Sometimes people will put bars on the windows when they live in unsafe neighborhoods.

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I've also seen stoppers, that stop the window from being opened more than a few inches. But those are unlikely to deter anyone committed to getting in.

They make sash stops that you can retrofit in many cases.

Bars work well but your neighbors may have a huge issue with it. Think about it, you are basically telling anyone that passes your house that you have a crime issue. That could certainly hurt real estate values. I would be careful with that unless bars are common in your neighborhood.

I wouldn't do that without a couple of dogs in the house, and that's in a relatively safe neighborhood.