Aerogel has the highest R-value per inch of any commercially available insulation, at roughly R-10 per inch, and vacuum insulated panels (VIPs) go higher still at R-30 to R-45 per inch. But for real roofs, attics, and walls, closed-cell spray foam (R-6 to R-7 per inch) and polyiso rigid board (R-5.6 to R-6 per inch) are the practical winners, because aerogel and VIPs cost far too much for whole-house use. The material with the highest R-value per inch is rarely the one you should buy. This page ranks every common insulation by R-per-inch, then shows where the dollar-per-R math flips the ranking.
Which insulation has the highest R-value per inch?
Ranked by thermal resistance per inch of thickness, aerogel and vacuum insulated panels lead, followed by the rigid and spray foams, then the fiber insulations. R-value per inch (also written R/inch) is the fair way to compare materials because it strips out thickness: a material at R-6 per inch does the work of a material at R-3 per inch in half the depth.
| Insulation material | R-value per inch | Typical form | Relative cost per R |
|---|---|---|---|
| Vacuum insulated panel (VIP) | R-30 to R-45 | Rigid panel | Very high |
| Aerogel | R-10 (approx.) | Blanket, board | Very high |
| Closed-cell spray foam (ccSPF) | R-6 to R-7 | Sprayed on site | High |
| Polyisocyanurate (polyiso) | R-5.6 to R-6 | Rigid board | Medium |
| Extruded polystyrene (XPS) | R-5 | Rigid board | Medium |
| Graphite EPS (GPS) | R-4.7 to R-5 | Rigid board | Medium |
| Expanded polystyrene (EPS) | R-3.6 to R-4.2 | Rigid board | Low |
| Open-cell spray foam (ocSPF) | R-3.5 to R-3.9 | Sprayed on site | Medium |
| Mineral wool (rock/slag) | R-3.0 to R-4.2 | Batt, board | Low |
| Fiberglass batt | R-2.9 to R-4.3 | Batt | Lowest |
| Cellulose (blown) | R-3.2 to R-3.8 | Loose fill | Lowest |
| Blown fiberglass | R-2.2 to R-4.3 | Loose fill | Lowest |
The ranges are wide because density, additives, and lab conditions shift the number. Graphite-impregnated EPS (sold as Neopor or GPS) pushes standard EPS up by about 20%. Blown fiberglass varies most, because installed density changes its performance. For a full breakdown by climate zone and target depth, see our insulation R-value chart.
Does the highest R-value insulation mean the best insulation?
No. The highest R-value per inch does not make a material the best choice, because R-value ignores three things that decide real-world performance: air sealing, moisture behavior, and cost. A cheaper material installed to the same total R-value, with good air sealing, often outperforms a premium foam installed poorly. R-value measures conductive heat flow only. It says nothing about air leakage, which can account for 25% to 40% of a home’s heating and cooling load.
Closed-cell spray foam wins on more than R-value: it air-seals and acts as a vapor retarder in one pass. That combined function, not the R-6.5 number alone, is why it commands a premium. Fiberglass and cellulose need a separate air-sealing step to match it. Before adding insulation to an attic, sealing the leaks first matters more than the material you pick. See our guide on air sealing an attic for what to seal and in what order.
Aerogel and vacuum panels: highest R-value, wrong tool for most jobs
Aerogel (R-10 per inch) and vacuum insulated panels (R-30 to R-45 per inch) hold the top of the R-per-inch chart but rarely make sense for a house. Aerogel blanket runs several dollars per square foot per R, many times the cost of foam board, and is used mainly for thin retrofit spots, pipe wrap, and industrial applications where space is the hard constraint. VIPs are fragile: puncture the panel and it loses most of its R-value, so they are used in appliances and specialty construction, not open framing.
The lesson: pick the highest R-per-inch material only when thickness is the binding limit, such as a low-slope roof assembly or a retrofit where you cannot lose interior space. When you have room to build up depth, a lower R-per-inch material at greater thickness reaches the same total R-value for far less money.
