Replies

1. Expert Member
   Dana Dorsett | Apr 03, 2015 06:15pm | #1

   The density required to eliminate settling over time is a function of seasonal humidity cycling into & out of the cellulose, as determined by Rasmussen's research performed at Aalborg University in Denmark. Since colder climates result in higher wintertime moisture accumulation, climate zone matters. At 3.5lbs you'd be good in most lower-48 US locations, but US climate zone 7A or eastern Canada it might need something closer to 4lbs (which delivers better air tightness but a slightly lower R than at 3.5lbs). Even 3-3.2 lbs would be fine for zone 5 & lower.

   Don't sweat the variability between vendors on those air-flow tests- at 3lbs+ they're ALL "good enough" from a thermal performance point of view and NONE of them are good enough to be considered a true air barrier.

   Make the exterior sheathing your primary air barrier by caulking the framing to the sheathing in each stud bay. For horizontal seams in the plywood or OSB sheathing ideally they would be supported by a structural element like 1x strapping sealed with caulk, but you can probably skate with housewrap tape painted over with duct-mastic if you have housewrap on the exterior detailed as an air barrier.

   It's still worth caulking or can-foaming every wiring penetration, including lateral runs through studs.

   If the electrical boxes are already mounted to the framing it's tough to air seal the backs & wiring penetrations, but with the wiring roughed to the box with the box not attached you can can-foam any leak points on the back side. In Canada they have "vapour hats" designed to fit over the backs of electrical boxes, with flanges on the interior side designed to be taped to the a flexible interior side air/vapor barrier. (With cellulose using a smart vapor barrier such as Intello or MemBrain as an interior side air barrier works fine in most climates. If you use Intello Plus as the interior air barrier it's tough enough to use as blowing mesh- not sure about MemBrain.) There are also air-tight electrical boxes out there, but I've never seen them in box stores, but they can be source over the web.

   The Force-1 is a single stage blower that might be difficult (and slow) to dense pack as high a 3.5lbs/ft^3, but it'll do better than 3. You should be able to get there easily with a Krendl 450a.

   It's going to take you multiple days to dense-pack a house no matter which blower you use. I'm not sure it's worth paying extra for the Wasp.

   Most rentals don't come with dense packing hoses, but as long as it comes with a reducer it's not tough to fab up your own.

   When you're done dense-packing it you can roll the pillowed out mesh (or air barrier) and detail any punctures or holes (if blown in a vapor retarder) before hanging the interior layers.

2. 
   nmschulte | Apr 03, 2015 06:45pm | #2

   Thanks Dana. The interiors are already finished, so it sounds like my best bet is to caulk the holes in the elctrical boxes as best I can. Fortunately there are few if any horizontal electrical and plumbing holes.

   I wonder about cutting sheets of membrane or tyvek and draping them down the insides of the walls behind the mortar/plaster and lath before packing. The exterior has some nails protruding but it might be candidate too. I'm not sure the vapor/air permeance of plaster/lath, but maybe it's not worth it.

   Membrane is air imperm, but vapor molecules can pass? Tyvek housewrap functions similarly correct?

3. 
   nmschulte | Apr 07, 2015 02:45pm | #3

   I understand now the difference between Intello and Membrain and e.g. Tyvek wraps; thank you for the suggestion Dana. It seems the answer to my questions really depends on what the exterior of the house looks like with regard to water plane ("moisture barrier") and vapor barriers.

   Looking underneath a corner of siding above the foundation I found an inch or so of fiberglass insulation immediately behind the siding, and then some more metal behind that. I know somewhere underneath is OSB sheathing and plank board, but I'll have to take off a whole strip of siding to tell. I'm not sure if the metal is another layer of siding, or if it's from the interface between the structure and the foundation.

   Finding fiberglass "batt" underneath the metal siding was a huge surprise, and has me worried as far as the water/drainage plane goes. I definitely am not wanting to re-side the home, as that changes the whole picture in terms of my options for insulating goes.

