Some feedback on my RV8 rear duct
You may find my rear ducting thread after you search....but now that the aircraft is flying (in -15 to -20C) I have some feedback from my rear seater.(
http://www.vansairforce.com/community/showthread.php?t=55773 )
I ran a duct under the right hand sill that directs heat at the rear seater with the outlet just aft of the roll bar. Actually, I had more complaints during the summer that some hot air was managing to blow back if the butterfly valve wasn't completely closed. As others have mentioned, seal that canopy skirt as best you can. Making an opening for outlet air is important too.
A rear duct does require a bit of preplanning though.
1: You need to go for a 4 pipe exhaust so you can put in two heat muffs...this of course leads to two heat boxes on the firewall with the associated scat tubes. (This is for a fwd inlet IO360...there are other engine configs that you can get a muffler on, so you would have other options...)
2: Make sure your sight-line to your breakers aren't obstructed by the duct. My duct just fits this criteria. I have been 'annoyed' by its presence a few times, but the trade off is worth it for a happier G.I.B.
3: I ran the scat through my fwd baggage compartment at the corner. It hasn't been a problem with the baggage space there, since I fill the lower area with my tie down bag, my toolset, and a spare quart of oil.
3: You need to run another heat control to the rear seat (that you can also reach from the front seat...I put mine just aft of the roll bar below the duct outlet.