How Much Thermal Resistance Does Polyiso Really Provide?

Editorially reviewed August 6, 2026. This article distinguishes manufacturer data, industry guidance, and historical independent testing. It provides general reference information, not project-specific engineering or code advice.

The short answer: approximately R-6 to R-6.5 per inch

Polyisocyanurate insulation—usually shortened to polyiso—is commonly listed at approximately R-6 to R-6.5 per inch. That is a useful starting point for comparing materials and estimating space requirements, but it is not a universal rating for every polyiso board. Hunter Panels identifies approximately R-6 to R-6.5 per inch as an initial range that varies by product.

R-value measures resistance to heat flow. A higher R-value indicates greater thermal resistance under the conditions used to establish the rating. A board rated R-12 therefore resists heat flow more than one rated R-6 when both figures were determined on a comparable basis.

The qualification—under the stated conditions—matters. A reported polyiso R-value can vary with:

• Foam formulation
• Board thickness
• Facer type and permeability
• Product age and conditioning
• Test method
• Mean test temperature
• Moisture exposure
• Intended roof or wall application
• Installation quality

Manufacturers may publish an initial R-value, a conditioned value, or Long-Term Thermal Resistance (LTTR). These figures do not necessarily describe the same point in the product’s life or the same test conditions.

For preliminary planning, the following rule of thumb is reasonable:

Estimated polyiso R-value = thickness in inches × approximately R-6 to R-6.5 per inch

Do not treat the result as a product specification. Before ordering, check the current technical data sheet for the exact manufacturer, product name, thickness, facer, and application. Where available, use the published total R-value for the selected board thickness instead of assuming every polyiso product supplies the same value per inch.

A board’s material rating must also be separated from the effective thermal performance of a roof or wall. A complete assembly includes joints, fasteners, penetrations, membranes, framing or decking, air leakage paths, moisture conditions, and workmanship. Adding board ratings produces a nominal insulation total; it does not by itself prove whole-assembly performance or code compliance.

Polyiso thickness and R-value chart

The following table converts the common R-6 to R-6.5-per-inch range into nominal totals. It is a planning range, not a guaranteed installed result or a substitute for product data.

Polyiso thickness	Rough nominal R-value range
1/2 inch	R-3 to R-3.25
1 inch	R-6 to R-6.5
2 inches	R-12 to R-13
3 inches	R-18 to R-19.5
4 inches	R-24 to R-26

These ranges are consistent with a published general chart that qualifies polyiso values as approximate ratings at a 75°F mean temperature. See the supporting nominal polyiso thickness chart.

The arithmetic is straightforward. At R-6 per inch, a 2-inch board is estimated at R-12. At R-6.5 per inch, it is estimated at R-13. The same multiplication produces the other ranges.

That calculation is appropriate only when the applicable product data support a reasonably proportional relationship between thickness and R-value. Some products publish slightly different per-inch values at different thicknesses, so a generic multiplier can differ from the current labeled total.

A product-specific schedule illustrates the distinction. IKO publishes the following thermal-resistance values for Enerfoil, its foil-faced polyiso product:

Enerfoil thickness	Published R-value	Published RSI
1/2 inch	R-3.1	RSI 0.54
1 inch	R-6.2	RSI 1.08
2 inches	R-12.4	RSI 2.16
3 inches	R-18.6	RSI 3.24
4 inches	R-24.8	RSI 4.32

IKO identifies these as product-specific values based on conditioning requirements and testing for foil-faced polyiso under ASTM C1289 and ASTM C518. It also qualifies the figures by referring to appropriate treatment of joints and penetrations and directing users to investigate and verify the information. Review IKO’s Enerfoil thickness, R-value, and RSI schedule.

This schedule should not be transferred automatically to another brand, a permeable-faced roof board, or a high-density cover board. Board construction, facer, conditioning basis, and published rating all matter.

