How Much Thermal Resistance Does Polyiso Actually Provide?

By the Mortar Desk editorial team · Updated August 4, 2026

Editorial note: This source-checked guide synthesizes manufacturer data, trade-association guidance, and independent technical research. It is preliminary specification and procurement information, not project engineering or code approval.

The short answer: polyiso is commonly about R-6 to R-6.5 per inch

Polyisocyanurate insulation—usually shortened to polyiso—is commonly reported at approximately R-6 to R-6.5 per inch under specified laboratory or product-rating conditions. For early comparison before a product has been selected, R-6 per inch is a reasonable preliminary estimate.

That estimate is not a universal specification.

R-value measures resistance to heat flow. A higher R-value indicates greater thermal resistance, meaning heat moves through the material more slowly under the stated conditions. Total board R-value generally increases with thickness, but the correct value depends on the exact board and the basis used to report its performance.

Manufacturer-published examples illustrate the range:

• IKO lists its one-inch, foil-faced Enerfoil product at R-6.2 in its Enerfoil R-value chart.
• Rmax claims R-6.5 per inch for polyiso in its manufacturer overview of polyiso performance.
• Hunter Panels describes approximately R-6 to R-6.5 per inch as an initial-value range that varies by manufacturer and product type in its explanation of R-value and LTTR.

These are attributed manufacturer examples, not proof that every one-inch polyiso board has the same rating. Product category, facer, thickness, test conditions, and reporting basis all matter.

The applicable R-value can vary with:

• Product formulation and category
• Board thickness
• Facer material
• Initial, aged, labeled, or LTTR reporting basis
• Test method and mean test temperature
• Service temperature
• Moisture conditions
• Joint, penetration, fastening, and assembly details

Use R-6 per inch only for rough comparison and early estimating. For purchasing, bid preparation, code documentation, or final design, use the current technical data sheet and approved submittal for the exact product and thickness being supplied. Confirm that the rating basis applies to the intended roof or wall application.

Polyiso R-value by thickness: product ratings and LTTR examples

A generic “polyiso R-value by thickness” table can be misleading because different tables may describe different products and reporting procedures. The tables below are deliberately separated: the first contains product-specific values for one foil-faced board, while the second presents LTTR examples for applicable roof insulation.

IKO Enerfoil product-specific example

The following values are manufacturer-published figures for IKO Enerfoil foil-faced polyiso. IKO attributes them to ASTM C1289 conditioning and ASTM C518 testing and qualifies them by requiring joints and penetrations to be detailed appropriately. IKO also says its webpage data are based on periodic internal testing and production measurements at the time of manufacture, so the current product data sheet and approved submittal should control procurement.

Enerfoil thickness	Published R-value	Effective R-value per inch
0.5 in.	R-3.1	R-6.2/in.
0.625 in.	R-3.9	R-6.24/in.
0.75 in.	R-4.5	R-6.0/in.
1 in.	R-6.2	R-6.2/in.
1.5 in.	R-9.3	R-6.2/in.
2 in.	R-12.4	R-6.2/in.
2.5 in.	R-15.5	R-6.2/in.
3 in.	R-18.6	R-6.2/in.
3.5 in.	R-21.7	R-6.2/in.
4 in.	R-24.8	R-6.2/in.

These figures come from IKO’s product-specific thickness and R-value table; they are not generic ratings for every polyiso board.

The thinner entries show why looking up an actual thickness is preferable to multiplying a rounded number. A three-quarter-inch board is listed at R-4.5 rather than the R-4.65 that strict multiplication by R-6.2 would produce. The documented product table—not an independently rounded calculation—should control.

LTTR examples for applicable roof insulation

A GAF-hosted technical review reports the following PIMA QualityMark minimum LTTR examples for applicable polyiso roof insulation:

Board thickness	Example LTTR	Effective LTTR per inch
1 in.	R-5.6	R-5.6/in.
2 in.	R-11.4	R-5.7/in.
3 in.	R-17.4	R-5.8/in.
4 in.	R-23.6	R-5.9/in.

