Rockwool exterior insulation is a rigid mineral wool board that goes on the outside of your wall sheathing, under the siding. It wraps the whole house in one unbroken layer, so heat can’t sneak out through the wood studs. The common product, ROCKWOOL Comfortboard 80, gives about R-4.2 per inch. A 1.25-inch board gets you a little over R-5, and 2.5 inches gets you a little over R-10. Unlike foam board, it lets water vapor pass through, so a wet wall can still dry to the outside.
Builders call this kind of layer continuous insulation, or “ci” on the plans. Our Grizzly Insulation Co crews see it more and more on new Denver homes and big remodels, so here’s how the boards actually work, what they cost you in thickness, and when they’re worth choosing over foam.
What Is Rockwool Exterior Insulation?
It’s stone wool pressed into stiff boards. The fibers are spun from melted basalt rock and recycled slag, then packed together at about 8 pounds per cubic foot. That makes the board dense enough to hold its shape on a wall but still soft enough to cut with a long serrated knife.
The board most people mean is ROCKWOOL Comfortboard 80. It comes in thicknesses from 1 inch up to 5 inches, mostly in 24 by 48 inch sheets. If you’re newer to the numbers, our blog on what R-value means is a quick learning exercise.
How Does Continuous Mineral Wool Insulation Work on a Wall?
It covers the studs. In a normal wall, insulation sits between the studs, and every stud is a little strip of wood with far less R-value than the batt beside it. Heat finds those strips and walks right out. Builders call that thermal bridging.
A layer of board on the outside runs across every stud without a break. So the whole wall, studs included, gets the extra R-value. That’s why the code gives you credit for even a fairly thin layer of continuous insulation. Our guide to the best wall insulation for cold climates shows how this compares with other wall setups.
What R-Value Does Rockwool Exterior Insulation Give per Inch?
About R-4.2 per inch, according to ROCKWOOL’s own test data. Here’s what the common thicknesses work out to:
Board thickness
Approximate R-value
1 inch
R-4.2
1.25 inches
R-5.2
1.5 inches
R-6.3
2 inches
R-8.4
2.5 inches
R-10.5
3 inches
R-12.6
Foam boards pack in a bit more per inch. Polyiso is around R-6 and XPS is around R-5, so mineral wool needs a little more thickness to reach the same number. Our blog on the highest R-value insulation lines up the options side by side.
Does the Building Code Require Exterior Insulation in Denver?
Not by itself, but it gives you an easy way to meet the wall rule. The Department of Energy’s summary of the 2021 IECC insulation requirements lists four options for wood frame walls in climate zone 5. You can do R-30 in the cavity, R-20 plus R-5 continuous insulation, R-13 plus R-10 continuous insulation, or R-20 of continuous insulation alone.
Denver sits in zone 5, and the city’s energy code is based on the 2021 IECC. So a 2×6 wall with R-20 or better batts plus 1.25 inches of Comfortboard meets the code. A 2×4 wall with R-13 batts plus 2.5 inches of board does it too.
How Is Rockwool Exterior Insulation Installed?
The boards go over the sheathing and the weather barrier, then get held in place by wood strips screwed through to the studs. A typical order looks like this:
Sheathing and weather barrier first. The house wrap or taped sheathing is your air and water barrier. The board doesn’t replace it.
Boards hung tight together, with joints staggered so no seam lines up with the one above it.
Furring strips over the boards, usually 1×4 wood strips, screwed through the insulation into each stud with long screws.
Siding fastened to the furring strips. The strips also leave a small air gap behind the siding, which builders call a rainscreen.
Window and door openings built out so the trim lines up with the new, thicker wall.
ROCKWOOL publishes its own attachment guide with screw sizes and spacing, and your installer should follow it. The thicker the board, the longer the screws have to be.
Which Rockwool Board Should You Use on Exterior Walls?
For most houses, Comfortboard 80 is the one. It’s the residential board, at about 8 pounds per cubic foot. ROCKWOOL also makes Comfortboard 110, which is denser at about 11 pounds per cubic foot. Both are listed for walls, foundations, under slabs and stucco, and both get held on with screws through strapping.
The denser board is stiffer, so it handles more weight and resists getting crushed when furring strips are screwed down tight. That makes it handy under slabs or behind heavier cladding. On a regular wood-framed house with lap siding, the 80 does the job for less money. Ask your installer which one the siding maker’s guide calls for.
Does Mineral Wool Board Trap Moisture in Walls?
No, and this is its biggest selling point. ROCKWOOL lists Comfortboard 80 at about 31 perms, which is very vapor-open. Water vapor can move through it and out of the wall.
You’ll sometimes hear that any insulation on the outside of a wall “traps moisture and rots the studs.” Nobody has shown that a vapor-open board does that. The real risk with outside insulation is a layer that’s too thin to keep the sheathing warm, combined with leaky walls, and that’s mostly a foam-board question.
