Blown-In Wall Insulation: Dense-Pack R-Value & Cavity Math

Figure blown-in insulation for walls the right way: by cavity volume and density, not attic-floor depth — with the dense-pack vs loose-fill distinction, the two install pathways, and cavity R-value by stud depth.

Blown-In Wall Insulation R-Value by Stud Depth

A wall cavity’s R-value is its depth times the material R per inch — there is no settled-depth lookup like an attic floor, because a dense-packed cavity is full top to bottom. The table below uses cellulose at 3.7 R per inch (the value our Blown-In Insulation Calculator engine uses for blown cellulose) across the common framing depths. The wall total adds any exterior sheathing and continuous insulation on top of the cavity figure.

These cavity figures are nominal material R-values before whole-wall thermal bridging through the studs is accounted for. The framing itself conducts more heat than the insulation, so the assembly’s effective R is lower than the cavity number — which is exactly why continuous exterior insulation is added in colder zones. Confirm the wall R-value your climate zone calls for with your local building department.

How the Blown-In Wall Coverage Math Works

Attic blowing is a floor-area-÷-coverage-chart problem. A wall is a volume × density problem, because you are filling a fixed cavity to a target pounds-per-cubic-foot rather than blowing to a depth. Here is the chain the Blown-In Insulation Calculator follows in wall-cavity mode:

The framing fraction (~15% at 16-inch on-center, more at 24-inch or with extra studs) is the wall-specific wrinkle the attic math never has: studs, plates, and headers occupy cavity the insulation cannot fill. The density term (3.5 lb/cu ft for dense-pack per DOE) is what turns cubic feet into pounds and then bags. Because the labeled R is reported on a settled-density basis under the FTC R-Value Rule, a properly dense-packed cavity holds its rated R without a settling buffer.

Why Walls Are Dense-Pack, Not Loose-Fill

Density Is the Wall Variable

In an attic, gravity is your friend — loose-fill cellulose at about 1.5 lb per cubic foot sits flat on the floor and stays put. Stand that same material up in a vertical wall cavity and it slumps, leaving a cold void at the top of the bay within months. The fix is density: per DOE, wall and cathedral cavities are blown to 1.5 to 3.5 lb per cubic foot, and 3.5 lb/cu ft or higher is the common dense-pack target. At that density the cellulose forms a semi-rigid plug that resists settling, fills the cavity corner to corner, and slows air movement through the wall.

Settling in a Vertical Cavity vs an Attic Floor

This is the single biggest reason wall jobs differ from attic jobs. An attic floor never faces slump — its material settles a few percent downward into a layer that is still continuous. A wall cavity packed too lightly settles away from the top plate, and that gap is a thermal short circuit you cannot see behind the drywall. Dense-pack at 3.5 lb/cu ft is the engineered answer: the install density is high enough that the column supports its own weight. This is why a wall is specified by a density target while an attic is specified by a settled depth — two genuinely different problems.

Open-Wall Net-and-Blow vs Drill-and-Fill Retrofit

There are two ways to dense-pack a wall, and which one applies depends on whether the framing is open. New construction / open framing: a fabric netting (a spun-bonded mesh stapled across the stud faces) is fastened over the open bays, and cellulose is blown through a slit in the fabric until the cavity is packed tight — the “net-and-blow” method. Finished / closed walls: DOE describes the drill-and-fill retrofit — installers remove a strip of exterior siding about waist high, drill a row of three-inch holes (one into each stud bay) through the sheathing, insert a filler tube to the top of the cavity, and dense-pack the insulation, then plug the holes and replace the siding. The attic guides cover neither pathway, because an open attic floor needs no netting and no drilling.

Wall Materials: Greenfiber SANCTUARY Cellulose vs Owens Corning AttiCat Fiberglass

Two blown materials dominate at the big-box stores. Greenfiber SANCTUARY cellulose is the workhorse for dense-pack walls — at 3.7 R per inch it reaches more R in the same cavity, and its fiber readily dense-packs to 3.5 lb/cu ft. Owens Corning AttiCat is a blown fiberglass at 2.79 R per inch; in closed walls the equivalent fiberglass approach is a netted blow-in-blanket system rather than the loose AttiCat attic product. The table contrasts them at the common wall cavity depths.

R/in values are the calculator engine constants (cellulose 3.7, AttiCat fiberglass 2.79). At R-20 in a 2x6 wall, cellulose hits the target in the cavity while fiberglass at the same depth lands near R-15 — a meaningful gap when the cavity depth is fixed by the studs. Use the Blown-In Insulation Calculator for the bag estimate by wall area, depth, and density.

Blown-In Walls vs Batts vs Spray Foam

Fiberglass batts are the cheapest cavity fill (often under $0.75 per sq ft in materials) but only work in open framing and are easy to install with gaps and compression around wiring — both of which drop real-world R below the rated R-13 or R-20. Dense-pack blown-in at 3.5 lb/cu ft flows around wires, boxes, and blocking to fill the cavity completely, and it is the only one of the three that retrofits a finished wall without opening the drywall. Spray foam adds an air seal and (closed-cell) a vapor retarder in one step but costs the most — typically $2.50 to $5.00+ per sq ft — and gives up the drying capacity that a vapor-open cellulose wall keeps. For a closed wall, dense-pack is usually the practical pick; for new open framing, it competes with batts on a cost-vs-completeness basis.

