I've used a small rucksack a few times when on the bike, but really only for family rides when we'll going at "Little L-C speed" and so I'm pretty sure I won't be sweating: and need to be able to answer the call of "daddy I'm hungry".
Other than that, far too sweaty .. and I have the same hassles when hill walking as well. You wear all your technical wicking layers and so on and then your carefully wicked away sweat runs up against a layer of foam.
I've used an over-sized bum / waist bag type affair that Mrs-LC found for me last year. It's got a couple of big old foam bits that hold it quite well off your back, and it only really covers your kidney area so that's OK. It's made by Lifeventure and is a lot like
this one. Oh, they call it a "hip pack" I see.
For the light days commuting this year, I used an
Altura bar-bag .. but that clashes with my lights for space on the bars and so is now resting until next Spring.
I'm thinking about getting a Topeak Tri Bag to carry my spare tube, multitool and - just might fit it in - a small cable lock for the Winter commute*, which will reduce the weight in the "hip pack" a bit. Or I might go for a frame pack .. can't really fit a conventional saddle bag under my seat and allow space for mudguard, light and all the other old rubbish.
*reasonably secure parking with CCTV covering the route to / from where my bike is parked.