If we were building our own house here, we’d like mineral wool outside for exactly this reason. It forgives small mistakes. A wall that gets a little wet from a leaky window has a way to dry out. If you’ve ever seen condensation spots on walls, or wet-looking spots on walls, you know how much drying matters.
Is Rockwool Exterior Insulation Fireproof?
It’s about as close as building insulation gets. ROCKWOOL lists the board at a flame spread of 0 and smoke developed of 0, and it doesn’t melt until roughly 2,150°F. Foam boards, by contrast, need to be covered and protected because they burn.
That matters on homes built close to the neighbors, which describes a lot of new Denver infill. It also matters in wildfire-prone foothill areas, though siding, vents and roofing matter more for wildfire than wall insulation does.
Is Rockwool Exterior Insulation Better Than Foam Board?
It depends on what you care about most. Here’s how they stack up:
Drying: Mineral wool lets walls dry outward. Foil-faced polyiso and thick XPS mostly don’t.
Fire: Mineral wool is non-combustible. Foam isn’t.
Sound: Mineral wool’s density cuts outside noise a bit better. For better understanding, check out our blog on whether insulation helps with sound.
Thickness: Foam reaches the same R-value with a thinner layer.
Cost and labor: Mineral wool boards usually cost more and are heavier to handle.
Our honest take is that mineral wool is worth it when you’re already re-siding, when fire or noise matters, or when you want a wall that can dry. If budget and thickness are the main limits, foam board is still a fine choice.
Can You Add Rockwool Exterior Insulation to an Existing Home?
Yes, but only when the siding comes off. That’s the natural time to do it. Once the old siding is off, the crew can fix the weather barrier, hang the boards, add furring strips and put new siding on top.
Doing it at any other time means tearing off good siding, which rarely pays back. While the walls are open, it’s also smart to check the stud cavities. Empty or slumped batts can be fixed with batt insulation from the inside or blown-in insulation from the outside before the boards go on.
Where Else Is Rigid Mineral Wool Used Around a Home?
The same boards work below grade and in some roofs. ROCKWOOL lists them for basement walls, around foundations, under concrete slabs and in wood-framed cathedral ceilings. Dry crawl space walls can take them too. On larger buildings they’re common behind stucco and metal panels, which our commercial insulation team works on regularly.
Conclusion
Rockwool exterior insulation gives your walls a continuous layer that stops heat at the studs, lets moisture escape, and won’t burn. It takes a little more thickness than foam, but it forgives more, and it’s one of the more energy efficient insulation choices for walls. If you’re building new or re-siding, it’s one of the smartest upgrades you can make. To see where walls fit in your whole-house plan, check how much insulation you need.
Planning new siding or a new build in Denver? Grizzly Insulation Co can help you choose the right board and thickness.
How Rockwool Exterior Insulation Boards Work as Continuous Insulation on Your Walls
Rockwool exterior insulation is a rigid mineral wool board that goes on the outside of your wall sheathing, under the siding. It wraps the whole house in one unbroken layer, so heat can’t sneak out through the wood studs. The common product, ROCKWOOL Comfortboard 80, gives about R-4.2 per inch. A 1.25-inch board gets you a little over R-5, and 2.5 inches gets you a little over R-10. Unlike foam board, it lets water vapor pass through, so a wet wall can still dry to the outside.
Builders call this kind of layer continuous insulation, or “ci” on the plans. Our Grizzly Insulation Co crews see it more and more on new Denver homes and big remodels, so here’s how the boards actually work, what they cost you in thickness, and when they’re worth choosing over foam.
What Is Rockwool Exterior Insulation?
It’s stone wool pressed into stiff boards. The fibers are spun from melted basalt rock and recycled slag, then packed together at about 8 pounds per cubic foot. That makes the board dense enough to hold its shape on a wall but still soft enough to cut with a long serrated knife.
The board most people mean is ROCKWOOL Comfortboard 80. It comes in thicknesses from 1 inch up to 5 inches, mostly in 24 by 48 inch sheets. If you’re newer to the numbers, our blog on what R-value means is a quick learning exercise.
How Does Continuous Mineral Wool Insulation Work on a Wall?
It covers the studs. In a normal wall, insulation sits between the studs, and every stud is a little strip of wood with far less R-value than the batt beside it. Heat finds those strips and walks right out. Builders call that thermal bridging.
A layer of board on the outside runs across every stud without a break. So the whole wall, studs included, gets the extra R-value. That’s why the code gives you credit for even a fairly thin layer of continuous insulation. Our guide to the best wall insulation for cold climates shows how this compares with other wall setups.
What R-Value Does Rockwool Exterior Insulation Give per Inch?
About R-4.2 per inch, according to ROCKWOOL’s own test data. Here’s what the common thicknesses work out to:
Foam boards pack in a bit more per inch. Polyiso is around R-6 and XPS is around R-5, so mineral wool needs a little more thickness to reach the same number. Our blog on the highest R-value insulation lines up the options side by side.
Does the Building Code Require Exterior Insulation in Denver?
