A Denver homeowner planning new siding learns that the job is the right time to add foam board over the walls. At the supplier, three kinds of rigid foam sit side by side: pink or blue boards, white beaded boards, and boards with shiny foil faces. Each carries a different R-value on the label, and the highest number is not always the best buy for a cold climate.
Polyisocyanurate, usually called polyiso, has the highest label at about R-5.6 to R-6 per inch, but it loses R-value in cold weather. Many builders count it at about R-5 per inch for outside walls in cold climates. Extruded polystyrene, the pink or blue board, is rated R-5 per inch and slowly loses a little over the years. Expanded polystyrene, the white beaded board, rates about R-3.6 to R-4.2 per inch, holds that value for life, and usually costs the least per point of R-value.
Rigid Insulation R-Value per Inch at a Glance
The labeled R-value is measured at about 75 degrees Fahrenheit. Outside walls and roofs in Denver spend much of the winter far colder than that, so the table also shows how each board behaves in the cold and over time.
Board
Labeled R per inch
In cold weather
Over decades
Cost per R-value
Polyiso
R-5.6 to R-6
Drops, often counted as R-5
Small loss
Low to middle
Extruded polystyrene
R-5
Rises slightly
Slow loss
Highest
Expanded polystyrene
R-3.6 to R-4.2
Rises slightly
No loss
Lowest
No single board wins every row. Polyiso gives the most R-value in a thin layer, expanded polystyrene gives the most value for the money, and extruded polystyrene sits between them with strong moisture resistance.
Why Polyiso Loses R-Value in Cold Denver Weather
Polyiso is made with gases that slow heat flow very well at room temperature. When the foam gets cold, some of those gases condense inside the cells and stop doing their job. The Green Building Advisor guide to insulation for cold climates explains that polyiso loses ground as temperatures fall, while extruded polystyrene gains a little.
An Owens Corning lab study found a steep drop for one polyiso sample at very low temperatures. Owens Corning makes extruded polystyrene, so read that result with care, and note that blends vary by brand. The drop depends on the average temperature through the board, not the outdoor low. On a Denver wall, the inner part of the foam stays warmer than the outside face.
The practical rule is simple. For outside walls and roofs here, count polyiso at about R-5 per inch. On that basis, it still matches extruded polystyrene in the cold and beats it the rest of the year.
Extruded vs Expanded Polystyrene Over the Long Term
Extruded polystyrene starts at R-5 per inch because it traps a gas that insulates better than air. That gas slowly leaks out, so the board loses part of its R-value over the years. Expanded polystyrene traps only air, so its R-value stays the same for life, according to an Expanded Polystyrene Industry Alliance bulletin.
Over a few decades, the gap between the two narrows. Expanded polystyrene also lets a little water vapor pass through, which helps a wall dry. Extruded polystyrene blocks more vapor and shrugs off water, which is why it is common under slabs and on basement walls.
Expanded polystyrene also comes in several densities. Lighter grades rate about R-3.6 per inch, while denser grades reach about R-4.2 and handle the weight of soil and concrete below grade. A gray version made with graphite rates close to R-4.7 per inch and, like the white board, holds that value over time. Check the grade on the label, since two white boards can perform quite differently.
Which Rigid Foam Board Fits Each Part of a Denver Home
Each board has a home where its strengths count most. Four common spots cover nearly every residential project, and the right pick changes from one to the next.
1. Exterior Wall Sheathing in Denver
Foam board over the sheathing keeps the wall framing warmer and cuts heat loss through the studs. A warmer sheathing surface also resists condensation spots on walls. Foil-faced polyiso gives the most R-value per inch, and expanded polystyrene costs the least for thicker layers. Thicker foam also needs longer fasteners and wider trim around windows and doors.
2. Basement Walls and Under Slabs
Below grade, the board must handle constant contact with damp concrete and soil. Extruded polystyrene and high-density expanded polystyrene both work well there. Polyiso is a poor fit in ground contact. For basement walls, compare spray foam and rigid foam board before you choose.
3. Roofs During a Re-Roofing in Denver
Polyiso is the most common board on flat and low-slope roofs, since it packs high R-value into a thin layer. A roof replacement is the time to add it, and deciding whether to insulate before or after a re-roof shapes the whole plan.
4. Crawl Space Walls in Denver
In a sealed crawl space, foam board on the foundation walls keeps the space near indoor temperature. Pair it with a vapor barrier on the crawl space floor so ground moisture stays out.
