Polyisocyanurate insulation, or polyiso, is rated around R-6 per inch at room temperature. When the board itself gets colder than about freezing, it loses some of that R-value. The gas trapped in its cells starts to condense and carries heat more easily. In cold places like Colorado, roofing pros plan around R-5 per inch instead of R-6. Pairing polyiso with a layer of a cold-tolerant insulation outside it keeps it warmer and working closer to its label.
That surprises people, because polyiso has the highest R-value per inch of any common foam board. Our Grizzly Insulation Co crews see it on Denver roofs and walls all the time, so here’s what really happens when the temperature drops.
What Is Polyisocyanurate Insulation?
It’s a rigid foam board, usually with foil or paper on both faces. You’ll see it on flat commercial roofs and as sheathing on walls. It’s light, easy to cut and packs a lot of R-value into a thin board.
Its label value is measured at a mild 75 degrees. That’s a fair test for comparing products, but it isn’t what a roof sees on a January night. Our blog on the highest R-value insulation shows how polyiso compares with other boards.
Why Does Polyiso Lose R-Value in the Cold?
It comes down to the gas inside. Building Science Corporation’s research on temperature-dependent R-values in polyiso found that every board tested lost R-value as outdoor temperatures fell below freezing. The likely cause is the blowing gas in the foam cells condensing into a liquid, which carries heat better than gas.
The same research found polyiso worked best when temperatures stayed roughly between the high 30s and about 108 degrees. So the board performs well in mild weather and slips as it gets cold.
How Much R-Value Does Polyiso Lose in a Colorado Winter?
Building Science Corporation points to roofing industry guidance that uses about R-5.6 per inch for warm climates and R-5.0 per inch for cold climates. That’s instead of the roughly R-6 on the label.
Denver sits in climate zone 5, with plenty of nights well below freezing. So a 2-inch polyiso board labeled R-12 may act more like R-10 when it’s coldest. That’s still good insulation. It just isn’t the full label number.
Does Polyiso Also Lose R-Value as It Ages?
Polyiso is sold with an LTTR rating, short for long-term thermal resistance. That number is meant to reflect the board after years of slow gas loss, not the day it left the factory. The cold-weather drop is a separate effect on top of that. If the terms are new, see our blog on what R-value means.
Do Other Foam Boards Lose R-Value in the Cold?
XPS and EPS foam boards actually hold their R-value or gain a little as they get colder. Mineral wool behaves the same. That’s why an R-5 per inch XPS board can outperform polyiso on the coldest nights. Our blog on whether insulation keeps cold in explains how heat flow changes with temperature.
You’ll hear that “polyiso is always the best board because it has the highest R-value.” That only holds in mild weather. On a 10-degree Denver night, the gap between polyiso and XPS shrinks or flips.
How Do You Keep Polyiso Working in Cold Weather?
Keep it warm. The trick is to put a cold-tolerant layer on the outside of the polyiso so the polyiso sits on the warmer side. Good approaches:
Hybrid layers. XPS, EPS or mineral wool board outside, polyiso inside.
Extra thickness. Plan around R-5 per inch instead of R-6.
Polyiso inside the wall. Use it where it stays warmer, not as the outermost layer.
How Do You Choose a Polyiso Board for Cold Weather?
Ask for cold-weather data, not just the label. Some polyiso makers publish R-values tested at colder mean temperatures, like 40°F or 25°F. Those numbers tell you much more about winter performance than the standard 75°F figure.
Newer formulas vary, so two boards with the same label can behave differently in the cold. If the maker can’t share cold-temperature numbers, plan around the R-5 per inch figure. Our guide on why it’s so cold in your house covers other reasons a home loses heat in winter.
Is Polyiso Still a Good Choice for Denver Homes?
Yes, when it’s used smartly. On a flat roof, a mix of polyiso and another board works well. On walls, polyiso sheathing is fine as long as you size it with the cold-weather number in mind.
If your roof were ours, we’d use polyiso for the bulk of the layer and top it with a board that doesn’t mind the cold. In Denver, that small change keeps the roof closer to its rated value through the winter. Our commercial insulation crews plan flat roofs this way, and our blog on ceiling insulation R-value shows the targets for roofs over living space.
Plan Polyisocyanurate Insulation Around Real Colorado Winters
Polyisocyanurate insulation is excellent, but its label number is a mild-weather number. Plan for about R-5 per inch, keep it on the warm side with a second board, and it will serve you well for decades. For more ways to tighten your home, see our guide to energy efficient insulation.
Planning a roof or wall upgrade in Denver? Grizzly Insulation Co can help. We’re BPI Certified and an Xcel Energy rebate partner.
Why Polyisocyanurate Insulation Loses R-Value in Cold Colorado Winters
Polyisocyanurate insulation, or polyiso, is rated around R-6 per inch at room temperature. When the board itself gets colder than about freezing, it loses some of that R-value. The gas trapped in its cells starts to condense and carries heat more easily. In cold places like Colorado, roofing pros plan around R-5 per inch instead of R-6. Pairing polyiso with a layer of a cold-tolerant insulation outside it keeps it warmer and working closer to its label.
