Posted By Dana1 on 14 Mar 2012 12:24 PM
Lbear:   At 30% framing fraction a 2x6 wall with 5" of  R6.9/inch ccSPF  (can't go full-depth- it's too hard to trim flush) ends up with a whole wall R of ~R13.5.

The same wall with R3.6/inch cellulose at 5.5" ends up at R12.1.

That's a very expensive extra R1.4 compared to putting 3/8" of XPS underlayment (~R1.9) under the siding of the cellulose insulated wall, eh?

The lifecycle greenhouse gas potential of the 5" ccSPF blown with HFC245 is several times greater than the energy sources that it offset.   It will be getting somewhat better soon, but even the newer blowing agents are less enviro-friendly than the pentane used for blowing rigid iso.

Rodents nesting in cellulose- even low-density open-blown attic stuff is actually pretty rare, (unlike low-and mid density fiberglass.)  The fire retardents used are eye and lung irritants.  There is no sulfuric acid or formaldehyde- the MOST toxic stuff in it is the printers-inks (being primarly recycled newsprint) which it the only material in cellulose that outgasses.  (Unlike ccSPF, which outgasses the HFC blowning agents for decades.) It's important (but not difficult) to avoid cellulose products with sulfate fire retardents, since those become fairly corrosive to metals should it ever become wet, but there are several vendors that only use borates. Borates are relatively benign to humans, used in everything from soaps to cosmetics and medicines inconcentrations much higher than could be ingested by living in a cellulose insulated house (even if you slept buried in the attic cellulose!) but it destroys the gut flora of termites/ants/bees and other wood-boring insects, rendering the digestion of wood impossible of the host insect, killing them.

Good points. While treated cellulose is an irritant to rodents, cellulose is not 100% rodent proof. It is rodent "resistant" but there are cases where rodents will still nest in it.

A lot depends on WHERE you live. Most urban areas will never have issues but rural areas are a different story. There are cases of rodents nesting in treated cellulose. Rodents like pack rats will nest in steel wool, tumbleweed, materials that are supposedly jagged and irritating but it doesn't really phase a rodent looking for a dry and warm place to nest.

So while rodents "don't like" cellulose, it is not 100% rodent proof and rodents will still occasionally nest in it, especially in rural areas.

Treated cellulose does work well on most insects, like you stated.

Insulation Studies

Show me a single case of rodents nesting or tunneling in dense-packed cellulose. I haven't seen any, but I'm just one guy- can't see everything first-hand.

Mouse crap on (and some tunneling in) open-blow attic stuff, sure, that I've seen, but not an actual nest. Almost all homes with low density fiberglass (including mine) have mouse tunneling issues in wall and cathedralized ceiling cavities though, and I've seen mouse nests not in insulation layers that had bits of fiberglass insulation (apparently actively scavenged for use in) the nest.

Also, do you have any references younger than 20 years old, or field evidence (where the mice aren't intentionally constrained with an insulation panel as a barrier) or a lab-test that actually has a cellulose example? The samples tested in your referenced documented experiment with the trapped mice included:

Dow Styrofoam (=XPS)

rock wool

Cellotex Tuff-R (=foil-faced iso)

fiberglass

vermiculite

With only references in the text to prior studies by other experimenters regarding cellulose,with no actual data or description of those 1980s studies.

Rats and mice can and will gnaw through damn-near anything eventually- even fiber-cement board. Closed cell foam is by no means immune- it's similar in density & hardness to the Dow Styrofoam XPS in your referenced experiment, and apparently just as tasty:

http://listserv.repp.org/pipermail/greenbuilding_listserv.repp.org/2010-July/016270.html

I'm sure there are mouse nests in cellulose insulation out there, but it's not super common compared to most fiber insulation, and I've not seen so much as a tunnel in dense-pack (but I'm sure there an exception to that rule too, eh. ;-) )