Kitchens – Mounting a microwave w/o the upper cabinet

The counterop weighs less than 300 lbs. The cabinets probably weigh less than 100 lbs. This is about 400lbs spread over a floor area of almost fifteen square feet. this is less than 30 lbs per square foot.

An adult standing still is about 150 lbs on one square foot.

You should have no problem with the load regardless of the placement with regard to the joists.

In almost all cases you can do this. The difference between the microwave models is more about the mounting and how the front looks. There are basically three general types. Your typical countertop microwave which some people just stick in cabinets - which is fine given that the electrical is done right. The you have your over the range which usually have two extra functions - an undercarriage with range venting and lighting, and simple mounting brackets on the top. Then you have your cabinet or built-in models. These are the least sold and usually cost a bit more because they have a nice mounting plate in front.

Any ventilation space that you currently have should be adequate for whatever you get. In your particular case my "problem" would be the undercarriage. These microwaves are generally not made for sitting on their bottoms which have a fan and lights and usually a plastic casing. This casing may break or crack over time. I don't see an "issue" with doing this - although I would simply double check by calling Whirlpool - but I wouldn't do this unless I was 100% certain that the bottom of that microwave wouldn't be damaged because of movement and rubbing. Also I can't see how this aesthetically looks the best of the three options.