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Ask the Contractor, Do You Have Resident Bats?

Ask the contractor
By David McNutt
Published: August 20, 2010
Editor’s Note: This is the second installment of a new Cabin Life feature. Send your questions to: Ask the Contractor, Cabin Life Magazine, 1001 E. Ninth St., Duluth, MN 55805. Or e-mail:, with the subject line: Ask the Contractor.

Problem: We have bats in our cabin. Help!
Solution: As a builder and owner of a lakehome, I have some firsthand bat experiences. Our business gets many calls when customers hear things in their soffits or even worse – a bat flying around in the house at night. These are the first (and in the latter case, obvious) clues that you could have a bat or bats moving in.
Bats can be a nuisance and if left uncontrolled can cause damage, usually in a non-living space like the attic. If they move in and the colony becomes large, you’re faced with a major and unpleasant task: evicting the bats and cleaning up behind them.
On the other hand, bats do serve a great purpose for the natural world. The most common bat in the northern states is the little brown bat, which in one hour can consume over 1,000 mosquitoes and other mosquito-sized flying insects. (Pregnant and nursing bats eat even more.) And we can all agree that the fewer mosquitoes the better.

The roof ridge vent is a common place for bats to enter the cabin. Install a hardware screen underneath the ridge vent to prevent bats from entering your place. (The screening shown at the bottom of the photo will be covered by the vent cover.)
Photo by David McNutt
Where do they roost & how did they get there?
There are many places for bats to roost in a home, including: attics, soffits, around windows, behind shutters, behind loose siding and in the spaces where dormer eaves meet the main roof.
As a builder, I have found that one of the most common places bats enter a cabin is through a ridge vent that has come loose or was not properly installed. Another very typical entry point is through soffits – either at a dormer, or where the soffit meets at the peak. These are common places where materials loosen up and spaces can open.
A bat can enter through a hole as small as ½ inch in diameter (about the same size as your thumb). The thing with bats is that they have very good homing instincts and once they have found a new hangout, they tell their friends. Every year they return to the same place, and as they raise their young, the colony becomes larger and larger.

How do you know if you have bats?
Many times, you will find one flying around in the house at night. You may see bat droppings (guano) on the floor; bat droppings resemble small rodent droppings. You may see bats in the ceiling space of a covered porch. If they are roosting in a piece of loose siding, you may see black or brown stains below the roosting area.
Another sign: You may hear scratching in your soffits during the day. The most common time is just before they take their evening flight for dinner.
If they are in your attic, at dusk you can walk around outside your cabin and you may see where they are exiting your structure. If you see them flying out of your attic venting or soffit areas, you can get a good idea on how large of a colony there is. (I can remember sitting in the backyard of a good friend’s lakehome, and we counted over 300 flying out from his soffits. I said, “Tom, I think you have a problem.”)

Evicting & Prevention
Buttoning up your cabin so pests can’t enter is called “exclusion.” Exclusion for an established colony must be done after mid-August, unless there is direct human contact or danger is involved. This is so the bat pups can leave. If it is before mid-August, a qualified excluder should be contacted to help remove the colony more directly and check for pups.
You can exclude or evict the bats yourself in the autumn. But a common mistake is to just seal the bats out once they leave in the evening. The problem is that most of the time, they do not all leave. So when you seal up the entry points, you will need to make one-way valves so the bats can get out, but can’t get back in.
Once you have evicted the bats, preventing them from re-entering your cabin is the key. Inspect your place by looking for potential roosting or entry points. Caulking, flashing or screening will bat-proof most spots of entry.
We have started installing a ¼-inch hardware screen at the ridge vent to prevent bats or other unwanted guests from entry. I remove the vent cover and install a ¼-inch hardware screen along the entire length and then install the vent and ridge cap shingles.
If you do have a large colony, you may want to seek help from a certified bat removal professional. You can try to locate one using these websites:

• Bat Conservation International (BCI),
• Organization for Bat Conservation,
• Bat Conservation of Wisconsin,
Once you have excluded bats from your cabin, another good way to prevent them from moving back in is to install bat houses around your property to give them the proper homes they need. Click here to see the free plans available on the Cabin Life website

David McNutt, owner of Wooden Craft & Design, sees the world in terms of solutions, not problems.


Excluding Bats With Tubes

In most cases, tubes make the best bat-exclusion devices, even for openings on buildings with rough exterior walls, such as brick or stone houses and log cabins. Tubes also work best for holes at corners where walls meet and on horizontal surfaces such as soffits.
Exclusion tubes should have a diameter of 2 inches (5 centimeters) and be about 10 inches (25.4 centimeters) in length. Exclusion devices can be purchased commercially or made from PVC pipe or flexible plastic tubing.
Bats are unable to cling to the smooth surface of these tubes, so the tube should project no more than ¼ inch (6 millimeters) into the opening. This will ensure exiting bats can easily enter the tube.
Laura Finn of Fly By Night Inc., says empty plastic caulking tubes also work well after caps at both ends have been cut away. Caulking tubes must be thoroughly cleaned before they can be used for exclusions because dried caulk forms a rough surface that could allow bats to reenter. These flexible, plastic tubes let you squeeze one end so it fits into a crevice. Or you can cut one end into flaps that fit over an opening, and the tube can be caulked, stapled, nailed or screwed into place (see diagram).
Once the tube has been secured over the hole, a piece of lightweight, clear plastic can be taped around the tube’s outside end (see diagram) to further reduce the likelihood of bats reentering, though this is usually not necessary.
The added plastic sleeves collapse on themselves, preventing bats from reentering once they have crawled out through the tube. After the tube has been secured into or over an opening used by bats, any spaces between the outer rim of the tube and the building must be sealed shut. Also be sure to seal any other openings in the building that bats could use. Leave the tube in place for a minimum of five to seven days to ensure all bats have left. After the bats have been excluded, the tube should be removed and the opening permanently sealed.

Source: This excerpt courtesy of Bat Conservation International,

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