Dana1 also misunderstands the building science behind my Riversong Truss wall system.

He says:

Without exterior sheathing with only clapboards & houswrap holding the dense-packed cellulose the housewrap is packed tight to the backside of the clapboards, giving it far less back-ventilation than a typical clapboard, which puts them at a higher risk of wintertime moisture accumulation. This can be further aggravated if the 18" roof overhangs aren't maintained (or increased) to further limit wetting from the exterior. His rule of thumb on overhangs appears to be an inch for every vertical foot of siding, but that's not necessarily good enough for wind-driven rain, nor does it deal with splash-back of drips from the edge of the eaves. Without back-vented siding a good portion of moisture that makes it past the clapboard will end up in the cellulose. Whether the drying rates would be quick enough that the cellulose could tolerate it without settling (or worse, saturating), or whether the clapboards would eventually suffer is local-climate dependent.

Back-ventilation behind wooden cladding is necessary only in the most extreme rain/wind zones (such as coastal hurricane zones) and by the use of non-breathing (vapor impermeable) wall materials, such as petrochemical insulation. It can be eliminated in most climate zones if the wooden cladding is back-sealed and end-grain-sealed during installation.

I have no such "rule of thumb" for roof overhangs, which are largely determined by summer solar shading requirements by latitude. For weather protection, more is better though anything more than 24" typically requires engineered structural support. The amount of roof protection required is also a function of the micro-climate and amount of local wind protection from topography, trees and other structures.

Back-splash at the ground can be mitigated by proper ground clearance, ground texture, and ground grade, and virtually eliminated by the proper installation of gutters and downspouts, which are standard on all my homes. A 1" rainfall on a 1,000 SF roof will drop 600 gallons of water at the foundation, which is foolish no matter the building methodology.

The settling of cellulose over time is a function of moisture cycling, and the manufacturer-specified 3.1lbs density stated in the article may work in a US zone 6 climate in wall structures better protected from rain intrusion, I'm not convinced it's going to be an adequate density in his buildings, and may not be adequate for colder climates.

Moisture cycling under normal conditions has no effect on cellulose settling, which is solely a function of density, surface friction and proper application. Cellulose insulation can absorb and release up to 30% of its weight in water without effect and redistributes local moisture concentrations faster than any other insulation or construction material, thereby protecting wood framing from dangerous levels of moisture accumulation.

All cold-climate homes must have interior relative humidity controlled by local and whole-house ventilation and avoidance of the common bad construction practices (such as lack of roof gutters or improperly drained and water-proofed basements) that create moisture intrusion issues. But an envelope with a large moisture buffer can accommodate relative humidity fluctuations better than non-hygroscopic materials and systems.

I aim for a minimum of 3.1 pcf cellulose density, but almost always exceed that. Contrary to Rasmussen's calculations, the gravity settled density of cellulose in an attic loose-fill application is 1.4 pcf, so any density exceeding that is like compressing a cellulose sponge in a confined space and doubling that density virtually eliminates any possibility of settling. I've seen a dense-packed demonstration wall section with clear and smooth plexiglass coverings that was trailered at highway speeds to multiple home shows with not a trace of settling.

For a colder/wetter/windier climates adding an exterior sheathing layer to hold in the cellulose then furring-out a proper rainscreen gap to limit rain intrusion & enhance the drying rate toward the exterior seems prudent. It doesn't hurt to add another 10% to the over hangs either.

A properly-installed weather-resistant barrier (housewrap) prevents the intrusion of liquid water and condensation on the inside of the barrier material is possible only if there are voids (which are impossible with dense-packed cellulose. Moisture accumulation, either diurnal or seasonal, is a function of the balance between wetting rates and drying rates, with drying direction reversed when solar heating occurs on the outer cladding. If you use a poly vapor barrier on the interior, which prevents drying to the interior, then you may need a rainscreen to enhance drying to the exterior and prevent solar-forced moisture migration.

But, if you follow the International Residential Code, which mandates only a vapor retarder on the interior (such as latex VB primer) and maximize the drying potential to the outside by minimizing the number of layers, then you will optimize the drying potential of your envelope, as is the case with my Riversong Truss Wall System.

There are other ways in which you'd run afoul with Canadian building codes with Riversong's approach- the lack of interior vapor barrier is just the beginning. SFAIK cellulose does not qualify as a fire-blocking material, but you may be able to meet that with sheets of rigid high-density Roxul rather than horisontal planking.

If you can't convince your code authority that the IRC vapor retarder is an improvement over the now-obsolete vapor barrier standard, then you may have to compensate for that foolishness by an otherwise unnecessary modification such as a rainscreen.

And densepack cellulose has been third-party approved as a firestop, and three different building code officials I've worked with have approved it for that purpose. A 1994 Research Council of Canada study found that cellulose increased the fire-resistance of wall systems 40% over rock wool, and Omega Point Laboratories found that cellulose of minimum 14.5" depth outperformed solid wood blocking as a firestop.

Posted By jonr on 02 Dec 2012 09:54 AM
I agree that a siding that holds moisture and no rain gap isn't a good idea in many climates. The general lack of concern about interior/exterior pressure differentials doesn't help. Use stabilized cellulose and there won't be any settling.

When the walls were disassembled at the conclusion of the experiment, the sheathing and framing showed remarkably little evidence of wetting damage or mold growth. No visible mold growth or evidence of staining or water rundown was found. The damage was limited to some limited grain raise of the interior surface of the oriented strand board at the cellulose wall, and slight corrosion of fasteners and staples.

Based on the data, calculations, and analysis, all three walls should be at high risk of failure; the analytic tools used indicate that these walls should have failed. However, disassembly showed that the walls were essentially undamaged by the monitored moisture exposure. This suggests that the walls, at least in the configurations tested, were far more robust than current analysis tools would indicate. Various theories were proposed on what protective mechanisms might be at work in these assemblies, in particular, the effect of cavity fill insulation. The cellulose walls were likely protected by borate preservatives (which inhibit mold) and the ability to safely store moisture.

it is entirely likely that many double-stud walls are insulated with cellulose with only Class III vapor control and provide fine service. In either case, a Class I vapor retarder (polyethylene) is not recommended, because it completely eliminates inward drying.

The vapor permeance of 1x plank sheathing is about 1-perm when dry, less than 2 perms when wet

"Robert's presentation on moisture mechanics was the best presentation I have seen on the subject. I would highly recommend this workshop to all builders, architects and building trades people. I believe the issue of moisture as it relates to residential (and commercial) construction is one of the most important pieces of building science a builder today should have a strong working knowledge of." - Jay Walsh, Energy Analyst, Energy Star Homes and LEED-H Rater, Center for Ecological Technology