Blown-In Wall Insulation Cost

Wall dense-pack is priced differently from attic blowing because the labor is higher — drilling, tubing each of the stud bays, and patching siding add hours a flat attic never needs. DIY cellulose materials run about $10–$16 per bag, while a contractor drill-and-fill runs about $1.50–$2.50 per sq ft. Typical 2026 ranges, all to be verified before you commit, are compared below:

Costs are approximate as of 2026 and vary by region, retailer, wall access, and siding type; verify before purchasing. A two-story exterior may add staging/lift cost to a drill-and-fill quote. The dense-pack density you specify drives bag count directly — a wall packed to 3.5 lb/cu ft uses more material than one packed to 2.0, so confirm the target density with your installer.

Wall R-Value Targets by Climate Zone

How much wall R you need varies by climate zone. DOE publishes wood-frame wall R-value recommendations aligned with the 2021 IECC, and many jurisdictions express the wall requirement as a cavity-plus-continuous combination (for example, cavity R-20 or R-13 cavity plus R-5 continuous). The exact figure and whether continuous exterior insulation is required depend on your zone and the IECC edition your jurisdiction has adopted.

Blown-In Wall Insulation Checklist

Steps in order for a dense-pack wall job. Items marked code-dependent should be confirmed with your local building authority before permitted work.

1. Confirm the cavity depth and stud spacing — Measure the wall to know whether you have a 2x4 (3.5-inch) or 2x6 (5.5-inch) cavity and whether studs sit 16 or 24 inches on center; this sets both the cavity R and the framing fraction in the bag math.
2. Check for knob-and-tube wiring and moisture before filling — In older homes, active knob-and-tube wiring should not be buried in insulation; resolve it first. Look for any water staining on the sheathing — dense-pack traps a wet cavity, so fix the leak before you fill.
3. Pick the pathway: open-wall netting or closed-wall drill-and-fill — Open framing gets fabric netting stapled over the studs and is blown from the face; a finished wall gets the DOE drill-and-fill method (siding strip off, three-inch holes one per stud bay, fill tube to the top, dense-pack).
4. Set the density target, then size the bags — Specify dense-pack at 3.5 lb/cu ft, run the wall area through the volume × density math (or the calculator’s wall mode), and buy with a small overage; dense-pack uses more material than a light fill.
5. Dense-pack each bay to refusal — Pack until the bay stops taking material and the fill tube backs out under pressure; a bay that takes more than expected has a void or a leak, and one that packs short will settle.
6. Plug, patch, and confirm vapor control — Plug the drill holes and replace siding, then confirm the wall’s interior vapor-retarder strategy (often a Class III interior latex paint in colder zones) suits your climate zone and assembly.

Common Blown-In Wall Mistakes

These are the wall-specific errors that drive callbacks — each different from the attic-floor mistakes.

• Blowing wall cavities at attic (loose-fill) density — Filling a vertical cavity at ~1.5 lb/cu ft instead of dense-packing to 3.5 lb/cu ft guarantees slump and a cold void at the top of the bay within months. Walls are a density target, not a depth target.
• Figuring walls by floor area instead of cavity volume — Using attic sqft-per-bag math on a wall ignores both the cavity depth and the ~15% framing fraction, so the bag count comes out wrong. Walls are volume × density ÷ bag weight.
• Dense-packing a wet or knob-and-tube cavity — Sealing moisture into a cavity invites rot and mold, and burying active knob-and-tube wiring is a fire hazard. Resolve both before filling — a problem you pack in is one you cannot see.
• Ignoring vapor control for the assembly — A wall dries differently from an attic. Skipping the interior Class III vapor-retarder check (or installing the wrong class for the zone) can trap moisture at the sheathing; vapor-retarder class and placement are jurisdiction-specific.

Common Questions

How do you calculate blown-in insulation for walls?

By cavity volume, not floor area. Take net wall area (gross minus windows and doors), subtract about 15% for stud framing, multiply by cavity depth in feet for cubic feet, multiply by the density target (3.5 lb/cu ft dense-pack per DOE) for pounds, then divide by bag weight. The Blown-In Insulation Calculator runs this in wall-cavity mode.

What R-value does dense-pack cellulose reach in a wall?

Cavity R = depth × 3.7 R/in: a 2x4 (3.5 in) reaches about R-13, a 2x6 (5.5 in) about R-20, and a 2x8 (7.25 in) about R-27. Add any exterior sheathing or continuous insulation on top of the cavity figure for the wall total.

What is the difference between dense-pack and loose-fill in walls?

Loose-fill at attic density (~1.5 lb/cu ft) slumps in a vertical cavity; walls are dense-packed instead. DOE describes blowing to 1.5 to 3.5 lb/cu ft, with 3.5 lb/cu ft or higher the common dense-pack target that resists settling and fills the cavity. Density, not depth, is the wall variable.

How is blown-in insulation installed in existing walls?

For closed walls, the DOE drill-and-fill method: remove a strip of exterior siding, drill three-inch holes one per stud bay through the sheathing, insert a fill tube to the top of the cavity, dense-pack, then plug and replace siding. Open framing uses fabric netting blown full from the face instead.

Do blown-in walls need a vapor retarder?

It depends on climate zone and assembly. Many wall assemblies use an interior Class III vapor retarder (a coat of latex paint often qualifies) so the wall can dry inward, while the exterior sheathing face is the condensation plane to watch. Vapor-retarder class and placement are jurisdiction-specific — confirm with your local building department.

Estimates only — verify with your local building authority and a qualified contractor before construction. Brand names referenced for descriptive purposes (nominative fair use); Greenfiber® and Owens Corning AttiCat® are trademarks of their respective owners; CraftedCalcs is not affiliated with or endorsed by these manufacturers. See our full disclaimer.