Greetings!  Just stumbled across this forum while looking to find some answers on a recent issue with my house.
It looks like there are a lot of knowledgeable people here so... here goes:

   I am in the final stages of re-habbing a 50+ year old farm house in Massachusetts.  My goal is to keep this house as efficient as possible (but stay in budget of course).
Here's the prob.  Since the house was demo'ed to the studs, I chose to have Icynene installed.  Unfortunately the installer did a hack job.  There were several voids and some walls only had an inch of coverage.  Luckily this was noticed before the plaster went up.  The installer said that it was caused because the heat was not turned on in the house (the walls were too cold) and the Icynene did not expand correctly.  He apologized, promising to make it right.  
   The next day he sent back his crew and they fixed some of it, but there are still several areas that are not filled to the edge of the studs.
   He has offered to install foam boarding (at his expense) to the outside of the house to compensate for any shortages in the Icynene.  This would be installed between the Tyvek and the vinyl siding.  Does this make sense?  Also, I was told that this might allow too much flexibility behind the vinyl allowing it to break easier.  I have three kids and there is bound to be a stray ball hitting the house on a regular basis :)

Sorry for the long rant. 

Thanks for reading,
Alexander

Firstly, it sounds like the installers intent is good, but he (or his crew) is not very skilled at their jobs.
Secondly, I doubt that a ball thrown against the vinyl siding (when the outside temp is above about 40-50 F) would cause any major damage to a good quality vinyl. Vinyl siding is very flexible at these temps. And the foam board behind it will allow for more give, which would actually increase the vinyl's ability to survive balls,etc.. TIP--go for a better quality siding that is at least .044" in thickness. It has better appearance over a longer lifespan.

I've never used Icynene, so I can't comment directly, but I do know that cold surfaces cause adhesion problems with some foams and poor expansion sounds logical. Are most bays almost filled? If so his suggestion sounds reasonable, with a few "ifs". Builders and building scientists (see "Building Science Corporation at www.buildingscience.com/) have found that foam board will thermally isolate the framing and increase the efficiency of the home. A 2" (or more) thickness is good; 1" will help, but thinner sheets (1/4" or 1/2") won't give you much benefit. Also, avoid the aluminum foil covered foam - that may cause condensation and rot in the walls since moisture going through the Icynene could condense on the foil.

By the way -make sure there aren't gaps where the Icynene pulled away from the studs - if so they should be filled with spray foam.

Most half-pound foam installers going for a stud fill will blow some excess and trim it flush. Not so in this case, eh?

Sheet goods on the exterior are a good option (and rightfully shoulda been in the retrofit plan in the first place). By putting the foam on the exterior of the sheathing it more than doubles the R-value at the studs & framing, which would otherwise have been a thermal short-circuit. Assuming the wall is 2x4, 16" o.c. construction, the center-cavity R-value of the Icynene is ~R13, but at the studs it's only ~R4 (max). Add ~R1 for the sheathing and it's ~R5 at the studs, R14 center cavity. But if you add 1" of XPS (R5) to the exterior it adds R5 to the studs (cutting the heat loss there in half) and brings the center-cavity to R19-ish.

The "whole wall" and "clear wall" R-values go up considerably when you get rid of those sub-R5 short-circuits from framing elements. Even if some of the Icynene is only half-depth, and inch of XPS will more than make up for it. I'd insist on R5 as a minimum, since there are voids and gaps, but you'll end up with decent perfromance if you do- at least as good as if they'd done the job perfectly in the first place. R2.5-3 (half-inch XPS) might work too, depending on just how much of it is screwed up. Push for R5-R7.5, but don't settle for anything less than R4 (1" of EPS).

Play around with this calculator developed by the Oak Ridge National Labs, which should give you a good feel for what the exterior foam will do for you. 

Select either cellulose or R13 fiberglass for the cavity insulation type- the polyurethane foam in their calculator is the higher density closed cell stuff, not Icynene. With "no foam" selectected in the calculator this would be more or less where you would have been if the job had been done right initially.  Select R11 for  a best-case of how it probably is as-patched-with-voids, etc.  If they don't want to spring for more than R3 (which may be reasonable, depending...) it's still cost-effective in MA to offer to pay a bit more for the material costs to bump it to R5.  The labor costs will be the same for either, but the material will roughly double. (Make them show you their material/labor breakdown if you go this route.)

Stick with XPS if you can, but if they want to use EPS or ISO, insist on un-faced or fiber-faced breathable stuff, or you may end up with condensation issues within the cavity.  Icycene at 3.5" is pretty vapor-permeable, and best-practice in most of MA would be to use at least vapor-retardent paint on the interior finish wall surface and let the assembly dry toward the exterior.  If they use foil or poly-facers on the rigid board the sheathing layer can't dry in winter, since it's inside the vapor -envelope, and colder than the dew point of the interior air much/most of the winter.  XPS is semi-permeable, which allows the assembly pass water as vapor out to the drier winter outside air.  Fiber faced iso or un-faced EPS is even more vapor permeable than XPS, but not as structurally rigid, easier to damage in handling, etc.

When I got Icynene foam insulation about a year ago, I too was concerned about the depth. Mine was 2.5 to 3 inches. I did some research, and somewhere on Icynene's site I found text that listed the companies suggested instillation depth. I think it said that the suggested depth was 3 inches. And it seems like they said anything over 2.5 was acceptable.

Posted By glien on 01 Mar 2010 11:01 PM
When I got Icynene foam insulation about a year ago, I too was concerned about the depth. Mine was 2.5 to 3 inches. I did some research, and somewhere on Icynene's site I found text that listed the companies suggested instillation depth. I think it said that the suggested depth was 3 inches. And it seems like they said anything over 2.5 was acceptable.

Acceptable is in the mind of the beholder, eh? At 3.5" the center-cavity value of Icynene is about R13.  At 2.5" it's about  R9.  

Is R9 an acceptable substitute for R13?

At R9 I'd be insisting on some rigid foam, or filling that last inch with a flash of 2lb foam. YMMV.

At 1" it's no more than R4, best case (assuming it's thin due to slightly higher density.)