Insulation materials that is…
We had cellulose blown into the walls of our house about three years ago. The contractors did a nice job and I checked over everything with an infrared scan afterward. The most interesting part was when the sales manager visited the site that afternoon.
The manager, both members of the work crew and I spoke for a bit. I was a bit surprised when the sales manager asked (and the crew didn’t know either) what the difference was between blown cellulose and blown fiberglass. It was an enlightening conversation … I mean if the contractor has questions about blown in insulation, imagine how many questions homeowners might have. So what are the options for blown in insulation?
Blown In Insulation
Blown in insulation comes in two broad categories with a few variants falling into those groups: cellulose and glass fiber insulation. Cellulose is shredded organic material (either paper or denim) treated with fire retardant borates. Mineral wools (that’d be rock wool and fiberglass) are fibrous insulation where the rock/glass are melted and spun out like cotton candy. The fibers are captured and for blown in insulation, kept as individual fibers. I’ll write a comparison between the different subtypes in a later article, but for this one they are substantively similar.
Price – Whether blowing onto an attic flat or into a wall cavity, the prices of blown in cellulose and blown in fiberglass are very comparable. Whether you have a contractor or do it yourself, the equipment and labor costs will be close to identical. The only difference is the material costs which are also quite close.
Ease of Use – If you’re a DIY type (and we don’t run into that much in Maine), simplicity is a big selling point of any project. If you don’t have the tools, you can’t do it. Blown in insulation for attics requires about three molecules of training with a rental blower. Not terribly onerous. However, we’re talking about attic flats here. See below for wall cavities as dense pack, netted or wet spray insulation are more difficult.
R-Value – Both blown in fiberglass and cellulose (even dense pack or wet spray) produce comparable R-values. One can quibble about 0.1 or 0.2 R-value difference but we’re not talking the 3 or 4 R-value difference per inch like comparing it to polyisocyanurate foam board. The true difference is in effective R-value where fiberglass performs poorly in a number of circumstances like extreme cold that do not affect cellulose as adversely.
Air Barrier – To perform most effectively, your thermal barrier (aka insulation) needs to be paired with an air barrier. However, there is no form of blown in fiberglass or cellulose that meets the technical definition of an air barrier.
Moisture – Both blown in fiberglass and cellulose are pretty lousy dealing with moisture. They both trap moisture in their fibrous structure and if paired with an impermeable vapor barrier, can cause serious moisture accumulation. The major difference is that blown fiberglass has no organic bits to foster mold growth, unlike Kraft paper faced fiberglass batts or cellulose.
Wind Washing – Both blown in insulation can be adversely affected by wind washing. In attic flats, wind can blow through the soffit and pick vents. At low velocities, cold air can strip some of the warm air pockets blown in insulation trap, reducing effective R-values. At higher velocities, wind can blow the insulation around the attic unless the eaves are effectively blocked and sealed. If wind persists in a single direction, blown in insulation can be stripped away, leaving bit portions of the attic floor uninsulated.
Wall cavities are affected similarly by persistent wind. If the insulation is loose filled, the effective R-value can be affected by air leakage. However, when insulation wall cavities with cellulose, it is usually dense packed or wet spray cellulose; both approaches which retard air flow and effective R-value.
Air Retardant – I mentioned a few paragraphs ago that neither blown in fiberglass or cellulose meets the technical standard of an air barrier. And they don’t. But the most common retrofit approach, dense pack, and with new construction or a gut rehab, wet spray cellulose both very effectively retard air flow.
Flammability – The history of residential building is entwined with not having them go up in flames. Blown in fiberglass and cellulose deal with fire issues differently. A sufficiently hot fire will only melt fiberglass (rock wool, with its stratospheric melting point, may not even get that) and blown in fiberglass has the added benefit of no Kraft paper. Cellulose has fire retardant borates added. Older cellulose had fire issues because of erratic borate content between batches though that isn’t a problem with modern quality controls.
Installation Redux – Remember how easy I said insulating attic flats can be? Not so with insulating wall cavities. Walls have cross braces, hurricane braces, electrical wires, pipes, etc. etc. It is very hard, requiring some considerable skill to ensure the entire cavity is uniformly insulated. And if you’re attempting to dense pack the wall cavity, that can only be done with cellulose or certain brands of fiberglass and a much more powerful insulation blower than can be publicly rented.
Embodied Energy – Embodied energy is the sum of energy required for a project or material. Fiberglass has a much higher embodied energy than cellulose insulation. Fiberglass is glass that is melted and spun into fibers like cotton candy. There are fiberglass brands which use recycled content but more often they use new raw materials.
Most cellulose brands use a high recycled content and the production process (shredding paper and adding fire retardant borates) uses much less energy.
Extreme Cold – Cellulose, whether loose fill, dense pack or wet spray, performs better in extreme cold. Fiberglass works by trapping pockets of warm air in its millions of microscopic fibers. Very cold air quickly strips out the thermal energy and glass itself is a poor insulator. Cellulose doesn’t suffer as acutely since cellulose fiber is a naturally better insulator.
Weight – Weight of insulation would never be a concern for wall retrofits as the vertical load sits over the building foundation. Blowing insulation over attic flats and slopes is a different matter. Cellulose weighs considerably more than fiberglass of comparable R-value. Achieving the recommended R-49 to R-60 for our climate zone means 18+ inches of insulation. If you have concerns about the roof’s weight load, a civil engineer should be consulted to calculate the structural load.
Flexibility of Use – Both blown in fiberglass and cellulose can be blown over attic flats into netted wall cavities or into wall cavities as a retrofit. However, cellulose can be wet sprayed into an open cavity (sprayed with water and allowed to dry before adding drywall) or dense packed (blown in to a density of 3.5 lbs/cubic foot). Standard loose fiberglass cannot be applied in either of these ways though there are specialized variants.
Dust and Debris – Both blown in cellulose and fiberglass require respiratory protection during installation. Dry cellulose is very dusty (though there are low dust brands) while the fiberglass micro particles can become easily air borne and inhaled. Like I said, both require protection.
I strongly lean toward cellulose because it helps produce a tighter building envelope. New building code insulation standards are including standards for building tightness. Every individual has their own criteria.