I'm getting down to the nitty-gritty of my build now, and wonder how to handle the transition from ICF walls to the roof structure. Usual method is to bolt a PT 2X down to the ICF, like any other sill plate, and then attach the trusses to that. It seems like somewhat of a thermal short-circuit, though. I would use a 2X that was narrower than the ICF wall, of course. How do you insulate it, though? Strips of 1-1/2" foam, inside and out? Or move the wood to the outside and have more foam inside, or? I was just wondering how others deal with this.

A raised-heel truss and a bit of exterior iso sheathing aligned with the exterior foam of the ICF sill would give you a decent thermal break at the sill. On the inteior you can either cut'n'cobble in some XPS from the sill-inward to the inner EPS plane of the ICF, or seal & insulate that strip with closed cell SPF.

http://electrobus.com/images/passive_solar_design_trusses.jpg

(Ignore the fact that it's a stick-built wall- treat the top plate of the studwall as your foundation-sill plate. Sorry I can't dig up a better picture for you.)

Even if you install a wide 2x top plate that covers from outside to inside of the foam, it still isn't that much of an energy hit because wood is a much better insulator than concrete.

Assuming you have 6" ICF with 2.5" of EPS on both sides, that is 11" overall width.

Pine lumber is ~R1.4 per inch, so 11" of 2x lumber for the top plate is ~R15 which is better than most stick built homes.

If you rip the 2x down to 10" wide and install a strip of 1" polyiso insulation on the inside or outside, that brings the overall top plate R value up to roughly equal with the ICF assembly. Better yet is to install a 2x10 top plate which is 9-1/4" wide. Install a strip of 3/4" insulation on the inside and a strip of 1" insulation on the outside for an overall width of 11". If your ICF forms are 11-1/4" overall width, install 1" strips on both sides. Then, the R value at the top plate will be equal to or greater than rest of the wall.

Note that it is easy to cut strips of rigid insulation. Just install an abrasive type concrete or metal cutting blade on your table saw and it will slice through the foam like hot butter and leave a nice clean edge.

Okay, that makes sense. I'm using 6" Fox, so I have 11-1/4" to work with. I'll go with a 2X10, and 1" rigid inside and out. I do plan on using raised heel trusses, although my contractor says they're way overkill in South Carolina. But then, so is ICF! I'll be retired living on a small fixed income in this house some day, so I need to keep my utility bills as low as possible. I guess I should make the raised heels shorter than the outside dimension so I can run the foam up and over the raised section as well? I would think make them flush with the outside of the 2X10 sill.

Arkie6- Thanks for the tip on cutting the foam!

Posted By jdebree on 30 Mar 2012 04:52 PM
I'm getting down to the nitty-gritty of my build now, and wonder how to handle the transition from ICF walls to the roof structure. Usual method is to bolt a PT 2X down to the ICF, like any other sill plate, and then attach the trusses to that. It seems like somewhat of a thermal short-circuit, though. I would use a 2X that was narrower than the ICF wall, of course. How do you insulate it, though? Strips of 1-1/2" foam, inside and out? Or move the wood to the outside and have more foam inside, or? I was just wondering how others deal with this.

Many installers screw a 2x6 to a plywood shoe and use it to level the the wet concrete an inch and a half lower than the top of the wall. Place a 2x6 mud sill in the trough with the anchor bolts already attached. Alternately you can cut the inside panel down 5.5" and substitute a 2x6 on edge. This time the anchor bolts go horizontally and the assembly goes in prior to the pour. I normally place a horizontal rebar at this level and tie the anchor bolts to it. Make sure the nuts on the anchors are right at the end of the tread so that you don't have to cut the bolts latter. Spacer block are used and the the 2x is attached to the strongbacks. Tack the trusses to the top of the 2x6 and add framing clip for a high wind lift. After the pour is cured tighten the bolts and tack a piece of 3/4" foam to the face of the 2x6 to flush up the wall. You can also leave the concrete down 3/4 and cap the concrete with another piece of foam. Many different ways to do the same thing. Some better than others.