The reported values and their increasing effective per-inch rates appear in GAF’s manufacturer-authored technical review of polyiso testing and LTTR.

These two tables should not be merged. The Enerfoil figures describe one foil-faced product under its stated conditioning and testing basis. The LTTR table presents longer-term roof-insulation examples under a different reporting framework. Neither should automatically be substituted for a wall board, high-density cover board, composite panel, tapered product, or another manufacturer’s roof insulation.

If the selected product lists a two-inch board at R-12.4, two such layers have a nominal board total of R-24.8. That arithmetic does not confirm product compatibility, fastening, adhesive selection, facer suitability, joint treatment, or whole-assembly performance. Verify the proposed combination against current product literature and project requirements.

How many inches of polyiso are needed for R-20, R-30, or R-40?

The basic planning formula is:

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

The arithmetic is simple; selecting and disclosing the correct assumption is the important part.

Target	At R-6.2/in. product example	At R-5.6/in. planning assumption	At R-5.0/in. older cold-climate assumption
R-20	3.23 in.	3.57 in.	4 in.
R-30	4.84 in.	5.36 in.	6 in.
R-40	6.45 in.	7.14 in.	8 in.

The first column uses the cited one-inch Enerfoil product example. The second uses the one-inch LTTR rate reported in the cited GAF technical review. The third uses an older conservative assumption from Building Science Corporation guidance discussed below. These are theoretical calculations before selection of available board thicknesses.

Assumption 1: R-6.2 per inch

Using the R-6.2-per-inch Enerfoil example from the product table above:

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

These results are mathematical estimates, not available board selections. A calculated thickness of 4.84 inches does not mean a 4.84-inch board exists or that every 4.84-inch polyiso configuration delivers R-30.

The next step is to round up to available thicknesses or choose a multilayer arrangement, then total the documented ratings for those boards. For example, two two-inch Enerfoil layers have a nominal sum of R-24.8 because each documented layer is R-12.4. That total remains subject to appropriate treatment of joints, penetrations, and other assembly details.

Assumption 2: R-5.6 per inch

Using the one-inch LTTR example of R-5.6 reported in the GAF technical review:

• R-20 ÷ 5.6 = 3.57 inches
• R-30 ÷ 5.6 = 5.36 inches
• R-40 ÷ 5.6 = 7.14 inches

R-5.6 per inch also appears in older design discussions, but it is not a universal value for every polyiso product or thickness. In the cited LTTR table, thicker boards have different effective rates, ranging from R-5.7 to R-5.9 per inch. Once a product is known, use its thickness-specific table rather than extending the one-inch rate indefinitely.

Assumption 3: R-5.0 per inch

Using R-5.0 per inch gives:

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

Building Science Corporation summarized R-5.6 and R-5.0 per inch as warm- and cold-climate design assumptions in guidance dated 2013. The same guidance reported variation by manufacturer and service temperature. These figures are dated, conservative assumptions—not current universal requirements or code rules that supersede present product documentation. See Building Science Corporation’s temperature-dependent polyiso guidance.

Turn the calculation into an orderable configuration

After calculating a theoretical thickness:

1. Identify the rating basis used for the estimate.
2. Obtain current literature for the exact product.
3. Check available board thicknesses.
4. Select a board or multilayer combination that meets or exceeds the target.
5. Add the documented values for the selected layers.
6. Confirm product, facer, attachment, adhesive, and assembly compatibility.
7. Check the result against the project specification and applicable compliance method.

Do not round to the nearest available thickness if doing so would put the documented total below the target. Round up or revise the layer arrangement.

If using an external or future polyiso calculator, require it to display the selected assumption—such as R-6.2, R-5.6, or R-5.0 per inch—beside every result. Calculator output is appropriate for preliminary estimating; it does not establish code compliance or create a project specification.

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

Different polyiso articles and data sheets can report different figures without necessarily contradicting one another. They may describe different products, facers, thicknesses, test conditions, or points in the material’s aging process.

