I was just cyber-browsing through the blog archives and realized I’ve written about regular ol’ fiberglass but never just about blown in fiberglass insulation. 10 years ago this might have been no big deal but the box stores have adopted blown in fiberglass in a huge way. One of my best friends is a sales manager at one of the big box hardware stores and they regularly stock dozens of pallets of loose fiberglass bundles during the heating season. What is blown in fiberglass and what are the comparative advantages and disadvantages.
Loose Fill or Blown in Fiberglass Insulation
Loose fill or blown in fiberglass is more or less what you’d get if you stripped the kraft paper off a fiberglass batt and fed it through a shredder.
Fiberglass is a silica that has been melted and spun out into fibers like cotton candy. The fibers are caught and pressed flat for roll and batt style fiberglass. Loose fiberglass obviously collects the individual fibers and stuffs them into a plastic bag which is stacked on a pallet at Lowe’s.
Blown in Fiberglass – Advantages
Price – The biggest advantage of fiberglass insulations are the big two: price and ease of use. Fiberglass along with cellulose is the least expensive way outside of strawbales to add R-value to a building.
Ease of Installation – Loose blown in fiberglass is very easy to work with and unproblematic in most applications. Most insulation supply venues will provide training on use of the blower machine. Insulating attic flats are as simple as ensuring you have enough product to achieve desired insulation depth.
This ease of installation does not apply to retrofitting existing walls. Dense packing cellulose or blowing in fiberglass into wall cavities involve too many variables to do so confidently without considerable experience.
Flammability – One of the big selling points on fiberglass is that it will not burn. Under sufficiently hot flames, it may melt but burning is not an issue. Blown in fiberglass goes one step better since the paper on fiberglass batts can ignite. Loose fill fiberglass has no paper content so even less ignition potential (aka none).
Moisture – Fiberglass can be a real problem with moisture but on the whole merits a slight net positive. Fiberglass being spun glass won’t absorb water and because of its light weight and air permeability, dries out quickly. If installed against wet concrete or other high moisture situations, it can be another story. The millions of glass fibers can easily trap moisture if not allowed to dry, for example if installed with a vapor barrier.
Weight – Fiberglass has a low density and light weight meaning the addition to the roof’s weight load is minimal. This can be a concern in very snowy climates where unmelted snow can accumulate. If there are weight load concerns for your roof, a structural engineer can do load calculations for safety purposes.
Blown in Fiberglass – Disadvantages
R-Value – Blown in fiberglass has a generally lower R-value than cellulose which is denser or fiberglass batts which have factory specified fiber density and spread. Loose fill fiberglass has a settled R-value around 2.5-2.8 per inch. This means achieving an attic R-value of 49-60 for a cold weather zone requires a very deep layer of fiberglass.
Air Barrier – Always a fun topic with fiberglass. Loose fill or blown in fiberglass must be paired with an effective air barrier. The loose, low density glass fibers don’t in any way stop or even slow down stack effect or wind wash driven air flow.
Stack effect will transport heat right through the fiberglass and wind will strip heat from the fibers. This is particularly egregious with loose fiberglass as it doesn’t even have the kraft paper barrier. Loose fiberglass is also susceptible to being blown around by wind if the eaves aren’t sealed.
Extreme Cold – Fiberglass insulation of any stripe works by trapping tiny pockets of air in its millions of spun glass fibers. The colder the outside air, the more quickly those warm air pockets may be stripped away. This can significantly impede fiberglass cold weather R-value.
Particles – Fiberglass like I’ve said ad nauseum are microscopic glass fibers … like asbestos they are very fragile, friable (and alliterative) fibers. Fiberglass by its nature produces loads of airborne bits of glass fiber. Manufacturers have addressed this with batts and rolls, adding a vented plastic facing to prevent stray fibers from becoming airborne.
The flying glass fibers can be prevented in batt form. Blowing in loose fill fiberglass is another story. Any installer using blown fiberglass should wear full HEPA filtered masks. Inhalation of fiberglass particles can have significant health risks.
Utility – Blown in fiberglass is utilized quite similarly to cellulose with less success (sorry guys). When blown into wall cavities, loose fiberglass doesn’t have the density of dense pack cellulose or achieve similar air retardancy.
Blown fiberglass is a cost effective approach to insulation well suited for attic flats. It has a few minor drawbacks that shouldn’t be an issue as long as you’re aware of them.