What is Insulation and How Much Do You Need?
Insulation is any material used to slow the transfer of heat between the inside of your home and the outside environment. Properly insulating walls, attics, floors, and roofs keeps your home comfortable year-round and significantly reduces heating and cooling costs.
What is R-Value?
R-value is a measure of a material's resistance to heat flow. The higher the R-value, the better the thermal resistance, and the more effective the insulation. It is expressed in units of °F·ft²·h/BTU in the US system, or m²·K/W (RSI) in the metric system.
R-value depends on the type of material and its thickness — doubling the thickness doubles the R-value.
How to Calculate the R-Value of a Barrier
The total R-value of a multi-layer barrier is simply the sum of the R-values of each individual layer:
- Rtotal = Rlayer 1 + Rlayer 2 + … + Rlayer n
- Each layer's R-value = R-value per inch × thickness in inches
This calculator supports up to 5 layers of different materials, so you can model complex wall assemblies (e.g., drywall + fiberglass batt + OSB sheathing).
Understanding the U-Value
The U-value (or U-coefficient) is the reciprocal of the R-value: U = 1 / R. While R-value measures resistance to heat flow, U-value measures the rate of heat transfer. Lower U-values mean better insulation performance.
Material R-Values Reference
| Material | R-value per inch (Imperial) | RSI per cm (Metric) |
|---|---|---|
| Fiberglass Batt | 3.14 | 0.218 |
| Fiberglass Blown | 2.20 | 0.153 |
| Mineral Wool / Rockwool Batt | 3.14–3.20 | 0.218–0.222 |
| Cellulose Blown | 3.13 | 0.217 |
| Spray Foam (Closed-Cell) | 6.25 | 0.433 |
| Spray Foam (Open-Cell) | 3.70 | 0.257 |
| XPS (Extruded Polystyrene) | 5.00 | 0.347 |
| EPS (Expanded Polystyrene) | 3.85 | 0.267 |
| Polyisocyanurate (Polyiso) | 6.50 | 0.451 |
| Drywall / Gypsum Board | 0.90 | 0.062 |
| Plywood | 1.24 | 0.086 |
| OSB | 1.59 | 0.110 |
| Softwood Lumber | 1.25 | 0.087 |
| Concrete | 0.08 | 0.006 |
| Brick | 0.20 | 0.014 |
| Still Air Gap | 0.17 | 0.012 |
Recommended R-Values by Location
- Walls (2×4 framing): R-13 to R-15
- Walls (2×6 framing): R-19 to R-21
- Attic (minimum): R-30
- Attic (recommended, cold climates): R-49 to R-60
- Floors over unheated spaces: R-19 to R-30
- Basement walls: R-10 to R-19
Frequently Asked Questions
What R-value do I need for my walls?
This depends on your climate zone. In the US, the Department of Energy recommends R-13 to R-21 for exterior walls in most climate zones. In Russia, the recommended thermal resistance (RSI) for walls is typically 2.8–3.5 m²·K/W depending on the region.
Can I combine different insulation materials?
Yes! Combining materials is common and effective. For example, a typical wall assembly might include: drywall (R-0.45) + fiberglass batt 3.5 in (R-11) + OSB sheathing 0.5 in (R-0.80) for a total of about R-12.25.
Is more insulation always better?
Generally yes — up to a point of diminishing returns. Adding more insulation reduces your heating/cooling costs, but the savings decrease as R-value increases. Most building codes specify minimum R-values, and exceeding these often provides good cost/benefit returns.
What is the difference between RSI and R-value?
They measure the same thing — thermal resistance — but in different unit systems. R-value uses US customary units (°F·ft²·h/BTU), while RSI uses SI metric units (m²·K/W). To convert: 1 RSI = 5.678 R (US).