RSI expresses thermal resistance in metric units, while the R-values used here are inch-pound values. If project documents use RSI, use the manufacturer’s published metric figure where available rather than converting a rounded R-value and introducing unnecessary rounding differences.

For purchasing and design review:

1. Identify the exact product and facer.
2. Find an available thickness.
3. Record the published total R-value or RSI for that thickness.
4. Confirm the conditioning, aging, and test basis.
5. Check that the rating applies to the intended roof or wall use.
6. Evaluate the complete assembly separately.

A per-inch figure is useful for screening options. The labeled total for an available board configuration is the better basis for final material calculations.

Initial R-value, aged R-value, and LTTR are not the same

Different sources publish different polyiso R-values partly because they may be describing different stages of the product’s life.

An initial R-value, sometimes called a fresh R-value, is measured before the full effect of foam aging has been represented. Polyiso is a gas-filled, closed-cell foam. Its blowing-agent gas generally resists heat flow more effectively than ordinary air. As the material ages, blowing agent leaves or redistributes while air enters the cells, and thermal resistance can decline.

The rate and amount of change are not identical for every board. Foam chemistry, cell structure, thickness, facer permeability, temperature, and manufacturing details can influence aging. The essential point is that a fresh measurement and a long-term measurement answer different questions.

Long-Term Thermal Resistance, or LTTR, is a standardized metric intended to account for this aging behavior. Industry guidance describes it as a 15-year time-weighted average and also characterizes it as corresponding closely to predicted performance at five years. A 180-day conditioned value represents a different point in the aging process. PIMA explains the distinction among LTTR, conditioned values, and other R-value factors.

The following measurements should not be interchanged:

• Initial or fresh R-value: Measured near manufacture, before long-term aging is fully represented.
• 180-day conditioned R-value: Measured after a defined conditioning period.
• LTTR: A standardized long-term aging metric.
• Service-temperature performance: Thermal resistance under a particular operating temperature or temperature gradient.
• Whole-assembly performance: Performance of the complete construction rather than an insulation specimen alone.

A board can have a valid LTTR rating without that figure predicting its performance at every winter or summer temperature. Likewise, a cold-temperature test does not replace the product’s standardized aging metric.

ASTM C1289 and related procedures are commonly cited for polyiso product ratings. Seeing “ASTM tested” is not enough by itself: check the exact procedure, conditioning basis, thickness, and facer identified in the product documentation.

For permeable-faced roofing polyiso, PIMA operates the voluntary QualityMark program for participating manufacturers. PIMA states that verification includes independent facility visits, random board selection, approved laboratories, and third-party administration. That process verifies published LTTR values for participating products; it does not create one R-value for all polyiso. See PIMA’s description of LTTR measurement and QualityMark verification.

When two data sheets appear to conflict, first determine whether they report the same metric. Compare:

• Exact product and board construction
• Facer permeability
• Thickness
• Initial, conditioned, or LTTR basis
• Test method
• Mean temperature
• Publication date
• Intended application

An apparent disagreement may disappear once unlike measurements are separated.

How temperature can change polyiso performance

A standard or label rating does not necessarily describe every installed temperature condition. Polyiso has received particular attention because historical testing found reduced thermal performance in the roof boards tested both below freezing and at high temperatures associated with solar-heated roofs.

A Building Science Corporation article published in 2013 discussed testing by BSC and the National Roofing Contractors Association. It reported cold- and high-temperature performance reductions in the tested boards while emphasizing substantial variation among samples and manufacturers. Those results do not justify assigning every modern polyiso product one fixed winter value. Review BSC’s historical temperature-dependent polyiso testing.

Test-temperature context is important. The label-testing discussion used a mean temperature of 75°F, commonly produced with a 50°F cold side and a 100°F warm side. A winter roof may experience a substantially different temperature profile, helping explain why a value established under standard conditions may not precisely predict cold-weather performance.

