[Greenbuilding] Weight of spray cellulose

[John Straube]

There are numerous reasons to ventilate attics and cathedral ceilings: removal of moisture is the most important.
Another reason is to limit ice damming in climates with sufficient snow and weather that cycles near to freezing temperatures. If you insulate enough, you dont need to ventilate for that reason.  
BUT, there is no way you can build cathedral ceilings with cellulose and NOT ventilate without taking serious risks.  Some cellulose companies have been selling this idea for years, and we (and most other forensic consultants) have seen sufficient failures to realize that it is too risky in most cases.  If the house below is super air leaky and hence very dry in cold weather you can often “get away with it” but modern houses in cold climates with higher RH levels (e.g. not 15%!) cannot tolerate small air leaks.
Do a great job of air sealing, live in a climate that is not too cold (e.g. Zone 3 or 4) and you might be able to pull off a cellulose unvented cathedral. Still risky, but must less so.
An alternate design is to put enough insulation outboard of the sheathing and cellulose. This is a good retrofit option. Fill up the available rafter space, then put an air barrier membrane on the sheathing, and add sufficient R-value on the exterior to avoid condensation (this amount varies ratiometrically with the amount of cellulose R-value, the outdoor climate, and the indoor winter humidity.

PS I have R-100 in my house… roughly.  about 28” of cellulose in a flat ceiling under a well-ventilated attic. No furring, just 2x4 bottom chords of trusses supporting 1/2” GWB with standard screw patterns. R-100 does not make a lot of sense, until you see the price difference between R-60 and R-100 is so small
But Frank’s experience is worth a lot…. 30 houses over more than a decade.  That is worth more than any experiment in my books.


> On Feb 19, 2017, at 9:32 PM, Reuben Deumling <9watts at gmail.com> wrote:
> 
> R80, huh? Wow. 
> I'm curious, Frank, what you make of the article circulated here recently that suggested that (and I'm paraphrasing) above R30 the ventilation of cathedral ceilings could be skipped?
> 
> On Sun, Feb 19, 2017 at 6:05 PM, Frank Tettemer <frank at livingsol.com> wrote:
> I agree with David's suggestion, to alleviate concern,
> However, for over a decde we have in 22 inches of blown in place cellulose = (R-80?). this had held up consistantly each year, We put up MemBrain by Certainteed stapled to the ceiling joists @ 24" o.c..
> Then, 1x4 spruce strapping on 12" o.c.
> 
> Then 1/2" drywall, double screwes into 1x4 on 9" o.c..
> 
> At least 30 ceilings and insulation installations over 14 years,
> with no failures except in my own garage ceiling where I stepped off the joists with my big bad foot.
> 
> Question?: Do you have a plan to block off the vented soffits, and allow ventilation channels from the vented soffit, above the cellulose, to the vented ridge, or, maximum vent box, or similar?
> More opportunities and good use of corrugated cardboard?
> Or inexpensive Moore vent/rafter vents, and vertical cardboad baffles, from outside face of the wall plate, up to the rafter vents?
> 
> Cheers,
> Frank Tettemer
> 
> On 2017-02-18 12:29 PM, David Wentling via Greenbuilding wrote:
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Dr John F Straube, P.Eng.
jfstraube at uwaterloo.ca
www.JohnStraube.com