I have posted before about our exterior insulation of using polyiso 2" on our new house. We have nearly completed the installation of the foam and taped all seams. Getting ready to prepare for the next steps and wanted to throw out a couple questions to everyone. The majority of our new home will be bricked, but we have a couple of small areas that will have vinyl siding and an area that will have hardie plank. With the 2" foam we feel that we need something on the exterior to mount vinyl and hardie plank to. The most recent thought was to sheet the exterior foam with OSB to mount the the vinyl and hardie plank to. In these areas it would mean that we would have a stackup of OSB sheathing then 2" polyiso and then another layer of OSB sheathing. I'm fairly confident this should work fine for the vinyl siding doing nothing more than mounting siding directly to OSB, but have heard that hardie plank may need some air space behind it to dry out and not cause problem with the OSB. Would installing tar paper over the OSB and then the hardie plank suffice?

Also, am beginning to look into wall cavity insulation options. I had been planning to use a wet dense pack cellulose in the wall cavities to finish out my wall stackup of insulation, but have been warned by many that even with the wet dense pack cellulose over time it will still have a tendancy to settle down the wall cavity and create a void near the top. I don't know if this is true, but am looking to others if this is a valid concern. Thought about open cell foam cavities, but seems to be very pricy. Another option that was presented to me was 1" of closed cell sprayed with R11 fiberglass batts applied to complete the cavity fill, which logically to me sounds like a worthy idea, especially considering that I've already got R13 polyiso on the exterior side of the wall. This should keep the dew point away from the interior side of the wall where the fiberglass batting is done. Thoughts please???

I can't really comment on the cellulose - I considered that option & went another direction, partly for the same reason. Everyone says that it doesn't settle, but I have yet to see any 10+ year tests that prove it. I wish I could find something, because I think that overall it's a very cost effective solution.

When we built, we did the flash & batt, that you're describing. A couple benefits that made us go that route.. We did xps foam on the exterior, & in the non-structural areas ONLY used the foam (no osb). The inch of spray foam adds some rigidity to the wall. Also, that along with caulking between the studs really helps to create a good air-sealed house.

Use 1x furring strips (mounted vertical) long screwed through the foam board into the 2x studs instead of OSB on the exterior of the foam for a couple reasons:
-provides air space & drain plane between the foam board and your siding
-provides solid material to attach your siding to... you really need more than 1/2" OSB to nail your siding to (I believe this is code also... Can someone verify? Or you nail the siding through the OSB and into your 2x studs that are 16"oc or 24" oc)

For insulation, since you have 2" polyiso (this is a vapor BARRIER) on the exterior over your OSB you should be able to spray foam up to 1" closed cell spray foam (Dana can verify, I believe up to 1" would be a vapor RETARDANT not BARRIER) and fill the rest of the cavity with spray in cellulose/dense packed cellulose (this would be my first choice) OR fiberglass batts. How do you plan on insulating the rim joist? I suggest spray foaming the rim joist too, maybe use open cell as thick as you want so you don't have a situation where you have more than 1 vapor barrier. You could also use open cell in the wall cavity instead of closed cell.

The most careful study I've seen to date on cellulose settling is Rasmussen's work on dry-blown dense-pack, showing that there is a calculable definable minimum density for stability that is climate dependent-  non-elastic creepage of the material is a function of it's duration and levels of moisture during annual humidity cycling. Colder & drier climates can get away with lower density than areas with high humidity levels.

But with wet-sprayed the adhesives keep it from sagging, even at lower density.

Almost all settling issues with cellulose are seen in open-blow attic situations at densities roughly HALF that of a minimum dense-pack in a wall cavity.  Low-density 2-hole method dry blown wall cavity retrofits have settling issues too, but we're talking ~5% over 20 years (after which it's rate of settling drops dramatically.)  In practical terms that reduced performance at the 2-decade mark is comparable to the performance of typical batt installations on day one, since the latter are prone to compressions gaps & voids leading to convective losses.  Dense-packed to ~3.2lbs/ft² (52kg/m³) there essentially IS no settling to speak of except under VERY humid conditions, where you might have to go as high as 3.75lbs/ft² (52kg/m³)  See table 1, p4 in that document.  But even at 3.0lbs/ft² WITH the wet-spray adhesive (a minimum density dense-pack) or even 2.2lbs/ft2 (open sprayed wet-sprayed) it is stable pretty much forever.