Historical NRCA roof-design guidance used these assumptions:

• R-5.0 per inch for predominantly heating conditions
• R-5.6 per inch for predominantly cooling conditions

These are dated, roof-specific planning assumptions—not current universal ratings, automatic cold-weather penalties, or code requirements. The NRCA discussion also recommended specifying roof polyiso by desired thickness rather than assuming published LTTR represented every service condition. See the 2014 NRCA discussion of aging and climate-dependent design values.

PIMA reported the following average LTTR results from its 2015 QualityMark testing of permeable-faced roofing products:

Tested board thickness	Reported average LTTR per inch
1 inch	R-5.78
2 inches	R-5.74
3 inches	R-5.85
4 inches	R-5.95

PIMA reported that the program tested 33 samples at each listed thickness. These historical averages were higher than the NRCA R-5.0-per-inch heating-condition assumption, but the figures address different questions. Certified LTTR is an aging-based standardized rating; the NRCA figure was conservative service-design guidance for roofs. Review PIMA’s 2015 QualityMark results and qualifications.

It is more useful to treat the difference as a distinction between rating methodology and service-design judgment than as a dispute over which number is the single “real” R-value. Neither figure calculates the performance of every roof in every climate.

For a current project:

1. Start with the current product-specific rating.
2. Identify whether it is LTTR, a conditioned value, or another rating.
3. Review the manufacturer’s service-temperature information.
4. Check the roof-system requirements and design documents.
5. Obtain project-specific analysis where unusually cold or hot conditions are important.

Historical cold-weather evidence is a warning against blind reliance on a standard-temperature label. It is not evidence that every current board falls to one precise R-value in winter.

Calculating polyiso thickness for R-20, R-30, and R-40

For preliminary sizing, use:

Required thickness = target R-value ÷ selected R-value per inch

For flat, homogeneous insulation with parallel surfaces, R-value increases proportionally with thickness when thermal conductivity remains constant. Product-specific data must still establish the appropriate value to use. See the general calculation method for flat insulation.

The multiplier must be disclosed. Saying “R-30 requires about five inches” is incomplete unless the calculation identifies the assumed rating. The result changes materially depending on whether the basis is R-6.2, R-5.8, or R-5.0 per inch.

Target	At R-6.2 per inch	At R-5.8 per inch	At R-5.0 per inch
R-20	3.23 inches	3.45 inches	4.00 inches
R-30	4.84 inches	5.17 inches	6.00 inches
R-40	6.45 inches	6.90 inches	8.00 inches

The first basis reflects the published Enerfoil example; R-5.8 is an illustrative assumption near the middle of PIMA’s historical QualityMark averages; and R-5.0 is the dated NRCA heating-condition assumption described above. These are calculation bases, not interchangeable product ratings.

Product-specific R-6.2 example

Using IKO’s published R-6.2-per-inch Enerfoil value:

• R-20 ÷ 6.2 = 3.23 inches
• R-30 ÷ 6.2 = 4.84 inches
• R-40 ÷ 6.2 = 6.45 inches

These are mathematical estimates based on a particular foil-faced product. They are not recommended board schedules and should not be transferred automatically to permeable-faced roofing insulation. IKO publishes Enerfoil at R-6.2 per inch for the listed schedule.

Illustrative roofing LTTR assumption of R-5.8 per inch

Using R-5.8 per inch as an illustrative roofing LTTR assumption:

• R-20 ÷ 5.8 = 3.45 inches
• R-30 ÷ 5.8 = 5.17 inches
• R-40 ÷ 5.8 = 6.90 inches

This is close to the middle of PIMA’s historical 2015 averages, but it remains a planning assumption. The current data sheet for the selected roofing board should determine the final calculation.

Historical heating-condition assumption of R-5.0 per inch

Using the historical NRCA heating-condition assumption:

• R-20 ÷ 5.0 = 4 inches
• R-30 ÷ 5.0 = 6 inches
• R-40 ÷ 5.0 = 8 inches

The difference illustrates why an undisclosed multiplier can cause ordering errors. For R-30, the calculated difference between R-6.2 and R-5.0 per inch is more than one inch.