Initial or fresh R-value

An initial R-value, sometimes called a fresh R-value, describes thermal resistance measured near the beginning of a foam product’s life under stated test conditions. It can characterize newly manufactured material, but it does not automatically describe long-term or field performance.

Polyiso is a gas-filled, closed-cell foam. Gas within the cells contributes materially to thermal resistance. Over time, some blowing agent can diffuse out while air enters the cells, generally reducing performance from its initial level.

Long-Term Thermal Resistance

LTTR means Long-Term Thermal Resistance. It is a reporting method intended to approximate longer-term performance rather than stating only an initial result. The cited technical literature describes LTTR for applicable roof insulation as a 15-year time-weighted average.

Rmax states that PIMA member manufacturers use LTTR for permeable-faced polyiso roof insulation. It also describes the PIMA QualityMark program as a voluntary, independent third-party certification program administered by FM Global. Those details and that roof-specific scope appear in Rmax’s explanation of LTTR and QualityMark.

That scope matters. An LTTR value for an applicable roof-insulation product should not automatically be assigned to:

• A foil-faced wall board
• A different manufacturer’s board
• A high-density cover board
• A composite panel
• A tapered board at an unspecified thickness
• Any product whose data sheet uses another reporting method

The 180-day aged approach

A 180-day aged R-value is a separate reporting basis. It states resistance after a sample has undergone the specified conditioning period. It is not another name for LTTR, and it is not necessarily equal to an initial value or a measured field result.

The Polyisocyanurate Insulation Manufacturers Association explains that reported performance can depend on age, temperature, installation, testing standard, product design, and facing material. Its overview of polyiso R-value reporting distinguishes LTTR from the 180-day aged procedure and advises using product-specific data.

A practical data-sheet rule

Before copying an R-value into a comparison or calculation, identify what the number represents:

• Initial or fresh R-value
• Labeled R-value
• LTTR
• Aged value after a stated conditioning period
• Thickness-specific product rating
• Laboratory result at a stated mean temperature
• Field or assembly estimate

Compare products only when the relevant conditions are equivalent. An initial value from one wall board and an LTTR value from a permeable-faced roof board do not form an apples-to-apples comparison merely because both are expressed as R-value per inch.

It should not be treated as interchangeable with a center-of-board laboratory rating.

Temperature and the gap between label ratings and in-service performance

Polyiso can show lower thermal performance under some cold conditions, but the evidence does not support one fixed penalty for every current product, thickness, manufacturer, or climate.

What a 75°F mean test temperature means

Testing discussed by Building Science Corporation commonly uses a 75°F mean temperature with a 50°F temperature difference. Mean temperature is the average of the warm- and cold-side temperatures across the sample; it is not outdoor air temperature.

For example, a test with a 50°F cold side and a 100°F warm side has a 75°F mean. Its reported result averages performance across that temperature gradient. It does not reveal the material’s thermal resistance at every individual temperature between the two surfaces.

That distinction limits simplistic conversions. A board rated using a 75°F mean-temperature test cannot automatically be assigned a precise different R-value merely because the forecast outdoor temperature is 32°F, 25°F, or 0°F. Temperatures at the insulation’s surfaces and through its thickness depend on the complete assembly and indoor and outdoor conditions.

What older cold-temperature testing found

Building Science Corporation’s 2013 guidance summarized testing by BSC and the National Roofing Contractors Association. Samples from multiple manufacturers were tested, results varied among products, and performance in the cited programs decreased as mean temperature moved away from 75°F. BSC also cautioned that the relationship appeared nonlinear, limiting simple extrapolation.

Those results support a general conclusion: some tested polyiso products performed below their labeled or expected values under cold conditions. They do not establish a universal percentage loss.

The same source proposed R-5.6 per inch for warm-climate design and R-5.0 per inch for cold climates. These values can be used as sensitivity assumptions to see how an estimate changes, but they remain older conservative guidance. Current product documentation, project specifications, and jurisdictional requirements must be checked before either value is adopted.