The long term performance stability of wet-spray cellulose hasn't gone un-noticed- fiberglass blowing wools are now available with similar adhesives, applied for similar reasons.

-Thanks to all -
Dana1- What are you thoughts on the furring strips vs solid OSB? I figured the solid OSB would be fine for vinyl siding see it done that way without any protection between siding and OSB all the time. Hardie plank probably at least needs tar paper between OSB and hardie board or maybe furring strips is the better way to go here? What about using the closed cell on the inside of my wall stackup with the polyiso, OSB, closed cell, then batts? Am I setting the OSB up for failure being sandwiched between the polyiso and closed cell? 2" polyiso should keep duepoint within the foam for my climate in Cincinnati. Thanks for your help.

Mounting the siding on furring would be fine, even preferable since it enhances the rain-screen effect by giving it more volume. Cement board siding takes on water, and the exterior OSB would benefit from at least 1/4" of back-ventilation between it and the housewrap/felt layer (3/8" of rainscreen gap would be even better- and is now required by code in Canada.)

With 1" of closed cell on the interior of the interior OSB you won't create a moisture trap- it'll still be semi-permeable, and can dry toward the interior. But if you go with R11 or R13 batting you'll be better off going with UNFACED batts, since kraft facers run slightly under 0.5perms. From an R-stackup, with R13 iso on the exterior and R6 of foam inside the sheathing the temp of the OSB would have to stay below the dew point of the interior air for MONTHS through the 1-2 perms of closed cell foam (as opposed to the brief intermittent few hours you'll experience in your climate during cold-snaps). But if you put a 0.5 perm (or lower) facer on the interior you've reduced the drying capacity of the studs and sheathing by something like 3/4 or more in that stackup, since you have effectively ZERO drying capacity toward the exterior. Since you don't need the interior facer to protect the sheathing due to the R-stackup, keeping the drying capacity high by leaving it out is preferable.

Dana- OK so the closed cell looks like a safe bet in my stackup. It seems a little better to me vs the cellulose, but cellulose may be most cost effective and yeild similar results. I'm having it priced various ways and like to get unbiased opinions from someone not trying to sell a product.

As for the furring strips. Are we safe to use solid OSB on the outside of the polyiso to mount vinyl siding to? Hardie plank I think we will just use the furring to give us adequate rainscreen/drying, but was thinking the vinyl siding would do just fine directly connected to the exterior osb sandwiched foam. In that particular area of the house we would have stud wall, OSB, 2" polyiso, OSB, then vinyl siding. Just want to make sure this does not set us up for failure down the road.

I'll give you a builder's perspective on the straping question: I'd go with the solid wood strapping as it has much better nail holding capacity than OSB, especially OSB strips.

I'm not sure I'd use nails with Hardie- screws would be better. The furring itself has to be through-screwed to the studs with sufficient length to handle the higher loading of hanging the weight of cement-board siding out that far from the stud too.

[edited to add] Be sure to use cement-board-compatible materials for the fasteners too. (Hardie documentation specifies that fairly well.)

So are we saying that solid OSB sheething even for just vinyl siding is not the best idea? Would it work? Or are we advising to only use 1x furring? Thanks.

Vinyl siding is inherently back-ventilated, works fine with OSB without adding rainscreen gaps almost all cases.

But with cement board siding, at least 1/4" of space is a good idea. It may not be absolutely necessary- the moisture burden depends on overhangs, average wind, drip-splash, etc., but a rainscreen gap is protective- it allows the OSB to dry quickly even if it got a good dousing during a wet windstorm and the cement board was hanging onto 50 galllons of extra water. With 1x furring it'll be PLENTY of gap, and a sturdier material for mounting the (fairly dense) cement board than cut-down OSB strips.

With either vinyl or cement siding, use either 15# felt or a housewrap on the exterior side of the OSB as a drain-plane.

