I am contemplating a quite small house. In fact if all goes well, this house will actually be a guest cottage for a "main house" to be built in the future. I would like to try a few building systems out on the first project and would like comments. Climate is an island off the Rhode Island coast - 120 mph winds in winter, low winter temp of 0 to 5F, Summer highs of maybe 90F tops but mostly pleasant and breezy so no AC. Humidity is very high most of the year - lots of fog - mold and mildew are always an issue. Elect. costs are among the highest in US (currently $.45/kWh) and other fuels almost as high proportionately soon energy savings are huge. I am thinking of an ICF foundation/basement to contain services for this house and future (well tank, water heater, elect panel. etc) first floor will be bar joist/metal deck/ and 3.5" slab with radiant in it, polished for finish. Walls will be 2x4 @ 24" structural wall with 2x4 horizontal "strapping" installed "on the flat" at 24"oc to the interior of the structural wall with 5" long screws at each intersection. This is sort of a "Mooney Wall". Total framed wall thickness will be 7". The plan is to "flash" the interior side of the plywood sheathing (1/2" CDX) with cc foam about 1" to 2" thick then blow in dense cellulose to fill the remainder of the 7". The only "thermal bridging" would be the 1 1/2" squares at 24" where the 2x4's intersect and the foam could enclose the exterior, structural 2x4's (the interior 2x4's are essentially blocking. Roof is still a question. Roofing will likely be wood shingles or standing seam. I want to avoid SIP's for the walls as I have concerns (founded or unfounded) about OSB and high humidity however I'm toying with SIP's for the roof (which will be steep 10/12 or 12/12). This will be a small one story affair but may have an open "loft" space so think "cathedral" not attic. All mechanicals are to be in the basement. Here are my questions: Siding: The traditional, and my preferred, siding is white cedar shingles unpainted (think Nantucket). Normally these are installed over a house wrap or felt over CDX (maybe with cedar-breather). I prefer felt due to the cedar surfactants allowing bulk water through the house wrap. Is there some way to introduce a continuous layer of rigid insulation on the exterior of the CDX and carry the shingles without screwing up the drying flow of the wall? Any ideas for drainage planes/rain screens? Is there a formula or modeling system for determining the required thickness for the spray foam "flashing" to keep the dew point out of the cellulose? I'm super concerned about the walls accumulating moisture. Anyone have any other ideas for boosting efficiency and/or saving money? I plan to GC this myself (I'm an architect with lot's of hands on experience.) so labor is less valuable than materials. Also, on an island specialized crews (like spray foamers) are extra costly as the crew needs to be ferried and housed - so think conventional materials and methods where possible.

Birdman
You can install 2" of XPS (tape & seal joints) over the CDX & house wrap, then install strapping for the rainscreen & shingle over that. There s house shown on the Building Advisor website with horizontal strapping where they notched the back of the strapping to allow vertical drainage. Seems like a lot of work, but the rainscreen is valuable in providing the air space where any escaping moisture can disapate. It may be that a horiozontal air space behind the shingles will provide enough air to vent or drain the cavity without the notching; haven't read any tests on this. (But the shingles are certainly not airtight) As for SIPS on your roof (and walls), the main problem over the last 30 years of SIPS use seems to be moisture escaping the building & condensing, destroying the OSB. Most manufactureres recommend venting roof SIPS with strapping another layer of sheathing, and using a rainscreen technicque on the walls, but no one seems to actually do this, citing costs and the "we haven't had any problems" excuse. I intend to use SIPS incorporating venting on walls and roof, as well as with curtainwall panels on our upcoming timber framed homes. It does add to the cost, but it seems inexcusable to take chances with an excellent product.

Let me throw out some ideas.
This is a small house. Idea #1 is use ICF all the way to the rafters and then use Metal scissor trusses to get the cathedral effect for the roof. Use a structural attachment with a snot load of uplift capability to tie the trusses to the wall. Alternatively use litedeck for the roof.
When you are done the little house is the hurricane shelter for the big house to be built later.
Then afterwards the little house holds the building crew that rebuilds the big house after the storm.
 
I think you will have a problem in a 120 mph wind zone getting a stick framed 2X4 OVE house approved by an engineer.
I would start with 2X6 OVE and strap horizontally with 2X2. Almost as efficient, not quite.
 You could also just use I-joists vertically, this gives very minimal thermal bridging. I think the real problem from a durability standpoint is that Wind driven rain will be a big problem.
I would think you need a rainscreen design with some extra redundancy and lots of drying potential to the inside and outside if you stick frame. Of the top of my head it would be cedar shingles-cedar breather (the thickest one made)-30 lb felt- zip sheathing( with taped seams). I might use the roofing version of zip roofing panel on the walls for extra wracking strength. This assembly gives you 3 layers that stop water intrusion Into the walls. Insulation would be Cellulose for lots of hygric buffering, walls would all be mold resistant drywall with the ability to dry to the inside. 