Closed-cell spray foam: the highest R-value insulation you would actually spray
Closed-cell spray foam delivers R-6 to R-7 per inch, the highest R-value of any insulation sprayed into a house on a routine basis. Most 2-lb products rate near R-6.5 to R-7 as installed and settle around R-6 to R-6.5 per inch after thermal drift, the slow decline as the blowing agent diffuses out over the first years. It also stops air movement and slows vapor, which is why it is specified for rim joists, cathedral ceilings, and unvented roof decks.
The tradeoff is cost. Closed-cell runs roughly $1.00 to $1.50 per board foot installed, several times the price of fiberglass or cellulose for the same total R. It is the go-to where space is tight or where the assembly needs air and vapor control built in. For the open-cell comparison and attic-specific detail, see spray foam attic insulation.
Polyiso: high R-value board with a cold-weather catch
Polyisocyanurate (polyiso) rates R-5.6 to R-6 per inch at standard test temperatures, making it the highest-performing rigid foam board by the label. It is the default insulation on commercial low-slope roofs and a common continuous-insulation layer on walls. But polyiso has a quirk the label hides: its R-value drops as temperature falls.
In cold weather, polyiso’s real R-value can fall to R-4.5 to R-5 per inch or lower once the board temperature drops below about 25 to 40 degrees Fahrenheit. That means a polyiso roof rated R-6 per inch in the lab may deliver closer to XPS-level performance on a January night. In cold climates, some builders layer polyiso with XPS, or simply add thickness, to hedge the drift. XPS (R-5) and EPS (R-3.6 to R-4.2) hold their rated R-value more steadily across temperature but start lower.
Fiberglass, cellulose, and mineral wool: lowest R-value, lowest cost per R
Fiberglass, cellulose, and mineral wool sit at the bottom of the R-per-inch chart (R-2.2 to R-4.3 per inch) but at the top of the cost-efficiency chart. For a vented attic where you can pile insulation as deep as you want, these fibers hit a target R-value for the least money, which is why blown fiberglass and cellulose dominate attic floors.
- Fiberglass batt: R-2.9 to R-4.3 per inch. Cheapest per R, but only performs if cut and fitted tightly with no gaps or compression.
- Blown cellulose: R-3.2 to R-3.8 per inch. Recycled paper, fills irregular cavities well, settles about 10% to 20% over time so installers over-blow to compensate.
- Mineral wool: R-3.0 to R-4.2 per inch. Fire-resistant, water-repellent, holds shape better than fiberglass, costs more.
Because these need depth, thickness math matters. Reaching R-38, R-49, or R-60 with fiber means a lot of inches. See our thickness guides for R-38 insulation thickness, R-49 insulation thickness, and R-60 insulation thickness by material.
The cost tradeoff: highest R-value per inch versus lowest cost per R
The material with the highest R-value per inch is usually the most expensive per unit of R-value delivered. The decision is a tradeoff between space and money. If you have depth to fill, fiber wins on cost. If depth is capped, foam wins because it packs more R into fewer inches.
| Priority | Best pick | Why |
|---|---|---|
| Thinnest possible layer | Aerogel or VIP | Highest R-per-inch, cost no object |
| Tight space plus air sealing | Closed-cell spray foam | R-6.5 per inch plus air and vapor control |
| Rigid board over roof deck | Polyiso or XPS | High R-per-inch board, XPS steadier in cold |
| Deep attic floor, lowest cost | Blown cellulose or fiberglass | Cheapest per R when depth is free |
| Fire and moisture resistance | Mineral wool | Non-combustible, water-repellent |
A simple rule: convert every quote to dollars per R per square foot before comparing. A foam at R-6.5 per inch and a cellulose at R-3.5 per inch can both hit R-49, but the cellulose usually costs a fraction as much when the attic has room for the extra depth.
How much R-value do you actually need?
The right total R-value depends on where the insulation goes and your climate zone, not on chasing the highest number. The U.S. Department of Energy recommends attic insulation of R-30 to R-60 for most homes, with R-49 to R-60 in cold zones (6, 7, and 8) and R-30 to R-49 in warm zones (1 through 3). Walls typically target R-13 to R-21.
Match the material to the assembly, then buy enough thickness to hit the code or DOE target. Picking the highest R-value insulation and then under-installing it, or leaving air leaks, wastes the premium. Get the total R-value and the air sealing right first, then optimize cost per R.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.