Blown-In Insulation for Existing Walls: How Drill-and-Fill Works
Drill-and-fill is how insulation gets into a wall that is already finished. A small hole is made near the top of each stud cavity, from outside or from indoors, blown-in insulation is packed into the cavity through it, and the hole is plugged and patched. Nobody opens the wall. Done well, the only signs of the job are the patches, and on the outside those can usually be hidden behind the siding.
This guide walks through the whole thing: how to find out what your walls hold now, each step of the work, how the holes are closed, when open walls should get batts instead, and the reasons a wall should wait. Insulation Pros blows fiberglass into closed walls, so that is the material described here. Many older homes in North Carolina went up with little or nothing in their outside walls, and this is the usual way to fix that.
What Is Drill-and-Fill Wall Insulation?
Every outside wall of a wood-framed house is a row of hollow bays, one between each pair of studs. In a finished wall those bays are closed on both sides: drywall or plaster indoors, sheathing and siding outdoors. Drill-and-fill reaches each bay through a hole and fills it with loose insulation from a hose. The Building America Solution Center, a U.S. Department of Energy resource, describes it as a fairly low-invasive way to insulate an existing wall.
The material is loose-fill, the same kind of insulation that gets blown across attic floors, but it goes into a wall very differently. In an attic it lies loose. In a wall it has to be packed tight, to the density its manufacturer gives for filling closed cavities, so it fills the bay from bottom to top and stays there.
Why packed fiberglass
Federal guidance lists two materials for this work, fiberglass and cellulose, and names no winner between them. What matters more is the fill. A Building America guideline on wall insulation warns that blown fiberglass or cellulose put in too loosely, below its prescribed density, can slump and leave the top of the bay empty. That is why the density is the number to ask about. We blow fiberglass, packed for walls, and we do not install foam of any kind.
Is There Insulation in Your Walls Now? How to Check
Before anyone drills, find out what is already there. ENERGY STAR describes a simple home check. Switch off the breaker for an outlet on an outside wall and check with a tester that it is dead. Take off the cover plate and shine a flashlight into the gap beside the box. You will usually see insulation, or the back of the siding, or nothing at all.
Check more than one wall. Walls on different floors can differ, and an addition built decades after the main house often has insulation that the original walls do not. A guideline written for the Department of Energy's Building America program says houses built before 1980 often have little or no insulation in their walls, but age alone proves nothing, so look.
What you might find
Empty bays are the best case for drill-and-fill: the cavity is open and the insulation can fill it completely. Old loose fill that has slumped to the bottom of the bay can usually be topped up through a new hole. Batts are the hard case. The Building America Solution Center notes that blown insulation often cannot get past old batts to fill the bay, so a wall that already holds batts in fair shape is often better left alone.
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How Drill-and-Fill Works, Step by Step
The Building America Solution Center's guide to blowing insulation into existing walls sets out the sequence. In plain terms, the job runs like this:
- Pick the side. The holes go in from outside or from indoors. That choice comes first, because it decides how the holes are closed later.
- Find the framing. Every stud bay is located. So is anything that splits a bay: blocking, a brace, the framing over a door and under a window. Each piece of a split bay needs its own hole.
- Make the holes. The guide calls for a hole of an inch or two across, sized to the blowing nozzle, near the top of each bay. A second, lower hole is added where one is not enough to pack the whole bay.
- Fill each bay. The insulation is blown in until the bay is packed to the manufacturer's dense-fill density.
- Check before closing. The guide has every bay checked before the holes are closed and suggests an infrared camera to do it. Gaps show up best on a day with a big difference between indoor and outdoor temperature.
- Close the holes. Indoor holes are plugged and patched. Outdoors, anything behind the siding that was opened is repaired before the siding goes back.
Inside or outside: which side the holes go
It depends on what covers the walls. A Building America guideline favors the outside for houses clad in wood, vinyl or fiber cement, since the rooms stay undisturbed, and the inside for brick, stucco or stone. Working from indoors leaves the weather barrier behind the siding alone. Working from outside avoids dust in the house but means lifting siding and repairing that barrier afterward.
The Holes, the Plugs and the Patching
This is the part you will see afterward, and the honest answer is that it depends on the wall.
From outside, siding boards are lifted or removed so the holes go through the sheathing behind them. The holes are plugged, the weather barrier is patched, and the siding goes back over the spot. When that is done carefully, nothing shows. From indoors, the holes go through the drywall. The Building America guide describes plugs of drywall or rigid foam set into the holes and finished with joint compound. After that come sanding, primer and paint.
The Department of Energy's homeowner insulation guide is frank that patches can show unless new siding or trim goes over them, so ask before you agree to anything. Your estimate should say which side the work is done from, how the holes are closed and who does the painting.
