Flue materials

I'm about to embark on building my first RMH, and have everything gathered up except the bench flue. I'm intending to use 8" but it seems that this is potentially the expensive bit. This is a 4.5m (14' bench, so a there-and-back run with 2x 90 degree elbows should be plenty of length. Doing it in stainless single wall is pricey though, and the alternative is flexi flue liner, which I presume will offer more resistance (and potentially trap moisture along its length?) but which is a hell of a lot less expensive.
Is there a cheap(er) flue material that people are using, or is the best bet to go for smooth, straight single wall? Seems hard to get second hand, but again, any suggestions for sourcing?
cheers
Steve

Hi steve; Not sure where you are located,but if you are in the us you can buy 5' sections of 8" hvac pipe to bury in your mass. I think that i paid apx. $11.00 a stick for it at home depot. You need to use at least one stick of heavy wall 8" wood stove pipe to exit your transition area but then you can switch to hvac , i used one more stick of heavy wall where the pipe rises up out of the mass. Make sure that you coat the pipe well in cob as it is expected to rot away after awhile leaving a smooth bore for the heat to follow. hope this helps

Hi Thomas, I'm in the UK,specifically Northern Ireland, where choice is sometimes a bit more limited than mainland UK especially for used stuff. I've just discovered spiral ducting, which is dirt cheap, can be jointed properly and is largely smooth - certainly a lot better than flexi flue. Would it do?

Going back and forth? Make a half barrel bell!

http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build/