We're looking to improve the insulating performance of our walls and windows on our 1959 ranch on a slab in central Pennsylvania. The current windows are very leaky single-pane aluminum-frame windows, probably from 1959, so just about anything will be an improvement! We're thinking Pella Impervias, just due to availability here.
The walls are 2x4 stud filled with (guessing) R-2 fiberglass batts, with foil-backed drywall inside and 1/2" crumbly yellow foam outside, and vinyl siding over that. No sheathing. I think there's tar paper in there somewhere, but they were pretty casual about putting it on (at least 4" of the bottom of the walls exposed in most places). We get a lot of mildew on the inside walls, both in the humid part of the Summer and the cold part of the Winter, which really drives up the wife-says-do-something factor...

Our plan is to take off the current siding and foam board, spray in foam into the cavities, put 1" or 2" foam board on the outside of the studs for a thermal break, then housewrap and hardie plank (fiber cement plank) siding. Now the questions...

We've found a builder who's done fiber-cement board siding before. He wants to put up OSB sheathing (not 100% certain it's necessary, but I'm not against it), and nail the planks into the studs. If the foam board is put on, however, he doesn't know how he should be attaching the planks through the board. The other issue is whether to use furring strips to create an airspace behind the planks; he's never done that before, and is again wondering how to attach the planks. James Hardie Inc. has installation guides for attaching through foam, and for attaching with furring strips, but they don't show what to do with both. So:

- How necessary is an air space behind the siding in Central PA? My thought was it might help our internal dampness issues, but I can't figure out if it's required by the IRC or not.

- Can anyone point me to details for fiber-cement planks on furring strips over foamboard? Nailing schedules would be great, too

- builder's sub for the spray-in foam is using open-cell - should it be closed-cell for this application?

- Since the builder hasn't worked with foam board under siding before, should I just be moving on? So far he's willing to work with me on this, but he's obviously frustrated to be doing it differently from his usual method. There's another builder in town who I know specializes in green building techniques, but their quotes are jaw-droppingly high (3x to 4x other builders' quotes for the same work and materials). Yes, I do get what I pay for...

Suggestions, comments, and abuse cheerfully accepted, thanks!

Yes, I do get what I pay for..

Not necessarily. My best work this year on new and uncommon (read: Green) things has been off the lowest bids.

With 2" of foam board it's EASIER to install with the rainscreen gap than without it, even though it's not necessary for IRC complianance on a foam-clad building.  In your climate zone (US zone 5) 1" of XPS or iso is sufficient for dew-point control.

But 1" foam-board with 10,000 nails holding the fiber-cement to the sheathing, the thermal bridging of those nails substantially reduces the R-value of that foam layer due to the much higher thermal conductivity of steel compared to wood or foam.

If the wall is already X-braced with cut-in timbers or steel you might get away with foam-only but even if it's braced to structural code a layer of OSB detailed as the primary air barrier would be more robust and add structural rigidity to the wall.  Bulk water incursions causes more moisture issue in walls than air leakage, and air leakage is an order of magnitude bigger problem than vapor-diffusion. Get the flashing around windows & doors right, and make the walls air tight!

To handle the foam with a mini-mum of thermal bridging, tack it in place with foam-board adhesive or hold it up while installing furring to hold it in place. Use pancake-head timber screws (FastenMaster HeadLok or similar) 24" o.c. that penetrate the sheathing & stud by 1.5".  For 1" foam and 1x furring that would take a 3.5" screw, for 2" foam 4.5".  They're available at that orange-flavored home center store. (For a flatter wall some prefer to use 2x furring, but that adds to your fastener budget- they're not cheap.)

Details on how to deal with the foam, housewrap, furring, flashing, etc can all be found here, if you click on all the highlighted links:

http://www.greenbuildingadvisor.com...-sheathing

Thanks for the link; lots of good stuff over there! I think I'll have a hard sell to get the builder to accept only nailing 3/4" deep into the furring strips to hold up the siding, but at least I can show him some sources and prescriptions to back it up. Right now he thinks I just came up with this to torment him...

