When considering methods for insulating exterior walls in an existing home, one common and somewhat uninvasive method is known as drill and fill. In this method, small holes are made in the interior or exterior wall near the top of the wall at each wall cavity, then the wall cavities are filled with blown-in insulation. See Figure 1 and 2.) An ideal time to do an exterior installation is when a house is being re-sided. Re-siding offers the opportunity to check and install or re-install a drainage plane like house wrap if necessary. Re-siding also offers the opportunity to install exterior rigid foam insulation, in addition to or instead of cavity insulation. (See Rigid Foam Insulation for Existing Exterior Walls).

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Siding has been removed so cellulose insulation can be dense-packed into the exterior walls of this home

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If the blown-in insulation is to be installed from the exterior as part of a siding replacement, an ideal opportunity is presented to improve the airtightness of the wall assembly with minimal effort. Exposed structural panels (plywood or OSB) can be effectively air sealed with fluid-applied air barriers, low-expansion foam, caulk, or quality tapes. Board sheathing should be covered by a house wrap, with all laps sealed with appropriate tape. Continuous rigid foam sheathing can be applied over existing sheathing to provide additional R-value, air sealing, and a continuous drainage plane.

It is critical for water management and durability that the wall assembly’s drainage plane be maintained. When blown-in insulation is added from the exterior, the drainage plane (if existing) will be breeched and it must be made whole by appropriate patching or full replacement. The chosen method will depend on the amount of access and the state of the existing drainage plane material. If the insulation is to be installed from the interior, the drainage plane will not be affected.

Depending on the level of disturbance to existing exterior walls (interior and exterior sides), be aware of lead safety rules for homes built prior to 1980. In addition, if a home has knob and tub wiring, the National Electrical Code (NEC) Article 394.1 scope does not allow insulation to be applied to or surround the wiring.

This guide primarily focuses on wood framing construction with exterior wood sheathing and interior gypsum wall board. For more information on drill and fill installation in wood-framed walls as well as in other exterior wall materials, see the Drill & Fill Installation Guide.

For other approaches to insulating exterior walls in existing homes, see the guides Spray Foam Insulation for Cavities of Existing Exterior Walls and Rigid Foam Insulation for Existing Exterior Walls.

How to Blow In Insulation into Existing Exterior Walls

1. Determine if the existing exterior wall cavities are insulated. Options for assessing insulation levels include making a small cut-out in the interior gypsum wall board, viewing cavity insulation from the side of an electrical junction box cutout (shut off power to electrical outlets to avoid potential safety hazards), or using an infrared thermal imaging camera to qualitatively assess insulation levels.
   1. If the wall cavities are empty proceed to step 2.
   2. If the wall cavities have some existing insulation:
      1. Existing blown insulation that has deteriorated can remain (or be vacuumed out) and additional blown insulation can be installed in the wall cavity to the desired fill and density.
      2. Existing batt insulation that has deteriorated can block blown insulation, preventing it from completely filling the cavity. Cut out a 1-foot strip along the wall near the floor on either the interior or exterior side of the wall, then pull out the old batt insulation to allow proper installation of the blown insulation into the wall cavities. Alternatively, use an infrared camera to verify that the blown insulation has adequately filled the wall cavities around the existing batt insulation.
2. Decide if insulation is to be blown from the interior or from the exterior.
3. Verify that the exterior wall framing is not balloon framing. If the walls are balloon framed, blocking will need to be installed at the top and bottom of the walls to provide a barrier between the wall cavities and the basement below and attic above to allow for dense packing of the cavities.
4. Identify the wall studs and use painter’s masking tape to mark them.
5. Using a hole saw, 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 (see Figure 3). It may be necessary to also drill a lower hole to ensure the full cavity is packed with insulation. If there is blocking or other obstacles in the wall cavity (identified using infrared camera), make an additional hole below the blocking (see Figures 4 and 5). Remember to drill in to any cavities above door headers and above and below windows, as well as wall cavities behind cabinets on exterior walls. Also if a bathroom is on an exterior wall, bathroom tiles may need to be “popped” off prior to drilling a hole through the wall board.
6. Blow in new insulation to the desired density. Confirm manufacturer’s lbs/ft³ recommendations to achieve “dense” packing. The common insulation type is a dense-packed blown cellulose or fiberglass insulation. For information on blowing machines and nozzles, see the Drill & Fill Installation Guide.
7. Fill all exterior wall cavities then re-verify with an infrared camera to ensure that all cavities are completely filled before patching.
8. Once verified, patch the holes. For interior penetrations, patch the holes with gypsum board or rigid foam plugs and joint compound. When dry, sand smooth then paint. For exterior penetrations, patch requirements will differ depending on whether siding was removed or remained in place. Repair the drainage plane and air barrier as needed before installing siding.