Why not just go with 2x6 walls and add 2" of rigid insulation on the outside? It would seem to achieve the same goals. The lumber and framing costs should be significantly lower.

I also wonder about your pricing. We compared pricing for traditional stick framing and SIPs. To take the builder mark-up out of the equation, we priced it with the walls built off-site and shipped in just as they would be with SIPs. We were looking at 2x4 walls with 3 1/2" closed cell insulation compared with 6.5" SIPs. The stick framing of the walls in this case are really quite cost effective, something like $2/foot for the framing, plus the cost of the lumber - actually amazingly cost effective, I can't imagine that a builder can compete. The blown in foam was the shocker at something like $5000 for ~200ft of wall. Thus this is at least comparable with SIPS, and probably slightly more and, of course, the energy performance is not as good.

We could have saved some money by going with 2x6 walls and cellulose. Even there the saving was not that great.


Your design certainly doesn't look like a 50% cost saving over SIPs. I don't really see how there could be a saving when I look at the complexity of the framing you're proposing, I suspect that Chris's comment from 2/17 may be applicable to your case - the building is charging you for SIPs as if there's framing to be done otherwise he wouldn't make money on the SIPS. , You could trying to going back to whoever is providing you with pricing and tell them that you're going to have ALL the framing done off-site and shipped in (it should be more cost effective and certainly more accurate to use prefab'd wall panels) and see how the price comparison changes.

stone,
how would i attach building membrane to the foam that is applied to the outside? i am using rainscreen application with open joints and need really good weather barrier (vaproshield). also i will be attaching heavy hardwood siding over vertical strips, this just seems not to work with what you are proposing.

as for the pricing and material cost. quote for my sips is $10,00o roughly for R30 8" thick panels , plus some more for additional dimensional lumber needed for openings, plates and so on. labor charged is $4000,

for double staggered 2x4 walls, $2500 for framing material, $1000 labor, $3000 2" closed cell foam, $500 cellulose for the rest of the cavity. so together $7000.
also quote for sips i included is the lowest one i got, i also have quote for $15000 and crazy quote for $30000.

to be clear, i am open to go with SIPs, i actually was set with them for a while, but $7000 is quite a difference that i can use for upgrades somewhere else.

We're adding 2 sheets of 2" polyiso(4" total)  on the outside of our existing house.

We're attaching it with 6" deck screw through 1x lumber so that we too have a rain plane.  The siding is fiber cement attached to the battens.  So from the outside of the house you have:

HardieBoard - 1x3 battens - 2" polyiso - another 2" polyiso - vapor barrier - wall sheathing - wall

We basically followed this:

Figure 2. Wall with rigid foam on exterior of sheathing.
https://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/96/961110.html

There is a discussion about whether the vapor barrier goes inside or outside the polyiso.  There is no consensus and since the foam itself is a vapor barrier so I suspect that it makes very little difference.

We're adding a new SIP second storey.  The walls are 8¼" SIPs.  We're going with a truss roof with energy heel (cantilever) trusses.  We want the storage.  SIP roofs are expensive and while you can make quality, cool, high catherderal ceilings, it seems a little excessive when we'd just be storing stuff.

Without giving away anyone's trade secrets, the raw panels should run about $3.50-$4.00/sq.ft.   Figure about 10% less than the total wall area since you'll have windows, if someone does a good takeoff.  For $2500 in lumber costs, I assume that your looking at a couple of hundred feet of walls, say 200ft of 8' walls, so you should be looking at about $5500 or so for the raw panels.

If you're paying the panel builder to cut and frame openings, then expect more BUT if you're paying the panel builder to cut the openings then the $4000 on-site cost seems way out of line compared to the $1000 framing estimate.  If you're not paying for the panels to be cut by the manufacturer, then the $10,000 sounds high (but you didn't say how long or how high your walls are). 

If you've got a really complex build then the stick framing may be easier, but I doubt you do if you have a $1000 framing estimate.

The other reason that I think you have an estimate from someone not interested in SIPs is that SIP panels come with wiring chases already cut, so wiring is certainly no more difficult than standard framing - even for an electrician with no SIP experience.  From what I hear, if the elecrician has SIP experince it's lower cost than stick framing.

It does sound, from the framing costs, as though you may have a very hungry builder,  so maybe the stick framing will end up being your most cost effective option.

There are some real horror stories of contractors around here (and probably everywhere).  One of the reason we were going to prefab all the framing was we didn't want a  contractor taking off (part of) our existing roof and then standing there with a skillsaw and sledgehammer saying that he needed more money.  You can always find excuses to up the estimate if you low balled it - unexpected jobsite complications - and if the rain was coming down we wouldn't be in much of a position to argue.

stone, sounds like we are building pretty similar exterior system. i would like to know more about those polyiso panels. do you have more info, or link to manufacturer maybe. i would be intersted in this system. what is R value for 1" of material. what about cost per s.f., what colors does it come in(need black)?

as for the sip quote, it is for panes with openings pre cut in factory. i guess my buiilder is worry about sips as he has never done it. i will be meeting with him tomorrow and discuss our options.
my house is pretty small, 1500 sf, + 500sf garage(not insulated). you can check out my blog here: http://aurhaus.blogspot.com/ , it has some images of house and layout.

We have a little different project since the PolyISO is on an existing 1950's house and the SIPs are a new addition (second storey).

I was in the same position as you. I got one estimate for SIPs, early on, that was SO far out of line with stick framing that I eliminated SIPs as an option. It wasn't until very late in the design cycle that someone suggested other vendors and the price became reasonable. There's a private message for you

Have you considered just getting a package from one of the vendor, cut and numbered and just install it yourself?

I got an amazingly good deal on recycled PolyISO. You can sometimes get new/surplus PolyISO in the Denver area for $15-$20 for a 4x8 sheet, check Craigslist. If you can get sheets at a decent price it should be better than the $3K for foam. PolyISO is _about_ R7/inch from just about any vendor, blown or foam. You might hear higher numbers but it does age a little. As far as I know PolyISO is always a light yellow color, but you can get paper or foil covering. I have paper that is a dark green color.

Any news from the builder? Do you have a SIP convert? BTW, for 2" of PolyISO - if you decide to go that way - you might be able to use 4" nails - you'd have about 1 1/2 inches into the wall. You can shoot these with a (big) air nailer.