In a US home, cavity wall insulation means filling the hollow space between the wall studs, behind the drywall. That gap is the wall cavity. In new construction it gets batts, blown-in fiberglass or cellulose, or spray foam before the drywall goes up. In an older house with empty walls, it’s usually filled by blowing insulation in through small holes. It’s worth doing when your walls are empty or nearly empty, the wiring is safe, and the walls stay dry.
The term trips people up because most of what you’ll read online comes from the UK. There, a “cavity wall” is two layers of brick with a gap between them. Most houses here are wood framed, so the cavity is the stud bay.
What Is a Wall Cavity in a US Home?
It’s the space between two studs, closed in by sheathing on the outside and drywall on the inside. In a 2×4 wall the cavity is 3.5 inches deep. In a 2×6 wall it’s 5.5 inches deep. Studs usually sit 16 or 24 inches apart.
Every exterior wall in a wood-framed house is a row of these cavities. When they’re empty, the only thing slowing heat is a thin layer of still air, and that air rarely stays still.
How Does Cavity Wall Insulation Work?
It fills the space with material that traps air in tiny pockets. Trapped air slows heat far better than an open gap, where air can swirl around and carry heat from the warm side to the cold side.
The deeper the cavity, the more insulation fits, and the higher the R-value. If R-values are new to you, our blog on what R-value means makes it easy for you to understand.
What R-Value Can Cavity Wall Insulation Reach?
It depends on the wall depth and the material. The Department of Energy’s guide to types of insulation lists R-13 or R-15 batts for 2×4 walls and R-19 or R-21 batts for 2×6 walls. Here’s a rough comparison:
Material
2×4 wall (3.5 in)
2×6 wall (5.5 in)
Fiberglass batt
R-13 to R-15
R-19 to R-21
Mineral wool batt
R-15
R-23
Dense-pack cellulose
About R-12 to R-13
About R-19 to R-20
Open cell spray foam
About R-13
About R-20
Closed cell spray foam
About R-21 to R-23
Higher, but rarely needed
For a closer look at the two most common 2×4 batts, check out our blog on R13 vs R15 insulation.
Which Materials Are Used for Cavity Wall Insulation?
There are four main choices, and each one fits a different situation:
Batts. Pre-cut fiberglass or mineral wool, pressed in before drywall. Cheapest for open walls. Our batt insulation crews do this on new builds and remodels.
Blown-in fiberglass or cellulose. Loose fill blown into the cavity. In open walls it’s held behind netting. In closed walls it goes in through small holes.
Spray foam. Sprayed into open cavities. It seals air leaks and insulates in one step, but costs more.
Injection foam. A slow-expanding foam pumped into closed walls through small holes.
How Do You Insulate a Wall Cavity Without Removing Drywall?
You drill small holes and blow or inject insulation through them. The crew drills a hole near the top of each cavity, either from outside under the siding or from inside through the drywall. Then they fill the cavity with dense-pack cellulose, blown fiberglass or injection foam, and patch the holes.
It makes the most sense when your walls are empty or nearly empty. Plenty of homes built before the 1970s went up with nothing in the walls at all. You’ll usually notice:
Exterior walls that feel cold to the touch in winter.
Rooms on the north side that never warm up.
Drafts at outlets on outside walls.
High heating bills even after the attic was topped up.
There are a few times we’d hold off. This part isn’t guesswork. The Department of Energy warns against putting insulation around active knob-and-tube wiring, the old cloth-covered wiring on porcelain knobs. That wiring was built to shed heat into open air, and burying it can make it overheat. An electrician should check it or replace it first.
Other times to wait:
Walls with active leaks. Fix the water first. Wet insulation does more harm than good.
Solid brick walls. Many older brick homes have solid masonry walls with no stud cavity to fill.
Brick veneer air gaps. The gap behind brick veneer is meant for drainage, not insulation.
What Does a Properly Insulated Wall Cavity Look Like?
Full, top to bottom, with no gaps. The Department of Energy’s checklists for wall insulation say blown-in insulation should completely fill each cavity at the maker’s listed density, so it doesn’t settle and leave an empty band at the top. Holes get patched and cracks get sealed afterward.
A good crew also checks a few things before drilling. They look for knob-and-tube wiring, water stains and pest damage, and they tell you about anything that needs fixing first. In older balloon-framed houses, where the wall cavities run straight from the basement to the attic, they add blocking at the top and bottom so the insulation doesn’t pour away.