I had our home 100% insulated with open cell spray foam. Absolute positive waste of money!

Please elaborate, why do you dislike the open cell foam? What would you recommend? I'm leaning towards closed cell foam flash n' batt or cellulose.

I will start at the beginning. A few years ago I began planning our new home. I am an HVAC pro and have always been anal about energy efficiency! My first choice was SIPS as we have a very reputable manufacturer right in my home town, however getting a quote from these guys was like pulling teeth (they finally got back to me the very day my framers started). Most of the new construction homes my company deals with are insulated with blown cellulose (parpak), they seem to work well and are very quite,and have never heard heard any complaints. This was the road I was planning to follow, however the roof truss design of my 1 1/2 story Cape Cod made any insulation other than spray foam very dificult. I wanted closed cell foam, but the contracter talked me into open cell ( my fault I take complete blame). While he was doing the ceiling I deciced he may as well do the rest of the house . my  heat bills are not any less than any of my neighbors. My biggest complaint is on cold mornings when there is frost on the roof you can clearly see the truss lines on the shingles, and the frost melts off my roof before it does any others. My inlaws live down the street in an older ranch house of comparable square footage, 4" fiberglass walls and they have cheaper gas bills than I do. I designed this house to be efficient, I have good heating and cooling equipment ( I am not here to sell any thing) , good windows they all face the south except one. 
 One thing I did of which I am very pleased with is my ICF basement. Even in my northern climate I have yet install a heat source in my infloor radiant tubing and the basement stays very comfortable with no other heat source!
 My next house I  may use a double wall (Laursen Truss ?) conventional framing with 2 to 4" fo EPS over the sheating or I have not given up SIPs yet.

Slowroadster,

Did you have a blower door test done to see if you have air leaks? If not, I think I would start there and seal up any air leaks that you may have. Have you checked the foam insulation in the attic access areas that was sprayed on the underside of the roof deck (behind the kneewall areas)? I have heard of instances where the spray foam job wasn't done well (due to inexperience, equipment malfunction, etc.) and the foam didn't completely cover/seal the framing due to shrinkage.

Jere

Half-pound foam in stick framing walls is usually tighter, but a somewhat lower R than cellulose, and MUCH lower thermal mass than cellulose. If you sprayed the roof deck with half pound foam (rather than the attic floor/truss chord level) the temperature differences due to the thermal bridging become obvious. But if it melts before your neighbor's houses insulated with fiberglass on attic floor it probably means you have an overall lower-R due to either gaps or insufficient thickness.

Depending on climate zone & construction you may be able to rectify that by adding R at the attic floor (or roof deck, depending on where the currently insulation lives. With 2" of 2lb polyurethane applied to the roof deck you can get away with a lot of sins below. DO put at least an inch on the rafters themselves too, both for protecting the structural wood from condensation, and for reducing the thermal bridging. Alternatively, putting 2-4" of iso or EPS above the roof deck and sealing any venting when it's time to re-roof adds sufficient R against the thermal bridging effects of the trusses, raising the total R significantly. (How much foam you need to protect the roof from interior moisture with an unvented roof is climate and interior-R dependent.)

2x6 24" o.c. + 3-4" of exterior iso can put you in the R40 clear wall range. Any time you go to double-studwalls or Larsen Trusses you're nearly doubling your framing costs, and force a lot of detailing to reduce the thermal bridging, and only cost effective for truly high-R (R50+) if you're paying for the labor, and you'd be wise to cost-compare it to super-R SIPs.

Dana1 - You seem to be a big proponent of the cellulose, I'm finding that as I talk to more people in my area there is not alot of love for cellulose out there. In attics they claim it's dusty/dirty and just simply undesirable. In wall cavites they claim that even wet sprayed cellulose they have seen settle over a period of several years. Maybe that's right or maybe not. I know as you get out and talk around everyone is going to have their opinion some based on experience some based on perception. You seem to be very knowledgable on the topic and I truly respect and value your explainations and experiences. On my project I'm leaning towards 1" closed cell with cellulose/fiberglass fill or a full cellulose wall fill. Do you feel one of these senerios to be better than the other? Do you like cellulose best in attic applications?

kschweitzer69, any product is only as good as the installation. You could have the best closed cell spray foam available, but if the installer does a poor job, then it defeats the purpose of paying a premium price for a premium product. This is why it is very important to use professional contractors that have great reputation. Get references from other people, builders, contractors, etc. that have used them. Maybe check with a 3rd party energy analysis consultants to get recommendations. You have 1 shot to getting the insulation done right, don't go with the lowest price... because in the end it could end up the highest cost (increased heating/cooling bills, having to re-do insulation, etc.).

