As I wrap up my build, one item still on the list is attic insulation. I was going to have the ceiling flashed with foam, and then blown-in insulation, but after being up there a while, I realized I can detail the attic myself and skip the foam. I'm getting quotes, and each vendor has a strong preference for either blown fiberglass or cellulose. Most are pushing fiberglass, and it is a bit cheaper. I think I would prefer stabilized cellulose, but after searching for hours, there doesn't seem to be much of a consensus.

The small house (1250 sq ft) is ICF, and I'm in a mild climate (South Carolina). I'm planning on R-49 in the attic, which some vendors are telling me is ridiculous and not needed here. Most seem to think R-30 is plenty. In my research, I found a lot of horror stories about 'insulation fraud' with all sorts of cheating being very common, especially in the South. It seems to me that if I take my laser level and make 'markers' all over the attic, they're going to have a hard time shorting me on cellulose. I guess with fiberglass they resort to 'fluffing', which would be a lot harder to detect for a home owner.

Any remarks? Recommendations? Do I need to guard against 'insulation fraud'? What questions should I ask a vendor? Thanks!

Cellulose is more air-retardent and more opaque to infra-red radiated heat coming from hot roof decks, and it's harder to "fluff" than fiberglass (but not impossible.)  Stabilized cellulose that is damp-applied will not be moved around by high wind currents through soffit vents either.  A fluffed cellulose job will settle very dramatically in just a few years, whereas fluffed fiberglass can just sit there looking pretty but under performing for decades. 

With the cellulose if it sinks 20% in 2 years you'll know, and can call them back on it. Stabilized cellulose at R50 will still settle some, but if blown at proper density it's more like 8-10% over ten years, after which it stops. If you start out with ~15-16" of initial depth, and it should still be more than 13.5" in 10 years (and thereafter.)   If it drops to a foot in 2 -3 years, it's under-blown, but can be topped off.

The higher air retardency of cellulose means that during the heating season it doesn't lose performance by convective currents of warmed air at the ceiling level bubbling up into the cold attic, and being displaced by the denser cold attic air, as happens with low-density open blown fiberglass.

The higher infra-red opacity of cellulose means that during the summer the hottest layer of the insulation is at the top, rather than an inch or two into the  insulation layer the way it is with fiberglass, where under a hot roof deck that hottest layer of the fiberglass is insulated from the attic air by an inch or two of insulation, which means you're insulating against a higher temperature with less insulation(!).

R30 is fine from the point of view of meeting current code, but under IRC 2012 code min would be R38 in zone 3A (which is your climate zone):

http://publicecodes.cyberregs.com/i...sec002.htm

And R50 isn't insane from a lifecycle cost point of view. The marginal cost of bumping from R30 to R50 just isn't that much (or shouldn't be), since it's tying up the same crew and truck for about the same amount of time, but applying 67% more material.  Most of the labor is in the drive time, set-up & breakdown time, and the raking flat- it's just not a huge uptick in overall installation time.  A more nuanced R-value recommendation can be found in table 2 p.10 of this document:

http://www.buildingscience.com/docu...mate-zones

Note for zone 3 they are recommending R50 as a rational starting point for zone 3. To be sure R50 done with cellulose or fiberglass has a better rationale than with spray foam (any density), since the latter has a much higher cost/R-foot than open-blown cellulose or fiberglass.

I did install catwalks about 18" above the ceiling plane. I can't get everywhere, but I can get close enough to inspect for trouble. There is no attic storage. All that's up there are some wires, vent pipes, and bathroom vent ducts. There is a single 22X32 access hatch (code required) that I'm going to make an insulated cover for. I hope to never open it in my lifetime.

While we're talking attic penetrations, I will admit that I have some of those evil can lights. They are ICAT, and I caulked them to the drywall. Next, I built R-Board boxes, which I put over them and foamed in place. Finally, I'm using LED fixtures that are quite airtight themselves. Since they are supposed to last many years, I think I am going to caulk them to the drywall when I install them. I don't see how any air is going to get past all that.

Walking around up there, it's amazing how many opportunities there are for air leaks. Things such as electrical boxes not only have gaps around them, but there are openings where the cables go in, screw holes for back mounting, and even KO's that are slightly ajar. Even penetrations that are inside the interior walls, like pipes and wires provide conduits for unconditioned air to enter those uninsulated wall cavities. I never realized how leaky houses are until I started sealing one up.

Posted By Alton on 12 Mar 2014 01:43 PM

One problem that I have run into with thick insulation is that a catwalk will be needed to access attic storage and to make roof inspections for leaks and rot, check pipes, wiring, etc. If insulation is only between the ceiling joist or bottom chords of trusses, then one can walk on them.  But this way of insulating does not insulate the ceiling joists/chords. 

Dana1, can mineral wool be substituted for cellulose/fiberglass without incurring problems?  Would it be more air tight than cellulose and less transparent than fiberglass?   Would the mineral wool batts be much harder to install than blown cellulose.  As you can see I have never used mineral wool.  Your thoughts will be appreciated.

Blown rock wool is also opaque to IR, and more air retardent than open-blown fiberglass, but not as air-retardent as cellulose. The higher air retardency of cellulose is due to the fact that it's macroscopic form factor if flakes of newsprint-grade paper- the fibers are bonded into a highly air-resistant piece of paper. The macroscopic form factor of fiberglass & rock wools are blobs of loosely connected thin fibers, at a density that is more air-filter than air-barrier. Packed tightly together (as when dense-packed into a wall cavity) those fibers become pretty air-retardent, but that's not how it's installed on attic floors. The fiberglass vendors have improved it a bit since the 1980s- the blobs of newer attic blowing wools are a bit more tightly knit, sort of like a shredded R13 batt, but still nowhere near as air-retardent as cellulose, and it's still possible to fluff the stuff.

Rock wool batts are sufficiently high density to be pretty air retardent, and easy to install (as are high density "cathedral ceiling" fiberglass batts.)  But all batt solutions have the fit issues at the edges to attend to to ensure that it doesn't have convective leakage at the seams.  Rock wool seems to have fewer issues than fiberglass with friable fiber ending up suspended in the conditioned space, but I haven't seen much in the way of hard comparative data on that. The stone fiber itself is denser than glass, which may help it settle out of the air more quickly(?), and perhaps likely to get picked up by infiltration currents.

The fire retardency of rock wool batts is as good as it gets- it's essentiallys spun slag from steel-making. Fiberglass melts at a much lower (and still relevant) temperature, and transmits far more heat as radiated infra-red from one side of the assembly to the other.   Cellulose will burn in a sustained fire (it's paper, after all), but is treated with borates (and too often, sulfates) as a fire retardent, but under extended exposure to flame the fire retardents eventually vaporize, and it'll burn.  The high air retardency means that in wall assemblies  a vertical foot or so of cellulose in a stud bay is "worth" about the same as a 2x fireblock in terms of inhibiting the rate of fire spread when the sheathing is burning.  In an attic floor cellulose installed over recessed  lighting fixture not rated for insulation contact with a 75W halogen installed has some amount of fire risk since it can get hot enough to cook out some of the fire retardent over time, but that wouldn't be an issue with rock wool (thought the electrical insulation on the wiring might burn eventually.)  As batt solutions go, rock wool is at the top of my preference list.