I took a look at the manufacturer specs for the I-Joists that run above the bathroom. According the the manufacturer specs I should be okay to drill a 6-1/2" hole through the webbing and still be able to maintain minimum distance from the supporting ends, however, if I use smaller 4" ducting I would have a lot more options and would be able to go right through the I-Joist webbing and out through the soffit. In addition the smaller ducting would allow me to make the hole through the webbing large enough that the duct insulation will not be too compacted. I decided to go with the Fantech PB270-2 system, which works with either 4" or 6" ducting and provides 270 cfm of air flow.

look at broan-nutone fans like the ilfk series.

I don't have a lick of installation expertise so can't help you. But I love my fantech...one installed in shower one over tub. Probably overkill, but my closet is in bathroom so I wanted to make sure. I don't even get a hint of steam on the mirror or windows. I think you will love it.

Our bathroom is about the same size as yours and we used dual grilles with halogen lights, installing both in the 7x4 walk in shower. It's the only exhaust in the bathroom and it works great, even when it's the bathtub that's giving off steam.

We used a Lutron dual switch- dimmer for the lights and underneath is a countdown timer for the fan.

If I were you, I would go with a double... replace the existing fan and add the other one right outside your shower. Or, replace the existing shower light with the light/fan combo. I, personally, wouldn't install a new fan in the shower, right next to the existing light.

You could even replace the existing vent with a light combo on it's own switch/dimmer. The light shining down in that corner would look nice!

As for installation above, I don't have a clue, sorry.

--Just make sure you leave access to the motor if necessary in the future. Instead of cutting through beams and stuff, I would just hide everything in a pony wall or low cabinets. For guest bedrooms, it's nice to have a lot of surfaces for people to spread their stuff out. It's hard to tell from the pics if that would even be an option.

50 CFM removes the least amount of moisture, so that's definitely part of the problem. I recommend a 200 CFM with at least a 6" duct, and ensure the duct runs smoothly to the roof. Curves and bends in the duct reduces the flow of moisture out of the bathroom.

I will wholeheartedly agree with southernroots that 50CFM is not sufficient for your bathroom, but the 200cfm that he suggests is far too much.

The first step of the project is to determine exactly what size of fans you need.

Properly, the bathroom exhaust fans should produce 4 complete air exchanges per hour so the question then becomes how many cubic feet is four exchanges per hour?

The main part of the bathroom is 10'x 12' which is 120sq.ft. Now assuming an 8' ceiling we get 120sq.ft x 8' vertical equals 960cu.ft in the room and we desire four complete exchanges per hour so we need to move 960cu.ft x 4 = 3840cu.ft per hour. We then divide 3840cu.ft/hr by 60min/hr and we get 64cu.ft/min.

The shower is 4' x 5' so it is 20sq.ft and again assuming an 8ft ceiling we get 20sq.ft x 8' = 160cu.ft in the room.
Four air exchanges per hour would then be 160cu.ft x 4 = 640cu.ft/hr. Now dividing 640cu.ft/hr by 60min.hr = 10cfm.

For peak efficiency I would put a separate light/fan unit in each room sized to the nearest nominal trade size which is equal too or greater than the theoretical size from the computations. By example, you already know that 50cfm is not sufficient, but I doubt if you will find a 64cfm fan. The solution would be to then select a 75cm fan. Actually the slight increase would work to your advantage because it allows for line resistance of the ductwork, but to just arbitrarily install a 100cuft fan would result in way too much air flow, and keep in mind that you are paying to operate your HVAC system to condition all that excess air.

Now, while you can run the duct up and out through the roof, I would prefer to run it horizontal to the nearest external wall and install a wall vent to the outside. That not only reduces the line friction loss in the duct work, it totally eliminates the possibiity of a roof leak where the line passes through the roof.

Taking this to the next level, keep in mind that you are venting warm moisture laden air and as it passes through the ductwork it will cool and condensate moisture in the duct. For my house, I made the ducts with PVC pipe and installed them horizontal with a 1/4" per foot downward pitch to the wall outlet. In that manner I do not have to worry about metal ducting rusting or water pooling between the folds of flex duct.

Lazypup's last paragraph is especially important if you live a a cold climate area. I live in central Ohio where winter temperatures can get down to 0 degrees. We have 20" of blown-in insulation (R49) in the attic floor, so the attic space is below freezing for extended periods, especially during the morning when most showers are taken. Water from condensation pools in the insulated flex-duct running from the bathroom ceilings to the roof cap vent. I've spoken with Fan Tech's tech people twice about this. They don't have a solution other than managing the flow of water through a hole drilled in the in-line fan unit or weep-hole in the duct with a collection pan.

Increasing the insulation around the duct wouldn't do much because the temperature of the duct will drop to the temperature of the attic. And then the sudden flow of warm moist air from the fan will result in condensation.