Wall cavities look simple until you try to fill them. Electrical boxes, plumbing runs, outlet cables, and odd stud spacing create gaps that batt insulation rarely seals completely, and those gaps are where heat escapes in winter and pushes back in during summer. Proper insulation placement matters more than the R-value printed on the package, because a cavity that is only 90 percent filled performs closer to 60 percent of its rated value. A blown-in blanket system, usually called BIBS, tackles this problem by pumping loose-fill fiberglass behind a fabric membrane so the insulation expands into every corner of the cavity, around wires, and behind electrical boxes.
How a Blown-In Blanket System Works
A BIBS wall is built by stretching netting or fabric across the open face of each stud cavity, then blowing loose-fill fiberglass through small holes in that membrane. The fabric holds the fiber in place while it expands to fill the full depth of the cavity. The installer controls density by monitoring the air pressure and feed rate of the blowing machine, which lets the crew pack fiber around obstructions without leaving voids. The membrane stays in place for the life of the wall, usually covered by drywall, and it doubles as a barrier that keeps the fill from slumping or settling.
The system works best in new construction and in retrofits where the studs are already exposed, but it is not a do-it-yourself job in the way that unrolling batts is. The blowing equipment, the calibrated feed rates, and the netting technique are usually handled by trained crews. Wall insulation is only one piece of the building envelope. Slab edges and perimeter foundations follow different rules and use different materials, which is why slab insulation fundamentals get separate attention when you plan a whole-house insulation strategy; a warm floor slab changes how the walls above it behave.
- Frame the wall and staple the netting across each open cavity using cap staples.
- Cut a small access hole near the top of each cavity bay.
- Insert the blowing hose and fill the cavity from the bottom up at the specified density.
- Remove the hose, seal the access hole with tape or a netting patch, and inspect the fill level.
- Leave the netting in place and close the wall with drywall or paneling.
Netting Types and Fiber Choices
Netting comes in woven polypropylene and spunbond fabrics. Woven netting breathes and stretches slightly, which helps when cavities are irregular or studs are out of square. Spunbond fabric is denser, holds fine fiber more reliably, and resists tears at the staple lines. Both types must be rated for the blowing pressure the crew uses; cheap mesh that rips under the hose ruins a cavity fill faster than any other single failure.
Stabilized vs Standard Loose-Fill Fiberglass
Most BIBS installations use stabilized fiberglass, which carries a binder that locks the fibers together after installation. Stabilized fiber holds its shape against vibration and against the weight of fiber above it, and it resists settling in wall cavities for decades. Standard unbonded loose fill is cheaper but is more likely to pack down over time, so it is rarely the right choice behind netting.
The Installation Process Step by Step
Preparation happens before the blowing machine arrives. Air-sealing penetrations, caulking around electrical boxes, and blocking any chase that would let fiber spill into a neighboring cavity all come first. The netting goes up after the framing passes inspection, because once the membrane is in place, an inspector can no longer see the wiring and plumbing behind it. On a typical house the crew staples netting to every exterior wall bay, then runs the hose bay by bay.
The blowing machine pulls fiber from a hopper, fluffs it, and pushes it through a long hose at a controlled rate. A two-person crew handles the job cleanly: one person feeds the machine and monitors density while the other fills cavities and closes holes. The technique mirrors what dry injection systems do in Europe, where crews blow fiber into confined wall spaces through small ports. The ventilated dry injection system documented by BuildingGreen works on the same principle: inject fiber under pressure into a bounded space, let it expand, and judge the fill by watching pressure and port response. North American BIBS crews use the same feedback to know when a cavity is full, since overfilling bows the netting and underfilling leaves a cold spot.
Density Targets
Density matters more than raw thickness. Standard BIBS walls are blown to roughly 0.7 to 1.0 pounds per cubic foot in 2×4 cavities and 0.9 to 1.2 pounds per cubic foot in 2×6 cavities. Higher density raises the R-value per inch and slows air movement through the fiber, but it also increases material use and machine strain. A crew that blows every bay to the same density delivers a wall that performs consistently from one end to the other.
