the gaps left by connections near plumbing, sewer systems, pipes, phone lines, cable lines, electrical lines, meters, fireplaces, chimneys, wood stove pipes, gas connections, and washer and dryer vents. I’ve even seen mice in our circuit breaker panel. Another place to check is your HVAC condenser because mice will nest and chew wires there as well.
Where do mice go after they have gotten past the entry points? You might not want to know this, especially if you are easily freaked, but mice can be found in the following places:
- inside the frames of the walls
- inside pantries
- inside closets
- in the attic
- in the basement
- anyplace that they can fit in, especially someplace within 35 feet of a food source
Once mice or rats enter the attic, they can travel from room to room, or if it's an apartment, unit to unit. Now, I hear you thinking, "Is that what I've been hearing?"
Ever hear things scurrying about above you in the wee hours of the morning? Instead of freaking out, realize this, you can’t ignore it. Some people get used to the sounds and do nothing about it, thinking it’s either their imagination or just one mouse. You don’t want to pull down that box of your most prized holiday Christmas ornaments from the attic only to find that Stuart Little's cousin chewed on them.
Mice Climb Vertical Walls Like Spider-Man
To Vacuum or Not to Vacuum?
My HVAC tech got a service call from a homeowner who complained of a foul smell that got worse when the heat was turned on; she had paid an HVAC company to professionally vacuum the ducts a few days before. While the vacuuming may have helped, they weren’t very thorough. Dead mice still littered the ducts; the mice had eaten poison set out by the homeowner a few weeks earlier, which caused them to die but not in a good final resting place.
Some of the rotting carcasses breed bugs, possibly fleas as well. This is just one of the hazards of mouse poison; you never know where the mice will die. And to make matters worse, after believing the vacuum helped, every time the air system came on, she was still breathing feces, urine, rotting rodent carcass, mouse dander, possible mold, funky proteins, bacteria and other accompanying bugs. If you are susceptible to allergies, beware. This is not a DIY job, as the ducts need to be cleaned and disinfected as well.
Some HVAC professionals and mice exterminators say cleaning your ducts as a preventative measure has shown inconclusive and more to the point, for the DIYer, vacuuming yourself can cause the situation to worsen by breaking down the chemicals into smaller particles and you wind up inhaling them as you clean. A double-edged sword, damned if you do and damned if you don’t.
Does the term volatile organic compound sound scary? It should. VOC is an industry term for all those nasty things mice (or rodents in general) harbor, including some that the EPA considers harmful to humans.
Duct Tape Is Not for Ducts
Hardware stores carry several different insulation sealing kits for metal, plastic, or flex ducts. Here are some questions to ponder while you are up the attic poking around:
- Do you repair the insulation?
- Do you replace sections?
- Do you rip it all up and start from scratch?
- Is just taping with duct tape helpful?
Makes sense to use duct tape for ducts, but duct tape is the LEAST effective tape for ductwork! What a surprise.
Does 100% Rodent-Proof Insulation Exist?
I have been writing about rodent deterrents since 2011. Besides the usual rat traps, mouse traps, and assorted poisons, I have always been on the hunt for rodent-proof insulation.
Mice and rats love nesting in your insulation if they can find a nice quiet spot to call home, but if it happens to be a little busy for them, they will just take some of that insulation with them to build a nest elsewhere in your attic, walls, garage, or closets. Ever since one of my sons moved out of his bedroom upstairs, the attic above that room has been a little too noisy for my liking!
Researchers have yet to discover an insulating material that will act as a rodent deterrent 100% of the time. There is a product that advertises itself as "rodent-proof" and consists of a reflective type insulation, constructed with a sort of metalized film and nylon combination instead of the more common insulation types. I studied the site some more and found a rodent damage disclaimer on there. So, while this new product may be beneficial and perhaps better than products builders currently use, it's still not 100% rodent-proof.
How My Son Figured Out My Mice Problem
I’ve read two statements in HVAC forums: Duct problems are easier to find than to fix, and duct problems are easier to fix than to find. Well, guys, make up your mind. My son, who is an experienced technician, says they are easier to find than to fix because the damage may be hard to reach. We initiated our investigation without his usual tools because he just stopped by to say hi.
He used a flashlight and a 3m dust mask. I peeked my head into the attic entry and let him do all the dirty work. I gave him a spray bottle full of water to keep his hand wet for the duration of testing. With the HVAC powered up and pressurized, he dampened his hand with the spray bottle and ran it above and along the ducts, feeling for air leaks. I was worried he might cut his hand, as there are sharp things around, but he’s a pro, so I guess I should quit mothering. It seemed to me that this may not be the safest method, but again, I am an observer. He does normally use a smoke tool that's made just for this particular inspection, but they are kind of pricey for a one-time test.
What he found was leaks in the junctions, chewed seals, a chewed hole through some plastic part of the duct, insulation in the ducts (nests), and tunnels inside the insulation that we might have missed had we done it on our own. A mouse family was discovered in the end duct where little air flowed, along with mice urine and feces nearby.
Take note of the ducts you encounter as there are several types of ducts used: metal, flex, and plastic. Flex ducting is more common than metal, but metal is more expensive. Plastic is fairly new and not as desired as metal or flex.
When to Call a Professional
I’m not well-versed in the lingo of HVAC and being around it, but I do know that the ducts and return system that sends air to your rooms need to be tightly sealed or you are just leaking money as well as air.
Besides the damage to the insulation, mice chew wires and break seals and junctions. As my HVAC tech explained to me, damage from mice causes leaks in the ductwork which causes the pressurized air to escape. This can cost you plenty in energy bills or can even cause your central heating/air system to not work at all.
I'm lucky to have an HVAC tech/electrician in the family, but I highly recommend calling in a professional if you think you have found evidence of rodent damage (maybe even a plumber if you’ve found damage in that category). I stress this because unless you know what you are doing, you may be doing more harm than good—to your house or yourself. Remember that if you call a professional, be on guard and check references.
Electrical wires that have been gnawed by rodents could be a real fire hazard; if the wire has exposed copper, it could cause a spark leading to a house fire. Just taping it over with electrical tape may not be the solution, as the entire section of the cable may need replacing. This is not to be taken lightly; I read some news articles about rodents being the cause of house fires and another one about a mice-induced fire at a cat shelter.
Different Types of Mouse Traps
- Electronic mouse traps, set along the edge of the areas, are useful for maintaining control once the ducts are cleaned and repaired. Make sure you check them often, so they can be disposed of quickly.
- As much as I dislike snap traps, they do serve a purpose, especially when you run out of money buying electronic traps. Put them in the vent and tie a string so you can pull them out, but be careful of the droppings. Wear gloves and a mask.
- The CDC does not recommend glue traps since the mice urinate and poo in the traps, which can spread Hantavirus, which is just one of the 35 to 60 direct and indirect diseases caused by rodents.
I am in no way an expert in the fields I mention. I am only an observer, homeowner, and researcher because of my own mouse problems. Please call a professional if you have ANY question whatsoever about any of the situations I write about. One of the reasons I chose rodents as a topic is because I never took mice problems seriously. What I found in my search for the best mouse trap blew me away, grossed me out, and freaked me out.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2011 Pam Valentine