A GAF-hosted, manufacturer-authored technical paper acknowledged findings of unexpectedly low apparent R-values in cold testing but advised designers to continue using labeled values while low-temperature behavior was being characterized more fully. That manufacturer position illustrates the competing considerations: conservative assumptions can reduce the risk of overestimating cold performance, while labeled values remain the declared basis for an identified product.

Field-aged samples show variability, not a universal conversion

Research conducted by ROCKWOOL Building Science and RDH reported approximately R-3.5 to R-6.6 per inch across a small set of field-aged polyiso samples removed from buildings. The samples differed in age, thickness, facer, density, manufacturer, and service history. ROCKWOOL manufactures competing insulation, so that commercial context should also be considered. The field-aged polyiso research summary demonstrates potentially substantial variability; it does not establish a representative range for all polyiso.

Blowing-agent chemistry and cell-gas behavior have been proposed as explanations for temperature-dependent results. The available evidence does not establish one mechanism as the sole cause of every observed difference.

The practical conclusion is narrow but important: do not apply an automatic generic cold-weather deduction. For a cold-climate roof or wall, seek current product-specific temperature data where available and use the analysis method required by the project. Where the consequences are significant, have the complete assembly evaluated by a qualified building-envelope professional.

Standard insulation boards and high-density cover boards need separate treatment

“Polyiso board” can refer to several product classes with different functions and reporting conventions:

• Standard roof insulation
• Wall sheathing
• Foil-faced boards
• Permeable-faced roof boards
• Tapered roof insulation
• High-density cover boards
• Composite or specialty boards

Their R-values should not be blended into one generic chart.

A half-inch standard foil-faced board is not necessarily rated the same as a half-inch high-density roof cover board. Even when nominal R-values are similar, products can differ in density, facer, compressive strength, dimensions, intended position, and installation requirements.

High-density cover-board example

A typical 0.5-inch high-density polyiso cover board is reported at approximately R-2.5. That figure applies to the cited high-density cover-board class; it is not a universal rating for every half-inch polyiso product.

A high-density cover board can provide roof-system protection while adding thermal resistance. When installed above mechanically fastened insulation in an appropriate assembly, it may place a thermal layer over the fasteners and reduce some fastener-related thermal bridging. The PIMA cover-board bulletin hosted by Siplast describes the typical R-2.5 value and this assembly concept.

That does not mean all fastener effects disappear. A complete thermal analysis is needed to quantify the result.

Manufacturers offer high-density cover boards with different facers, compressive strengths, dimensions, and installation instructions. Confirm the exact product’s rating rather than entering R-2.5 automatically.

Tapered polyiso

A single generic total can conceal the difference between minimum, average, and maximum thickness.

Do not calculate a tapered system by applying one nominal thickness everywhere. Use the tapered layout, layer schedule, manufacturer’s documented values, and the project’s specified method for determining thermal performance. The available evidence does not support a universal tapered-board correction or averaging procedure.

Board R-value is not the same as whole-assembly performance

A board rating describes a tested insulation product under stated conditions. It does not, by itself, describe heat flow through the complete roof or wall.

Heat can bypass or partially bypass insulation through:

• Metal or wood framing
• Mechanical fasteners
• Aligned joints between boards
• Gaps and damaged edges
• Roof or wall penetrations
• Clips, girts, and attachment systems
• Perimeters and transitions
• Other conductive components

These paths are thermal bridges. Their impact depends on material conductivity, size, spacing, continuity, and location. There is no support for one generic percentage that can be deducted from every polyiso board total.

Why multiple layers and staggered joints matter

PIMA recommends multiple polyiso roof-insulation layers with staggered joints and reports that roof boards are commonly available in thicknesses from 0.5 to 4.5 inches, allowing different thicknesses to be combined. Its recommended polyiso thickness bulletin also directs users to manufacturers for product-specific thicknesses and system R-values.