Do not automatically round a calculated thickness down. If the calculation produces 4.84 inches, selecting 4.5 inches for convenience could leave the nominal total below R-30. Instead:

1. Calculate the minimum planning thickness.
2. Identify available board thicknesses.
3. Build a practical single- or multilayer configuration.
4. Check the published total R-value of every layer.
5. Confirm that the sum meets or exceeds the target.
6. Verify the configuration against project requirements.

Actual roof configurations may include flat boards, tapered insulation, crickets, saddles, sumps, cover boards, and existing insulation.

PIMA’s commercial-roof guidance presents example configurations from R-20 through R-40 and notes that typical colder-zone requirements may exceed R-40. It also explains that an equivalent U-factor approach may be used and directs readers to manufacturers for product-specific thickness relationships. These are industry-guidance examples, not universal legal requirements. The controlling value depends on the applicable code edition, climate zone, jurisdiction, approved design path, and project documents. Review PIMA’s qualified commercial-roof thickness guidance.

Board availability also changes the answer. A calculated requirement of 5.17 inches does not mean that one board of that thickness exists or suits the assembly. The specified configuration may instead use two or more available layers whose labeled values meet the target.

Keep two figures separate:

• Planning thickness: Derived from an explicitly stated per-inch assumption.
• Specified configuration: Based on actual available boards and their current published totals.

The first compares options. The second determines what is ordered and installed.

Roofing board, foil-faced wall board, and cover board values

“Polyiso” describes a material family, not one interchangeable board. Facer, density, compressive strength, intended use, conditioning, and test basis can change the relevant rating.

Permeable-faced roofing polyiso

Standard commercial roofing polyiso commonly has permeable facers and is frequently rated using LTTR. PIMA’s 2015 QualityMark results averaged approximately R-5.74 to R-5.95 per inch across the four thicknesses tested.

That range is historical program data for the tested products—not a guarantee that every roofing board currently sold falls within it. Current roofing data sheets may list total LTTR by thickness rather than one universal per-inch figure, especially where the per-inch value varies slightly with board thickness.

Foil-faced wall polyiso

Foil-faced or otherwise gas-tight products use different construction and may follow a different conditioning or reporting basis. PIMA has described foil-faced wall polyiso as typically R-6 per inch or greater while warning that impermeable-facer values should not be treated as equivalent to permeable-faced roofing-board LTTR.

IKO Enerfoil is one product-specific example, with a published schedule equivalent to R-6.2 per inch for the listed thicknesses. It is not a proxy for roofing polyiso generally.

Facers also affect how a board fits into the proposed air-, vapor-, and water-control strategy. A thermal rating alone does not establish compatibility with a complete wall or roof design.

High-density polyiso cover boards

High-density polyiso cover boards are a distinct product category. They provide a protective layer in a roof system while contributing some thermal resistance. Their density, compressive properties, facers, thicknesses, and intended function differ from ordinary insulation boards.

PIMA reports R-2.5 for a typical 1/2-inch high-density polyiso cover board and notes that products are available with different facers, compressive strengths, and board sizes. The exact manufacturer’s data must control. See PIMA’s technical bulletin on high-density polyiso cover boards.

Do not assign that R-2.5 figure to:

• Every high-density cover board
• An ordinary 1/2-inch roof-insulation board
• A 1/2-inch foil-faced wall board
• An unidentified polyiso offcut
• A product whose current data sheet lists another value

The contrast with the nominal chart is instructive. A standard 1/2-inch board estimated at R-6 to R-6.5 per inch would be roughly R-3 to R-3.25, while the typical high-density cover-board example is R-2.5. Both can be polyiso, but they are different product categories.

Before using any value, verify:

• Manufacturer and exact product name
• Roof, wall, or cover-board application
• Facer type
• Nominal and specified thickness
• Published total R-value
• LTTR or other conditioning basis
• Test method
• Compressive strength where relevant
• Current product approvals and system requirements

The rule is simple: match the number to the board, not merely to the word “polyiso.”

