Fiberglass Insulation, Plainly Explained
The go-to insulation in North America since the 1930s. What it’s made of, the shapes it comes in, the facings that turn it into a finished ceiling, and honest notes on when to pick something else.
How Buying Works — Three Steps
Fiberglass, in one paragraph
Fiberglass is spun glass. Molten glass gets pulled through tiny nozzles into hair-thin strands, cooled, and matted onto a conveyor. The matted fibers get a little bit of glue to hold their shape, then get pressed to a target thickness and cut into rolls, batts, or bags of loose fill.
The insulation doesn’t come from the glass. It comes from the millions of tiny air pockets between the fibers. Still air is what insulates — the glass just keeps the air from moving. Every insulation product on the market works on that same idea, no matter the material.
Owens-Illinois patented the modern spinning process in 1938. The big four U.S. makers — Johns Manville, CertainTeed, Owens Corning, and Knauf — ship over a billion pounds a year, most of it certified to NAIMA and ASTM C665 standards. That’s the material Insulation Overstock ships.
How thick is R-19? How thick is R-30?
R-value tells you how well the insulation slows heat from moving through it. Higher is better. Here’s what the common R-values look like and where they show up:
One important note: fiberglass only hits its rated R-value at its full thickness. Cram a 6″ batt into a 4″ cavity and you get closer to R-13, not R-19. Give it room to be fluffy.
Batts, rolls, and blown-in — what’s the difference?
Batts — pre-cut rectangles
Cut to fit standard house-framing bays: 15″ wide for 16″ framing, 23″ wide for 24″ framing, 48″ long. Fast in a normal wall. Wasteful anywhere the bays are odd sizes or full of pipes and wires.
Best for: stick-built house walls, floor joists, rafter bays.
Rolls — long continuous runs
Come in lengths from 16 to 100 feet, in 4-, 5-, 6-, and (for WMP-30 only) 8-foot widths. The go-to for metal buildings and pole barns because you unroll a whole bay in one pass. Almost always shipped with a facing already attached — that facing is your finished ceiling once it’s up.
Best for: metal buildings, pole barns, commercial roofs, ag buildings, workshops. This is what we ship. See our four facings →
Blown-in — loose bags, blown through a hose
Compressed loose fiberglass in a bag. A rental machine breaks it apart and blows it through a hose into an open attic. Fastest way to top up an existing attic without cutting anything.
Best for: attic retrofits, hard-to-reach spaces.
The thin sheet on one side changes what the insulation does
The fluffy fiberglass itself is the same across the industry. What separates products is the facing — the thin sheet laminated to one side. The facing gives you a moisture rating, a fire rating, tensile strength (how much pulling it can take during install), and for the vinyl-faced products, a clean finished interior surface.
We stock the four standard facings:
- VR-R Plus — the entry-price facing. White finish. Handles moisture well enough for most pole-barn and small metal-building jobs.
- WMP-10 — a small step up. Better tensile strength than VR-R Plus. Best pick for dry climates.
- WMP-30 — heavy-duty. The only facing that comes in 8-foot-wide rolls. Good pick for wider bays and humid climates.
- WMP-50 — warehouse grade. Highest strength, includes a metalized backing that bounces heat away. The commercial choice.
Full side-by-side of all four? See the facing comparison table.
The jobs it’s built for
Metal buildings & pole barns
Vinyl-faced rolls unroll across the purlins in one pass. The facing is your ceiling — no drywall needed. Costs a fraction of foam. This is the near-automatic pick.
Regular house walls and ceilings
Batts drop into standard 16″ and 24″ framing bays in minutes. Meets code. Costs less than half what spray foam would run for the same R-value.
Rooms where you want it quiet
Fiberglass absorbs sound better than any other common insulation. Home theaters, offices, walls next to a busy street — specify fiberglass, not foam. Closed-cell foam is dense enough to transmit sound; fiberglass eats it.
Attic top-ups
Blown-in fiberglass adds R-value to an existing attic faster than anything else. A rental machine and one morning gets you from R-19 to R-49 on a 2,000 sq ft attic.
The jobs where you should buy something else
We sell fiberglass. We make money when you buy fiberglass. That’s exactly why the next few paragraphs matter — if your job calls for foam, we would rather send you to the right product than take your money for the wrong one.
Cathedral ceilings and tricky roof shapes
When the ceiling is also the roof deck — no attic above — you need the insulation AND the air seal in one step. Fiberglass squeezed into a rafter cavity leaves air paths at every rafter. Warm air sneaks up to the cold roof deck and drips as condensation. Ten years later you’re looking at brown stains and rot. Closed-cell spray foam does both jobs at once. Read the spray foam guide.
A continuous outer layer on your building
If your code or your climate wants a solid sheet of insulation on the outside of the studs, fiberglass can’t do that. Foam board can. Read the foam board guide.
Hot climates with a metal roof
A metal roof in Texas or Arizona hits 165°F on a July afternoon. That’s a huge radiant heat load pushing down into the room. Fiberglass alone lets that radiant heat right through. The right pair is a reflective barrier at the roof plus fiberglass in the ceiling — each doing what the other can’t.
Anywhere it will get repeatedly wet
Wet fiberglass loses close to half its R-value, and it doesn’t fully recover when it dries. If the space gets soaked regularly — a leaky basement wall, a crawlspace with standing water, an attic under a bad roof — fix the water first, then use closed-cell spray foam or XPS foam board. Wet fiberglass turns into a mold farm.
Compare
The other three kinds of insulation
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