Blown-in wall insulation

A wood-framed wall is a row of tall narrow boxes. Studs close each box on two sides. Sheathing and siding close the outside, drywall or plaster closes the inside. Nothing gets in without a hole, and that is the entire reason this job looks the way it does.

The Department of Energy's Building America Solution Center sets out the pattern in its guide for existing exterior walls. It says to use a hole saw and drill "1-inch or 2-inch holes (depending on the size of the blower nozzle) in the center of each stud cavity, roughly 6 inches below the ceiling."

Studs sit at 16 or 24 inch centers, so the wall length sets the hole count. A 40 foot wall at 16 inch centers is about 30 cavities, and that is at least 30 holes before anybody adds a second one lower down.

The hose is not simply aimed at the hole. A dense pack crew feeds a flexible fill tube down the cavity, close to the bottom, then starts the machine. The tube backs out slowly as material packs in behind it. That is what makes the fill tight rather than loose. A crew that holds a nozzle at the hole and pulls the trigger is doing a different job with the same machine, and you will not be able to see which one you bought.

Where the holes go, and what you own afterward

This site holds no measured price for that repair work. Siding, brick and paint are other trades, and no figure for them has been read from a source we can point at, so none is printed here. The Insulation Report cost model prices blowing fiberglass into an attic. It does not price a wall and it is not used on this page.

What you can do is force the repair into the open. Ask for the fill and the making good as separate lines. Ask who owns a cracked siding piece. Ask whether paint is in or out. Two bids that look far apart usually differ on those three answers. Wall insulation cost takes the money question on its own.

An attic floor is flat. A wall stands up. That one difference drives everything on this page.

Blow material loosely across an attic floor and it settles a little. The pile stays where you put it. Blow the same material loosely into a standing cavity and gravity has a whole column to work on. The material creeps down over months and years. What is left behind is a strip at the top of the wall with nothing in it.

That strip is in the worst possible place. The top of the wall is where warm indoor air pushes hardest to get out in winter. That is the stack effect, and air sealing an attic explains it in full. So the part of the cavity that empties first was doing the most work.

Dense pack is the fix. The material goes in hard enough that the fibers lock against each other and cannot creep. DOE's Building America Solution Center says dense packing brings "the advantages of preventing insulation from settling, completely filling the cavities, consistent insulating values, and the ability to slow airflow." It publishes the densities to aim at, and they are the numbers to ask a bidder about.

Read the two fiberglass numbers again. DOE gives 4.2 per inch for the dense-packed wall product. This site carries 2.2 to 2.7 for blown fiberglass in an attic. Both are right. It is the same glass at two very different densities, and density is what changed. An attic number and a wall number are not swappable. R-value per inch by material carries the attic figures and how they are tested, under ASTM C518 and the FTC R-value Rule (16 CFR Part 460). The bag specifications are ASTM C739 for cellulose and ASTM C764 for mineral fiber loose fill.

This site publishes no whole-wall R-value, because it has measured none. R per inch times your cavity depth describes the cavity, and a wall is more than its cavity. Sheathing, siding, drywall and the studs all count, and the studs are a path around the insulation rather than through it. That is why the guidance later on this page points at sheathing outside the studs rather than at more material between them.

A stud cavity is not always one clear column from floor to ceiling. Older walls are full of things that split it in two. A horizontal fire block. A let-in diagonal brace at the corner of the house. A header over a window. A row of blocking somebody added to hang a cabinet. From outside, a divided cavity looks exactly like a whole one.

Fill a divided cavity from a single hole near the ceiling and you fill the top half. The bottom half stays empty. The machine sounds right, the wall takes material, the crew moves on, and half of that bay does nothing for the rest of the life of the house. This is the single most common way a paid-for job comes out half done.

DOE's guide says so plainly. "It may be necessary to also drill a lower hole to ensure the full cavity is packed with insulation." And where a scan finds an obstruction, "make an additional hole below the blocking." Short cavities have the same problem in miniature: the strips above and below a window are their own small boxes and each one needs its own hole.

Three habits separate the two, and all three are things you can ask about before you sign rather than discover afterward.

That last one is DOE's instruction, not an opinion. Its guide says to "fill all exterior wall cavities then re-verify with an infrared camera to ensure that all cavities are completely filled before patching." It also names infrared imaging as the way to find the framing and blocking in the first place. A bidder who has never used a camera on a wall is telling you something about how the job will be checked.

Ask for the density they work to, in pounds per cubic foot, and ask for it in the written scope. A crew that dense packs will answer without hesitating, because it is the number their whole method is built around. A crew that shrugs is quoting you a different job.

You cannot see this work once it is done, so the checking has to be planned before it starts. Three ways, cheapest first.

A thermal camera on a cold day

This is the real test and it costs an hour. Pick a cold morning, with the house heated and the sun off the wall. The bigger the gap between indoor and outdoor temperature, the clearer the picture, so deep winter beats a mild spell. Then read the pattern rather than the colors.

• A properly filled wall reads as a regular stripe pattern. The studs show as narrow lines at even spacing, because wood insulates less well than a packed cavity. Even spacing is the sign that you are looking at framing and not at a fault.
• A missed cavity is a wide stripe. It runs the full height of the wall and it is the width of a bay, not the width of a stud. One of those means one hole that never took material.
• A settled or half-filled cavity is a band, not a stripe. A band across the top of the wall is material that dropped. A cold block partway up, with warm wall below it, is a cavity that was filled from one hole above a brace.
• Cold at the plugs alone is nothing. A plug reads differently from the wall around it. That is the repair, not the fill.

