Aspirin helps (the person who's doing the cleaning). You can call a professional to steam clean it - easy, fast, and $$$. You can rent a steamer, not a lot of skill required but listen carefully to the how-to-use it. Not too many $$. You can hire a strong adolescent to read the following instructions and provide the labor. Probably the least expensive, but hand labor is time-consuming and teens need frequent feeding. You can do it yourself, providing you are in flexible shape and don't strain your back or arm muscles. Find the fuse (or mainbox switch) that protects the electricity and remove (or flip to off) to be sure there is no live current available to the fan; snicked fingers hurt. This is one of the rare times I would consider using TSP, a high-strength cleaning powder/additive, usually available at the hardware store. It is not environmentally friendly and so should be used only in the most extreme case. 50 years of grease and dirt sound pretty extreme to me. But *before* purchasing/using TSP, try the other way: Remove the mesh filters - if it has them. Pour boiling water through the mesh and then soak in hottest water that has 1/2 cup household ammonia added for each gallon. Let sit about an hour. Use a scrub brush to scrub the solution around the mesh. This will stink, so be sure the windows are wide open and use a portable fan to help move stinky air out the window. In a glass bowl, make a solution of 1/4 cup ammonia +2 Tablespoon baking soda mixed into 1 quart very warm water. Using a plastic scrubbee, dip it in the solution and start scrubbing the fan blades and then the housing, about a handsized area at a time and work inwards towards yourself (so dirty water doesn't drip on cleaned area). Wipe the scrubbed area frequently with a clean dampened towel/rag/sock (I don't recommend using anything you'll want after this chore). The scrubbing will have to be repeated several times. Do NOT scrub any wires or electrical connections. Use a paper towel to wipe off the motor housing -- okay, use a bunch of towels, it will eventually come clean. Go outside and clean both sides of the flap and that side of the fan blades. Oil the flap hinges and where the blades join the body. If there is a stove hood, it will require the scrub and rinse process same as the housing and blades, however, *IF* it is stainless steel you can use 0000 steel wool (from hobby or hardware store) instead of the plastic scrubber. Rinse the steel pad often in clear water. Reassemble the unit including the rinsed filters. Turn on the electricity. Turn on the fan. If it works, pat yourself on the back. If it doesn't work, don't be surprised because there is a good chance the motor was burned out - or maybe just plain wore out. Buy a new fan unit (measure carefully, they come in several sizes) and either install it yourself or have an electrician install. Try to convince your mum that the hood and blades should be wiped off with an ammonia-window cleaner once a week so this doesn't happen again. Remind her that grease build-up has a very serious potential to cause fires, and it's easier and cheaper to remodel an un-burned kitchen.

I thought about going straight up with it, but I don't have a lot of room due to the angle of the ceiling. Plus, I am going to be using a canopy style hood, so the chase would have to be rather large to be in proportion with the hood, and I'm not sure how that would look. I did think about placing cabinets on top and then a chase above that, but again I am probably too close to the rafters in the attic due to the slope of the ceiling. Consequently, out the wall and into the garage attic seems to be the simplest way to go. Plus, that gets the inline fan away from the house and makes everything more accessible. I don't mind enclosing the vent in drywall to bring it up to code. While the vent needs to be in a soffit, what about the inline blower? It seems like the blower should be left out in the open, with the remainder of the duct enclosed? That said, I can see what you are saying about direct penetrations from the garage into the house are not permitted due to fire concerns. However, would a solid duct running from the kitchen into the garage attic and out the roof fall within that logic? There are no breaks in such a duct run, since the only openings are at the hood itself and wall/roof cap. So as long as the penetration where the duct goes from kitchen to garage is sealed with fire caulking around the duct, how is there an issue? Again, I don't mind enclosing the duct run in a chase if need be, but it seems like overkill....

You are correct, the ducting should be rigid. Also, if the walls are open, and you have the opportunity to install new exhaust ducting, it should be 8" in diameter, not 6". People who have 6" ducts have them because they are stuck with them already-behind walls when they moved in, because builders just put in 6" ducts as the easiest thing. If you are installing new ones, they should be 8" in diameter. You create issues with a 6" duct, mostly a lot of noise as the air is pushed through too small of a space, and less efficiency of the exhaust, because of the smaller duct. See if you can switch to an 8" duct at the same time as you switch to a rigid duct. I am concerned about your installer. First because AFAIK, flex ducting for an exhaust is against Code everywhere due to fire hazard. Second because he is unable to provide a wall vent cap. These are available at places like Home Depot - they are hooded or lidded, and have a grate at the opening to prevent critters. They are also usually available from the hood manufacturer. You may want to stop with this contractor, and find another one who knows what he is doing.

On a side note, we are so blessed to know all our firefighter friends. These guys are all in different professions - both white & blue collar. We have nearly every trade represented on the FD! They’d be good friends regardless, but it is a relief to know these guys personally. This HVAC guy is now retired from both jobs, but he was the maintenance engineer at a major university here in Mpls, and his soldering skills are unmatched! He helped us take off all the old valves in the laundry room and half bath to make tiling easier and just to update them. Unfortunately, the caps they used didn’t come off when it came time to put the new valves on. Apparently they had some brass in them. IDK. So they had to cut them off and make the copper even shorter. The new floor was thicker and a couple of them were already really close. His plan was to put compression fitting on, but those two that were short were impossible. He finally decided to solder them, but he was working inside the tile! The hole was small and he was really concerned. We were dangerously close to having to rip out the basement ceiling. He managed to get them both on perfectly and there was a collective sigh of relief! He’s got skills! I think he deserves dessert when we take them out for dinner, ha!