Not by itself, but it gives you an easy way to meet the wall rule. The Department of Energy’s summary of the 2021 IECC insulation requirements lists four options for wood frame walls in climate zone 5. You can do R-30 in the cavity, R-20 plus R-5 continuous insulation, R-13 plus R-10 continuous insulation, or R-20 of continuous insulation alone.
Denver sits in zone 5, and the city’s energy code is based on the 2021 IECC. So a 2×6 wall with R-20 or better batts plus 1.25 inches of Comfortboard meets the code. A 2×4 wall with R-13 batts plus 2.5 inches of board does it too.
How Is Rockwool Exterior Insulation Installed?
The boards go over the sheathing and the weather barrier, then get held in place by wood strips screwed through to the studs. A typical order looks like this:
ROCKWOOL publishes its own attachment guide with screw sizes and spacing, and your installer should follow it. The thicker the board, the longer the screws have to be.
Which Rockwool Board Should You Use on Exterior Walls?
For most houses, Comfortboard 80 is the one. It’s the residential board, at about 8 pounds per cubic foot. ROCKWOOL also makes Comfortboard 110, which is denser at about 11 pounds per cubic foot. Both are listed for walls, foundations, under slabs and stucco, and both get held on with screws through strapping.
The denser board is stiffer, so it handles more weight and resists getting crushed when furring strips are screwed down tight. That makes it handy under slabs or behind heavier cladding. On a regular wood-framed house with lap siding, the 80 does the job for less money. Ask your installer which one the siding maker’s guide calls for.
Does Mineral Wool Board Trap Moisture in Walls?
No, and this is its biggest selling point. ROCKWOOL lists Comfortboard 80 at about 31 perms, which is very vapor-open. Water vapor can move through it and out of the wall.
You’ll sometimes hear that any insulation on the outside of a wall “traps moisture and rots the studs.” Nobody has shown that a vapor-open board does that. The real risk with outside insulation is a layer that’s too thin to keep the sheathing warm, combined with leaky walls, and that’s mostly a foam-board question.
If we were building our own house here, we’d like mineral wool outside for exactly this reason. It forgives small mistakes. A wall that gets a little wet from a leaky window has a way to dry out. If you’ve ever seen condensation spots on walls, or wet-looking spots on walls, you know how much drying matters.
Is Rockwool Exterior Insulation Fireproof?
It’s about as close as building insulation gets. ROCKWOOL lists the board at a flame spread of 0 and smoke developed of 0, and it doesn’t melt until roughly 2,150°F. Foam boards, by contrast, need to be covered and protected because they burn.
That matters on homes built close to the neighbors, which describes a lot of new Denver infill. It also matters in wildfire-prone foothill areas, though siding, vents and roofing matter more for wildfire than wall insulation does.
Is Rockwool Exterior Insulation Better Than Foam Board?
It depends on what you care about most. Here’s how they stack up:
Our honest take is that mineral wool is worth it when you’re already re-siding, when fire or noise matters, or when you want a wall that can dry. If budget and thickness are the main limits, foam board is still a fine choice.
Can You Add Rockwool Exterior Insulation to an Existing Home?
Yes, but only when the siding comes off. That’s the natural time to do it. Once the old siding is off, the crew can fix the weather barrier, hang the boards, add furring strips and put new siding on top.
Doing it at any other time means tearing off good siding, which rarely pays back. While the walls are open, it’s also smart to check the stud cavities. Empty or slumped batts can be fixed with batt insulation from the inside or blown-in insulation from the outside before the boards go on.
Where Else Is Rigid Mineral Wool Used Around a Home?
The same boards work below grade and in some roofs. ROCKWOOL lists them for basement walls, around foundations, under concrete slabs and in wood-framed cathedral ceilings. Dry crawl space walls can take them too. On larger buildings they’re common behind stucco and metal panels, which our commercial insulation team works on regularly.
Conclusion
Rockwool exterior insulation gives your walls a continuous layer that stops heat at the studs, lets moisture escape, and won’t burn. It takes a little more thickness than foam, but it forgives more, and it’s one of the more energy efficient insulation choices for walls. If you’re building new or re-siding, it’s one of the smartest upgrades you can make. To see where walls fit in your whole-house plan, check how much insulation you need.
Planning new siding or a new build in Denver? Grizzly Insulation Co can help you choose the right board and thickness.
Call (303) 854-6221 to talk it through.
Frequently Asked Questions
About R-4.2 per inch, per ROCKWOOL’s test data.
The board is vapor-open, and the wall still needs an air and water barrier on the sheathing.
At least 1.25 inches (about R-5) with R-20 cavity insulation. Use about 2.5 inches (about R-10) with R-13 cavity insulation.
Most siding goes onto furring strips that are screwed through the board. Follow the siding and ROCKWOOL install guides.
It’s water-repellent, not waterproof. The weather barrier behind it handles bulk water.
Yes, it cuts heat loss through studs, which batts alone can’t stop.