Insulation
Garage Insulation Calculator
Works out garage insulation, where which surfaces matter depends on whether you are heating it.
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 is the usual calculation, applied to whichever surfaces you are actually treating. The question that comes first is purpose. If the garage is to be a conditioned space, the exterior walls, ceiling and door all need insulating and it becomes a small building envelope. If the aim is to protect the house from the garage, only the shared wall and the ceiling below living space matter, and the exterior walls are irrelevant. Those are very different quantities for the same building.
What moves the number
- Whether the garage is being conditioned or just isolated from the house.
- The garage door, which is a large uninsulated surface and often the weakest point.
- Whether there is living space above, which makes the ceiling the priority.
- Fire separation requirements on the wall and ceiling shared with the house.
Where this one goes wrong
The wall and ceiling between a garage and a house are a fire separation, and insulating them does not permit disturbing that. Drywall on the garage side is there for a rated purpose, penetrations have to be sealed, and a door between garage and house has specific requirements. Insulating that assembly means putting it back correctly rather than leaving framing exposed. The other consideration is combustion and vehicle exhaust: tightening up a garage that contains a fuel-burning appliance or is used for running engines needs ventilation thought through, and a sealed garage attached to a house should not be transferring air into it.
Before you quote
- Decide whether the garage is being conditioned or isolated first.
- Restore the fire separation on any shared wall or ceiling you open.
- Insulate the garage door if the space is being conditioned.
- Consider ventilation if the space is being tightened.
Where the rules come from
Questions
About this calculator
Which garage walls should I insulate?
It depends on the goal. To make the garage comfortable, all exterior walls, the ceiling and the door. To stop the garage affecting the house, only the shared wall and any ceiling under living space, and the exterior walls do nothing for you. Those are very different jobs and costs.
Is the wall between my garage and house special?
Yes, it is a fire separation, and the drywall and sealed penetrations are part of that function. Insulating it is fine, but it has to be closed back up correctly rather than left open, and the door between garage and house has its own requirements.
Is insulating the garage door worth it?
If you are conditioning the garage, yes, because it is a large surface and usually the least insulated part of the envelope. If you are only isolating the garage from the house, it does nothing, since the door is not between the garage and the conditioned space.
The formula
thickness = (target R - existing R) / R-value per inch
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How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 22 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 24 ft.
- Target R-value. Enter your own figure; the page starts at R-13, walls, mild.
- Material. Enter your own figure; the page starts at Fibreglass batt (R-3.2 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 $1.05.
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) | 22 ft |
| Width (ft) | 24 ft |
| Target R-value | R-13, walls, mild |
| Material | Fibreglass batt (R-3.2 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.05 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 4.1 |
| Area | 528.0 |
| R-value still to add | 13.0 |
| Estimated material cost | $554.40 |
| Bags if blown | 7 |
You need 4.1 inches to add R-13.0 across 528.0 square feet, roughly $554.40 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.53 | 4.1 |
| $0.79 | 4.1 |
| $1.05 example above | 4.1 |
| $1.58 | 4.1 |
| $2.1 | 4.1 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 22 ft.
- Width (ft) (ft)
- Measured in ft, starting at 24 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.05.
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
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