The ducting must be installed based on the specs of the hood you purchase. And the size of the ducting must comply with the requirements for the hood. Most of the "professional" hoods seem to require 8" solid (NOT flex) ducting). Pic our your hood, determine the height off the range it will be (usually 30") and then figure out where the ducting must come to.

Maybe I should have opened up the email first. Check this out! My client asked me for a top designer and I told her to call Corey. I have know him for a long time and he is as anal about design as me. The big difference between us is that he is both a certified designer and NKBA design teacher/speaker - and I am not. I have opinions but am better building than designing I think - especially when it comes to Kitchens. This is going to be a Kick Ass project. Glad I have a small part in it. I love designers who can do these 3D drawings! Thanks Corey! http://www.houzz.com/pro/coreyklassen/corey-klassen-interior-design

Ah! you missed one of the versions of a comment I have added to quite a few threads. Here it is again. Because we do not get to close off parts of the hood not receiving cooking plume impact, all of the hood has to flow the amount needed at the most stressing part of the effluent plume to be contained at the hood baffle space. How much do we need? Well the actual issue is what velocity of air has to be pulled past the baffles to ensure that the rising plume is pulled into the baffles and contained, versus reflecting off of the baffles and potentially curling back into the room, thereby escaping the capture area and hence removal by the hood. Hot cooking plumes over gas burners can rise at velocities of 1.2 m/s, or almost 4 ft/s. The baffle gap is roughly half the baffle assembly area, so to entrain the plume flow, it seems that some portion of 4 ft/s or 4 x 60 CFM per sq. ft. of baffle area is needed for good containment. Actually, there is usually space under the baffles larger than the baffles proper, so adequate flow is needed all across the hood capture aperture. I believe that I have about 900 actual CFM at home, pulled through a 10 sq. ft. Wolf Pro Island hood. This is equivalent to 90 ft/s. This works over induction hobs, including searing and wok cooking with a 3500W Cooktek wok hob. As it happens, Greenheck also looked into the requirements for CFM, perhaps like me doubting that 100 CFM per ft. of hood edge had a mathematical connection to the process of cooking plume containment. They have published a range of values, and I have taken the liberty of copying their relevant table below. So take your hood area, multiply by some value you believe in for actual needed flow, then multiply by another factor to account for blower ratings not actually applying to the conditions in which they are used. (Another topic written about here more than a few times.) http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf No one has complained here that aiming for 90 CFM/sq. ft. of hood aperture allowed smoke and steam to escape into the kitchen, so long as the hood overlapped the cooking area by enough to capture the expanding plumes.

GreenDesign's comment, supplemented by weedmeister's comment is the most succinct. I was imagining using a four-sided duct cover and tiling the entire space including behind it. Then no grout to cover interface is needed. Note that the tile grout will not bond well to the stainless steel, and with different thermal coefficients of expansion, will crack there anyway. Some grout colored silicone would be needed. One could tile to the ceiling or not. Unless the duct cover is kept away from the wall, island hood style, a close fit will avoid having to clean behind it. If the tile stops before the ceiling or soffit, then there will be a gap on the wall side that will be difficult to clean without an air gun. To avoid that the three-sided solution with tile installation last can be used.