How to dense-pack cellulose in a double-stud wall?
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What is the process to dense-pack cellulose in a double-stud or Larson-truss wall? Is this something that an owner-builder can do with the cellulose blowers that can be rented from the local building supply store? What is the best way to insure the proper density in the wall cavity?
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google the house wright, and/or robert riversong. he has the steps on his web site. j
The typical rental blowers are inadequate for proper density. A minimum of 80" WC (2.9 psi) is required at the hose end.
Also, the proper material/air mixing ratio and proper technique is necessary for good density in any wall. In an open-framed double wall, special techniques are required and few professional installers have the expertise.
You need a cellulose contractor with experience with this type of insulation cavity.
To insure proper density, you have to calculate actual cavity volume and count the number of bags per envelope section. A 25 lb bag should fill 7 to 8 cubic feet of cavity.
Is the primary purpose of packing the cellulose to prevent settling? If so, would it also work to nail light, horizontal battens to the gussets between the inner and outer wall to help support the cellulose? I suppose you would loose some of the R-value having that extra wood in the wall, but compressing the cellulose must also reduce the R-value somewhat compared to using fluffy cellulose.
The purpose behind my question is to determine if there is a way a relatively unskilled cellulose installer (me) could adequately insulate a double wall with the tools available at a typical building supply store.
Cellulose, like most fibrous insulations, increases in R-value per inch with increased density). At 3.5 pcf installed density, cellulose can provide R-3.8/inch. At normal gravity-settled density in an attic, cellulose offers between R-3 and R-3.4 per inch, depending on depth.
I would stronly advise against doing the installation yourself. As I already stated, it requires equipment and techniques that are beyond the reach of a DIYer. I teach owner-builders, but I also teach how to know when you're in over your head.
By the way, the more insulated a wall is, the more significant becomes any void.
An R-47 wall (12" of dense-pack cellulose) with a 10% void yields an R-14.5 wall. With 5% void, the wall would be R-22. Proper technique and proper density matters more than you might think.
By the way, the more insulated a wall is, the more significant becomes any void.
An R-47 wall (12" of dense-pack cellulose) with a 10% void yields an R-14.5 wall. With 5% void, the wall would be R-22. Proper technique and proper density matters more than you might think.
John I'm glad you raised this question. I am hoping to design a buidling that either uses a double wall or larsen truss with cellulose fill. I've recently had a dense pac cellulose drill and fill install on my own home and found few contractors even doing this in my area. I doubt anyone locally has experience filling dense pac cellulose in thick new construction wall assemblies.
Robert, can you suggest where someone could get trained in filling larger assemblies in or near Minnesota?
I think I recall from other posts that you feel walls over 12" thick start to make less and less sense. Do you see a limit to how thick you can prudently go with a double wall/ larsen truss cellulose fill assembly?
I'm surprised to hear dens pac has more R-value than non-dense pac cellulose. I read an explanation once about how fiberglass has LESS r-value at higher densities because at higher densities more contact is made between the fibers and therefore more conductive thermal transfer happens.
J,
I don't know anyone out in MN, but you might contact Bill Hulstrunk, the Techical Manager for National Fiber, the best cellulose manufacturer in New England (http://www.nationalfiber.com/): [email protected]
Fiberglass is a very poor insulation, in part because of the extremely low density of most batts. If you think about it, you would realize that increasing the density increases the R/inch. The old 3½" batts were R-11 (0.6 pcf) but the new higher density batts are R-13 (0.8 pcf) or R-15 (1.5 pcf). Semi-rigid fiberglass foundation insulation can be as high as R-4.3/inch.
The common misperception that you're referring to is that when you compress a thick batt into a thinner space, though the R/inch increases, the total installed R-value is less than the uncompressed batt, so the $/R increases.
For instance, a conventional R-19 batt intalled in a KD 2x6 wall will yield R-18 (R-3.3/inch), but compressed into a KD 2x4 wall will yield R-13 (R-3.7/inch). This makes it evident that the new R-13 batts are nothing more than a compressed R-19 batt.
But, because fiberglass batts can never achieve the density of blown cellulose, they can never function as well as insulation, acoustic barrier, air barrier or fire stop. And, of course, it does nothing to prevent insect or rodent infestations which destroy any fiberglass installation.
There is evidence as well that the new small-fiber blown fiberglass, such as JM Spider, does not often achieve either the densities or R-values or air-resistance claimed.
Robert,
On the fiberglass issue. The new R-15 batts for 2x4 walls are clearly very heavy and stiff. At summercamp 2 years ago, the rep from one of the fg manufacturers spoke. He said the higher density R-15 batts were no more airtight, but the "extinction coefficient" for IR emittance through the was much better. So basically the conduction and convection part is unchanged, but the radiation part is better.
And FYI for anyone that will attempt to densepack with a lumberyard rental machine. Your best odds will be with a Krendl 200. And it can work well enough for 2x4 or 2x6 retrofit construction but will be very slow. Don't even think of the two hole nozzle fill method!
Our Krendl 1000 was still boringly slow packing 8" and 10" cavities on pole barns. I can't imagine a 12" cavity.
We had a Meyer 1400 that would blow a bag every 40 seconds but never got the chance to densepack any large cavities with it.
For planning purposes, can anyone suggest a ball-park figure for the cost per sq ft (or cubic ft) to install dense-pack cellulose in a double wall, plus loose insulation in an attic, for an uncomplicated house in VT?
The Force/3 blower is probably the best portable unit on the market. It can push 80 bags an hour to high densities.
Robert S, I'm not sure how the FG folks can explain the increased R-value of dense batts purely by IR extinction coefficient (the inverse of radiative transmittance), since ASTM C518 tests all insulation materials with hot plate technology that measures total thermal transmission and does not segregate radiative and conductive constituents. All insulation materials vary in thermal transmittance with variations in density, each having an optimal density for minimal thermal transmission.
John Hess, there are no "ball-park figures" since they depend on which ball park you're in and the current market and the complexities of the installation and which contractor you use. Get a quote, or two or three. I'd recommend Murphy's Cell-Tech in St Johnsbury.
Have you or anyone else had good luck with the Force/2 blower? I believe I can get that at my local rental center for low $s. I would be blowing a mix of 2x4, 2x6 walls, and even a few 2x8 walls, and need something that will work. Hopefully something that would work for a DIYer.
Thanks!
The Force2 blower is a good unit for smaller jobs. It should work fine as long as it's putting out at least 3 psi.
Martin Holladay published an interview with Bill Hulstrunk where he provides detailed instructions for blowing loose-fill cellulose on attic floors and dense-packing walls:
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-cellulose-insulation
great article.
For 12" double stud walls, is there a preferred type of install for it? Meaning dense packed or wet applied? I read on other forums wet spraying 12"+ had many installers fearful of the moisture content. It appears you do not have that same fear? If it were your choice, which method would you install in a 12" thick wall?
One installer I am getting quotes from is really pushing towards blown fiberglass instead. They much prefer working with it, and it has a slightly higher r of just over 4. I realize cellulose is a "greener" option, but any thoughts on this product? he is also saying the price difference is "minimal", but has yet to give me a price on a cellulose install.