What this calculator does
Thickness follows from the target R-value and the material thermal resistance per millimetre. Reaching R3.5 takes 144 mm of fibreglass batt but only 101 mm of rigid foam board, because the foam resists heat better per millimetre.
That difference is what you are paying for with foam. It costs more per square metre but fits a given R-value into less depth, which matters when the cavity or rafter depth is fixed. Where space is not constrained, cheaper bulk insulation at greater thickness reaches the same R-value for less.
The formula
Thickness is the target R-value divided by the material R-value per millimetre. Roll or batt count is the area divided by the coverage per pack, rounded up. Note that the pack count depends only on area and coverage, so if you change the thickness you should also change the coverage figure to match the product actually being bought.
| Term | Meaning |
|---|---|
| R-value | Thermal resistance in m²K/W. Higher resists heat flow better. |
| R per mm | The material resistance per millimetre of thickness, which is what differs between products. |
| Thermal bridging | Heat bypassing insulation through framing, which reduces the effective whole-wall R-value. |
| Compression | Squashing batts into a shallow cavity reduces their R-value below the rated figure. |
The inputs explained
| Field | What to enter |
|---|---|
| Area to insulate (m²) | Area to insulate in square metres. |
| Target R-value (m²K/W) | Target R-value. Requirements vary by climate and by element, so check local building requirements. |
| Material | Material, which sets the R-value per millimetre. |
| Roll/batt coverage (m²) | Coverage per roll or batt pack, from the packaging. Change this if you change the thickness. |
| Price per roll ($) | Price per roll or pack. |
When to use it
Meeting a required R-value
Building codes specify minimum R-values, and the thickness needed depends on which material is used.
Working within a fixed cavity depth
Where the rafter or stud depth is fixed, a higher-performance material may be the only way to hit the target.
Comparing materials on cost
A more expensive material at less thickness can be the cheaper answer where space is constrained.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How thick does each material need to be?
The same R-value target in four materials.
| Material | Thickness needed | Material volume | Rolls/batts needed |
|---|---|---|---|
| Mineral wool batt | 152.8 mm | 6.11 m³ | 5 |
| Fibreglass batt | 144.0 mm | 5.76 m³ | 5 |
| Loose-fill cellulose | 136.7 mm | 5.47 m³ | 5 |
| Rigid foam board (XPS) | 100.9 mm | 4.03 m³ | 5 |
Questions
What R-value do I need?
It depends on the climate and on which element you are insulating. Ceilings generally need more than walls, and cold climates more than mild ones. Building codes set minimums, and those are the figures to work to rather than any general rule.
Is thicker always better?
Up to a point, and with diminishing returns. Going from R2 to R4 halves the heat flow; going from R4 to R6 cuts it by only a third again. Beyond the code minimum the payback lengthens, and other measures such as sealing air leaks often give more for the money.
Does compressing batts matter?
Yes, and it is a common mistake. Squashing a 150 mm batt into a 100 mm cavity does not give the rated R-value; it gives roughly what a 100 mm batt of that material would, sometimes less. Match the batt thickness to the cavity depth.
What is thermal bridging?
Heat bypassing the insulation through the framing, which conducts far better than the insulation does. Timber studs can reduce a wall effective R-value by 10 to 20% below the insulation rating, and steel framing far more. Continuous insulation over the frame addresses it.
For ventilation rates, see the air changes per hour calculator. For heating capacity, see the furnace sizing calculator.