Does Cavity Wall Insulation Cause Damp or Rot?
Not when it’s done right in a typical American wood-framed home. Then there’s the scary version you’ll find online. Most of the damp stories come from UK brick cavity walls in rainy, exposed spots, where wind-driven rain can wick across filled cavities. That’s a very different wall from ours.
Here in Denver, our dry climate helps a lot. The real risk in our walls is warm, humid house air leaking into a cold cavity in winter. If your walls were ours, we’d seal the outlets and gaps first, use dense-pack so the fill doesn’t settle, and make sure gutters and flashing keep rain out.
Which Cavity Wall Insulation Works Best in Denver's Climate?
For closed walls in older homes, dense-pack cellulose is our usual pick. It’s blown in tight, about 3.5 pounds per cubic foot, so it fills around wires and pipes and doesn’t slump. Blown fiberglass works too, and it won’t absorb water the way cellulose can. Our blog on the pros and cons of cellulose insulation and blown fiberglass insulation compare the two.
For open walls during a Denver remodel, batts are the budget choice and spray foam is the premium one. Spray foam earns its price on walls that leak a lot of air, like ones facing the wind or next to a garage. Otherwise, well-fitted batts and good air sealing do the job for much less.
Is Cavity Wall Insulation Worth the Cost?
Usually, when the walls start out empty. Going from an empty wall to a filled one is a much bigger jump than topping up an already-insulated one. The rooms feel warmer, the drafts at outlets fade, and the furnace runs less. In most Denver homes the attic gives a faster return, because it’s easy to reach and heat rises into it. If your attic is thin, start there with attic insulation and air sealing, then look at the walls.
How Long Does Cavity Wall Insulation Take?
A typical house takes one to three days. New-construction batts can go in within a day. Blowing insulation into closed walls takes longer, because each cavity has to be found, drilled, filled and patched. Painting over the patches is usually the homeowner’s job, so plan for that.
Conclusion
Cavity wall insulation turns hollow, drafty walls into real barriers against the cold. Check the wiring, fix any leaks, seal the gaps, then fill the cavities with the material that fits your walls. Your rooms will hold heat better and your furnace will thank you.
Not sure what’s inside your walls? Grizzly Insulation Co can check and recommend the right fill for your Denver home. We’re BPI Certified and an Xcel Energy rebate partner.
How Cavity Wall Insulation Works in US Homes and When It Is Worth Doing
In a US home, cavity wall insulation means filling the hollow space between the wall studs, behind the drywall. That gap is the wall cavity. In new construction it gets batts, blown-in fiberglass or cellulose, or spray foam before the drywall goes up. In an older house with empty walls, it’s usually filled by blowing insulation in through small holes. It’s worth doing when your walls are empty or nearly empty, the wiring is safe, and the walls stay dry.
The term trips people up because most of what you’ll read online comes from the UK. There, a “cavity wall” is two layers of brick with a gap between them. Most houses here are wood framed, so the cavity is the stud bay.
What Is a Wall Cavity in a US Home?
It’s the space between two studs, closed in by sheathing on the outside and drywall on the inside. In a 2×4 wall the cavity is 3.5 inches deep. In a 2×6 wall it’s 5.5 inches deep. Studs usually sit 16 or 24 inches apart.
Every exterior wall in a wood-framed house is a row of these cavities. When they’re empty, the only thing slowing heat is a thin layer of still air, and that air rarely stays still.
How Does Cavity Wall Insulation Work?
It fills the space with material that traps air in tiny pockets. Trapped air slows heat far better than an open gap, where air can swirl around and carry heat from the warm side to the cold side.
The deeper the cavity, the more insulation fits, and the higher the R-value. If R-values are new to you, our blog on what R-value means makes it easy for you to understand.
What R-Value Can Cavity Wall Insulation Reach?
It depends on the wall depth and the material. The Department of Energy’s guide to types of insulation lists R-13 or R-15 batts for 2×4 walls and R-19 or R-21 batts for 2×6 walls. Here’s a rough comparison:
For a closer look at the two most common 2×4 batts, check out our blog on R13 vs R15 insulation.
Which Materials Are Used for Cavity Wall Insulation?
There are four main choices, and each one fits a different situation:
How Do You Insulate a Wall Cavity Without Removing Drywall?
You drill small holes and blow or inject insulation through them. The crew drills a hole near the top of each cavity, either from outside under the siding or from inside through the drywall. Then they fill the cavity with dense-pack cellulose, blown fiberglass or injection foam, and patch the holes.
