Wall Cavity Insulation
user-2069108
| Posted in Energy Efficiency and Durability on
I know that most people on this web site rave about dense packed cellulose, but why? If the air barrier characteristic is not relevant (as in a REMOTE or PERSIST wall), will any equal R-value insulation do?
Would the most significant advantage of dense pack be the lack of settling over time? Is that a problem with other blown in types of insulation, even the glue/wet cellulose? Our Habitat for Humanity homes in the area use blown in rock wool with a glue/water spray. The stuff seems moderately stuff when they are done, but will that change over time> Will it settle?
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Replies
I don't know if this qualifies as a rave but...
Preventing settling is just one aspect of dense (cellulose or other fiber). Damp sprayed cellulose or other fiber with adhesives in the mixture won't settle, but their air retardency isn't the same.
Air retardency is still important. Low density cellulose is substantially more air retardent than other fiber insulations (unless dense packed), which prevents convection loops from robbing performance, and limit the performance hits from any incidental air leaks in the sheathing or wallboard. But dense-packed they all have similar air retardency.
Cellulose has many times the moisture buffering capacity of fiberglass or rock wool, and will "share" the moisture burden to protect the structural wood without loss of thermal performance. (It's a double edged sword though- it can mask & retain bulk water leaks, well enough that small leaks may not be detected for some time.)
Cellulose has a lower carbon footprint than other fiber insulation. By some carbon accounting methods it's carbon negative- the material itself is sequestered carbon, after all.
Cellulose has substantially more thermal mass than other fiber insulation R for R, inch for inch, enough to make a measurable difference in high-R assemblies.
Cellulose (particularly damp-sprayed) is often less expensive than other decently air-retardent solutions.
Cellulose uses borates for fire retardent, which offers wooden assemblies some protection from carpenter ants, termites, and other wood boring insects.
Steve,
Assuming that air leakage has been well controlled by one or more air barriers, then R-value is R-value.
Dana has done a good job summarizing the advantages (including the environmental advantages) of cellulose insulation. Of course, there are many projects where other types of insulation make more sense -- especially in regions of the country where insulation contractors are unfamiliar with cellulose.
PERSIST walls, properly speaking, never include cellulose. The PERSIST system puts all of the wall insulation on the exterior side of the wall sheathing. When the PERSIST system was developed, only rigid foam insulation was used for this type of wall. In recent years, some PERSIST builders have begun to use semi-rigid mineral wool panels for PERSIST walls. But you can't use cellulose for a PERSIST wall.
If you read the contractor forums, cellulose fails in the most spectacular ways when the house eventually leaks...
Anon3,
For a regular GBA reader with strong opinions, it remains surprising that you won't share your name.
If a cellulose-insulated wall takes on enough water to "fail in spectacular ways," then the wall would certainly have failed in similarly spectacular ways, even if it had been insulated with fiberglass.
If you read the contractor forums you can find all sorts of stupid-attacks committed by contractors using all sorts of products, but that doesn't mean they should be emulated.
A rare but spectacular contractor stupid-attack specific to cellulose insulation starts with installing copper plumbing in the exterior wall (where it is not so freeze protected), then insulating the wall using a cheap cellulose product that contains sulfated fire retardents (instead of borate-only). When the first sweat-soldered joint starts to seep (from freeze/thaw, or poorly soldered joint) the cellulose soaks it up & redistributes it, often without showing surface wetness or failing interior paint right away. The wet cellulose keeps the wet sulfates in contact with the copper over weeks or months or even years, spreading along the pipe, causing it to corrode and eventually leak even more. By the time the reek of wet sulfates are detected indoors whole sections of drywall & plumbing may need to be replaced.
This won't happen in quite the same way with fiberglass- the leak will be detectable sooner as liquid water seeping at the baseboard trim or failing paint near the floor, and without the corrosive fire retardents the plumbing would likely be repairable without outright replacement.
But it takes multiple errors to end up in that boat:
A: Don't install plumbing runs in exterior walls! Whether the plumbing is copper or PEX or anything else, no matter what type of insulation is used, installing it in an exterior wall is a lousy practice.
B: Don't use products with sulfated fire retardents! (Those are even banned in some countries, but not the US.) This isn't really much of a cost-adder (truly "in the noise" relative to the total installed cost), and most competent cellulose contractors use borate-only products.
Dana,
As usual good advice, but I'm a bit more equivocal than you about running plumbing in exterior walls.
If it's a short run, like stub outs for a kitchen sink, I'd still rather run them in the wall than through the base of the cabinets. Assuming 2"x6" framing, with 4" of foam board against the sheathing and the rest of the stud bay to the interior left empty, you don't run much of a risk. Burying them in any type of batt or loose fill is asking for trouble though.