Humidity will move through the bag with ease. A humidifier inside the bag or sound hole is kind of a waste as it won't really do anything (IMO). Now if you put a humidifier in the soundhole and seal the instrument in a hard case with a good or better seal, now you have something. You can also measure the humidiy inside the case to see if it is doing any good (too much or too little). If your winter months are dry, then you should strongly consider a hard case. If like me,you live where the winter humidy still stays about 45% then you could even leave the instrument on a stand in the house and it should be fine. We however have cats in the house, so the guitars are always cased when not being played.
Hope this helps.
You might try this for your Grand: http://www.amazon.com/gp/product/B001QOGLWK
There's no pic of this bag, and I couldn't find a pic anywhere on the Internet before ordering it. However it's essentially just a smaller version of the Seagull Dread & Folk gig bags -- without the embroidered logo. The padding is infinitely better than on the bag that came with my old A & L Ami, but it's not a substitute for a hardshell case.
I thought it was a very good deal
I bought a Musicians Gear hard shell case for my Seagull mini jumbo. It's the dreadnought case and depending on your budget it might fit in for you at $49.95. You can get them at Music 123 and it's free shipping so it's $49.95 to your door.
It's a decent case for the money, certainly not a TKL or a TRIC but it's great protection for a very reasonable price. My 1970 Grammer dreadnought fits perfect in the case too, besides the Seagull.
I've even thought of switching the cases since my Grammer HSC is 40 years old, but it was a fine case in its day so I keep the Grammer in it since that's where it's called home the last 40 years. :-)
I think it is a very good buy for the money for cheap hard shell protection and would work well for the humidity factor too since it's lined very well.
I got this:
it's a polyfoam soft shelled (nylon) case that can best be described as a hybrid soft/hard case. It's very light, seems kinda durable although I'd not stack anything on it, and fits my S6 origonal like a glove. It seals well enough that the soundhole humidifier does work (I have an incase hygrometer to verify internal/external humidity). I would have gotten a hardshell but there wasn't one in stock at GC when I purchased my S6.
Good luck and i'm sure you'll be happy w/ knowing you are providing a safe home for your treasured Seagull!