Hmmm, to me venting is most critical for stovetop use and you mention that the stovetop is not a high use item for you. If the mwave is your principal tool, do you experience a lot of flavor smells or build up of condensate now? If not, then the charcoal filter vents that come in ranges with OTR mwave seems sufficient and outside venting might be unnecessary

Thanks, Urbicoastal. Actually, I don't experience smells/build up now (and there's not even a MW over the stove), and probably would not have even thought to consider venting if the helpful folks over on the Kitchen forum weren't such strong advocates for it. I do hope that, at some point, (despite my lateness in life...) I will decide to cook more - either that, or maybe the outside vent will help resale (though I don't plan to sell for 20 years, unless the man of my dreams decides to drop down from the sky & take me away from all this...)

Not sure what part of country u r in or what type of construction or housing. In SoCal where I have lived, airing out a house is nearly daily even on chilly days up in the mtns. In the northeast, essential to have some outside air circulation in summer because of humidity and heat , essential in winter because of stuffiness and dry air. So in both cases we have managed with recirculating internal vents. THERE IS NO QU that externally vented are faster and more effective. ALL are noisy. In condos and apts we have lived in, there are restrictions on external venting... Can't vent within "x" feet of someone window, for ex., so do be aware of local code, Lastly, the effort and cost are a consideration depending upon where the vent will penetrate to the outdoors. There are split units btw, where the actual motor and blower are mounted exterior or remotely from the actual hood and fan... This is to reduce interior noise. Most don't come this way and the split is more costly to buy and to install... AND if it is a high volume (CFM?) I can tell you that the split is not that much more quiet.

Good luck. You never know who and when you might be swept off into the rain bowed horizon

Hey, Urbicoastal. I'm in Philadelphia, Pennsylvania, In an old brick rowhome (with no AC) windows are wide open all summer, but winters I'm sealed up as tight as can be.
You raise some really good points. Once I get the contractor out here (next week) I'll hopefully get some sense of cost. The expenses are creeping up....

Depends on the options you may have for ducting and venting to the outside. That would be your highest cost (running the necessary ductwork and egress from the wall. If the microwave is against an exterior wall, it may not be much, although I can't speak to the complexity of brick construction. That not withstanding, there isn't a big difference in performance or price between models that only vent outside, vs. those that can be configured for either recirculating or vented. In my previous home, I installed a unit that could be configured to vent up, out, or recirculate. Usually, you pick one of those options and stick with that for as long as your unit will be there.

I can tell you that the efficiency of venting outside will be much greater than recirculation, but it depends on your usage patterns.

Thank you, Sonny. I should be able to vent directly outside - the MW will be on an exterior wall. I'm also planning to put a window in on that same wall - though several feet away. Hopefully, it won't add that much to the cost. Having done with no vent/circulation of any kind, I do think I could get by just fine with recirculating, but I kinda figure if I'm spending a lot of money anyway... I might as well do it as right as possible.

M1506.2 Exhaust openings.

Air exhaust openings shall terminate not less than 3 feet (914 mm) from property lines; 3 feet (914 mm) from operable and nonoperable openings into the building and 10 feet (3048 mm) from mechanical air intakes except where the opening is located 3 feet (914 mm) above the air intake. Openings shall comply with Sections R303.5.2 and R303.6.

International Residential Code