Comparing Netting-Based Systems with Other Wall Insulation Methods
A BIBS wall competes against fiberglass batts, dense-pack cellulose, spray foam, and rigid foam boards. Each method fills cavities differently and leaves a different thermal and moisture profile. The table below compares typical installed performance in a 2×6 exterior wall.
| Method | Nominal R-value (2×6 wall) | Air sealing | Settling risk | Installed cost per sq ft |
|---|---|---|---|---|
| Blown-in blanket (BIBS) | R-20 to R-23 | Good with air sealing prep | Low with stabilized fiber | $1.50 to $3.00 |
| Fiberglass batts | R-19 to R-21 | Fair, gaps common | Moderate if friction fit fails | $1.00 to $2.00 |
| Dense-pack cellulose | R-20 to R-22 | Very good at high density | Very low | $1.75 to $3.25 |
| Closed-cell spray foam | R-26 to R-30 | Excellent | None | $3.50 to $6.00 |
Rigid boards occupy a different role in the assembly. For exterior sheathing, foundations, and continuous insulation layers, rigid foam insulation delivers its own vapor control and uninterrupted coverage, which no cavity fill can match. High-performance walls often combine a cavity fill like BIBS with an exterior rigid board to break the thermal bridge created by the studs themselves.
Performance Numbers: R-Value, Air Tightness, and Settling
Loose-fill fiberglass delivers roughly R-2.2 to R-4.0 per inch depending on density, and a properly blown BIBS cavity performs near the top of that range because the fiber fills the full cavity depth. A 2×4 wall lands around R-13 to R-15, while a 2×6 wall reaches R-20 to R-23. Those numbers assume the cavity is completely full; the real advantage of the netting method is consistency from bay to bay, because the membrane makes a partially filled cavity visually obvious before the drywall goes up.
Air tightness follows density. Loose fiber at low density lets air circulate through it, which erodes the effective R-value in cold climates. At the densities used behind netting, fiber fills the cavity edge to edge and slows convection, and that matters because air movement through voids bypasses insulation entirely. Field testing of wall assemblies shows complete filling beats a one-point R-value difference every time. The same loose-fill logic applies to attics and other open spaces, where blown-in insulation is the standard retrofit method for topping up existing coverage.
Settling is the classic failure of blown insulation, and stabilized fiber is the answer. Unbonded fiber can lose 5 to 10 percent of its height in a wall over the first few years; stabilized fiber holds its volume within about 1 to 2 percent. That stability is what keeps an R-21 wall at R-21 a decade after installation.
Cost Breakdown and Project Planning
Material cost for loose-fill fiberglass runs about $0.40 to $0.80 per square foot of wall area at BIBS densities. Netting adds $0.05 to $0.10 per square foot. Labor dominates the total: an experienced two-person crew can insulate 1,500 to 2,500 square feet of wall in a day, and machine setup, netting, and cleanup add the better part of another half day on a typical house. Installed prices of $1.50 to $3.00 per square foot are common, and the spread depends on cavity depth, wall height, and how many obstructions the crew has to work around.
- Cavity depth: 2×6 walls use roughly 30 percent more fiber than 2×4 walls.
- Obstruction density: kitchens and bathrooms with heavy plumbing cost more to fill carefully.
- Access: second-story walls need taller ladders or scaffolding, which slows the crew.
- Machine logistics: the blowing machine needs 120-volt power and a clear path for the hose.
Weigh the total against the alternatives. Fiberglass batts cost less up front but leave gaps that show up as cold spots. Spray foam costs more but adds air sealing. The right choice depends on the rest of the assembly, and understanding how each insulation material performs inside a full building envelope keeps the comparison honest.
Quality Checks and Common Installation Mistakes
A BIBS wall looks finished before the drywall goes up, so the quality check happens on site. Homeowners and inspectors should verify that every bay is filled to the same level, that the netting has no tears large enough to pass fiber, and that the crew sealed every access hole. Density is harder to verify by eye, which is why experienced crews monitor feed rates and material usage as they work.
- Blowing too fast, which bridges fiber and leaves a void near the bottom of the cavity.
- Skipping air sealing around boxes and penetrations before the netting goes up.
- Overfilling, which bows the netting and makes drywall installation harder.
- Leaving access holes unpatched, creating a path for drafts.
- Mixing standard loose fill with stabilized fiber, which cancels the settling protection.
The last decision is which system belongs in which wall. A BIBS wall suits standard framed construction, but a partition that carries plumbing, a foundation wall that sees groundwater pressure, and an exterior wall under a vented roof each place different demands on the insulation. Working through wall insulation types and systems before the crew shows up prevents the most expensive mistake in insulation work: installing the right material in the wrong place.