New siding is the easiest time
If you are re-siding the house anyway, the holes disappear behind the new siding. For an uninsulated wood-framed wall, ENERGY STAR suggests drilling the sheathing and blowing the empty bays full while the old siding is off. We don't hang siding, so call before the siding crew starts, and the two jobs can be timed together.
Open Walls: When Cut-to-Fit Insulation Is the Better Fit
When the studs are bare, nobody needs to drill. In an addition, a remodel or a new build, and in any wall opened up for other work, batts go in before the drywall. The Building America Solution Center notes that batts are rarely an option in an existing wall unless it has been opened up for an addition or a full gut of the room.
A batt only works when it fills its bay side to side and top to bottom. That means cutting it to length, splitting it around wires instead of stuffing it behind them, and trimming it around outlet boxes and pipes. A batt that is folded, crushed or short of the top plate leaves a gap you will never see once the drywall is up.
What R-value an open wall needs
For a new wood-framed wall, North Carolina's 2018 energy code asks for R-15 in the cavity, or R-13 plus R-2.5 of continuous insulation, in climate zones 3 and 4. In zone 5, the mountain counties, it asks for R-19, or R-13 plus R-5, or R-15 plus R-3. Pick the batt made for the depth of the studs. The code's own example of what not to do is an R-19 batt squeezed into a 2x4 wall, which does not meet its wall requirement. Our R-value guide lists which climate zone each county is in.
Savings vary. Find out why in the seller's fact sheet on R-values. Higher R-values mean greater insulating power.
Faced batts get covered
Kraft paper and standard foil facings cannot be left exposed, the Department of Energy's insulation fact sheet says, because of how they behave in a fire. In a living space the drywall covers them. In a garage or any unfinished space, plan the covering before the batts go up.
Knob-and-Tube Wiring Means an Electrician Comes First
Knob-and-tube is the earliest kind of house wiring: single cloth-covered wires run on ceramic knobs and through ceramic tubes. It turns up in houses wired before about 1940. Wrapped in insulation, those wires can overheat, and the National Electrical Code does not allow insulation to be applied to or surround them. The Building America guide on blowing walls repeats that rule.
So a wall with live knob-and-tube does not get filled. An electrician checks the wiring first, and the walls wait until it has been replaced. Rewiring is an electrician's trade, not ours, but once it is done the same walls are often good candidates for drill-and-fill, and the rewiring holes and the insulation holes can sometimes be planned together.
Other reasons a wall should wait
Old wiring is the most common reason, not the only one. These walls need something done first, or should not be filled at all:
- Walls that let water in. Stains under windows and peeling paint point to a leak, and insulation does not fix leaks. Repair them first.
- Siding that may contain asbestos. Old cement-like shingles are not drilled until an asbestos professional has checked them: the EPA says never to drill holes in asbestos-containing material, and that you usually cannot tell by looking.
- Paint from before 1978. The EPA's lead renovation rule covers paid work that disturbs it, so ask how the dust will be kept in.
- Fragile plaster. The Department of Energy's homeowner guide warns that drilling can break the plaster keys that hold it to the lath.
- Balloon framing. Older walls open from one floor to the next have to be blocked at the top and bottom first, so the insulation stays in the wall.
- Solid brick, block or stone. With no stud cavity, there is nothing to fill.
Walls or Attic First?
In most houses, the attic. It is the easiest place to add insulation, and the Department of Energy's insulation fact sheet starts there while calling existing walls the difficult part. The National Park Service's guidance for older buildings also puts walls after the attic. If your attic is thin, more attic insulation, with air sealing done first, is usually the better place for the money.
Walls earn their turn when the attic is already in good shape and the outside walls are empty. That is often the case in a house whose attic was topped up years ago but whose walls were never touched. The signs are cold outside walls in winter and corner rooms that never feel right.
What Drives the Cost, and What the Estimate Should Say
We don't print prices for walls, because a closed wall is priced by its bays and its access, not by square feet. These are the things that move it:
- How much wall there is. Two stories mean ladders or staging.
- Outside or inside. Taking siding off and putting it back is slower than drilling drywall.
- How many bays are split by blocking, braces and windows, since each piece needs its own hole.
- The finish: a plug, or a plug plus patch, primer and paint.
- What has to happen first, such as blocking an open wall top or working around old insulation.
Get it in writing
After the patching, nobody can see inside the wall again, so the paperwork is your record. The Federal Trade Commission's R-value Rule requires the installer to give you a contract or receipt with the area covered, the insulation's thickness and its R-value. Ask for those lines, plus which walls are included, which side they are filled from and how the holes are closed. A wall is harder to price from a number than an attic, so for walls we sometimes make a special visit before pricing. The estimate is free either way.
Get a Free Estimate
Outside walls that feel cold all winter? Tell us roughly when the house was built and what covers the outside walls, and we'll tell you whether drill-and-fill fits, which side we'd work from and what the estimate would cover.
Prefer to talk? Call (336) 870-3893.