Moisture, Fire, and Code Rules for Rigid Foam in Denver
Each board handles water vapor differently. Foil-faced polyiso blocks nearly all of it, extruded polystyrene blocks most of it, and expanded polystyrene lets a little through. In a wall, that choice affects how the framing dries, so match the board to the rest of the wall. Closed-cell spray foam behaves much like extruded polystyrene, and whether closed-cell foam absorbs water follows the same logic.
All foam boards burn and must be covered by a fire-rated layer, such as half-inch drywall, inside living space. Polyiso tends to char in place, while polystyrene melts. Foil faces reflect radiant heat only when they face an air gap, and radiant barriers compared with insulation show the limits of that effect.
For new walls, Denver adopted the 2021 International Energy Conservation Code in 2023. Its wall options for climate zone 5 include R-13 in the cavity plus R-10 of continuous insulation. Two inches of polyiso counted at R-5 per inch meets that, while expanded polystyrene needs closer to three inches. A foam layer also eases the load on the furnace on cold Denver nights.
Polyiso has the highest label at R-5.6 to R-6 per inch, but counts it near R-5 for cold outside walls and roofs. Extruded polystyrene starts at R-5, slowly loses a little, and handles water well below grade. Expanded polystyrene rates R-3.6 to R-4.2, never loses R-value, and costs the least per point. Match the board to the job, and pick wall insulation built for cold climates that works with the rest of the wall.
Not always. Extruded polystyrene has a higher R-value per inch and handles water well, but it loses some R-value over time. Expanded polystyrene keeps its value and costs less.
It can go on the inside of a dry basement wall, but it should not touch soil or sit under a slab. Extruded or high-density expanded polystyrene is the safer choice below grade.
Yes. Foam board in living space needs a fire-rated cover, such as half-inch drywall. Some foil-faced products have special approvals, so check the label.
Each fits different spots. Boards suit flat surfaces like sheathing and slabs, while spray foam seals odd shapes and gaps. Choosing the right foam insulation for a Denver home usually means using both.
Rigid Insulation R Value per Inch: XPS vs EPS vs Polyiso Compared
A Denver homeowner planning new siding learns that the job is the right time to add foam board over the walls. At the supplier, three kinds of rigid foam sit side by side: pink or blue boards, white beaded boards, and boards with shiny foil faces. Each carries a different R-value on the label, and the highest number is not always the best buy for a cold climate.
Polyisocyanurate, usually called polyiso, has the highest label at about R-5.6 to R-6 per inch, but it loses R-value in cold weather. Many builders count it at about R-5 per inch for outside walls in cold climates. Extruded polystyrene, the pink or blue board, is rated R-5 per inch and slowly loses a little over the years. Expanded polystyrene, the white beaded board, rates about R-3.6 to R-4.2 per inch, holds that value for life, and usually costs the least per point of R-value.
Rigid Insulation R-Value per Inch at a Glance
The labeled R-value is measured at about 75 degrees Fahrenheit. Outside walls and roofs in Denver spend much of the winter far colder than that, so the table also shows how each board behaves in the cold and over time.
No single board wins every row. Polyiso gives the most R-value in a thin layer, expanded polystyrene gives the most value for the money, and extruded polystyrene sits between them with strong moisture resistance.
Why Polyiso Loses R-Value in Cold Denver Weather
Polyiso is made with gases that slow heat flow very well at room temperature. When the foam gets cold, some of those gases condense inside the cells and stop doing their job. The Green Building Advisor guide to insulation for cold climates explains that polyiso loses ground as temperatures fall, while extruded polystyrene gains a little.
An Owens Corning lab study found a steep drop for one polyiso sample at very low temperatures. Owens Corning makes extruded polystyrene, so read that result with care, and note that blends vary by brand. The drop depends on the average temperature through the board, not the outdoor low. On a Denver wall, the inner part of the foam stays warmer than the outside face.
The practical rule is simple. For outside walls and roofs here, count polyiso at about R-5 per inch. On that basis, it still matches extruded polystyrene in the cold and beats it the rest of the year.
Extruded vs Expanded Polystyrene Over the Long Term
Extruded polystyrene starts at R-5 per inch because it traps a gas that insulates better than air. That gas slowly leaks out, so the board loses part of its R-value over the years. Expanded polystyrene traps only air, so its R-value stays the same for life, according to an Expanded Polystyrene Industry Alliance bulletin.
