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newbiejohn
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Posts:141
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| 04/19/2009 10:57 AM |
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Was at the home show this weekend and a cellulose contractor told me that there is now no reason to use Wet dense pack cellulose in my walls because they use a fiber product that is stapled to the walls and the dry cellulose is pushed into the cavity this way to a certain density.
He says with this method im guaranteed to never have any compacting of the cellulose and therefore no spots up top in years to come with no insulation.
Do you believe this is fact or was he giving me fiction in trying to 'sell me'
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aardvarcus
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Basic Member
Posts:176
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| 04/20/2009 12:14 PM |
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Everything settles. As seasons change and humidity conditions change, cellulose will settle. If they are packing this stuff in to try to reduce settling, that will diminish the R value, because if it is packed in too tight, it will not develop the proper air gaps. I have seen dry cellulose used in walls before, especially wide double stud walls, but every time they always seem to have a way of topping the stud bays off with cellulose again for when it settles. Not if it settles, when it settles. They usually drill holes in the top plate and have removable bottom window jambs so they can come back with a blower in a few years and top everything off. This is one reason I prefer batt insulation to dry spray insulation, there is no maintenance. If I had to use a dry spray in insulation, I would use fiberglass, not because it won't settle(everything settles, remember), but because it will settle less***.
***Based of my experience in my climate with my family's homes. Please spare me the “fact sheets” I have seen them before.
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Dana1
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Posts:702
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| 04/21/2009 1:42 PM |
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At 3lbs+/ft^3 the only way cellulose will settle is with repeated actual-wetting/drying cycles (not mere humidity changes within normal ranges.) I suppose in a couple of centuries it MAY sag and need topping off under "normal" non-wetting circumstances... but not in a coupla decades (or even five.)
Two-hole method deliverers densities in 2.0-2.5lbs/ft^3 range, low enough to sag a bit over a decade or three, but FAR less sag than an open-blow attic floor situation (~1.5lb/ft^3 settled-density). R-value of cellulose doesn't begin to diminish with density until you're in the 2lbs/cubic foot range, and then drops only very slowly with density:
http://www.builditsolar.com/References/DensityvsRValue.htm.
Unlike mineral & glass fiber insulations, cellulose fibers are hollow, and do NOT rely solely on inter-fiber air trapping for their R-value. At densities of 3lbs/ft^3 or higher it limits convection currents within the material, and adds considerable thermal mass to the system. The total-performance "sweet spot" for cellulose is between 3-4lbs/cubic foot. Below that density infiltration & convection creeps up, and above that density R value slowly drops without much performance gain from further marginal drops in infiltration/convection issues. Most manufacturers spec 3.2 or 3.5lbs for achieving the rated dense-packed performance, but once you're over 3lbs cubic foot the differences are academic- you might be able to measure it in a lab, but not likely in the real world. (Some argue that 2.5lbs/ft^3 is good enough from a measured-performance POV, but it it may be more likely to run into settling issues than at slightly higher densities.)
The wet-spray stuff is guaranteed not to settle even at lower densities, since it has a water activated adhesive, but it's inflltration & convection factors aren't nearly as good as dense-packed. "Blow-in-bag" technique (using the fibrous netting) can be used for dense-packing, but it makes applying the finish wall tougher since it bows it out from framing. Blow-in-bag using wet-spray (at any density) can at least be trimmed flush to the studs after the adhesive cures, but not dry-blown, which retains it's shape primarily from spring tension against the retaining surfaces- if you trim it it won't fall out, but it'll lose density slightly.
Dry blown is preferred in very thick walls, since it can take weeks (or months!) for wet-spray to fully dry if applied deeper than standard stud depths. I'm not sure what the point of blow-in bag mesh would be in a Larsen Truss type design except for using the mesh as separators between the truss-bays so as to guarantee full & uniform density from one bay to the next. In a PassiveHouse engineered-beams-for-studs design the OSB webbing performs that containment function.
Batt & blown fiberglass can get you the same R-values, but they have their own issue (the requirement for fire-stop blocking in the stud bays, for instance). Batts are nearly impossible to install perfectly when dealing with plumbing & electrical schtuff, won't stuff properly into the small & nonstandard width sections of framing, and unless it's the high-density "cathedral ceiling" type, loses R value significantly with increased delta-T (not a problem in TN or CA, but a real issue in MN where it can be -10F outside for way more than a few hours out of the season.) Blown fiberglass need similar or somewhat higher densities as dense-packed cellulose to not sag, but that can be done if desired. |
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