I am interested in these drop-heel trusses you have described. Please feel free to email me some pictures or drawings, I am not familiar with this configuration but if it something that I'm missing from the plugin I would definitely be interested in adding it in.
I've lost the references to drop chord trusses. I do a lot of searching on my phone while kind of watching TV. The phone doesn't keep the records for long and I researched this weeks ago. But I remember the description and the points -- although I'm not technically able to evaluate their correctness, so I'd appreciate it if someone else did.
Description: A chord below the top of the wall that the ceiling is attached to. Think of a long squared U (e.g. └───┘). Of course, some spans would require additional vertical members for support.
Benefits claimed:
1. No bracing or blocking needed (vs. energy heels, particularly more than 5.25") as the wall provides the necessary structure.
2. Air sealing should be easier as the complexity of top plates, etc. is above the sealing level; just the ceiling and ceiling to wall seals.
3. Avoids ceiling lift in winter because the structural bottom chord (my terminology) is the same temperature as the top chord. Note that this requires the drop to contain enough of the insulation to expose the structural bottom chord.
My potential application:
* To be certain of all three benefits I'd need a drop equal to my insulation depth -- 15+" for R50 cellulose -- to expose the entire structural bottom chord to the same temperatures as the top. This adds that same 15+ inches to the top of my SIP walls to leave the bottom chord above the insulation. My estimated cost of doing this is $515/inch, I doubt that it would be worth it.
* If I use a drop of 15 - 4.625 (minimum 9/12 heel) = 10 then the top of the bottom chord would be close to the surface of the insulation, and might yield benefit 3. But this would cost about $5K, which is probably not worth it to me.
* If I use a drop the same as the energy 5" heel I get the first two benefits. This might be worth the cost of $1575.
Note that my cost analysis is entirely dependent on my use of SIP walls. Other construction has much lower material costs, which is what effected my costs. This analysis makes me more interested in investigating non-SIP walls. (Particularly after I found InsulationDepot dot com this morning, which sells recycled insulation.
Implementing drop chords in the plugin will require more point identification than a common truss. Obviously, the inner points of the walls will need to be marked -- as these are the ends of the dropped chord. But overhangs are possible too: my design has an overhang above the front door. (This is easy with a Fink truss, as i just extended the roof line.) I don't see any easy way of doing this with a drop chord truss; I hope you do.