From board R-value to a complete roof assembly

Adding labeled insulation values produces a nominal component total. It does not necessarily produce the effective R-value of the complete roof.

Suppose insulation layers have published values totaling R-27.5 and are combined with a typical 1/2-inch high-density polyiso cover board rated R-2.5:

R-27.5 + R-2.5 = R-30 nominal component total

That arithmetic is useful for a material schedule. It is not a whole-roof thermal model.

PIMA’s commercial low-slope-roof guidance recommends two or more polyiso layers with staggered joints in the assemblies it discusses. The purpose is to improve insulation continuity so that material in one layer covers joints in another. See PIMA’s multilayer commercial-roof guidance.

Staggering joints does not increase the arithmetic sum of the labels. Two R-15 layers still total R-30 nominally whether their joints align or not.

Factors separating nominal component totals from effective roof performance include:

• Gaps or poorly fitted joints
• Mechanical fasteners and metal plates
• Penetrations and curbs
• Roof drains and sumps
• Deck and framing geometry
• Air leakage
• Moisture exposure
• Damaged or displaced boards
• Existing materials of uncertain condition
• Installation quality

Metal fasteners can conduct heat more readily than surrounding foam. An adhesively installed high-density cover board above mechanically fastened insulation may cover the fasteners and reduce that particular bridging path. The magnitude of any whole-roof improvement depends on the complete fastening pattern and assembly, so it cannot be inferred from the cover board’s material R-value alone.

Moisture and air control require separate attention.

PIMA states that high-density polyiso cover boards can contribute thermal resistance toward roof configurations, but whether a particular rating is recognized for compliance must be established from the applicable code path, approved assembly, jurisdiction, and project documents—not assumed from the board’s material alone.

For estimating, keep three totals separate:

1. Labeled insulation total: The sum of published board values.
2. Compliance value: The value accepted under the applicable approved design path.
3. Effective assembly performance: Performance after assembly-specific thermal bridges and installation effects are considered.

These values may be similar, but they are not automatically identical.

A verification checklist before selecting thickness

Before ordering polyiso, work through the following checklist.

Product identity

• [ ] Exact manufacturer
• [ ] Exact product name and model
• [ ] Current technical data sheet
• [ ] Data-sheet date or revision
• [ ] Intended roof, wall, or cover-board application
• [ ] Standard-density or high-density construction
• [ ] Facer type and permeability
• [ ] Relevant product or assembly approvals

Dimensions and thermal rating

• [ ] Nominal board thickness
• [ ] Actual or specified thickness
• [ ] Available stock thicknesses
• [ ] Published total R-value for each thickness
• [ ] Published RSI where required
• [ ] Manufacturer’s per-inch figure, if provided
• [ ] Initial, conditioned, or LTTR basis
• [ ] Applicable ASTM or other test method
• [ ] Test or rating temperature where stated
• [ ] Whether the per-inch value changes with thickness

Service conditions

• [ ] Climate and expected seasonal temperatures
• [ ] Roof color and potential solar heating
• [ ] Moisture and bulk-water exposure
• [ ] Air-barrier continuity
• [ ] Water-control detailing
• [ ] Vapor-control strategy
• [ ] Board joints and layer staggering
• [ ] Fasteners, plates, and thermal bridges
• [ ] Curbs, drains, penetrations, and transitions
• [ ] Existing insulation and its condition
• [ ] Tapered insulation and minimum-thickness locations

Project requirements

• [ ] Applicable code edition
• [ ] Climate zone
• [ ] Jurisdiction
• [ ] Prescriptive R-value or approved U-factor path
• [ ] Project drawings and specifications
• [ ] Approved roof or wall assembly
• [ ] Manufacturer installation instructions
• [ ] Roof-system or warranty requirements
• [ ] Fire, wind, and other assembly requirements
• [ ] Confirmation from the responsible professional or authority where needed

Commercial-roof examples from R-20 through R-40 help illustrate possible configurations, but they do not establish the legal requirement for a particular building. PIMA’s guidance itself qualifies such ranges by climate and recognizes that requirements and approved paths vary. Final compliance must be established from the edition and documents governing the project.

