A bat house is a box-like structure that you can affix to trees or other structures, giving bats a home. The idea is that you use a bat box to keep the mammals away from another part of the property — one in which they are causing a problem.
The theory is a good one, but it doesn't always work. As with other preventative and more “laid-back” approaches, it tends to only work well in the minority of cases, and only when a series of specific steps have been taken. These steps will include the sealing of your building, creating or buying one-way exclusion devices, thorough inspections of the property, and a very in-depth cleanup operation. If you miss any of these steps, the problem is likely to return.
If you don't clean away the bat guano, the corrosive material can seep through floors and ceilings, alongside causing mold and problems that then come with mold.
Bat guano and other bat material will attract flies, maggots, beetles, rats, mice, and other scavengers, and they also come with their own health/disease threats. The biological material that wasn't cleared away will also start to smell badly. The smell will get worse the longer it is left un-cleared.
Small holes — 3/8 of an inch across — will enable bats to get back into the property, so these must be sealed with the appropriate materials. You will need to perform thorough inspections to make sure that you find every single hole. If you have more than one pest control issue, such as bats and rats or mice, you will need to take into consideration their impact on the sealing materials you are using. Rats can chew through almost any material, including the expanding foam that you were probably advised to use on the bat holes.
In reality, a bat box or house is a nice bonus for the bats when you're solving the problem, but it doesn't solve the problem by itself. In order to get rid of bats in the attic, you will need to make sure that every single hole has been sealed or repaired. You will need to clear away every biological trace that the bats were ever there. You may need to replace attic insulation, or throw away/dispose of personal belongings that have become contaminated with biological material. Just adding a bat house to the bottom of your garden isn't going to do all of that for you. You will also need to use one-way exclusion devices to get them out — the bat box isn't just going to lure the colony from their existing, quite comfortable home. You will need to force them there, and that requires exclusion.
You don't need to give the bats another home when you exclude them from your attic, but it is a nice thing to do. Bats are very important to us, especially in terms of pollination and insect-eaters. If we were to lose bats, we would lose a great service because they eat so many of the insects that would eat our crops otherwise.
Go back to the Bat Removal
page, or learn about bats in the attic with my Bats in the Attic