Over a few decades, the gap between the two narrows. Expanded polystyrene also lets a little water vapor pass through, which helps a wall dry. Extruded polystyrene blocks more vapor and shrugs off water, which is why it is common under slabs and on basement walls.
Expanded polystyrene also comes in several densities. Lighter grades rate about R-3.6 per inch, while denser grades reach about R-4.2 and handle the weight of soil and concrete below grade. A gray version made with graphite rates close to R-4.7 per inch and, like the white board, holds that value over time. Check the grade on the label, since two white boards can perform quite differently.
Which Rigid Foam Board Fits Each Part of a Denver Home
Each board has a home where its strengths count most. Four common spots cover nearly every residential project, and the right pick changes from one to the next.
1. Exterior Wall Sheathing in Denver
Foam board over the sheathing keeps the wall framing warmer and cuts heat loss through the studs. A warmer sheathing surface also resists condensation spots on walls. Foil-faced polyiso gives the most R-value per inch, and expanded polystyrene costs the least for thicker layers. Thicker foam also needs longer fasteners and wider trim around windows and doors.
2. Basement Walls and Under Slabs
Below grade, the board must handle constant contact with damp concrete and soil. Extruded polystyrene and high-density expanded polystyrene both work well there. Polyiso is a poor fit in ground contact. For basement walls, compare spray foam and rigid foam board before you choose.
3. Roofs During a Re-Roofing in Denver
Polyiso is the most common board on flat and low-slope roofs, since it packs high R-value into a thin layer. A roof replacement is the time to add it, and deciding whether to insulate before or after a re-roof shapes the whole plan.
4. Crawl Space Walls in Denver
In a sealed crawl space, foam board on the foundation walls keeps the space near indoor temperature. Pair it with a vapor barrier on the crawl space floor so ground moisture stays out.
Moisture, Fire, and Code Rules for Rigid Foam in Denver
Each board handles water vapor differently. Foil-faced polyiso blocks nearly all of it, extruded polystyrene blocks most of it, and expanded polystyrene lets a little through. In a wall, that choice affects how the framing dries, so match the board to the rest of the wall. Closed-cell spray foam behaves much like extruded polystyrene, and whether closed-cell foam absorbs water follows the same logic.
All foam boards burn and must be covered by a fire-rated layer, such as half-inch drywall, inside living space. Polyiso tends to char in place, while polystyrene melts. Foil faces reflect radiant heat only when they face an air gap, and radiant barriers compared with insulation show the limits of that effect.
For new walls, Denver adopted the 2021 International Energy Conservation Code in 2023. Its wall options for climate zone 5 include R-13 in the cavity plus R-10 of continuous insulation. Two inches of polyiso counted at R-5 per inch meets that, while expanded polystyrene needs closer to three inches. A foam layer also eases the load on the furnace on cold Denver nights.
Grizzly Insulation CO helps homeowners and builders decide where foam belongs in the building. Many projects pair foam board, often installed by the siding or roofing contractor, with closed-cell spray foam where boards cannot reach, such as rim joists and odd framing. We can explain how spray polyurethane foam is applied before any work starts. Our team installs crawl space insulation in Denver and commercial insulation for shops and warehouses.
Conclusion
Polyiso has the highest label at R-5.6 to R-6 per inch, but counts it near R-5 for cold outside walls and roofs. Extruded polystyrene starts at R-5, slowly loses a little, and handles water well below grade. Expanded polystyrene rates R-3.6 to R-4.2, never loses R-value, and costs the least per point. Match the board to the job, and pick wall insulation built for cold climates that works with the rest of the wall.
Frequently Asked Questions
Polyiso, at about R-5.6 to R-6 per inch on the label. In cold weather, it drops, so many builders count it at about R-5 per inch for outside walls.
Not always. Extruded polystyrene has a higher R-value per inch and handles water well, but it loses some R-value over time. Expanded polystyrene keeps its value and costs less.
It can go on the inside of a dry basement wall, but it should not touch soil or sit under a slab. Extruded or high-density expanded polystyrene is the safer choice below grade.
Yes. Foam board in living space needs a fire-rated cover, such as half-inch drywall. Some foil-faced products have special approvals, so check the label.
Each fits different spots. Boards suit flat surfaces like sheathing and slabs, while spray foam seals odd shapes and gaps. Choosing the right foam insulation for a Denver home usually means using both.