Is dense-packed cellulose an air barrier?
Tristan Roberts
| Posted in Energy Efficiency and Durability on
Numerous sources on the Web say that cellulose installed at 3.6 pounds per cubic foot is an air barrier, but in another Q&A item, Peter Yost said that it’s not. Can someone clarify this issue and provide data? Thank you.
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Tristan,
Dense-packed cellulose resists air infiltration better than fiberglass batts, but it's not an air barrier. Examples of air barriers include plywood, drywall, polyethyelene, metal flashing, glass, and rubber membrane.
If you hold a material up to your mouth and blow, you can tell whether it's an air barrier. If air moves through the material, it's not an air barrier.
ASTM E 283 defines an air barrier as a material having a maximum allowable air leakage rate of 0.02 liters/second/square meter at 75 Pascals pressure difference. As far as I know, dense-packed cellulose does not meet this standard, although I'm willing to be corrected if anyone is aware of testing that contradicts my understanding.
The confusion over this (and other similar) questions is that there are several different meanings of "air barrier". As Holladay correctly points out, there is an ASTM standard accepted by the American Air Barrier Association for an air barrier material (0.02 l/sec-m2 or 0.004 cfm/sf @75Pa), an air barrier assembly such as a wall or ceiling (0.20 l/sec-m2 or 0.04 cfm/sf @75Pa) and an air barrier system, which is an entire enclosure (2.00 l/sec-m2 or 0.4 cfm/sf @75 Pa).
In addition, The U.S. Department of Energy Building America Program sets an enclosure air permeance requirement of 1.65 l/sec-m2 @75 Pa for residential buildings. ASTM E-1677-00 Standard Specification for an Air Retarder Material or System for Low-Rise Framed Building Walls currently calls for an assembly air permeance requirement of 0.30 l/sec-m2 @75 Pa. The Building Services Research and Information Association (BSRIA) of Great Britain recommends 1.8 l/sec-m2 @75 Pa as “best practice” for the air permeance of residential building enclosures.
These standards are for the barrier system which is meant to control both air leakage and air diffusion into or out of a building envelope - for efficiency, comfort and control of moisture problems - by wind, stack effect, or mechanical pressure.
As such, the air barrier may or may not be identical to the vapor barrier (or retarder), which is meant to control moisture diffusion, and may or may not be identical to the weather-resistant barrier (WRB), which is meant to control the intrusion of environmental moisture (including wind-driven moisture) into the building envelope. And it certainly is not the same as fire-stopping, which must control the movement of air within and between elements of the structural envelope.
Because there are at least three building elements which are intended to control air movement - WRB, air barrier, fire-stopping - there is confusion about standards and functions.
An effective WRB, for instance, must also be highly wind-resistant, but it is not necessarily THE air barrier and does not have to meet the strict ASTM/AABA requirements. Ideally, effective cavity insulation is also sufficiently impermeable to air movement that it won't be degraded by convection and will facilitate fire-stopping.
Because of the fiber characteristics and high density of blown cellulose insulation (3 to 3.5 pcf compared to mineral wool or fiberglass batts at less than 1 pcf), it is very effective at preventing internal convection and has been recognized as an effective fire-stop by third-party testing and code officials. While it may not meet the strict requirements for an air barrier material (0.02 l/sec-m2 or 0.004 cfm/sf @75Pa), it will significantly enhance the air-resistance of an assembly over other fibrous cavity insulations, maintain its rated R-value by preventing internal convection and dramatically reduce the flammability of a building structure.
In addition, because of its hygroscopic qualities, dense-pack cellulose will serve as a moisture buffer (much like thermal mass does with heat) to moderate (by storage and release) fluctuations in indoor relative humidity, and may protect wood framing and sheathing from excessive moisture accumulation and consequent mold and rot.
Lets see someone post as clearly as possible - apples to apples - the various ratings of the various insulations available today, ie: vapor transmission or absorption, air transmission or block(convection), water absorption, R value(conduction), radiant heat blockage, fire stop, flamability, voc emmision of toxins such as fire retardant additive, mold block additive, etc.
