Hi all,   

I have been struggling to determine a solution for matching exterior foundation insulation to the outside of a sip panel.

http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/5/tpage/1/view/Topic/postid/50157/Default.aspx

this thread is close but no cigar (well its unfinished)

Anyway, I want to insulate the exterior of my basement with 2 inches of xps from footing to sill plate. I will also need 1/2 inch of EIFS on top of the xps above grade. This will leave a foundation 2.5 inches larger than the outside of the osb panel on a sip. I would prefer to have the outside of the parging match the outside of the osb because I would like to have the vertical siding extend slightly below the sill plate (effectively covering some of the top portion of the  finished (parged) foundation). SimonD responded in the above thread with a  picture of a slightly cantilevered  double sill plate. The difference is his drawing was for balloon framed construction and I will be using platform construction. My thoughts were what about using a double rim joist slightly offset (platform construction) >>> the second joist (laminated to the first) would give 1.5 inches beyond the sill plate bearing area and if you can cantilever the sill plate over say 1 inch then I will have acheived the 2.5 inches I need to match the sip to the foundation. This way the inner rim joist is resting completely on the sill plate and the sill plate itself is overhanging the concrete of the foundation by only 1 inch. Would a double sill plate still be advisable? The above thread's author was going to resort to xps on the exterior of the sip and with strapping I could use that to move the siding out to match the foundation but I'm not that interested in adding 2+ more inches of insulation to the building. I could comprimise and double the rim joist (gaining 1.5 inches) and add an inch of strapping/ insulation to the sip exterior before the siding (that would help keep costs down) and leave the sill plate flush with the foundation.

Also, in the previous thread what was the general consensus on where to place the insulation over a sip versus the membrane (typar/tyvek) ? would it be better to strap/insulate/wrap then side or to wrap/strap/insulate/side. Since xps foam doesn't effectivel absorb moisture it can be exterior to the house wrap -- right?

Any ideas anyone has would be appreciated.  I can't be the only one who doesn't want the foundation to have a 2 inch lip all the way around the building and covered by a piece of flashing.


p.s. I'm hoping to use 6.5 inch sips and no termites live here

If you use platform framing, then just extend the floor joists out over the walls to the desired distance, attach your rimboard and proceed with your framing. Use spray foam insulation in the rimboard area. Your joists can safely cantilever a couple inches, and it will be far stronger than doubling the rimboard, and cantilevering the sill plate. This is a common technique for squaring up framing on out-of-square foundations.

I think this detail could work. A studded angle embedded into the edge of the foundation wall.

What is this, drywall, plywood, or ?

Here is the whole picture. metal SIPs on ICF w/ wood floor system

What Wes said will work, but if you're uncomfortable cantilevering your floor joists out, add a steel angle like JeffD shows for support.

Yes thats fine but I plan on using platform construction with concrete blocks for the foundation. The whole ICF thing is entirely too expensive in this area. You can hire a block layer then insulate the inside and outside with rigid foam (r-22 (r10 + r2 + r10) for considerably less cash. Full length exterior foam will also supply a slip surface to guard against adfreezing and most studies show the foam supplies a primary drainage surface which virtually eliminates the concerns over foundation leaks (with properly installed footing drains). A third bonus is the foam layer helps protect against side loads on the block from improper back filling. That being said, angle iron mounted to the top of a concrete block wall is probably not a good idea.

Wes mentioned on the second post that this is relatively normal for correcting out of square foundations which I have no doubt is true but what we are talking about is up to 2.5 inches around the entire foundation. On the walls parallel with the joists the stringer (end) joist will be hanging in space where it butts the next stringer joist and on nails where meets the rim joist. To support the butt joint where 2 joists meet you would have to extend the supporting beam out through the foundation wall.

Here are some pics for better clarification. In the first one only 1/2 inch of the inner doubled joist is bearing on the foundation (via the sill plate). Do you think its structurally sound? I could easily laminate the doubled joist's together or add a second sill plate for further strength. It all depends on how the sips bear on the platform (vertically through the osb to the subfloor to the joists or vertically & laterally to the 2 x 6 plate to the subfloor to the joists?) 

In the second pic I have added 1 inch of foam and furring strips to the building exterior. Some of the posts here recommend an air gap behind the siding so this does this along with perhaps other advantages >>> better nailing through the furring strips >>>> increased r value via additional insulation >>>better insulation at the header area.

I have left the doubled rim joist in the second pic (for illustration purposes) but there is still only 1/2 of joist bearing on the foundation albeit it is the exterior one.

Any thoughts are welcomed >>> the budget is tight so I'm not interested in overkill but if the results regarding cost versus benefit are reasonable I'm interested.

Drawing 2 incurs the additional cost of furring strips/foam insulation on the exterior but might save on double perimeter joists. Maybe I could go with a cheaper type of foam? (4'x 8' white stuff) or use smaller furring strips and no foam (which should move the inner joist back over the foundation more) larger strips with thicker cheap foam etc...

Anyway thanks for the replies.

Exackery,

I really like your detail drawings. Well done.

I don't know all the details of your project, but below is an efficient way to frame it and still be structurally sound. You have to alter your floor framing at the ends and turn it perpendicular for about 1-2 floor joist spacings to create 'lookouts' to accomodate the slight cantilever. Not hard to do. Notice how in my detail the foundation insulation runs up to insulate the sill plate and meets the insulation you will have to put in between your joists to insulate your rim board.

Here is how the floor framing would go to make the detail above work.
 
As a disclaimer, I am not an engineer and not familiar with all the particulars of your project, but you can present the 'framing concept' to your design professional for evaluation. 

Don't forgot your 'termite sheild' (sheet metal flashing). You don't want a clear path through the foam from the gound to your framing.

a good idea to use treated foam anyway below grade for termites and carpenter ants

SimonD's last drawing shows the concept that will work very well to do what you want to accomplish. Floor joists of 2x10 or larger (or eq. engineered wood) are routinely cantilevered up to 24". This should not require any major engineering for approvals since it is routinely done with 2x's
and enigneered I joists all have this data available.

Thanks for your replys >>>  I was looking for a straight answer which you have given. I'm not that familiar with sips and it looks like a lot more load is applied to the subfloor along the edge (and consequently the rim joist) than with stick construction which is applied across the width of the bottom plate. Once the rim joist leaves the sill plate in a cantilevered design it is being held in place by nail shear alone. I wasn't sure how sips distribute vertical load to the platform.

I hope this didn't seem like a stupid question!

Again thanks for the replys



G. T.