That surprises people, because polyiso has the highest R-value per inch of any common foam board. Our Grizzly Insulation Co crews see it on Denver roofs and walls all the time, so here’s what really happens when the temperature drops.
What Is Polyisocyanurate Insulation?
It’s a rigid foam board, usually with foil or paper on both faces. You’ll see it on flat commercial roofs and as sheathing on walls. It’s light, easy to cut and packs a lot of R-value into a thin board.
Its label value is measured at a mild 75 degrees. That’s a fair test for comparing products, but it isn’t what a roof sees on a January night. Our blog on the highest R-value insulation shows how polyiso compares with other boards.
Why Does Polyiso Lose R-Value in the Cold?
It comes down to the gas inside. Building Science Corporation’s research on temperature-dependent R-values in polyiso found that every board tested lost R-value as outdoor temperatures fell below freezing. The likely cause is the blowing gas in the foam cells condensing into a liquid, which carries heat better than gas.
The same research found polyiso worked best when temperatures stayed roughly between the high 30s and about 108 degrees. So the board performs well in mild weather and slips as it gets cold.
How Much R-Value Does Polyiso Lose in a Colorado Winter?
Building Science Corporation points to roofing industry guidance that uses about R-5.6 per inch for warm climates and R-5.0 per inch for cold climates. That’s instead of the roughly R-6 on the label.
Denver sits in climate zone 5, with plenty of nights well below freezing. So a 2-inch polyiso board labeled R-12 may act more like R-10 when it’s coldest. That’s still good insulation. It just isn’t the full label number.
Does Polyiso Also Lose R-Value as It Ages?
Polyiso is sold with an LTTR rating, short for long-term thermal resistance. That number is meant to reflect the board after years of slow gas loss, not the day it left the factory. The cold-weather drop is a separate effect on top of that. If the terms are new, see our blog on what R-value means.
Do Other Foam Boards Lose R-Value in the Cold?
XPS and EPS foam boards actually hold their R-value or gain a little as they get colder. Mineral wool behaves the same. That’s why an R-5 per inch XPS board can outperform polyiso on the coldest nights. Our blog on whether insulation keeps cold in explains how heat flow changes with temperature.
You’ll hear that “polyiso is always the best board because it has the highest R-value.” That only holds in mild weather. On a 10-degree Denver night, the gap between polyiso and XPS shrinks or flips.
How Do You Keep Polyiso Working in Cold Weather?
Keep it warm. The trick is to put a cold-tolerant layer on the outside of the polyiso so the polyiso sits on the warmer side. Good approaches:
Our guide to the best wall insulation for cold climates shows how these layers stack.
How Do You Choose a Polyiso Board for Cold Weather?
Ask for cold-weather data, not just the label. Some polyiso makers publish R-values tested at colder mean temperatures, like 40°F or 25°F. Those numbers tell you much more about winter performance than the standard 75°F figure.
Newer formulas vary, so two boards with the same label can behave differently in the cold. If the maker can’t share cold-temperature numbers, plan around the R-5 per inch figure. Our guide on why it’s so cold in your house covers other reasons a home loses heat in winter.
Is Polyiso Still a Good Choice for Denver Homes?
Yes, when it’s used smartly. On a flat roof, a mix of polyiso and another board works well. On walls, polyiso sheathing is fine as long as you size it with the cold-weather number in mind.
If your roof were ours, we’d use polyiso for the bulk of the layer and top it with a board that doesn’t mind the cold. In Denver, that small change keeps the roof closer to its rated value through the winter. Our commercial insulation crews plan flat roofs this way, and our blog on ceiling insulation R-value shows the targets for roofs over living space.
What About Spray Foam Instead of Polyiso?
Closed cell spray foam is another option where boards are hard to fit. It seals air leaks at the same time and sticks to odd shapes. See closed cell spray foam R-value for depth charts. For attics, our blog on the best attic insulation for cold climates compares the choices.
Plan Polyisocyanurate Insulation Around Real Colorado Winters
Polyisocyanurate insulation is excellent, but its label number is a mild-weather number. Plan for about R-5 per inch, keep it on the warm side with a second board, and it will serve you well for decades. For more ways to tighten your home, see our guide to energy efficient insulation.
Planning a roof or wall upgrade in Denver? Grizzly Insulation Co can help. We’re BPI Certified and an Xcel Energy rebate partner.
Frequently Asked Questions
About R-6 at mild temperatures. Plan on about R-5 in cold climates.
The losses show up as the board drops below freezing.
On the coldest nights, XPS can match or beat polyiso per inch.
Yes. The drop is tied to temperature, so it comes back as the board warms.
Yes. Pair it with a cold-tolerant board or add thickness.