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We have sash windows and got broken into last week due to poor security (just internal latches).

I want to install better locks on the windows to prevent the windows being opened past certain amount, but am having trouble finding a reference for how much of a gap is considered practically impossible to fit through.

Though I understand that people, even burglars, come in different sizes and shapes, is there an acknowledged security-industry standard distance to limit such a window-lock installation?

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Although I understand your concern because your situation sucks. Know that if someone is going to break into your home through a window that a small max opening isn't going to stop them. Once they get that far they are basically all in unless it is just some kids. –  DMoore Mar 5 at 5:33
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Cavers (one of which I am not) manage to squeeze through 6-7 inch gaps, so that's too large. Aside from people and people's arms, think about tools reaching through. –  Ecnerwal Mar 5 at 5:36
    
@DMoore, I was thinking of pin locks with a removable key, or for some windows permanently affixing some wood above the movable pane (a la the dowel in the sliding door trick.) These would need a lot of time, tools and noise for a burglar to remove. Hopefully they wouldn't bother. –  HumanCondisean Mar 5 at 22:45
    
@Ecnerwal, Thank you that is handy to know. Those numbers fit with the head breadth figures I have found. –  HumanCondisean Mar 5 at 22:48

2 Answers 2

up vote 4 down vote accepted

Most code states that all railing balusters must be spaced no more than 4" apart to prevent a child from inserting their head.

So, that is one measurement you could use...if you don't want anyone's head to get through: 4".

Of course, also make sure you can't get an arm through that could reach the window lock.

Edit by OP:

According to http://upload.wikimedia.org/wikipedia/commons/6/61/HeadAnthropometry.JPG Men's heads range from 5.1" to 6.5" at their widest point (and women's 5.2" to 6.1"; 1st to 99th percentile).

As such, choosing a maximum gap between 4" to 5" would prevent somebody ranging between a child and adult from entering through the space.

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I don't think there are adults that can fit their head through 4". That rule is for infants and toddlers. I'd wager if a guy will send his kid through a window to defeat the lock, he'd defeat the lock with a hammer as well. –  Keith Hoffman Mar 5 at 6:00
    
Kids can be sly! –  DA01 Mar 5 at 7:20
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You gave me the idea to search for head sizes and I came up with this: upload.wikimedia.org/wikipedia/commons/6/61/… Hope you don't mind me editing your answer –  HumanCondisean Mar 5 at 22:27

Unfortunately, I don't think this approach to your security is going to help that much unless you are just trying to deter casual entry. If your neighborhood is that bad, you may need to consider window bars but these have very negative implications to life safety in the event of a fire. On the non home improvement side, get a big dog.

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or even a small dog. They cost about as much as an alarm system but are a bit more personable. Granted, there is poop involved. –  DA01 Mar 5 at 7:21
    
Thanks for your answer. I think deterring casual entry is important as a first step. Our current situation is low-difficulty:low-return for burglars, so if we can make it medium-difficulty:low-return then they may just try the next house along. Dog would be lovely but we are too busy to take care of one right now. –  HumanCondisean Mar 5 at 22:18
    
@HumanCondisean but note that those measurements are at the 'widest'. I think you'd want to find the 'narrowest' measurements to be absolutely sure. :) –  DA01 Mar 5 at 22:42

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