When comparing insulation products, compare like with like:

• Equal thickness
• Equivalent aging basis
• Comparable mean temperature
• Equivalent facer category
• Same intended application
• Comparable moisture exposure
• Same material or assembly level

A fresh nominal value from one product should not be compared directly with an LTTR value from another without acknowledging the difference. Likewise, a foil-faced wall-board figure should not be treated as interchangeable with a permeable-faced roofing-board LTTR.

For the final material schedule, use the current published total for the selected board configuration. If the target is R-30 and the chosen layers total only R-29.6 according to current data, do not relabel them as R-30 because a generic multiplier produced a higher estimate.

Approximately R-6 to R-6.5 per inch remains a useful first-pass answer. Reliable selection requires three steps:

1. Identify the exact polyiso product and intended application.
2. Use its current published total R-value or LTTR under the relevant rating basis.
3. Verify the resulting assembly against current project documents and local requirements.

Temperature, aging, facers, joints, fasteners, moisture, and installation all separate a board’s label from whole-assembly performance.

Mortar Desk is an independent general-reference site, not a contractor or engineering adviser. Specifications change and codes are local, so project decisions should be checked against current product documents, the edition used by the local authority, and qualified professionals responsible for the work.

Frequently asked questions

Is all polyiso insulation R-6.5 per inch?

No. R-6.5 per inch is a commonly quoted nominal figure, not a universal rating.

Depending on the product and reporting basis, polyiso may be listed around R-6 to R-6.5 per inch, around the historical R-5.7 to R-5.9-per-inch range for tested permeable-faced roofing products, or at another product-specific value. Facer, thickness, formulation, age, temperature, test method, and application can all affect which figure is relevant. PIMA explains why polyiso values must be compared under equivalent conditions.

Use R-6.5 only when current documentation for the exact product and thickness supports it.

What is the R-value of 2 inches of polyiso?

As a rough nominal estimate, 2 inches of polyiso is approximately R-12 to R-13 under the common R-6 to R-6.5-per-inch planning range. Published general charts qualify that range by temperature and product conditions. See the supporting 2-inch nominal range.

A specific product may differ. IKO Enerfoil, for example, is published at R-12.4 for 2 inches, while permeable-faced roofing products may have different LTTR totals. For ordering, use the current published value for the exact 2-inch board.

Should I use LTTR or nominal R-value when calculating roof insulation?

For permeable-faced polyiso roofing insulation, use the rating required by the current product documentation, approved design path, and project specifications. In many cases, that will be the manufacturer’s LTTR value.

Do not substitute an initial R-value for LTTR without justification. LTTR is intended to account for aging, while initial and 180-day conditioned values represent different measurements. LTTR still does not describe every service temperature or whole-roof thermal bridge.

How much polyiso is needed for R-30?

The examples in this article produce approximately:

• 4.84 inches at R-6.2 per inch
• 5.17 inches at R-5.8 per inch
• 6 inches at R-5.0 per inch

For flat insulation, R-value equals thickness divided by thermal conductivity HOW TO CALCULATE R-VALUES OF INSULATION.

Do not round down automatically. Select available boards whose current published totals meet or exceed R-30, then verify the complete configuration against the project requirements.

Does a 1/2-inch polyiso cover board add R-value?

Yes. A high-density polyiso cover board can contribute thermal resistance. PIMA reports R-2.5 for a typical 1/2-inch high-density polyiso cover board, while directing users to the selected manufacturer’s data for exact product performance.

The R-2.5 figure is not universal and should not be assigned to every 1/2-inch polyiso board. Whether it can be counted toward compliance must be established from the applicable approved assembly, project documents, and local requirements.