It's a frequently asked question, "Can I use both LATCH and a Seat Belt to install a car seat?".
Many parents try to use BOTH seat belt and lower LATCH anchors, believing that they are adding another level of protection. However, this is a misconception, because there is no evidence from NHTSA studies that shows any extra benefit from using both. There are however, potential dangers in this.
Firstly, it is important to use either the lower LATCH or seat belt, depending on which one gets the better installation for that seat, vehicle and position.
Because car seats and seat belt systems are engineered and tested to handle crash forces in a specific way, it is always important to refer to the manuals for both, the seat and the vehicle. Most will tell you that you need to use one OR the other, never both. Some seats and vehicles do allow you to use both, but you can only do so when BOTH the car seat manual AND the vehicle manual says it is OK to do so (though this is uncommon).
So here is why we can't use BOTH, the Seat Belt and lower LATCH anchors together to install a car seat:
The manuals tell you to never use both because, either they didn't test the seat using both, or they tested it and it failed. NHTSA does not release all of their internal testing data, so we really cannot know for sure what is the exact reason behind the car seat and vehicle manufacturers' recommendations. Also, there is not much data from actual crashes about this type of misuse, because at the site of an actual collision, usually only very major car seat misuse might be noted in the reports.
Here are some explanations for what could potentially happen when using both the lower LATCH and the seat belt to install the seat (keep in mind this is not from any testing data, rather speculations):
1. Because you would be installing through the same belt path, it could put stress on the car seat shell from two different angles, causing it to break during a crash.
2. It could concentrate more crash force on a small area of the car seat.
3. It can alter the way the seat is designed to absorb the crash forces and transfer more forces to the child instead.
4. Both belt webbings overlapping each other might change the way the belt is designed to stretch and take up crash forces, so instead transferring more of the forces to the child.
The bottom line, is that you don't want to use your car seat and vehicle safety features in a way that goes against the way the manufacturers intended.
More on the topic of LATCH : When forward facing, make sure to always use the top tether (it is part of the LATCH system) regardless of using the lower LATCH or the seat belt for installation. Also, keep in mind that you cannot use the lower LATCH in the middle seating position, unless indicated otherwise.
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