Fasteners, transitions, deck conditions, and other components remain.

For the Enerfoil values discussed earlier, IKO specifically conditions its figures on joints and penetrations being detailed appropriately. That qualification should not be overlooked when using its published table.

Other installed-performance factors

Nominal board R-value can remain unchanged while assembly performance changes because of:

• Air leakage: Moving air can carry heat around insulation instead of through its center.
• Moisture: Wet materials and moisture pathways can alter thermal behavior and durability.
• Facer selection: Facers affect compatibility, vapor behavior, attachment, and product classification.
• Layer placement: The same nominal insulation total can behave differently depending on where conductive components cross it.
• Joint quality: Open, damaged, or aligned joints weaken thermal continuity.
• Compatibility: Membranes, adhesives, fasteners, substrates, and adjacent components must work as the specified system.
• Workmanship: Gaps, misplaced boards, and poorly detailed penetrations can reduce installed effectiveness.

Simply multiplying thickness by R-6 cannot quantify these interactions. Where whole-assembly performance controls compliance or equipment sizing, use the required assembly calculation, approved modeling method, project specification, or professional analysis rather than inventing a correction factor.

A verification checklist before specifying or ordering polyiso

Use the following workflow to turn a preliminary estimate into a defensible product selection.

1. Identify the application

Determine whether the board is intended for:

• Low-slope roof insulation
• Exterior wall continuous insulation
• Residential wall sheathing
• Tapered roof insulation
• High-density cover-board service
• Another listed use

Do not compare a roof cover board directly with standard wall sheathing merely because both contain polyiso.

2. Select the exact product

Record the manufacturer, product name, intended application, and current data-sheet revision. Generic descriptions such as “two-inch polyiso” are not sufficient for final ordering.

Confirm that the quoted product is the same one used in the calculation. A proposed substitute may have a different facer, thickness range, reporting basis, or R-value.

3. Record thickness and facer

Write down:

• Actual board thickness
• Facer type on each side
• Board dimensions
• Edge configuration, if relevant
• Density or product class where stated
• Whether the product is flat, tapered, composite, or high-density

Facers are not cosmetic labels. They can distinguish product categories and affect where and how a board is used.

4. Identify the R-value basis

Determine whether the data sheet reports:

• Initial R-value
• Labeled R-value
• LTTR
• A 180-day aged value
• Another aged value
• A thickness-specific rating
• An assembly result

Do not convert one basis into another unless the manufacturer or applicable method expressly supports that conversion.

5. Check the test and temperature basis

Record the cited test standard, conditioning method, mean temperature, temperature difference, and qualifications attached to the rating.

A result generated at a stated mean test temperature is not automatically a prediction for every service temperature. If cold-condition performance matters, request current temperature-specific information for the selected product instead of applying an unsupported universal penalty.

6. Compare equivalent products

For a meaningful comparison, hold these variables as constant as possible:

• Product category
• Thickness
• Facer
• Aging method
• Test method
• Mean temperature
• Reporting units
• Intended application

If one manufacturer publishes LTTR and another publishes an initial value, the larger number does not necessarily identify the better long-term option. First establish whether the figures measure the same thing.

7. Confirm the multilayer total

List every insulation and cover-board layer separately. Record each layer’s documented R-value, then show the nominal sum.

Also verify:

• Whether the selected products may be combined
• Whether joints must be staggered
• Required fastening or adhesive patterns
• Facer compatibility
• Maximum and minimum approved thicknesses
• Requirements for joints, edges, and penetrations
• Whether the cover board has a separately documented R-value

Do not rely solely on “total inches × R-value per inch” once the actual board schedule is available.

8. Establish the project requirement

Before treating R-20, R-30, R-40, or another target as mandatory, identify:

• Project jurisdiction
• Adopted code edition
• Local amendments
• Climate zone
• Roof or wall assembly type
• New construction, alteration, or replacement scope
• Prescriptive or performance compliance pathway
• Project drawings and specifications

Industry summaries may report broad insulation requirement ranges, but those summaries are not authoritative code text and do not establish what a particular jurisdiction accepts.

