I donâÂÂt have all the answers, maybe I donâÂÂt have any. I was just trying to provoke some useful analysis of the problem. The attic and everything up there should be as cold as possible. You mentioned that the new addition has most of the problems and the older attic has none. Might this be because the newer section is too tight and the older one âÂÂbreathesâÂÂ? You should not have to clear snow off the roof with proper roof vents. See link below for roof vents designed for a cold climate. TheyâÂÂre very common here where snow can be quite deep on the roof, I donâÂÂt know if theyâÂÂre common in your region. They always extend above the snow. SR Here is a link that might be useful: 'Maxie Vent'

All of this is backwards! To determine requirements, one starts with the cooktop size and based on cooking plume expansion determines the hood entry aperture (capture area) size. From this the flow rate (CFM) is determined, and thence the duct diameter. Also, based on the flow rate and local ordinances, personal preferences, house architecture, and exterior environment, one establishes the requirements for the make-up air system. When requirements are established the hunt for systems or components of systems can proceed. Details of the requirements process can be found in many "hood" and "MUA" threads on this forum. Absorbing the first dozen or so pages of Greenheck's commercial perspective on this topic may be helpful. See: http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf

@sarah hossain I'm not an expert like @kaseki is on vent hoods, but here's my best try at answering your questions: Is makeup air (MUA) absolutely necessary if your house isn't tight? Well, my house was built in 1985 before more modern insulation methods were used and isn't tight at all, but I still get better exhaust through bathroom fans when I open a window, and those are only 150 CFM. When you get into 300 CFM or higher for a vent hood, you'll probably want to open a window or door to help with airflow if you don't have MUA regardless of how leaky your house is. Lack of MUA whether ducted or via something like opening a window will actually restrict the effectiveness of your blower at any speed. I don't have MUA and am about to install a ducted hood like you want to do. I chose a Wolf hood for the premium under-hood capture space with tilted baffles and a 300 CFM blower. In my city, I am required to keep the CFM of my blower 400 or less unless I have makeup air. This is very common because it complies with the International Residential Code (IRC) and helps prevent dangerous gasses from being pulled from the water heater, fireplace or other appliances into your breathing space when a high-CFM blower is being operated. If you live outside city limits, you won't necessarily need to abide by any code, but I wouldn't put anything more than a 600 CFM blower in a house without MUA for safety reasons. Most cities will require a permit for inspecting the new ducting work, and the inspector is usually going to also be checking that your new hood meets whatever code has been established. In northern climates where homes are built tighter, some cities have adopted a 300 CFM limit without MUA. Other cities have made exceptions to the IRC and allow up to 600 CFM without MUA. The requirements could be anywhere in between, but I've never seen mention of any city's code that exceeds 600 CFM for any reason without MUA. Keep in mind that while ideally you would want to open a window if you don't have MUA, that isn't always going to happen due to convenience or outdoor temperatures year-round. Does a large home and open floor plan reduce my need for a make-up air system? Having a large home such as yours as well as an open floor plan probably does help with exhaust issues if you don't have a MUA system. However, at some point if you are running your blower at the highest speed setting for a long period of time, you will need some way to provide make-up air even if it is through opening a window. A blower rated for 600 CFM won't provide that much actual exhaust if the interior airflow is restricted due to lack of make-up air. Limited options for ducting out of a centralized kitchen. It sounds as if you're bounded on all sides with that kitchen. Staircase to the right, bedroom upstairs, sun room on another side, all of this leads me to wonder if you've considered a totally different option which I'll try to explain. In Germany and some other European places where air efficiency is critical, it is actually more common to install a good recirculating vent hood with filters and then a heated air exchange system about 6 feet away from the hood that helps to exchange air with the outside. This helps to preserve the heating/cooling in your home since the air that is put back inside the house is pre-heated, and it actually does a pretty effective job of removing the things you want removed from the kitchen airspace. I found this early in my search on hoods and don't know if I can re-find the links where I read about it, but it might be a really good option in your case. You could keep the range where it is, install an island vent hood with a deep under-hood capture area and replaceable filters and washable baffles. Then install this other air-exchange system somewhere else, and you may have what you want. If I can locate the link to more info, I'll try to post again here later.

julieste --- Adding to what kaseki said, the hood & soffit idea will be fine. I say this having had to do something similar in my 110 y.o. house. I do a lot of cooking. The set-up looks acceptable and works well enough. Of course, my "good-enough" solution was not the best possible venting set-up I could have had. Some here would say (and do say) that the "merely good" is the enemy of "the best" and only the most optimal will do. Others of us will say that, If you have the the time, interest and budget for "the best possible," have at it. For me, the best possible venting solution would have required reconstructing the back end of my old house. "Good enough" was what I had the time, interest and money for. I'll offer a couple of ideas you could consider in designing your project. One is to consider transitioning the ducting (at the kitchen-den wall) to run across the den ceiling in 3"x10" rectangular ducting. That could make for a shallower, less obtrusive-looking soffit. A second idea to consider might be recessing some shallow LED "can" lights in the soffit as that end of your den looks rather dim in the photo above (although that might be an artifact of the photo.) Maybe hang a couple more paintings at that end of the room. That could make a functional necessity look like an aesthetic design choice. Of course, some find recessed lighting and soffits to be aesthetically abhorrent. YMMV. An alternative to the soffit-and-vent design might be one of the Vent--a-Hood ARS units. (ARS stands for "air recovery system.") These seem to be one recirculating hood system that actually works reasonably well. These are expensive but might be less expensive that the cost of the soffited vent system you are considering. If interested, check out the thread "Vent-A-Hood Ductless ARS RANGE hood Update" which started here a decade ago and was still getting posts a month or so ago.