Insulation
Fiberglass Insulation Calculator
Works out fibreglass, the most common insulation and the one most sensitive to how it is fitted.
Result
Required R-values are set by climate zone and by local energy code, and they differ for attics, walls and floors. The figures offered here are common code minimums rather than recommendations, and a minimum is a floor, not a target. Air sealing comes first: insulation slows heat conduction but does nothing about air leaking through gaps, and an uninsulated but sealed attic often outperforms an insulated leaky one.
Get a Free Business AuditHow the calculation works
Thickness from R target divided by R per inch, then quantity from batt coverage or the blown coverage chart depending on form. Fibreglass comes as batts and rolls sized to standard framing and as loose fill for blowing. Its R per inch is modest compared with foams, which matters where depth is limited and does not where it is not, and it tolerates moisture better than cellulose in the sense that it does not absorb and hold water the same way.
What moves the number
- Batt or blown, which suits different assemblies and has different failure modes.
- Cavity depth, since fibreglass needs more inches than foam for the same R.
- Installation quality, which affects fibreglass more than any other material.
- Faced or unfaced, and which side the facing goes on for your climate.
Where this one goes wrong
Fibreglass depends on still air in the material to work, so anything that lets air move through or around it reduces performance substantially. That is why a fibreglass filled wall that leaks air underperforms its rating badly, and why air sealing before insulating matters most with this material. Gaps, compression and voids have the same effect. Handle it with respirator, eye protection and long sleeves: the fibres irritate skin, eyes and airways, and that is a routine precaution rather than an optional one. Blown fibreglass in an attic should be installed to the chart's depth and bag count like any loose fill.
Before you quote
- Air seal the assembly before installing, since fibreglass does not stop air.
- Fill cavities completely without compressing or leaving voids.
- Use respiratory, eye and skin protection when handling.
Where the rules come from
Questions
About this calculator
Why does fibreglass perform worse than its rating in practice?
Because it relies on still air within the material, so air moving through or around it carries heat straight past. Gaps, compression and an unsealed assembly all reduce real performance substantially. Air sealing matters more with fibreglass than with foams, which seal as they cure.
Is fibreglass or foam better?
Foam gives more R per inch and air seals as it cures, which matters in shallow cavities and leaky assemblies. Fibreglass is far cheaper per R and perfectly effective in a well sealed assembly with depth available, such as an attic. The constraint decides, rather than one being better.
Do I need protection when handling fibreglass?
Yes. The fibres irritate skin, eyes and airways, so a respirator, eye protection, gloves and long sleeves are standard rather than cautious. Working overhead in an attic is where exposure is highest, and it is also where ventilation is worst.
The formula
thickness = (target R - existing R) / R-value per inch
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 40 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 30 ft.
- Target R-value. Enter your own figure; the page starts at R-38, attic.
- Material. Enter your own figure; the page starts at Blown fibreglass (R-2.5 per inch).
- Existing R-value in place. Enter your own figure; the page starts at 0.
- Cost per square foot at that depth ($). Enter your own figure; the page starts at $1.1.
The result updates as you type, so you can try a figure, see what it does to thickness needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Length (ft) | 40 ft |
| Width (ft) | 30 ft |
| Target R-value | R-38, attic |
| Material | Blown fibreglass (R-2.5 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.1 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 15.2 |
| Area | 1,200.0 |
| R-value still to add | 38.0 |
| Estimated material cost | $1,320 |
| Bags if blown | 40 |
You need 15.2 inches to add R-38.0 across 1,200.0 square feet, roughly $1,320 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.55 | 15.2 |
| $0.83 | 15.2 |
| $1.1 example above | 15.2 |
| $1.65 | 15.2 |
| $2.2 | 15.2 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 30 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.1.
What the results tell you
| Result | What it is |
|---|---|
| Thickness needed | The headline result. A decimal figure. |
| Area | A decimal figure. |
| R-value still to add | A decimal figure. |
| Estimated material cost | A money figure. |
| Bags if blown | A number. |
Keep going
Related calculators
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.