9. Ask the supplier specific questions

Before issuing an order, ask:

1. Is the quoted board the exact manufacturer and product used in the calculation?
2. What thicknesses and board sizes are currently available?
3. Which facer is supplied?
4. Is the quoted R-value initial, aged, labeled, or LTTR?
5. What is the documented R-value for each ordered thickness?
6. What is the documented nominal total for the multilayer configuration?
7. Are the layers, facers, adhesive, fasteners, membrane, and substrate compatible?
8. What joint-staggering and penetration details are required?
9. Does current third-party certification apply to this product and manufacturing source?
10. Could substitutions, minimum quantities, lead times, or thickness tolerances change the schedule?

Keep the approved submittal, current data sheet, supplier quotation, and board schedule together. This reduces the risk that the calculated product and delivered product differ.

10. State the estimate’s limitation

This guide is preliminary reference and procurement information. Its stated scope and professional-advice limitation notes that specifications change and codes are local.

Use approximately R-6 per inch only while screening options. Once a product is selected, replace the generic multiplier with the applicable documented rating and verify the complete configuration against project requirements.

Frequently asked questions

What is the R-value of one inch of polyiso insulation?

One inch of polyiso is commonly described as approximately R-6 to R-6.5, but the exact figure is product-specific. For example, the cited Enerfoil table lists one inch at R-6.2, while other manufacturers may publish different initial, labeled, aged, or LTTR values.

For rough estimating, use approximately R-6 per inch and label it as an assumption. For ordering, design, and compliance documentation, use the current data sheet for the exact one-inch product.

How thick does polyiso need to be for R-30?

As shown in the cited calculation section above, the preliminary answer depends on the selected rating basis:

• At R-6.2 per inch: 30 ÷ 6.2 = 4.84 inches
• At R-5.6 per inch: 30 ÷ 5.6 = 5.36 inches
• At R-5.0 per inch: 30 ÷ 5.0 = 6 inches

These are calculated thicknesses, not necessarily available configurations. Round up to an available board or multilayer arrangement, then recalculate the total using each selected board’s documented rating.

Is LTTR lower than the initial R-value of polyiso?

LTTR is intended to account for changes from a foam product’s initial thermal resistance as it ages, so it may be lower than a fresh or initial value. It is a different reporting basis intended to represent longer-term performance.

The difference is not a fixed universal amount. Product formulation, facer, thickness, conditioning, and reporting method matter. Confirm whether each number is initial, aged, labeled, or LTTR before comparing products.

Does polyiso insulation lose R-value below freezing?

Testing summarized by Building Science Corporation and NRCA found lower thermal performance for some polyiso products under cold conditions. Results varied among samples and manufacturers, so the evidence does not establish one penalty for every current product.

Do not assume that outdoor temperature alone determines board R-value or deduct a fixed percentage at freezing. Laboratory results reflect temperatures across the sample, while actual insulation temperatures depend on the complete assembly. Cold-climate projects should use current product-specific information and the analysis method required by the project.

Can a polyiso cover board count toward a roof assembly’s R-value?

A high-density polyiso cover board adds thermal resistance when the selected product has a documented rating. A typical half-inch high-density polyiso cover board is reported at approximately R-2.5, according to the cited PIMA cover-board bulletin.

Whether and how that layer may be used in a project’s compliance calculation depends on the adopted requirements, assembly, and approved compliance method. Include the cover board as a separate line in the layer schedule, confirm its exact rating and placement, and verify its treatment with the project authority rather than assuming every half-inch polyiso board is R-2.5.

The decision rule is straightforward: use roughly R-6 per inch only for early estimating, state the rating basis used in every calculation, round up to an available board configuration, and verify the resulting total against the exact product data sheet and applicable project requirements. Climate, aging basis, product class, joints, fasteners, and other assembly details can make a generic per-inch value unsuitable for final specification.