We’ve written about this in detail already. See our guide to injection foam installation and our blog on insulating existing interior walls.
When Does Cavity Wall Insulation Make Sense?
It makes the most sense when your walls are empty or nearly empty. Plenty of homes built before the 1970s went up with nothing in the walls at all. You’ll usually notice:
A lot of older bungalows and ranches around here fit this list. Our checklist of signs your home needs better insulation covers more clues.
When Should You Not Fill a Wall Cavity?
There are a few times we’d hold off. This part isn’t guesswork. The Department of Energy warns against putting insulation around active knob-and-tube wiring, the old cloth-covered wiring on porcelain knobs. That wiring was built to shed heat into open air, and burying it can make it overheat. An electrician should check it or replace it first.
Other times to wait:
What Does a Properly Insulated Wall Cavity Look Like?
Full, top to bottom, with no gaps. The Department of Energy’s checklists for wall insulation say blown-in insulation should completely fill each cavity at the maker’s listed density, so it doesn’t settle and leave an empty band at the top. Holes get patched and cracks get sealed afterward.
A good crew also checks a few things before drilling. They look for knob-and-tube wiring, water stains and pest damage, and they tell you about anything that needs fixing first. In older balloon-framed houses, where the wall cavities run straight from the basement to the attic, they add blocking at the top and bottom so the insulation doesn’t pour away.
Does Cavity Wall Insulation Cause Damp or Rot?
Not when it’s done right in a typical American wood-framed home. Then there’s the scary version you’ll find online. Most of the damp stories come from UK brick cavity walls in rainy, exposed spots, where wind-driven rain can wick across filled cavities. That’s a very different wall from ours.
Here in Denver, our dry climate helps a lot. The real risk in our walls is warm, humid house air leaking into a cold cavity in winter. If your walls were ours, we’d seal the outlets and gaps first, use dense-pack so the fill doesn’t settle, and make sure gutters and flashing keep rain out.
Wondering what damp walls look like? Check out our guide about wet-looking spots on walls.
Which Cavity Wall Insulation Works Best in Denver's Climate?
For closed walls in older homes, dense-pack cellulose is our usual pick. It’s blown in tight, about 3.5 pounds per cubic foot, so it fills around wires and pipes and doesn’t slump. Blown fiberglass works too, and it won’t absorb water the way cellulose can. Our blog on the pros and cons of cellulose insulation and blown fiberglass insulation compare the two.
For open walls during a Denver remodel, batts are the budget choice and spray foam is the premium one. Spray foam earns its price on walls that leak a lot of air, like ones facing the wind or next to a garage. Otherwise, well-fitted batts and good air sealing do the job for much less.
Is Cavity Wall Insulation Worth the Cost?
Usually, when the walls start out empty. Going from an empty wall to a filled one is a much bigger jump than topping up an already-insulated one. The rooms feel warmer, the drafts at outlets fade, and the furnace runs less. In most Denver homes the attic gives a faster return, because it’s easy to reach and heat rises into it. If your attic is thin, start there with attic insulation and air sealing, then look at the walls.
How Long Does Cavity Wall Insulation Take?
A typical house takes one to three days. New-construction batts can go in within a day. Blowing insulation into closed walls takes longer, because each cavity has to be found, drilled, filled and patched. Painting over the patches is usually the homeowner’s job, so plan for that.
Conclusion
Cavity wall insulation turns hollow, drafty walls into real barriers against the cold. Check the wiring, fix any leaks, seal the gaps, then fill the cavities with the material that fits your walls. Your rooms will hold heat better and your furnace will thank you.
Not sure what’s inside your walls? Grizzly Insulation Co can check and recommend the right fill for your Denver home. We’re BPI Certified and an Xcel Energy rebate partner.
Call (303) 854-6221 to set up a free home visit.
Frequently Asked Questions
It’s insulation that fills the space between wall studs. In the UK, the term usually means filling the gap in a double brick wall.
For open walls, batts or spray foam. For closed walls, dense-pack cellulose or injection foam.
Usually not without opening the wall. Blown insulation needs room to fill.
Yes, filled cavities soften outside noise noticeably, as covered in our blog about whether insulation helps with sound.
Take the cover off an outlet on an outside wall (power off first) and look in with a flashlight.
Yes, when the wiring is safe and the wall stays dry.