What is the R-value of fibreglass insulation?

About R-3.14 per inch for standard batts and R-3.8 for high-density batts. A 3.5 in batt gives R-13 and a 5.5 in batt gives R-21 in the standard density. Loose-fill fibreglass is lower, around R-2.5 per inch, and settles over time.

What does R-value mean?

Thermal resistance — how well a material resists heat flow, in h·ft²·°F/BTU. Higher is better. R-values add when materials are layered, so a wall's total R is the sum of its layers plus the surface air films, before thermal bridging is accounted for.

How do I convert R-value to RSI?

Multiply by 0.176110. R-13 is RSI 2.29. RSI is in K·m²/W and is the SI equivalent. Watch the basis: some metric tables give RSI per metre of thickness and others per 25 mm, and confusing the two gives an answer off by a factor of 40.

Why is a wall's effective R lower than the insulation's R?

Mainly thermal bridging through the framing, which occupies about a quarter of the wall area with a material of less than half the R-value. Air leakage and moisture reduce it further. The rated R-value describes the insulation alone, dry, with no framing passing through it.

Is a higher R-value always better?

It reduces conduction heat loss, but the returns diminish and the cost does not. Air sealing and addressing thermal bridging usually deliver more per dollar than adding insulation beyond a certain point. There is also a point where extra internal insulation can create a moisture problem by making the sheathing too cold.

What is the difference between R-value and U-factor?

U-factor is the reciprocal of total R — it measures heat flow rather than resistance, so lower U is better. Windows are rated in U-factor because they are specified by how much heat they let through. R-20 corresponds to U-0.05.

Insulation R-Value Chart

R-value per inch of thickness for 29 insulation materials and building products, from aerogel at R-10 per inch down to concrete at R-0.08.

Data verified 2026-09-29 · based on ASHRAE Handbook, 2021 Fundamentals volume

Find a material

R-Value per Inch of Thickness

Material^[1]	Class^[1]	R per inch h·ft²·°F/BTU·in^[1]	RSI per 25 mm K·m²/W^[3]	RSI per m K·m²/W^[3]	Density kg/m³^[1]	Note^[1]
Aerogel blanket #	High performance	10.00	1.733	69.335	140	Highest R per inch available; costly and dusty to install
Polyurethane board (closed cell) #	Rigid foam	6.00	1.04	41.601	32	Highest of the common rigid foams
Polyisocyanurate board #	Rigid foam	5.70	0.988	39.521	30	Slightly lower than polyurethane; common in roofing
Closed-cell spray foam #	Spray foam	6.50	1.127	45.068	32	Also acts as an air barrier and can add structural stiffness
Extruded polystyrene (XPS) #	Rigid foam	5.00	0.8667	34.668	35	Moisture resistant; the standard below-grade board
Expanded polystyrene (EPS) #	Rigid foam	3.85	0.6673	26.694	20	Cheaper than XPS and nearly as good when dry
Open-cell spray foam #	Spray foam	3.60	0.624	24.961	8	Air barrier but not a vapour barrier; lower R per inch
Phenolic foam board #	Rigid foam	7.00	1.213	48.535	35	High R per inch with good fire performance
Mineral wool batt #	Fibrous batt	3.30	0.572	22.881	40	Non-combustible; better than glass at high temperature
Fibreglass batt (standard) #	Fibrous batt	3.14	0.5443	21.771	10	The baseline batt; R-13 for a 3.5 in wall cavity
Fibreglass batt (high density) #	Fibrous batt	3.80	0.6587	26.347	20	Fits the same cavity with higher R
Cellulose loose fill #	Loose fill	3.60	0.624	24.961	40	Recycled paper with fire retardant; must be installed at the right density
Mineral wool loose fill #	Loose fill	3.00	0.52	20.8	40	Blown mineral wool; non-combustible
Fibreglass loose fill #	Loose fill	2.50	0.4333	17.334	10	Settles over time; install above the nominal thickness to compensate
Vermiculite loose fill #	Loose fill	2.20	0.3813	15.254	100	Largely historical; may contain asbestos in pre-1990 installations
Perlite loose fill #	Loose fill	2.70	0.468	18.72	100	Lightweight mineral fill
Wood fibreboard #	Structural board	3.50	0.6067	24.267	250	Can serve as sheathing and insulation together
Cork board #	Structural board	3.60	0.624	24.961	120	Renewable, dimensionally stable
Cotton batt (recycled denim) #	Fibrous batt	3.70	0.6413	25.654	25	Similar performance to fibreglass; requires careful fire treatment
Sheep wool batt #	Fibrous batt	3.60	0.624	24.961	25	Moisture buffering; higher cost
Vacuum insulation panel #	High performance	25.00	4.333	173.34	200	Highest R per inch by far, but loses most of it if punctured
Softwood lumber #	Structural	1.25	0.2167	8.6669	500	Timber is a poor insulator but a structural necessity
Plywood and OSB #	Structural	1.25	0.2167	8.6669	600	Similar to solid wood
Gypsum board #	Structural	0.90	0.156	6.2402	800	Half-inch board is about R-0.45 total
Brick #	Masonry	0.20	0.03467	1.3867	1900	Poor insulator; adds thermal mass rather than resistance
Concrete (normal weight) #	Masonry	0.08	0.01387	0.55468	2400	Very poor per inch — an 8 in wall is only about R-0.6
Concrete block (hollow) #	Masonry	1.10	0.1907	7.6269	1200	Empty cores help; filled or grouted block is closer to solid concrete
Stone #	Masonry	0.08	0.01387	0.55468	2700	Similar to concrete
Glass (single pane) #	Glazing	0.02	0.003467	0.13867	2500	Essentially no insulating value; the air film does the work

