Blown In Fiberglass vs. Fiberglass Batts

by Erik North on December 29, 2011

Blown In Fiberglass vs. Fiberglass Batts


I suppose it was only time before insulation manufacturers realized they could forego the expense of all that kraft paper binding the fiberglass batts. At first blush, it seems like blown in fiberglass and fiberglass batts ought to be very similar. I mean they’re both fiberglass, right? However, it’s like saying glass shards are the same as a drinking glass. They’re both made from the same stuff but are completely different in practice.

What are the differences between blown in fiberglass and fiberglass batts? Are there any cost or performance advantages of one over the other? Are there particular health concerns? Let’s start with a boring definition:

What is Fiberglass?

Fiberglass is a man-made material used in construction as insulation and soundproofing. It is made with melted glass spun out into very thin fibers. The microscopic glass fibers are captured then either left loose or pressed into a layered matrix for batts and rolls. The process is very similar to that of cotton candy just at several hundred degrees (though I have never heard of cotton candy batts).

Blown In Fiberglass vs. Fiberglass Batts – Similarities

Because of their identical structure, there are many similarities between fiberglass batts and blown in fiberglass. They’re both very inexpensive, though blown in can vary greatly if you’re doing it yourself versus using contractors.

Both forms retain heat in the same manner by trapping pockets of warm air within their microfibers.

Both are highly air permeable making pairing them with an effective air barrier imperative. Air leaks through chimney chases, recessed lighting or plumbing chases flows right through fiberglass. Wind washing is also a salient problem with fiberglass and standard ventilated attics.

Because glass is a good conductor, fiberglass loses R-value when compressed. Remember fiberglass works by trapping tiny pockets of warm air. When smooshed down, it reduces the air pockets and brings the conductive glass closer together.

Lastly, when insulating around an older, non-IC recessed light both forms of fiberglass need to be kept away from the lighting fixture. Typically a protective dam is built around the light can.

Blown In Fiberglass vs. Fiberglass Batts – Differences

Fiberglass batts and blown in fiberglass do have their differences despite the previous long list of similarities.

Fiberglass batts have a slight advantage in R-value per inch versus blown in fiberglass. Fiberglass batts have a uniform factory determined fiber density, improving their ability to hold those pockets of warm air. Blown in fiberglass has an erratic fiber level and density and may settle or blow around.

Both forms of insulation are highly fire resistant but with a major difference (but hey, that’s why we’re talking). When subjected to burn temperatures the glass fibers will melt but not burn. However, most fiberglass batts are faced with kraft paper which most certainly burns. There are foil faced variants which resist fire spread but these are much less common.

Blown in fiberglass is much better at creating a uniform blanket of insulation. There are a lot of details to work around in an attic: light fixtures, fans, ducts, radon mitigation vents, trusses, etc. etc.

It takes a very conscientious contractor/homeowner to cut fiberglass batts to fit snugly around all those details. Blown in? Assuming there are no non-IC recessed lights, just feed the loose fiberglass into the blower and let it rip. You’ll have a uniform layer of insulation covering all the details.

Very generally speaking, blown in fiberglass is less expensive per square foot than fiberglass batts, especially for the weekend warrior taking advantage of Lowe’s special for free use of their blower machines.

Lastly, if the soffit/eaves are not air sealed (where a solid foam block is sealed against the attic baffles), loose fill fiberglass will be blown around by wind gusts.

Blown in fiberglass and fiberglass batts have many of the same qualities for the niggling detail that they are materially exactly the same. However, there are enough differences that may make one or the other more appropriate for a project.

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