After researching back and forth on the most cost effective method to build the roof assembly I came to the conclusion that wood roof trusses and blown-in insulation has the lowest overall cost. I am not in a hurricane area - seismic yes. The gables will be framed as well - no ICF there. In traditional frame construction the roof trusses are most of time supported on on internal bearing walls. In the case of an ICF structure the exterior walls are strong enough to support the the whole roof load by themselves. I am wondering if has anybody had experience with large clear spans for the roof trusses - such as 40 ft on a roof pitch varying from 3:12 to 4:12? I will have this reviewed and stamped by an engineer anyway but before I get there - do you see any issues with this approach?

We have built ICF homes as you indicated. You just need to make sure your window concrete headers are designed for the loads and you should use truss plates to attach the non-load bearing interior walls to the trusses so as to address truss uplift (i.e., allow trusses to move up/down without lifting the walls and cracking the drywall).

Thanks for the reply.

I am actually planning to have a 2ft vertical gap tween the trusses bottom chord and the top of the interior walls. The gap will be filled with insulation on top the interior walls.
I will have 2x4 or 2x6 joists that will be supported on the interior walls themselves and on ledger attached to the exterior ICF walls and that will create the ceiling structure supporting the ceiling drywall and the insulation.

In that way I don't have to worry too much about truss uplift and I also don't have to do a heeled truss.

Just to be 100% clear...truss uplift is NOT a problem unless you don’t understand it and you don’t properly address it.

Constructing a secondary ceiling structure as you indicated will add significant cost to the project for little benefit that I can imagine. And why spend the money building taller ICF walls and then reducing the living space ceiling height? You will still have to use lumber that can handle all the various spans given the loads of the ceiling drywall and insulation. I think you would be way better off just using trusses. Anyhow, just my two cents...

I agree, energy heal trusses were a free add-on to my truss order. That is the way to go. Truss manufacturers know what they are doing and do it with precision.

It is very exciting building a house. Glad there is the internet to have people help us. Good luck with your build.

Yes, it is very hard to beat a properly done energy heel truss. If you are building an energy efficient building, you will also want to keep your HVAC ducting inside the conditioned building envelope (i.e., NOT put it in the unconditioned attic like most contractors do). If you don’t want to use soffits to accomplish this, you can use a plenum truss:

Plenum Truss

I haven't done a home with 40ft trusses, but did a commercial shop 14 years ago with a 50ft span. No difference - still engineered. Clear span. Not one issue with it since. I don't think what you are proposing is difficult at all.
Good luck with your project, you will be happy that you chose ICF !

Thanks again for the feedback.

The way I see it the 2 extra ft on the ICF wall and the joists that will need to be placed on top of the framed walls are worth the effort.

I am not really taking away from the room height - it will be standard 8ft framed wall height and I don't need or want to have higher ceilings. A higher room ceiling means more generous space - true, but that means additional room volume that requires heating.

The reasons for building extra 2ft of ICF wall height and a separate ceiling are:

1 - The extra height will allow placing the ceiling insulation in that space. Essentially the extra ICF height will be replacing the heel of the energy truss so no need to use a heeled truss.
2 - Better noise insulation. The location is very prone to outside noise (train, planes) and having the ceiling "sunk" for about two feet inside the exterior ICF structure should reduce the noise coming from the outside through the roof structure. Having additional ceiling joists will allow an additional noise blocking board to be placed on top of the ceiling joists.
3 - The extra wall height will allow for easier future placement of an additional floor on top of the existing ceiling. I may add board ledgers on the ICF for that purpose so an additional framed floor could be placed on top sometime in the future.
4 - Extra space in the attic will allow easier access for maintenance, utilities, etc.
5- Better air sealing. By putting the ceiling drywall directly against the bottom chord of the trusses - the air barrier membrane placed on top of the drywall will be subject to puncturing in many place due to drywall screws, light fixtures, etc. By placing the air barrier membrane on top of the ceiling joists there is virtually no puncturing on it and can be taped and sealed a lot easier.

You will not have a easy way to insulate the top of the ICF wall. The inner core of the wall will be exposed to outside air at the top.

You are missing the greatness of energy heal trusses. With them you can insulate over top the wall, all the way to edge of the outside wall. I cut scrap osb and nailed it up all around the outside of the house covering the energy heal vertically just before we did the eve/facia detailing (some have the guys insulating do it with card board). This stops the insulation from falling into the eve and brings the insulation right to the very outside edge of the house.

As far as noise goes. Use Cellulose because mass absorbs sound and that stuff is heavy....I didn't use it in my house because it was an extra 4,000 pounds I didn't want sitting on the ceiling drywall. An extra 2 feet is a 20% increase in wall cost materials.

You are wasting a lot of time and money with this method. The extra steps in a ledger board are not needed and will waste time and lead to many unforeseen problems. Inspectors will freak out because it is not normal and you could fall outside the code book requiring a Professional Engineer to Sign off.

Maybe we are missing something you are trying to describe so a drawing might help us. But in other words, there is not way in hell I would use this method but I am very stubborn like yourself and have made costly screw-ups before that I had to deal with. Wish you all the luck on this.