I was planning on doing like others suggested and keeping the concrete down 1.5" from the top of the form. However this probably wont be an option for me since I am choosing a couple windows with transoms and I think I will need every inch of conrete above it I can get unless we engineer an angle iron in there or something. I have raised heal truss, so I will plan on just using a 2x6 or 8 bolted with the face aligned to the inside of the wall. Then I was planning on installing 1/2" osb from the plate on the face of the raised heal (16" heal) to serve as the insulation damn and better brace/connect the tails together and too the board. Obviously I would still use hurricane clips. (or in the midwest, "tornado clips") 2-3" of foam will go on the exterior of the osb to flush up to the exterior face of the ICF wall. Will just have to make sure the block hight plus the 1.5" works with the 54" gyp sheets.

Posted By lzerarc on 05 Apr 2012 04:19 PM
I was planning on doing like others suggested and keeping the concrete down 1.5" from the top of the form. However this probably wont be an option for me since I am choosing a couple windows with transoms and I think I will need every inch of conrete above it I can get unless we engineer an angle iron in there or something. I have raised heal truss, so I will plan on just using a 2x6 or 8 bolted with the face aligned to the inside of the wall. Then I was planning on installing 1/2" osb from the plate on the face of the raised heal (16" heal) to serve as the insulation damn and better brace/connect the tails together and too the board. Obviously I would still use hurricane clips. (or in the midwest, "tornado clips") 2-3" of foam will go on the exterior of the osb to flush up to the exterior face of the ICF wall. Will just have to make sure the block hight plus the 1.5" works with the 54" gyp sheets.

The simplest solution to additional support over windows is to have the truss designer add a LVL beam over the window and hang the trusses from it. Add foam to cover it and line up with the outside envelope. 'couse this only works with high heel trusses unless you drop the beam down the wall.

thats a good idea, have not thought of that. I will be sure to mention it if the engineer can not get the rebar to cover the span. The span is 92" with about 10-12" of concrete depth above. In your opinion, is the recessed plate the better way to go? I will still need to face the heals of the trusses with something however since they will be exposed some on the exterior with my 2' overhangs.

Posted By lzerarc on 05 Apr 2012 05:26 PM
thats a good idea, have not thought of that. I will be sure to mention it if the engineer can not get the rebar to cover the span. The span is 92" with about 10-12" of concrete depth above. In your opinion, is the recessed plate the better way to go? I will still need to face the heals of the trusses with something however since they will be exposed some on the exterior with my 2' overhangs.

I prefer the 2x6 on edge with horizontal anchor bolts because it allows me to have everything complete prior to the pour. However I usually use a loose web ICF so it is easy to cut the inside panel.

As far as the heels of trusses, just order your trusses with the bottom chord ending at the inside of the outside panel and again add foam to flush up the exterior envelope.

Posted By lzerarc on 05 Apr 2012 05:26 PM
thats a good idea, have not thought of that. I will be sure to mention it if the engineer can not get the rebar to cover the span. The span is 92" with about 10-12" of concrete depth above. In your opinion, is the recessed plate the better way to go? I will still need to face the heals of the trusses with something however since they will be exposed some on the exterior with my 2' overhangs.

lzerarc - another thought. Raise the height of wall the amount you would have increased the truss heel height. This should give you the depth of lentil you need for your window span. Have the truss bear at normal height but drop the bottom chord down inside the wall for a standard ceiling height. Now all your heel are protected by a full ICF wall. Probably a tad bit more coin but it answers both problems.

I'm with FBBP on this one. Putting the raised heel truss inside of the wall is what I did on a build last year. Used ICF connect to attach the heels to the inside of the icf wall with Simpson ties holding the rafter part of the truss on top of the wall. Just made sense to have the ceiling insulation in direct contact with the icf wall without interuption. Also from an engineering point of view it allowed the customer to use a 4" wall instead of 6".

I can look into this option. It appears like it would add roughly $1500 to the total price of the foam + concrete. Not sure if and how much it would affect the truss price. One thing I would worry about is if the house was off just slightly from the truss chords, it would be hell fitting the trusses between the concrete walls....

Not really you have 5" of foam to play with.