Insulating the Walls of a House That Is Already Built

The method is ordinary work. The density the cavity is packed to is what decides whether the wall actually performs.

Why walls get skipped

An attic floor is open. You can stand at the hatch, see the whole surface, measure it and add to it. A wall is a closed box behind drywall or siding, and there are as many of those boxes as there are gaps between studs.

So walls get left out of the conversation, even in a house where the attic has already been brought up to depth.

How the work is done without opening the house

The Department of Energy and ENERGY STAR measure guideline describes this approach as a fit for homes that have insufficient insulation and where the work needs to be minimally invasive, with small holes cut from the exterior or the interior to reach the framing cavities.

A hole is drilled into each cavity, a tube goes in, and material is blown until the cavity will not take any more. The Department of Energy's own job aid for this work adds two steps that matter more than the drilling: probe the cavities to confirm every one was actually reached, and fill all of them to proper density. Holes are patched afterwards.

The reason the probing matters is that a wall does not tell you where its cavities are. Framing is not always where the spacing suggests, and there are headers over openings. The job aid says the drill pattern should reflect knowledge of the framing.

Density is the whole job

The Department of Energy's wall guideline gives cellulose a recommended installed density of 3.25 to 4.0 pounds per cubic foot, and says that whatever the material, it has to reach the required density throughout the entire cavity in order to reduce the risk of convective air movement. Convective air movement means air circulating inside the cavity itself, carrying heat across a wall that technically contains insulation.

That is why a loosely filled wall can measure as insulated and still behave like a wall that is not. The material is present, and the air is still moving through it. Packing to density is what stops that, and it is also what stops the material settling later and leaving a gap at the top of the cavity.

What has to be right before the first hole

Two things.

The first is water. The Department of Energy's insulation guidance says to address mold before installing cellulose, and warns that accidental wetting and high humidity can cause cellulose to absorb large quantities of water. For exterior wall work its job aid says to repair the drainage plane where one exists, to stop water getting in. A wall with a water path into it is a repair job before it is an insulation job.

The second is air leakage. The same measure guideline recommends caulking or foaming penetrations, around windows and doors, and at the top and bottom plates of the wall where those are accessible. Filling a cavity that is open to the outside at the top plate puts material around a hole rather than closing it.

Walls are not attics, and one number does not transfer

There is a settling figure that gets quoted about cellulose. A Department of Energy document records loose-fill cellulose settling about twenty percent against roughly two to four percent for rock wool and fiberglass loose-fill. That figure is for attic installations.

It is not a wall number, and applying it to a wall gets the reasoning backwards. In an attic, material is laid loose on a floor and gravity compresses it over time. In a wall it is packed to density on the day, and proper density is itself the main defense against settling. The question in a wall is whether it was packed hard enough, not how much it will drop later.

Ceilings between floors, and rooms over a garage

The same method reaches places people do not think of as walls. The floor between two stories is a cavity, and so is the ceiling of a garage with a bedroom above it.

Those cavities are reached the same way, through small holes and a tube, and they are filled to the same density. What changes is the access route and how the holes are patched, which depends on whether the surface is drywall, plaster, siding or a ceiling someone is going to be looking at.

What this changes, and what it does not

A properly packed wall cavity slows heat through that wall and stops air circulating inside it.

What it does not do is fix a window, a door that does not seal, or a cooling system that is the wrong size for the house. It also does not reach a cavity that already has something in it.

What we look at before putting a number on it

How many cavities there are and what the walls are made of. Whether the access is from inside or outside, and what patching that surface involves. Whether anything is already in the cavities. Whether there is a water path to deal with first. Whether the top and bottom plates can be reached for sealing while the wall is open.

Once we get started, you get one number for the whole project. Our cellulose insulation page covers the material and the drill and fill method, blown-in insulation covers the wider service, and a free estimate is how the cavity question gets answered for your house rather than in general.