Range Hood Venting

[TDKPE]

My over the range microwave vents out the back to a 3-1/4" x 10" 90* then up the wall, thru the 2nd floor, continuing up thru the wall and into the attic to a 3-1/4" x 10" x 6" round 90* transition. From there it runs about 18' horizontally to a 6" (gable end) wall hood. I sealed the joints with foil tape and added some tek screws just in case. The vertical ducts are female up and the horizontal in the attic is pitched slightly down and has the males pointing outward. The 6" actually terminates outside of the siding and connects directly to the hood. When the fan is turned on high the damper is pinned in the open position.

Sounds similar to my last house, where the PO had a downdraft Jennair unit.  Being on the other side of a garage wall, he ran it up the wall inside the garage, and across the two car garage ceiling, then out the gable end.  I reworked it for the over range microwave, keeping the 5” round duct.  Not as long a run, but pretty long IMO.  With the blower on high, it would pin the damper open and when DW was turning food into smoke, you could see the aggressive venting out the side of the house.  It’s all about having large enough duct work to keep the static pressure down and the flow rate up.  

In the case of the OP, I’d run it straight out the roof.  Lots of other stuff exits the roof, especially plumbing vents, and one more hole (if properly flashed) isn’t going to be any different.  And the short duct run will keep the back pressure to a minimum, though I would put it in the back part where it can’t be seen from the street, which may require longer duct work.

No gable end?  Hip roof all around?

[Snipe Hunter]

Always take what an inspector says with a grain of salt. They like to say all this stuff isn't done to code. Well yeah not to current code... Lots of stuff on all houses were code when they were built and no it doesn't mean you have to change it because it doesn't meet current code. Just one of my annoyances of inspectors. Anyway...
 

Spoken like a true real estate agent.
I can't speak for inspectors in other states but here we do have to follow a standard operating procedure or get sued or shut down. That standard (The National Standard) has very little to do with building codes. It has to do with "Best Practices" and reporting what we see. If we see a "potential" problem like interior fumes being vented into an attic, we have to put it in the report because it's known to be problematic. We don't do it to prove how smart we are, we do it to save the home owner money and headaches. A rotted out roof from a non-reported vent terminating in the attic could easily land me in a courtroom and I will lose every time.
Again, not sure about every other state but a home inspector is generally not permitted to quote code. That is in the "National Standard" which is the criteria that most states have adopted for home inspections. We are not code inspectors.

I see a lot of mold in attics (like the picture above) and rotted sheathing caused by vents terminating in attics.

I'll also bet you a dollar the hood manufacturer's recommendations says to vent it outside.

[Robert Adams]

Spoken like a true real estate agent.
I can't speak for inspectors in other states but here we do have to follow a standard operating procedure or get sued or shut down. That standard (The National Standard) has very little to do with building codes. It has to do with "Best Practices" and reporting what we see. If we see a "potential" problem like interior fumes being vented into an attic, we have to put it in the report because it's known to be problematic. We don't do it to prove how smart we are, we do it to save the home owner money and headaches. A rotted out roof from a non-reported vent terminating in the attic could easily land me in a courtroom and I will lose every time.
Again, not sure about every other state but a home inspector is generally not permitted to quote code. That is in the "National Standard" which is the criteria that most states have adopted for home inspections. We are not code inspectors.

I see a lot of mold in attics (like the picture above) and rotted sheathing caused by vents terminating in attics.

I'll also bet you a dollar the hood manufacturer's recommendations says to vent it outside.

             Nope spoken like someone with enough time in construction that any time I see an inspection report I laugh since they rarely ever show anything I would worry about. It's usually all minor stuff not the real stuff I worry about. If I wanted an inspection done on a house I would have a structural engineer look it over and a bug inspector. The piddly stuff a typical inspection finds are easy fixes and often non issue items. 
         I have seen a few things found by inspectors but generally it isn't of much use to me. Now for those with little knowledge of houses etc I can see it as an aid for them.

[rwe2156]

Our house had 2 bathroom vents and a stove vent both into the attic. 

They've since been replaced and done properly, but I was like that for 35 years before we bought the house.

If its time to upgrade the exhaust hood, might as well do it right.

We upgraded ours to a 900CFM hood wow what a difference!