Wall insulation

This is a wall page that opens by sending you to the attic. There are four reasons, and none of them is a preference.

• You can make an attic as deep as you like. Depth is the whole game. Nothing stops you laying more on top of what is there. A wall is a fixed box.
• An attic floor is open ground. A crew walks in through a hatch and blows. A closed wall has to be drilled bay by bay, and then every hole has to be plugged and made to look right again.
• Warm air goes up. The ceiling is the surface it presses against hardest in winter. Air sealing the ceiling is the cheapest step in the house and it comes before adding anything anywhere.
• The numbers are not close. ENERGY STAR recommends R-60 for a bare attic in climate zones 4 through 8, and R-49 in zones 2 and 3. A 2x6 wall packed with cellulose gets to about R-17 to R-20. The wall is not failing. It just has less room.

So the order of work is the same in every house. Seal the ceiling, then bring the attic to its target, then look at walls. A reader who came here for walls and went and did the attic instead got the right answer. The guides hub sets out that order in full.

One thing does jump the queue. The rim joist in the basement is a wall of sorts, it is a heavy air leak, and it is a couple of hours of work. It is in the table below.

"Wall insulation" is really five jobs with different answers. Find yours here first.

Two of those have their own pages, because they are where the mistakes happen. Knee walls covers the backing detail, and a room over a garage covers the floor and walls of the coldest room in most houses. For the garage wall, the reason to seal it first is carbon monoxide, which the U.S. Environmental Protection Agency and the Centers for Disease Control and Prevention both treat as a home safety matter rather than a comfort one.

Every wall decision on this site starts here, so answer it before reading anything about materials or price.

The line that matters: if a wall is going to be open for any other reason, insulate it then. New wiring, new plumbing, a window swap that strips the wall, siding coming off. The material is a small part of that bill and it will never be this cheap again. Opening a wall just to insulate it is a different question, and the answer is usually no.

For a closed wall, the drilled retrofit is the whole trade. Blown-in wall insulation covers how the holes are set out, inside against outside, and how a job is checked. ENERGY STAR points homeowners at the same method, saying to "drill holes in the sheathing and blow insulation into the empty wall cavity before installing the new siding".

Four checks, in order of cost. None of them needs a contractor. Do the first two before you touch anything electrical.

1. Feel the wall on a cold morning

Put your palm flat on an outside wall in a heated room. Then put it on an inside wall in the same room. An empty outside wall reads clearly colder. A filled one reads close to the room. Do this in the morning, before the sun has been on that side. An infrared thermometer turns the same check into a number, and a thermal camera turns it into a picture.

2. Look at the outside of the house

Walk the siding and look for a line of round plugs, roughly the size of a coin, spaced about as far apart as the studs. One line high on the wall, sometimes a second line low. That is the signature of a drill and fill job somebody already paid for. On vinyl siding the holes are hidden behind a lifted course, so look at the sheathing joints and at any spot where a course has been unclipped and put back.

3. Look down the wall from the attic

From the attic hatch, look at where an outside wall meets the ceiling. The top of the wall is closed by a flat piece of framing called the top plate. Where it is not sealed you can sometimes see straight down into the top of the cavity with a flashlight. That same gap is one of the biggest air leaks in the house, which is why air sealing deals with it. The attic inspection walk-through covers what else to look at while you are up there.

4. The outlet check, and how to do it safely

An outlet on an outside wall is a ready-made hole into the cavity. It is also live. Read this part before you pick up a screwdriver.

• Switch the breaker off first. Find the breaker for that room and turn it off. This is not optional and there is no version of this check that skips it.
• Then prove the outlet is dead. Use a non-contact voltage tester on the outlet itself. A room can be fed by more than one circuit, so trust the tester, not the label on the panel.
• Take off the cover plate only. One screw. Leave the outlet where it is. Do not unscrew it and do not pull it out of the box.
• Nothing metal goes near the gap. Look with a flashlight. If you want to feel for fill, use a plastic straw or a wooden skewer, gently, in the gap beside the box.
• Old wiring means stop. Cloth-covered wire on ceramic knobs is knob-and-tube. Put the plate back and read knob-and-tube wiring under insulation before anything else happens to that wall.

What you are looking at is the edge of the stud bay. Bare wood and a dark space means the cavity is empty. Gray or off-white fluff pressed against the box means a blown material. A paper or foil face means a batt. A hard skin of foam means somebody sprayed or injected it. Check two or three outlets on different outside walls, because houses get insulated in patches, and one bay proves nothing about the rest.

The National Electrical Code, published as NFPA 70, is a model code. States and cities adopt and change it on their own timetables, so your building department is the authority on what applies at your address. If any of the wiring is not something you recognize, this check is one for an electrician. Identifying insulation by sight covers what each material looks like in more detail.