Count the bags before the crew leaves

Density is not a mystery. It is weight divided by volume, and both halves are countable on the day. Take the total pounds that went into the wall, then divide by the cubic feet of cavity behind it. Cavity volume is the wall area times the cavity depth, and the crew can read that depth off the first hole they drill.

Bags are sold by weight, so the pounds are printed on the label. A 25 lb bag is one common retail size. Greenfiber SANCTUARY cellulose blown-in insulation was listed at $15.97 a bag by The Home Depot on 2026-08-11. That is a shelf price at one store on one day, and material only. A contractor pays less and buys elsewhere, so use it for the bag weight rather than as a budget. Compare the density you work out against DOE's 3.5 to 4.5 pounds per cubic foot for cellulose. Well under it means loose fill went into a wall that needed dense pack.

Ask for the camera images

If the crew scanned the wall before patching, the images exist. Ask for them with the invoice. A contractor who checked their own work is usually pleased to show it.

Three conditions end this conversation, and none of them is a haggling point.

Knob and tube wiring

This one is a safety stop. Knob and tube is early wiring that runs bare through the cavity and is designed to shed its heat into open air. Burying it changes that. DOE's guide states that where a house has knob and tube wiring, the National Electrical Code, published as NFPA 70, does not allow insulation to be applied to or surround the wiring.

So the order of work is fixed. An electrician goes first and either confirms the wiring is dead and disconnected or replaces it. Only then does anybody talk about filling the wall. An installer who offers to blow around it is not somebody to hire. Knob and tube wiring under insulation covers how to recognize it and what the options cost.

A wall that is already wet

Filling a wet cavity seals the water in against the sheathing and takes away the air that was drying it. Wet material also loses much of the R-value it was sold for, and packed cellulose that gets soaked can slump. Find the source first. A failed window flashing, a roof leak tracking down inside the wall, a plumbing leak, or a sprinkler head that hits the siding every morning. Stains, soft drywall, peeling paint outside and a musty smell in one room are the usual signs. Fix the water, let the wall dry, then insulate it.

Siding that will not survive being opened

Some cladding does not come off and go back. Old vinyl gets brittle and cracks in cold weather, and a discontinued profile cannot be matched. Cedar shingles split. Fiber cement chips at the nail line. A stucco patch stays visible. And siding that might contain asbestos is a different trade with its own rules, so it does not get drilled by an insulation crew at all. Asbestos and vermiculite explains why that boundary is firm. Where the siding cannot be opened safely, the choice is the inside route, or waiting for the siding to be replaced.

Work down the table and stop at the first row that matches. The rows that overrule everything else come first.

The recommendation. The first three rows are stops, not decisions. For everybody else, two facts settle it. Is the cavity empty, and is the siding coming off? Empty cavity plus siding already off is the best buy in this whole subject, and it is worth chasing the re-siding date rather than the insulation date. Empty cavity with sound siding is a real job, worth quoting on its own merits. Anything already in the cavity moves you to the next section.

This page treats the work as something to buy for one reason. Density. The number that decides whether the fill lasts is one a rented attic blower cannot reach, and the failure is invisible on the day it happens. Do it yourself or hire runs that split for the attic jobs where it genuinely is a choice.

We do not sell insulation and we do not install it, which is why this section can tell you to keep your money. The wall that pays for this work is an empty one. A wall with anything in it already is a much weaker case, and the gap between the two is wide.

What an empty wall is worth. Going from nothing to a packed cavity is the biggest single step available in a wall, because heat flow falls as one over R. The first inches take out most of what there is to take. That is the same arithmetic that makes the first inches in an attic matter more than the last, and is attic insulation worth it works it through step by step. An empty cavity also usually means an old wall, which usually means air moving through it, and packing the cavity slows that too.

What a partly filled wall is worth. Much less, for three reasons that stack up. The step from something to a bit more is a small share of what the first step gave you. A fill tube cannot get past a batt, so the crew is pushing material into a cavity they cannot reach the bottom of. And you lose the check, because a wall with old material in it looks broadly filled on a camera whether the new work reached anything or not. You are paying for holes, plugs and paint to buy the smallest part of the benefit, with the weakest way of proving you got it.

ENERGY STAR sends a wall like that somewhere else. Its recommended R-value guidance treats wall work as something you do when the siding is off. For uninsulated wood-frame walls it says to "drill holes in the sheathing and blow insulation into the empty wall cavity before installing the new siding," and to add R5 insulating sheathing in zone 3, or R5 to R10 in zones 4 to 8. For a wall that is already insulated, the 2x4 advice in zones 4 to 8 is to "add R10 insulative wall sheathing beneath the new siding." There is no drilling in that sentence. Find your climate zone to see which line is yours.

That guidance is based on the 2021 International Energy Conservation Code, which is a model code rather than a rule in force anywhere by itself. The same warning applies here as everywhere else on this site. Energy codes cover new building and work done under a permit. They do not usually cover adding insulation to an attic you already have. Which edition applies where you live depends on your state, and often on your city. Your local building department is the only authority on it. The energy code tracker shows which edition each state has adopted.

Two more cases where the wall is not the project. If your attic is thin, the attic comes first. It is cheaper for what you get, it needs no repair trade afterward, and in most houses it is further from the R-value target. And if the room that feels cold is over a garage or behind a knee wall, the wall in front of you is probably not the problem. A room over a garage and knee walls both fail in their own particular ways.

• Wall insulation: the whole assembly
• Is it worth opening the walls?
• What wall insulation costs
• Cellulose against fiberglass
• How blown-in insulation is installed
• Air sealing, and why it comes first