Dense pack cellulose as an air barrier

[Sifu]

Just read an article in JLC that claims dense pack cellulose serves as an air barrier, though it gave no data or perm ratings. I was led to believe dense pack cellulose does provide a greater measure of air impermeability but that it did not reach the level to be considered "air-impermeable". This question comes up with regards to the required air-space between the sheathing and the insulation again. For my purposes I thought I had settled this, at least as far as spray foam but now JLC puts out the article claiming the cellulose is "air-impermeable". Anybody have more info?

 

[RLGA]

Perm ratings are for vapor permeance. An Aar barrier's air permeance istested per ASTM E 2178 and is measured in cfm/sq. ft. of surface area at a certain pressure differential measured in psf.

I doubt it would pass the requirements of an air barrier as established by the Air Barrier Association of America (ABAA), which is 0.004 cfm/sq. ft. at a pressure differential of 1.57 psf.

If they don't have testing to prove it, they don't have an air barrier.

 

[Sifu]

So did I mis-use the term perm rating? By definition air impermeable insulation has an air permeance rating of .02 or less. Is that not called a perm rating? Maybe I am confusing terms here so I'll back up and try again.

The article makes two claims:

1-areas can be effectively insulated and air sealed with dense packed cellulose alone

2-doesn't provide the sort of hermetically sealed air barrier you get with foam but at atmospheric pressure it does stop the movement of air and moisture

So on one hand it claims to be less effective than foam then on the other hand it claims to be both an air and vapor barrier. The basic question is has anyone seen any science or reports that support the claims made in the article.

The code requires an air impermeability rating of .02 or less and a vapor permeance rating (through the back door by requiring air impermeable insulation be a class II vapor retarder) of .1 to 1.0. By claiming that dense packed cellulose is an air and vapor barrier it seems the author may be taking some liberties which is why I am trying to find out if there is actual data to support the claim.

 

[RLGA]

The "perm" is an IP (U.S. inch/pound) unit of measurement and only applies to vapor permeance. 1 perm is equal to 1 grain of water vapor per hour, per sq. ft., per inch of mercury. A metric perm (SI equivalent) is equal to 1.51735 U.S. perms.

The 0.02 air permeance in the IRC is given in metric units equivalent (L/s x sq. m of surface at 75 Pa) to the IP units I gave in my previous post (0.004 cfm/sq. ft. at a pressure differential of 1.57 psf).

I seriously doubt dense-packed cellulose insulation is a vapor retarder by IRC definition. However, the 2012 IRC allows a Class III vapor retarder coating or covering in direct contact with the insulation if the insulation cannot meet the requirements of a Class II vapor retarder for Climate Zones 5, 6, 7, and 8 (Section R806.5).

 

[tmurray]

They are seriously saying that cut up newspaper is impervious to water migration because of how dense it is packed into the cavity?

 

[Sifu]

They are seriously saying that cut up newspaper is impervious to water migration because of how dense it is packed into the cavity?

That is what the industry and the article say. I am trying to separate fact from fiction, at least as far as code is concerned.

 

[conarb]

I don't know about code but I've seen some real disasters with cellulose insulation, especially the ground-up newspaper variety. Your asphalt and kraft paper based WRBs have a water-holdout period of 20 to 155 minutes, plastic products like Tyvek have never been able to pass the 20 minute water-holdout test so they got a new test approved. If the walls stay wet for a period of greater than the water-holdout period you've got a wet moldy mess; furthermore, it doesn't do any good when wet, I've seen it packed down to 12" on the sole plate and/or fireblocking. No insulation should be hygroscopic, like all cellulose products, be they newspaper, ground-up Levis, or cotton batting.

 

[Fisherman]

Sifu, if you haven't already seen it, there's a great discussion on this topic on GBA, albeit a few years old. I'd say it's quite relevant to your question and may help shed some light. Here's the link: Is dense-packed cellulose an air barrier? | GreenBuildingAdvisor.com. If the link doesn't work, just google "dense pack cellulose air barrier" and it should be the first one that pops up. Hope this helps.

 

[Sifu]

Thanks for the link Fisherman. It seems my questions are certainly not original. The discussion seemed to provide the same equivocation and muddled terms present today. My concern is the accumulation of moisture on the coolor underside of roof sheathing over an enclosed rafter space. If there is no air-space required then there is no ability to remove the moisture heavy air from the cavity. So while many report that that dense pack cellulose is more effective, and that it may retard vapor permeance, the standards still don't seem to be clearly met. I'll just keep asking and learning about this until it is settled, until then I can't see allowing the removal of the air-space with cellulose. I would pose this question as well; where does the moisture that meets the cellulose go if it can't reach the sheathing? Is it simply getting sucked up into the product? As conarb says, no insulation should be hygroscopic.