R-value is a per-inch figure here, and total R is that number times the thickness. A material's rated R-value only applies at its stated thickness and installed density — loose fill installed below its rated density performs worse than the bag claims, and batts compressed into a cavity lose R in proportion to how much they are squeezed.

Two things this table does not capture. The first is air movement: fibrous insulation works by trapping air, so any convection loop inside it or air washing through it destroys a large part of its value — which is why an air barrier and a wind barrier matter as much as the R-value. The second is thermal bridging: studs, joists and slab edges conduct heat around the insulation, so an assembly's effective R is always lower than the insulation's nominal R.

R-value also falls with moisture and, for foams, with the ageing of the blowing agent. Polyisocyanurate in particular loses some of its rated R over the first few years as the low-conductivity gas escapes, which is why long-term values are quoted separately from initial ones.

R-Value, RSI and U-Factor

Three quantities describe the same physics in different units, and mixing them up is the usual error.

R-value is thermal resistance in imperial units, in h·ft²·°F/BTU. R-values add: a wall with R-13 insulation, R-0.5 sheathing and R-0.45 drywall has a total of about R-14 plus the surface films, before bridging is considered.

RSI is the same quantity in SI units, in K·m²/W. The conversion is 1 R = 0.176110 RSI, so R-13 is RSI 2.29. Because R is per inch and RSI is per metre of thickness in some tables and per 25 mm in others, always check which basis a metric figure uses.

U-factor is the reciprocal of total R, in BTU/h·ft²·°F. It describes how much heat flows, so lower is better — the opposite direction to R. A wall of total R-20 has a U of 0.05. U is what appears in heating and cooling load calculations, and it is what window ratings quote, because glazing is rated on heat flow rather than on resistance.

Keep them straight by remembering which direction is good: higher R is better, lower U is better.

Why the Assembly Always Performs Worse

The R-value on an insulation bag is measured on the insulation alone, in a laboratory, dry and undisturbed. An assembly in a real wall performs measurably worse, for three reasons that compound.

Thermal bridging. Studs at 16 in on centre occupy about 25% of a 2×4 wall's area and have an R-value of roughly 1.25 per inch against the insulation's 3.14. The framing short-circuits the insulation, and a nominal R-13 wall performs closer to R-11. Continuous exterior insulation, or advanced framing with studs at 24 in on centre, addresses this directly.

Air leakage. Fibrous insulation does nothing to stop air moving through it. A leaky wall loses far more heat to air exchange than to conduction through the insulation, which is why air sealing is usually the highest-return first step — and why it should come before adding more insulation.