   Dana, your information about how a new wall would be constructed is helpful, thanks much. It looks like I'm on my own as far as ways to air-seal the existing finished walls; I'm seriously considering draping a vapor permeable air-barrier down the wall cavities, but I won't know if that's a valid approach until I see what's really behind the siding.

4. 
   STEPHEN SHEEHY | Apr 07, 2015 02:59pm | #4

   Nathan: we assumed the insulation sub could fill the stud cavities from the top, but it proved to be too difficult to get enough density. My GC called back the sub who filled through holes about waist high in the membrane and ended up doubling the amount of cellulose used. It proved to be quick and easy to tape the holes. Someone described the right density as like a firm mattress. If too squishy, it'll sag for sure.

5. Expert Member
   Dana Dorsett | Apr 07, 2015 03:20pm | #5

   If your only source for cellulose is box stores, see if they won't special-order the "stablilized formula borate-only" goods for you:

   http://www.greenfiber.com/images/popularlinks/1030201345943PMPM-6.3-34%20REV%20G%20INS735LD%20Fact%20Sheet%2010-13.pdf

   The standard dry blown GreenFiber contains ~10-11% ammonium sulfate as a fire retardent, which becomes corrosive to metals if it ever becomes wet. (It stinks pretty bad when wet too.)

   http://www.greenfiber.com/images/technicaldocuments/87201342946PMMS-6.4-057%20Rev%20F%20Loose%20Fill%20Blended%20Formula.pdf

   While the stabilized formula contains moisture-activated adhesives (to stop it from settling even at lower density) it's performance when dry-blown is about the same as the dry-blow formulation, but it contains no sulfates to rust out all the nails and corrode the metal siding should it somehow get wet.

   Call it cheap insurance. The up-charge for borate-only goods is small, but they may tack on a stocking fee for special ordering it.

   Simply draping sheet goods won't do a whole lot for air tightness beyond what dense packing would do. For sheet goods to become an effective air barrier they have to be detailed as such, sealing every penetration seam & edge.

6. 
   nmschulte | Apr 07, 2015 04:17pm | #6

   Stephen, I was concerned about filling bottom-to-top with an open-top cavity too. I'm not entirely sure the physics, but the information from Rick Karg (RJ Karg Associates) made it sound like being able to build pressure in the cavity was necessary or at least useful to the dense packing process.

   I have about a foot of exposed cavity at the tops of my wall cavities: http://i59.tinypic.com/1zpimhx.png

   I plan to build a test "cavity" to practice technique and calibrate the machine on (as per the referenced study), and I may try this with an open-top setup to see how it works. I'm guessing it will be more hassle than it's worth, in which case I'll just whip up a relocatable jig (read: sheet of plywood with a filling hole and some wood screw) to close the cavity for filling and finish it off when I finish the attic.

   Dana: I have in my notes to buy the borate-only (sulfate-free) formulation as I'd read the same about corrosion. Thanks for the confirmation.

   With the draped sheet, I was thinking that the dense packed cellulose would ensure a good seal against the cavity sides, contributing to the air-barrier. I was hoping that by ensuring the sheet wraps from stud-face to stud-face (or even beyond, onto the exterior side of the cavity) completely covering the interior side of the cavity and any seams (including the bottom plate) that the cellulose would hold things tight.

   You're telling me you don't think the pressure from the cellulose in such a setup would be inadequate? I appreciate your experienced input. Adding a sealant (sprayed on adhesive? bead of e.g. caulk? something that won't harm the sheet) to the sides of the studs or margins of the sheet might not be too hard before packing. The sheet doesn't need to lay flat either, so the only critical piece here is the pressure to ensure a quality seal: that could be done with a simple roller down the sides before packing.

   I'll focus my attention on the accessible bits: electrical boxes and exposed framing seams (sill plates).

   Any insight with the fiberglass directly behind the steel siding?