What is Dense Pack Cellulose Insulation?

by Erik North on October 1, 2011

What is dense pack cellulose insulation


And I’m totally not copying huge chunks of the previous cellulose article (I am). What is dense pack cellulose insulation? How is it different that loose fill or wet spray cellulose. What kinds of cellulose insulation are used in modern homes and buildings?

OK, I admit the names are a dead giveaway…one is packed densely in the wall cavity, one is filled loosely and the other is sprayed in wet. Simple enough. When is dense pack cellulose insulation used and what are the advantages and disadvantages compared to other insulation and techniques?

A Wikipedia-Style Definition of Cellulose

Cellulose is the cell structure of any plant fiber. Cellulose insulation is insulation made from some type of plant fibers. Pretty straightforward, no? For something so simple, cellulose has a long varied history with as many drawbacks as the obvious advantages.

Historically the kinds of plant fibers pressed into service as insulation have included straw, hay bales, newspapers, sawdust or cotton. Today cellulose insulation has been standardized as the gray fluffy stuff pictured above.

Cellulose is typically used as wall cavity or attic cavity insulation. There are several different approaches to application (dry blown vs. wet sprayed, loose fill vs. dense pack) which helps increase heat retention and usually dampens noise levels as well.

Modern cellulose is made from either ground up paper (usually recycled) or denim (also usually recycled). It is heavily treated (around 15% by volume) with boric acid, borax or ammonium sulfate. These chemicals are not hazardous for humans but are very effective flame retardants and help reduce issues with pests.

Modern cellulose use (meaning ground paper with added fire retardants) grew in the second half of the 20th century, massively accelerating during the 70s oil shocks. It gained popularity for its low cost and facility of use for retrofits.

However bad publicity spread for several reasons (poor quality product related to lack of regulation, boric acid shortages, possibly apocryphal stories about insulation based fires). Fiberglass nimbly stepped into cellulose’s place as the residential standard for insulation in the US. Over the last decade, cellulose use has risen though it occupies a much smaller market share than fiberglass.

Dense Pack Cellulose Insulation – Current Uses

So that’s a general picture of cellulose. Dense pack cellulose insulation is when cellulose is blown under heavy pressure into an enclosed wall cavity. In new construction, this can be done after rough wiring and plumbing and either before (netting the cavities) or after adding finish drywall. Dense pack cellulose insulation can be blown into open cavities making it ideal for retrofitting older construction (the process entails removing a horizontal row of siding, drilling access holes at the top and bottom of each bay and replacing the wood plugs once the cavity is filled).

When evenly packed, dense pack cellulose insulation effectively seals the whole cavity. Cellulose is considered dense packed when it is around 3.6 lbs per cu ft. Dense packed cellulose insulation provides excellent thermal resistance (an R-value of 3.0-3.5 per inch), sound proofs the wall and greatly retards air flow.

Important Note

Important Note!

Important Note!
It is very important to note that dense pack cellulose insulation greatly slows trans-shell air flow but is not an air barrier or a vapor barrier. Because it does reduce air flow through a wall cavity, mimicking the goals of an air barrier, dense packed cellulose insulation is occasionally seen as such. However, it does not meet any of the technical standards for an air barrier, leading us to…


A brief aside on technical building issues

Aggggh! Technical Mumbo Jumbo

Warning: Tech Talk!
The ASTM (American Society for Testing and Materials) standard used by the American Air Barrier Association for determining an air barrier material is 0.2 L(liters) per second per square meter (0.2 L/s/M^2) at 75 pascals of pressure difference from one side of the material to the other. No form of fibrous insulation meets this standard though drywall, closed cell spray foam and most foam board insulations do.
End of Warning: Tech Talk


Dense Pack Cellulose Insulation – Advantages and Disadvantages

Cellulose occupies a particular low end niche in the spectrum of insulation. The thermal resistance of cellulose is comparable to fiberglass but unlike fiberglass, cellulose impedes air flow (and air transported heat loss).

When blown into the stud cavities, cellulose gets everywhere (trust me…everywhere). It flows around wires, pipes and electrical fixtures, eliminating air pockets and restricting air transported heat loss.

Cellulose is very inexpensive, being made from shredded paper and low cost chemicals. Those chemicals (the boric acid, borax or aluminum sulfate mentioned earlier) provide superb resistance to mold, pests and fire.

Most of the volume (approx 80%-85%) in cellulose is recycled newspaper. Cellulose has more recycled material than any other commercially available insulation. Finally, cellulose doesn’t use any greenhouse gases as propellants like spray foam formulations.

Enough with the advantages…disadvantages:

Like I mentioned above, when retrofitting dense packed cellulose insulation into wall cavities it will get everywhere. Dense packed cellulose will spill into the house through any wall penetrations. Expect to be cleaning the dust and cellulose particles for a good while. Homes with furnace duct systems can expect some of this dust to be recirculated through the house.

Dense pack cellulose insulation weighs several times that of fiberglass or rock wool. This usually isn’t an issue unless insulating at the attic slope (applied directly to the roof). One would need to account for the added insulation weight in calculating roof weight bearing loads.

Installation of loose fill, dense pack and wet spray cellulose also require experience, specialized equipment and training. Fiberglass can be installed by any weekend warrior. The installers should be experienced with each cellulose form.

Why Dense Packed Cellulose Insulation? Because It’s Awesome

And I actually don’t sell it. Hard to believe, I know. The reality is that modern building codes are introducing more advanced standards and metrics for home performance than just R-value. Improving insulation standards was a great start but there are all kinds of shortcomings to material R-value like performance under weather or thermal bridging. Standards for minimum air leakage, construction techniques, and building materials are being introduced. Dense pack cellulose insulation is one way builders can meet these more stringent standards in new and retrofit construction.

Ultimately dense pack cellulose insulation is a great (and very green) tool for energy auditors and weatherization retrofits.

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