On our new sip house our builder wants to use 3/4" furring strip  between the drywall and sip surface to create space for ease of wiring.  At first I thought I would fill that space with foam boards but reading several confusing paragraphs on  various insulating products available I think I found that 3/4" foam has a R value around 3 (the cheaper stuff) and  3/4" airspace in an insulation stack has only slightly less.  This somehow doesn't seem right to me.  We will use sips with steel framing which is exposed on the surfaces.    Any guidance would be appreciated.  Thanks.

Dick

Dick - I built a house with SIPs, but I am not a SIP expert...but...I believe the above idea is with furring strips is great.

Your SIP wall is the insulator. Adding extra foam just adds more insulation, but do you really want it? You would have gotten the R equivalent of your SIP wall if you were to screw the drywall directly to the SIP wall.

3/4" XPS (not EPS) has a 4 R-value.

Dick,

Check with your contractor but if you are using steel clad sips or OSB, I guess it would also work, but I used 1 1/2 " High Hat furring strips on the walls. With 1/2 inch drywall this will give you a two inch space just the right size for using shallow electrical boxes. This will eliminate the need to perforate the was to set the box.

The foam R-value of about 3 is about right. An airspace(1"-4") is only around R-1 according to ColoradoEnergy.org. Also, something other than air would be a pseudo fire block.

After the electrician has done his thing, I would use the 3/4" foam. A little more R-Value never hurts!

Dick;

I don't blame your builder for wanting to do it that way.  It is the way we do it with steel SIPs , using 1-1/2" steel hi-hat as shown on the link

then use a 4" x 4" x 1-1/2" deep box with 1/2" plaster ring  it also saves from cutting skin

just leaving the dead air space will leave insulation value too

Thanks everyone for the info.  Counting the drywall, sip, siding, and air space the wall comes to about R 25.  With the increase of 2 in R value from foam replacing the air the new total would be about 27 or about 8% better.  The cost of the R 3 foam will be adding something like 0.35% to the cost of the job.  With the cost being in the noise and the benefit not insignificant, I think I'll go for it. 

I'll think about the bigger furring strips.  They would double the space lost from 17 to 34 sq ft.   It's a long skinny house, so it's got more wall length to lose inches from than if it were square.  It's going on the same footprint of the one it's replacing only longer in the long dimension and with a second floor added so it has to be skinny.  It has to fit between an existing 600 sq ft room on the back and a 3 car carport on the front.  I'll talk with the electrician and see what I'm trading for that 17 sq ft.
 
Dick

Chris, can you tell us more about those steel joists at that link? Who makes them? What do they cost per lineal foot?

Regular NM(romex) house wire must be 1" from the surface.
Better check with your electrical inspector or use BX before using 3/4" strapping.

that is 1" passing thru  a stud, where the romex passes thru a  furring strip it must be protected with a "cant-nail" plate.

We do not even need to to that when we use 1-1/2" hi-hat ... gives sufficient clearance as is and has been accepted even in commercial buildings