This is the table that surprises people. Half the materials sold as insulation cannot go into a finished wall at all, and no amount of money changes that. The R per inch figures are this site's aged ranges from its reference data.

So a closed wall is a two-horse race, and cellulose wins it more often than not. It packs tighter, it holds itself up in the bay, and it carries more R for each inch of a cavity that has no spare inches. Cellulose against fiberglass runs that comparison in full, including the manufacturer's own dense pack wall figures. Blown-in insulation covers how the material behaves, and mineral wool covers the open-wall case where sound and fire matter.

The same closed cavity problem turns up above your head as well. A cathedral ceiling is a sloped bay with drywall under it, so it takes the same drilled approach and the same materials, with a ventilation question on top.

Two labeling points travel with those numbers. R-value is measured on a sample in a lab, under test methods such as ASTM C518, and the FTC R-value Rule (16 CFR Part 460) makes every manufacturer state it the same way. Loose-fill cellulose is made to ASTM C739 and loose-fill mineral fiber to ASTM C764. None of those tests is run on your wall.

Because the box is a fixed size. A stud is a piece of lumber, and the cavity between two studs is exactly as deep as that lumber is wide. You cannot add depth without moving the wall.

Below is each material run across the framing depths this site's reference data carries. These are the dressed depths of nominal 2x stock under the American Softwood Lumber Standard PS 20. Read the columns as cavity depths, not as a claim about your house.

How this table was built. Depth times each material's R per inch range, from this site's reference data. Both ends round down, so the figures understate rather than flatter. They are estimates, not label values. Loose fill also gets denser as it goes deeper, so it does not track depth exactly, and the bag being blown is the authority on what it delivers.

Now put that against the attic. ENERGY STAR recommends R-60 for a bare attic in zones 4 through 8. The deepest framing in the table is 2x10, and almost no wall is built from it. Fill even that with the highest R material there is, closed-cell spray foam, and you land at R-55 to R-64. That is the top corner of the whole table. It takes the deepest framing and the dearest material to get there. Every ordinary wall is shallower. A 2x4 wall, which is what most older houses are framed with, is shallower than every column shown. This site's reference data does not carry its depth, so no figure for it is published here.

There is a second loss the table cannot show. The studs themselves are not insulation, and there is one about every 16 inches, leaving a clear bay of 14.5 inches between them. Heat takes the easy path through the wood. So a whole wall performs below its cavity number, always. Measuring a whole assembly rather than a sample takes a hot box test, ASTM C1363, and this site holds no whole-wall measurement, so it publishes none. That gap is also why insulating sheathing exists: a layer on the outside face covers the studs as well as the bays.

For a 2x4 wall in climate zones 4 through 8 that already has insulation in it, ENERGY STAR says to add R-10 insulating sheathing under the new siding when the siding comes off. That is guidance for an existing home, not a rule for yours. 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.

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

The recommendation. Most readers land in one of two rows. If the attic is not finished, the wall is not the job, and that stays true however cold the wall feels. If the attic is done and the outside cavities are empty, a dense pack retrofit is worth quoting, and the best moment for it is whenever the siding is due anyway. Everyone else is better off sealing the ceiling, doing the rim joist, and leaving the walls alone.

We do not sell insulation and we do not install it, which is why this page can tell you to keep your money. Five walls that look like jobs and are not.

• Any wall, while the attic is still thin. The attic is cheaper for each point of R-value and easier to reach. Doing walls first is paying more for less. If your attic is within about R-10 of the level for your zone, that step is done and this argument stops applying. That threshold is this site's own estimate, not a measured figure.
• A wall that already has fill in it. The first inches do most of the work. Adding to a filled cavity means opening the wall, and the extra R is small. The diminishing returns arithmetic is the same on a wall as on a ceiling.
• Interior walls, for warmth. There is no temperature difference across them. Fill one for sound if you like the quiet, and only while it is open.
• One cold room, before you have found out why. A cold bedroom is more often a duct that was never balanced, a knee wall with open attic behind it, or a floor over a garage. Insulating the walls of that room fixes none of those.
• A solid wall with no cavity. Old brick, stone and block have nothing to blow into. Adding a layer to the inside face costs you room size and can trap moisture in the wall. That is a design decision, not a quote.

There is also a cheaper thing that is nearly always available. Sealing costs little in materials and a lot in patience, and on most houses it buys more comfort than the next layer of anything. Do it yourself or hire works through which parts are yours to do.

• Blown-in wall insulation: the drilled retrofit, start to finish
• Is it worth opening walls to insulate them?
• What wall insulation costs, and what the price is made of
• Knee walls, and the backing detail they need
• The room over the garage
• Knob-and-tube wiring under insulation

The attic pages a wall reader usually needs next:

• How much insulation do I need?
• Insulation cost per square foot
• Air sealing, which comes before all of it
• Insulation types compared
• Measure what your attic has
• The order attic work happens in