Steve,
Some, but not all, of the information you seek can be found in the Building Materials Property Table put together by the folks at the Building Science Corp.:
http://www.buildingscience.com/documents/information-sheets/5-thermal-control/building-materials-property-table/
Further information can be found in the GBA Encyclopedia:
Insulation Overview
Insulating Roofs, Walls, and Floors
Insulation Choices
Steve, if you give me a day or two I can post the results of my blower door tests on over 1500 new homes in Western NY.
It is not a "component level" comparison, but does reflect very real-world ACH rates from new homes in our area organized by insulation type.
I just have to find it (I just updated our server and my desktop PC)
The results for cellulose vs spray foam are pretty surprising for cellulose being a :non-air barrier."
-Rob
Robert Susz
That sounds like very informative information. I would be interested in seeing your results. Please keep us posted.
How about an answer with a question? Retrofit and renovation of a ranch built mid '60's in SC...
With gyp board and insulation stripped out to the black fiber sheeting used back then, with serviceable brick on the exterior, what should be installed as something resembling a vapor barrier along with the choice of insulation at this point?? There is never any barrier of any sort between the brick and the sheeting...What to do??
Thanks, Mr. Norm
Norm,
You don't really want a vapor barrier between your wall sheathing and your brick cladding; you want a water-resistant barrier (WRB).
There really isn't any way to retrofit a WRB behind brick veneer.
Early on when I used to do training for the Weatherization program we trained on real houses. In one instance, the class installed dense pack cellulose under the floor in an attic and as part of the training did a leakage test before and after the work was complete. On this particular job, the reduction in the cfm50 was 1020 CFM. On many other wall insulation training projects we kept track of we would get a 20 to 55% reduction in the air leakage rate of the house that was just from wall insulation. I will admit that on some of those wall projects some of the reduction resulted from our repair efforts to keep the cellulose in the wall.
All the standards offered by the various standard setting organizations seem to me to be arbitrary but I guess they all have the right to examine their own navels. I am confused as to why the US DOE sets a standard in metric when everyone I know measures air leakage in cubic feet and at 50 pascals and house surface areas in square feet.
In my experience, densely packed cellulose is an air barrier.
Steve McCarthy / Starbright Energy Services
Wellsboro, PA
In a generic sense, perhaps, since there is no doubt cellulose - even loosefill - prevents a great deal of air movement and internal convection.
But, given today's emphasis on building science and quantitative standards for achieving high levels of energy efficiency, we can no longer use such terms as "air barrier" as cavalierly as we used to.
Just as the term "vapor barrier" now means "a class 1 vapor retarder with a perm less than 0.1", similarly an "air barrier material" now means "a material that will pass no more than 0.02 l/sec-m2 or 0.004 cfm/sf @75Pa".
In that light, dense-pack cellulose is not an air barrier. But cellulose insulation could arguably and probably demonstrably be part of an an air barrier assembly such as a wall or ceiling (0.20 l/sec-m2 or 0.04 cfm/sf @75Pa) and an air barrier system, which is an entire enclosure (2.00 l/sec-m2 or 0.4 cfm/sf @75 Pa).
So, when we ask "Is dense-packed cellulose an air barrier?" we must specify what exactly we mean by "air barrier".
Cellulose is not an air barrier, but it resists air movement both through the wall and withing the wall through convection. Fiberglass Batt insulation is poor at both resisting air movement both through the wall and withing the wall through convection. The newer dense pack fiberglass blown insulation is supposedly better at retarding this air movement.
Gypsum wallboard is only an air barrier if it is sealed to the floor and itself at corners using the air tight srywall approach. Adding a vapor retardent coating like a primer sealer increases the air barrier characteristics as well as creating a vapor barrier.
For your wall with black sheathing under brick veneer, a one inch spray applied layer of foam will both increase the R value of the wall and almost entirely eliminate air movement through the wall. The remainder of the wall cavity then can be insulated with cellulose for a less expensive convection free insulation for the rest of the wall.