What Jere said- the installation quality is the first 90% of performance, the material is the secondary (but still important).

Open blow in attics at standard densities, cellulose will outperform fiberglass at a given rated-R at high delta-Ts. It's only "dirty" if it's allowed to blow around- the so called "stabilized" cellulose pretty much stays put due to it's adhesive. It may LOOK dirty- it's mouse-gray and full of dusty gray borate powders, not pink/yellow/blue/titanium-white like fiberglass blowing wools & batts, but who's looking at it? (If you don't want to look a it, drape some Tyvek over it. ;-) )

I primarily like cellulose because it protects the structural wood from moisture damage, followed by modest cost, then thermal mass, then fire-retardency, then ant & rodent deterrence. No one factor is a decider for me, and I'm happy to use other stuff (even low density f.g. batts), where called for, but bang-for buck it's decent for such mousy lookin' cheap stuff.

The 1" closed cell + cellulose approach is superior in a couple of ways. An inch of closed cell on the sheathing protects the sheathing by slowing the moisture flux from the interior, and guaranteeing that moisture never condenses on the sheathing- the condensing surface is the foam instead. But it's not so vapor retardent that the cellulose can't dry toward the exterior as well as the interior as conditions allow. Foam is also more likely to approach perfection from an air-barrier point of view. An all-cellulose approach can work, but if low-density wet-sprayed you have to ensure air-tightness by other methods, and if allowed to hit high humidity levels may lose performance slightly over time, but never really sag unless they got the mix & density completely under spec. Rainscreen cavities behind the siding and modest vapor retardency on interior finishes will usually be enough to avoid deterioration. The colder & drier the climate, the fewer issues there are- there are homes in the Canadian midwest with 80 years of history on low density cellulose that is still working just fine.

In an all-cellulose approach dense-packing is measurably better- it'll have 5-10% higher R, very low air infiltration potential, and long-term stability even with moderate humidity cycling. Rasumussen's verified model of density vs. humidity cycling is something of the standard- in quite humid climates it may be necessary to dense pack up to ~4lbs/ft^3, but in most of the heating-dominated US 3.2-3.5lbs is enough. (National Fiber in southern New England specs 3.2lbs min for dense-pack performance.) But hitting those numbers requires due-diligence by the installer, and as Jere points out, not all installations are equal, and some can be downright fraud.

[edited to add] If dry-blowing it's a good idea to specify "borate only" or "sulfate free", since much of the cheaper dry-blow stuff uses aluminum sulfate as a fire retardent, which will corrode metals- particularly copper, but iron too,  under high humidity conditions, should they ever occur.  In Australia and a few other countries they outright banned sulfated fire retardents for that use, but not in the US.  All wet-sprayed stuff is sulfate free (for obvious reasons.)

Thanks to all for the thoughtful remarks. While I had the attention of the group another question popped up. We are installing a wood framed chimney that extends from the basement through the roof in the middle of the house. I plan to use double wall stainless pipe from basement upward to attach wood furnace add-on to. The double wall pipe requires a 2" clearance to combustables and I'm wondering how to properly insulate the chase so that this framed chimney does not become a chimney to let all the hot air out of my house. On the second half of the chimney a direct vent gas fireplace will be installed which requires specific venting pipe. Not sure if that pipe is zero clearance or not. My inital thought on the whole thing is to not worry about insulating until I get to attic space since the chimney is in middle of the house heat radiating from it will benefit me. I'm thinking a attic shield could be installed within the chimney chase and then I could have cellulose blown in around the pipe to hopefully seal things up. Any thoughts??