Moisture. Wet insulation conducts much better than dry, and most fibrous materials lose a large fraction of their R when saturated. The design response is not more insulation but a correct vapour and drainage strategy, so the assembly dries rather than accumulates.

The practical order is: air seal first, then address bridging, then add insulation. Adding R to a leaky, bridged assembly delivers much less than the arithmetic suggests.

Frequently Asked Questions

What is the R-value of fibreglass insulation?: About R-3.14 per inch for standard batts and R-3.8 for high-density batts. A 3.5 in batt gives R-13 and a 5.5 in batt gives R-21 in the standard density. Loose-fill fibreglass is lower, around R-2.5 per inch, and settles over time.
What does R-value mean?: Thermal resistance — how well a material resists heat flow, in h·ft²·°F/BTU. Higher is better. R-values add when materials are layered, so a wall's total R is the sum of its layers plus the surface air films, before thermal bridging is accounted for.
How do I convert R-value to RSI?: Multiply by 0.176110. R-13 is RSI 2.29. RSI is in K·m²/W and is the SI equivalent. Watch the basis: some metric tables give RSI per metre of thickness and others per 25 mm, and confusing the two gives an answer off by a factor of 40.
Why is a wall's effective R lower than the insulation's R?: Mainly thermal bridging through the framing, which occupies about a quarter of the wall area with a material of less than half the R-value. Air leakage and moisture reduce it further. The rated R-value describes the insulation alone, dry, with no framing passing through it.
Is a higher R-value always better?: It reduces conduction heat loss, but the returns diminish and the cost does not. Air sealing and addressing thermal bridging usually deliver more per dollar than adding insulation beyond a certain point. There is also a point where extra internal insulation can create a moisture problem by making the sheathing too cold.
What is the difference between R-value and U-factor?: U-factor is the reciprocal of total R — it measures heat flow rather than resistance, so lower U is better. Windows are rated in U-factor because they are specified by how much heat they let through. R-20 corresponds to U-0.05.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASHRAE Handbook — Fundamentals, thermal insulation and building envelope chapters	standard	ASHRAE Handbook, 2021 Fundamentals volume
[2]	ASTM C518 — Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus	standard	ASTM C518-21
[3]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining formula and verified against every row.

Data Sources

Standard	Revision	What it covers on this page
ASHRAE Handbook — Fundamentals, thermal insulation and envelope chapters	ASHRAE Handbook, 2021 Fundamentals volume	the R-values per inch of thickness
ASTM C518 — Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus	ASTM C518-21	the method behind the measured R-values
ASTM C168 — Standard Terminology Relating to Thermal Insulation	ASTM C168-19	the definitions of R-value, RSI and U-factor

Cross-checked against:

R-values cross-checked between two insulation-manufacturer published tables and the ASHRAE values
The R-to-RSI conversion recomputed at build time for every row
Five values held as known-value checks spanning the highest and lowest R per inch in the table
Foam values checked for the difference between initial and long-term aged ratings

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
RSI per 25 mm and RSI per metre	RSI = R × 0.176110, the exact conversion. The per-metre column is the per-25 mm column multiplied by 40. Both are recomputed at build time; the two bases are given separately because metric tables use each.

R-values are per inch of thickness for the material alone, dry and undisturbed. Installed performance is lower: loose fill installed below rated density underperforms, compressed batts lose R proportionally, foam loses some R as the blowing agent ages, and any assembly loses more to thermal bridging and air leakage. Total assembly R is not the sum of the insulation R-values.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Insulation R-Value Chart." https://refmanual.com/charts/insulation-r-value/ (accessed 2026-09-29). Data from ASHRAE Handbook — Fundamentals, thermal insulation and envelope chapters, ASHRAE Handbook, 2021 Fundamentals volume.

BibTeX

@misc{insulation-r-value,
  title        = {Insulation R-Value Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/insulation-r-value/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASHRAE Handbook — Fundamentals, thermal insulation and envelope chapters, ASHRAE Handbook, 2021 Fundamentals volume}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Insulation R-Value Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/insulation-r-value/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASHRAE Handbook — Fundamentals, thermal insulation and envelope chapters, ASHRAE Handbook, 2021 Fundamentals volume
ER  -

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