I Ordered this specific bathroom exhaust fan because the ceiling above the bathroom where the old exhaust fan was had just 9 inches between the joists in this area. So this 9” by 9” fan would work in the existing opening.I found that there was some omissions as well as some inaccurate information on installing this fan.1) In the Amazon description an important piece of information was left out when installing this fan as a retrofit. On page three of the instruction manual with the heading “Planning Installation” it states that “ if installing in existing construction, you MUST have access to space above and below the installation space”. That is something that should be added to the product description to help to know if this fan is a good choice for your situation before buying.2) In my case gaining access to this area of the attic would be extremely difficult however I found a workaround. Being the two joists above the bathroom were 9 inches apart and the fan was 9”x 9” I was able to securely install the fan from below to the two joists without using the included joist brackets. If you do not have some sort of support to attach your fan housing to without using the included brackets that need to be done from the attic side I would seriously consider some other fan as the fan housing is heavier than some others on the market.3) In the instruction manual when installing as a retrofit it does not mention that to install this fan in a 9”x 9” opening you need to remove the two screws located just above the bottom flange where the electrical connections and exhaust housing is located. By doing this you can remove that panel and slide it up into the existing drywall opening. Once this is done you can actually make the electrical connections to the existing electrical wiring from below if you have about a foot or so of extra wire to work with. I was also able to attach the duct work to the fan housing without getting into the attic as there was a short section of flexible duct already in place. If you are dealing with rigid duct that may not be possible to do without access to the attic. Once the duct and electrical connections are done you can slide the fan box into the opening making sure it snaps in place and reinstall the two screws previously removed.4) There is one statement in the installation manual about attaching the duct to the fan using “duct tape”. The instructions should be more specific and state to use a brand of Aluminum HVAC Tape, 2 inch wide and not the fabric household variety of duct tape used around the house. If you use regular duct tape in time it will fail and possible loose your connection between the exhaust duct and the fan itself.5) Word of advice, if you are not an experienced DIY person with patience, I would not recommend doing this project yourself it can be a bit challenging as a retrofit project.With all that I have noted above. I am happy with the new fan and how it turned out. I hope my comments along with others made about this product will help you decide if this is a good DIY project or one that should be left to a professional installer.