HI all.. I'd like to do 2x6 walls with 4" xps sheathing, but am wondering what kind of cladding I can use. I'd like to use real cedar bevel siding, but would need vertical furring strips. Anyone with any experience as to what kind of fasteners, how thick of furring strips etc to use?? Or....should I try to find a builder to do 2x8 walls and use 2" foam? Thanks mucho..

check these out:

http://www.insulfoam.com/images/stories/docs/JLC-finalfeature.pdf
http://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall

it'll give you an ideea about what needs to be done. I'd go with the 2x6 and 4" of foam

good luck
Adi

What Adi said.

Two inches of extra cavity depth buys you less than half the whole-wall R than the 2" of foam it displaces.

An alternative approach is to XPS & furring is to use nailbase iso panels (Hunter, Atlas, et al). A 3.5-incher runs ~R20, and has a half-inch OSB nailbase skin to it (a split-SIP) on which to hang the siding. On those you'd still want at least a quarter inch of rainsceen gap between the OSB skin and siding to keep the siding dry. (Code in Canada is for 10mm minimum, which is about 3/8".) Or, using iso panels without facers also works. At 2" of XPS on a 2x6 wall you're already slightly past the knee in the curve where the lifecycle greenhouse gas impact from the blowing agents is now rising with every inch of foam (due to the type of blowing agents used), whereas with high-R iso that knee only occurs at a much higher R. See:

http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-global-warming-impact-insulation

So with 4" of XPS you're quite a bit beyond the optimum. Exception: Using XPS scavenged from commercial roofing for re-use is pretty much GHG neutral, since you're extending the benefit-cycle length of a GHG hit already taken. Used XPS/EPS/iso is often available locally from commercial roofers or demolition operators at a small fraction of the cost of virgin goods. (I helped out on a small charitable project over this past weekend with 4x8' sheets of 4" used iso that cost only $15/sheet, about 20% of the cost of virgin goods. It had a few dings & drill holes to fix with 1-part foam but even with full-freight on the labor & spray foam it's money in the bank.)

Ideally you'd have full vents bottom & top on each furring-bay, (screened, to keep critters from calling it home) which will keep the siding much drier. With any amount of vented rainscreen gap the moisture content of the siding stays low, paints last much longer, and the wood is far less prone to rot. This means you can use less expensive wood than cedar and still have it last for a very long time. (If you were going for the un-painted weathered look you'd still want cedar though.) Primed both sides, and with all cut-ends sealed/primed, back-ventilated wood siding lasts pretty much forever.

Well according to me going for 2 sheets of insulation sheating would be enough......

Thanks for the responses and links. But I still am not sure how thick the furring strips have to be to support cedar siding. I've see differing opinions. I wonder if I need to alter my plans to accomodate the thick walls by extending the eaves..? Another thing is the high cost of the fancy Fastenmaster screws for attaching the furring strips...

4" sheathing / furring is hard to support with screws alone, your design needs to be supported at the bottom to carry the weight of the siding

intuitively it would seem that way but according to the REMOTE system guys is not necessarily true for all types of siding.

adi

Seems like someone needs to make a foam insert with a screw that distributes the downward force into a wider section of the foam. Imagine a 1" wide, 4" long piece of sheet metal (or plastic) that has a tube down the middle for a screw and is pushed into the foam. 4" of XPS foam is pretty strong. Glue would also help.

Posted By pbrane on 28 Jun 2011 10:04 PM
Thanks for the responses and links. But I still am not sure how thick the furring strips have to be to support cedar siding. I've see differing opinions. I wonder if I need to alter my plans to accomodate the thick walls by extending the eaves..? Another thing is the high cost of the fancy Fastenmaster screws for attaching the furring strips...

Cedar is very low density stuff- standard 1x3 or 1x4 (0.75" nominal thickness) is good enough from a mechanical strength point of view (even for fiber-cement), but 2x furring is stiffer & straighter, and sometimes easier to install (but requires longer $crews.) 

I would consider ripping strips of treated plywood and then glue and screw them on.

.

Posted By jonr on 29 Jun 2011 11:59 AM
I would consider ripping strips of treated plywood and then glue and screw them on.

Nice Idea....

jonr; PT plywood wants to warp before its cut, once its cut it quickly turns to a pile of green spagetti. It is so saturated that many suppliers refuse to rip for you, as they have burned up motors.

pbrane; a simple solution is to use a clip to attach the top of furring to the rafter tails in addition to the long screws will keep the stress off the screws and keep the siding from sagging off the face of the wall.

Maybe treated OSB? Chris, what would you use?

jonr; I would use 1 x 3 cedar furring which is naturally rot / pest resistant and is stable - not often warps. Attached as previous stated

this screw is ideal

1x3s (particularly cedar) are more likely to split- 1x4s are the usual recommendation. (Fir or douglas-fir would be better from a structural point of view.)

Pressure treated isn't usually necessary for vented rainscreen strapping due to excellent & rapid drying capacity.

Dana1 ; you are right, they should be pre-drilled not just to avoid splitting, but because it is too hard to start a 5-6" pancake screw without wobbling. I like cedar because of its resistance to rot & pests, PT is too hard to work with, fir and douglas fir is not readily available in our area, spruce and SYP is more common, but SYP same as PT is hard to deal with.

Posted By cmkavala on 02 Jul 2011 04:31 PM
Dana1 ; you are right, they should be pre-drilled not just to avoid splitting, but because it is too hard to start a 5-6" pancake screw without wobbling. I like cedar because of its resistance to rot & pests, PT is too hard to work with, fir and douglas fir is not readily available in our area, spruce and SYP is more common, but SYP same as PT is hard to deal with.

Are you sure about the stuff that what Dana1 wrote is correct..... ?