If you run a wire up in your attic, all other conductors such as ducting, house wiring, gutters, and so on are part of your antenna system. You want to avoid near-field capacitive coupling as your losses will definitely go up in that case. Therefore, a wire antenna close to other conductors that run parallel to the antenna conductor is not good.
I am a big fan of numerical modeling of antennas. I model every single wire antenna I have ever built in the last 10 years using NEC2 (National Electromagnetics Code). Actually, I bought the license for NEC4 and use that a lot but NEC2 is free and can easily be run on PC Windows, Mac OS X, or any Linux or Unix platform (assuming you build it yourself, source is available).
With NEC2, you can actually model the antenna wire as well as other near by conductors whether they be wire or metal ducting (typically modeled as a mesh). Doing it up right means spending time creating the model but you can learn a lot about an antenna.
What happens when you have nearby conductive surfaces?
- If they are close, you can have greater losses.
- Your matching impedance can be changed (important for resonant antennas).
- Your propagation pattern will very likely be changed in creating nulls and
higher gain lobes where you least expect them.
Items 2 and 3 are determined when you model with NEC2 and NEC4. I mean, that is the principle reasons I do modeling is to calculate the matching impedance and to determine the location of nulls and higher gain lobes and take off angle and so on.
To get started with NEC2 you can acquire the free version of EZNEC software available from ARRL. Or, you can possibly get a 30-day trial from the EZNEC vendor W7EL (at http://www.eznec.com). Or, you can go to http://nec2.org or merely google NEC2.