Again, if we're going to be able to communicate with each other without engendering more confusion, we have to agree on terms. The Air Barrier Association of America (ABAA) has defined these terms and set the qualitative standards for each.
Air Barrier – Primary material that prevents or reduces the passage of air through the building enclosure system.
Air Barrier Material – principal element installed to provide a continuous barrier to the movement of air through building enclosures.
Air Barrier Components – transitional element installed to provide a continuous barrier to the movement of air through building enclosures.
Air Barrier Assembly - a collection of air barrier materials and air barrier components installed to provide a continuous barrier to the movement of air through building enclosures.
Air Barrier System – a combination of air barrier assemblies installed to provide a continuous barrier to the movement of air through building enclosures.
Unpainted drywall is the standard against which the ABAA rates all other materials as an air barrier. Unpainted unfinished drywall is an air barrier because it is an air barrier material. The taped joints and caulk or gaskets of the ADA are the air barrier components, or transitional materials to maintain the integrity of the air barrier material as it is applied within an assembly.
Along with the windows and doors and skylights, the drywall and tape and caulk form an air barrier assembly, such as a wall or ceiling. And, along with the other walls, ceilings, floor and any other boundaries creates an air barrier system.
VR primer is not necessary in order for drywall to function as an air barrier, and neither does it create a vapor barrier, but - as the name implies - merely a class 2 vapor retarder.
With a common language, established by credible national institutions, we can have effective discussions about such topics. Otherwise, we are merely talking past each other.
Did Robert Susz ever post his results
Excellent discussion here and it points directly to the need of being clear in what we mean by terms such as air barrier, vapor barrier and the like. It underscores the need to rely on performance testing and standards to give us baselines for referencing and comparing information. It also is important to remember that individual building components are a part of a system (wall, roof, floor, etc) and we must consider the overall effects of their combination.
So we can say the following is true? That dense pack cellulose is not an air barrier, but is highly effective in resisting air movement. Is it effective enough to eliminate the need for a plastic film barrier on the inside?
Here's my real world dilemma - the client is seriously opposed to plastic products, especially on the inside of the house where she's worried about indoor air quality. I'm closing in a timber frame with a 2x8-thick double-2x4-stud wall(2' o.c. alternating interior and exterior so there aren't 2 studs side by side). Interior will be drywall, exterior will have advantec sheathing and WRB and rainscreen behind siding. Stick-framed wall will be held out from timber frame 9/16" so that drywall can be slid in(behind timber frame) after mechanicals are in place. Rather than stretch reinforced poly over inside of stick-framed wall, I'm going to talk to the insulation sub about cutting holes in the drywall for blowing in cellulose. The holes will need to be patched later, but it will be skim coat plaster so I don't see an issue.
My worry is that the dense pack installation will need to be practically perfect to insure we don't have moisture migration problems where moist indoor air leaks through and condenses inside the wall somewhere. Any comments? Thanks.
Ben,
Be careful you don't confuse a diffusion question (that is, the question of whether or not you need a vapor retarder to slow water vapor diffusion) with an air barrier question (that is, the question of how to prevent air from migrating through your wall).
First of all, there is no code requirement for interior polyethylene anywhere in the U.S. In cold climates, many codes require a vapor retarder (kraft facing or vapor-retarder paint), but there is no requirement for a vapor barrier (polyethylene).
If you are worried about air leaks through your wall -- and you should be -- you should probably use the Airtight Drywall Approach. For more information, see Airtight Drywall.
If by "air barrier" you mean a material that stops ALL air movement, then almost nothing meets that standard. If by "air barrier" you mean a material or assembly that meets the ABAA standards, then dense-pack cellulose does not meet those standards except when combined with air-tight drywall.
If you're worried about moisture accumulation in the walls (I assume this is a cold-climate house), then I would strongly recommend CDX sheathing rather than Advantek which is far less vapor permeable - and drying to the outside is the dominant mode in a cold climate.