As far as I can tell, the only things keeping the walls from racking are the inner drywall and the outer 1/2" foam. I'm kind of amazed it's stood for the past 53 years, and this is not the only place they cut corners! Maybe we should evacuate before we remove the siding, just in case?

Posted By flyingpig on 23 Aug 2012 04:48 PM
Thanks for the link; lots of good stuff over there! I think I'll have a hard sell to get the builder to accept only nailing 3/4" deep into the furring strips to hold up the siding, but at least I can show him some sources and prescriptions to back it up. Right now he thinks I just came up with this to torment him...

As far as I can tell, the only things keeping the walls from racking are the inner drywall and the outer 1/2" foam. I'm kind of amazed it's stood for the past 53 years, and this is not the only place they cut corners! Maybe we should evacuate before we remove the siding, just in case?

You could save the argument for the upcharge of going 2x4s for the furring and upsizing to 5" HeadLoks (assuming 2" foam)

You can strip the siding without racking by cutting vertically then stripping the siding nearest the corners sufficient to install single sheets of plywood or OSB sheathing at the corners.  That will be enough to keep it from racking when you pull the rest of the siding. You might put up all the sheathing after stripping the junk from the cavites, then drill & fill dry-blown cellulose from the exterior, sealing the blowing-holes with can-foam before putting up the housewrap.  Blown will fill-in and work better than batts, but for a small job there may be a premium to pay for cellulose vs. batts. If batts, make sure they split rather than compress behind electric wiring & plumbing, and trim to fit rather than compress around electrical boxes & other anomalies- it makes a difference. 

A bead of caulk/can-foam/or construction adhesive between the edges of the sheathing and the framing helps air-tightness immensely, and if you're using batts rather than blown cellulose the air sealing is more critical to performance.  (If it were my house I'd do the air sealing no matter what, but it's hard to say how much you can torture the contractor you're dealing with.)

[edited to add]

I just re-read the origingal description- forget the above fiber insulation recommendations- open cell foam would be more air tight, and probably isn't breaking the bank.  Air sealing the edges of the sheathing to the framing still counts though- the o.c. foam only seals the cavity portion, and a doubled top-plate has a seam that will leak.

So, I sent the builder the wall detail I was looking for, backed up with a batch of articles (some from Dana1's link, and some notes from a couple of Fine Homebuilding articles). Between my sources, and him talking with an insulation installer he sometimes works with, he's on board now. Now to see what he'll charge for the extra work...

I'm doing the same 2" rigid foam over steel stud house with rainscreen gap with Hardie Plank. Something that only came to light when I started, is how to seal the bottom rainscreen gap, and how to supply air to the top of the rainscreen. I ended up redoing my soffit, so the soffit provided the air to the top,(The 2x4 attached OVER the furing strips, not directly to house) and used/cut cheap cobra vent like stuff to stop the bugs from going into the rain screen. There are some products out there, can't remember the name but I know it wasn't cheap..

Cor-A-Vent makes stuff suitable for critter-restant bottoms of rainscreens of various thicknesses, and there are polypropylene mesh solutions out there too. Detailing venting the tops of rainscreen cavities under window casings/sills etc takes some creativity, but there are numerous ways to go about it.

I found this for Hardie. On page 3 for 1.5" ( up to 1.5" thickness ) there is a specification of the nails without the use of furring strips. More than 1.5" thickness of foam is not recommended unless furring strips are used.

I was considering 0.75" of polyiso in R5 and was concerned about dew point. Z5 needs R7.5. Polyiso is R6.5 for one inch. So one needs roughly 1.15"

Now with this info (not needing furring) I will probably do 1.5" (two layers of 0.75" unfaced polyiso) staggered on the outside with the siding right on top. or 1.25" (0.75 and 0.5) to save cost

http://www.jameshardie.com/pdf/technical-bulletins/19-jh-over-advanced-framing-or-continuous-insulation.pdf

Just spoke to jameshardie technical rep, he confirmed they still are ok with 1.5" on the outside...