I think I would build this one ICF if it were me.

cheers
Eric

Bob, Good points. I had thought of the strapping to receive the shingle - and even wondered about the vertical drainage issue. Shingles in our area are typically installed 5" exposure so that's a lot of strapping. My guess - as I think was yours - is that the numerous joints in the shingles would allow enough gaps for the moisture to escape without the grooves in the strapping. My options seem to be 1.) as you describe, all cellulose in the cavity, CDX, 2# XPS (taped), housewrap, strapping & shingles or 2.) 1" of cc foam flash in cavity with balance cellulose, CDX, 1/2" or 3/4" XPS (taped), housewrap, shingles with long nails. or 3.) 2" - 2 1/2" cc foam flash in cavity with balance cellulose, CDX, felt, (optional cedar breather), shingles. Here are pros and cons as I see it (I'd welcome others): 1.) Pros: Money saved on spray foam, Thermal bridges killed dead, probably highest R value of all. Cons: Money/labor spent on strapping, overall wall now 12" fin. to fin., tricky details at openings, longest . 2.) Pros: Eliminate strapping, wall down to 10"-11" thick, opening details still a bit tricky, cc foam gives good air seal. Cons: Expense for labor for two foam systems instead of "one and done", foam on both sides of CDX may inhibit drying of CDX, opening details less tricky, shingles out on the ends of long nails. 3.) Pros: Exterior details are "conventional", cc foam is good air seal, exterior studs completely "wrapped" in foam, probably fastest method. Cons: 2"+ of foam is high $$, perhaps lowest R. Love to hear other thoughts.....

I agree with Eric that your best bet for sheathing is ZIP sheathing taped.  XPS is available at commercial roofing suppiers for less than I've seen it anywhere else - .65-.75/sf for 2"  - $24/sheet.  Look at BuildingAdvisors.com or BuildingScience.com for window installation details - not that complicated.  Another "pro" for this system is Best Drying Potential (=longer siding life) which I think would be pretty high on the list in this location.

Good comments. I contemplated ICF's long and hard, and there are a lot of pluses. The big negative is the twelve miles of open ocean between me and the ready-mix plant. Concrete on this island is now well over $200/yard and the quality control is very sketchy. Self consolidating is unheard of. The joys of island life!! Also, not to open the past debates but I'd like to see better than R 22. I admit that when it's blowing 120 with gusts to 145 that ICF house sure sounds nice. I have a bunch of other ideas on how to make the framing work for the lateral loads and I think it can be done - but it is truly valid issue you raise. One thought is that the hor. 2x4's would help stiffen the whole wall assembly, but that may require ties at the intersections, not just a screw. Funny you brought up the TJI as stud (ala Passivhaus). I originally thought of essentially building sort of Larsen trusses only with the structural wall to the exterior. Just plywood gussets off the studs to carry 2x3's to carry the interior finish. I still like a lot about that idea but it won't stiffen the wall in a breeze.

Birdman,
I agree that R22 is not so great in a true heating dominated climate. There are ICF blocks that have thicker foam parts that perform better than R22.
 The one thing ICF does is really reduce the infiltration of the walls, which is super critical in a very exposed windy setting. I am not saying you can’t get there with Stick framing, just that is not easy.
 
I could see a wall with the outside wall being load bearing. One guy to talk to about that approach is Jessie at www.kaplanthompson.com.
They had a double wall structure that was featured in JLC with the interior wall non load bearing. Nice execution by the GC (Kolbert)that built it and documented the build. I am sure you can buy the article online for ideas

If you used gussets to tie the 2 walls together, you could probably still strap the diagonals of the inside face of the outside wall with let in bracing to stiffen it up a bit. Besides, using Zip roofing panels will stiffen walls up nicely, these should have plenty of resistance to wracking with a good nailing schedule. I would probably use an engineer to spec the uplift prevention in that kind of wind area.

I was on St Croix after Hurricane Hugo blasted it. It was amazing the damage it did to buildings. I also spent a lot of time on Cape Cod as a kid. All those old line Portuguese fisherman had it figured out, views of the water are bad.   Build houses  in the valley, down low and sheltered. Use lots of concrete and rebar and big arse hold downs, Keep the mechanicals out of the basement to prevent flooding damage. Use generator powered sump pumps. Maybe it only matters every 50 years, maybe next year block Island gets hit by a Cat 3 hurricane.


Cheers,
Eric

I'd lean towards ICFs - and away from materials that rot or corrode (like wood and steel nails). The high concrete cost doesn't add that much. Or possibly formed on site, tilt up concrete walls + your own interior insulation in a non-load bearing (foam studs + wood furring strip + cellulose in between?) interior wall.

If you let me know the exact wall buildup, I'll enter it into WUFI to model the moisture accumulation. But I've become convinced that leakage through the cracks (which all houses have) is far more important than through the materials themselves. High winds make it worse - non rainscreen walls will suck rain into the wall in the places where the interior pressure is negative.

If you want beautiful and to avoid the need for siding, take a look at this:

http://www.hollowtop.com/cls_html/tiltup.htm

ftp://imgs.ebuild.com/woc/C660001.pdf