Most cellulose manufacturers recommend or require no poly vapor barrier, because the hygroscopic properties of cellulose perform best in a wall that allows some breathing. And a more efficient way to blow cellulose through the drywall is by installing a 12" wide strip of Insulweb or filter fabric horizontally across the middle of the stud walls, leave a 4"-6" gap between upper and lower drywall sheets, blow through this gap and then fill with strips of 3/8" drywall for a seamless surface.
Comment/Question for Robert:
Yes, I've seen articles suggesting this, but most people I know have done the holes+patching route.
I like this suggestion. It would seem faster than patching a lot of small holes.
Two questions:
1) Does this single strip give enough access to adequately fill both above and below the center band?
2) You said to fill the middle gap with 3/8 drywall. Was that because the whole wall is 3/8" drywall, or is it 1/2 inch and you are using a thinner piece of drywall here so that it can be made flush and smooth with joint compound? I am inclined to believe you just said 3/8th drywall to mean the thickness of the rest of the wall drywall but want to confirm this.
Thanks
Rob,
Yes, you can easily fill first below and then above the slit, but you have to push the 3" hose all the way down to the bottom plate and slowly withdraw it as it packs, and then push it up to the top plate and do the same.
Use one size thinner drywall for the filler strip for a flat joint. If the wall is ½" DW then the filler is 3/8". Make sure that two factory tapered edges face the center slot on the top and bottom sheets.
I'd like to add that at least one manufacturer of cellulose insulation states clearly on their website that the use of a vapour barrier is not required when their product is dense packed.
https://www.greenfiber.com/support?documenttype=safety-performance-documents&filtertype=documenttype#technical-documents-brochures
Cellulose Insulation, Moisture, and Vapor Barriers
Cellulose is the only insulation in common use that actually manages moisture. Moisture moves by two transport mechanisms: air movement and diffusion. Of these two, air movement is the more significant; accounting for over 98% of the total, and it is the primary cause for moisture related building failures. When cellulose insulation is blown or sprayed into a cavity at a density above 3.2 lbs/cf, the cellulose will impede the movement of air generated by wind, stack effect, and mechanical imbalances within the building. By blocking the movement of moisture-laden air, cellulose will reduce moisture movement to manageable levels within the building assemblies. Any remaining moisture moving by diffusion will be further blocked by primers and paints used on the interior surfaces.
In our Northeast climate, a vapor barrier is not only unnecessary but can also be potentially harmful, especially during the summer months in air-conditioned buildings, when warm moist air passes through wall assemblies and condenses on the outside of the cool poly vapor barrier. The hygroscopic nature of cellulose insulation allows it to manage and wick moisture from areas of greater to lesser concentrations, thus preventing damaging amounts of moisture from accumulating, as long as the path is not blocked by a vapor barrier. During most of the year in the Northeast, a vapor permeable wall will tend to dry to the outside, while in the summer, this same wall will tend to dry to the inside.
The tens of thousands of homes weatherized with cellulose insulation since the 1970’s, with no vapor barriers and no evidence of mold or structural damage, are a testament to this claim. Sophisticated moisture modeling by Mark E. Kelley III, PE, demonstrates that cellulose insulated building assemblies without a vapor barrier show faster drying and lower overall moisture levels over identical assemblies with vapor barriers installed.
In summary, we do not recommend the use of vapor barriers with cellulose insulation, except in circumstances of exceptionally high moisture levels, such as an indoor pool facility, and we warrant our cellulose insulation for the life of the building when installed by an approved GreenFiber insulation contractor.
In this article from Building Science Corporation, they put dense packed cellulose to the test and measure actual air leakage to see if it meets requirements. Spoiler alert: It doesn't.
https://buildingscience.com/documents/insights/bsi-043-dont-be-dense
"Dense pack cellulose and dense pack fiberglass are not air barrier materials, and they do not result in air barrier assemblies. Although, with respect to air barrier assemblies, they sometimes come pretty close."
The bottom line: "Dense packing a wall does not eliminate the need for an air barrier in new construction."