Estimate how much insulation to add to reach a target R-value for an attic, walls or a blown-in job, with a rough count of bags or batts and a cost estimate.
Insulation Guidelines
Recommended R-Values by Climate Zone:
- Attic (Warm climate): R-30 to R-49
- Attic (Cold climate): R-49 to R-60
- Walls (All climates): R-13 to R-21
- Floors: R-25 to R-30
- Basement walls: R-10 to R-15
Insulation Types & R-Values per Inch:
- Fiberglass batts: R-3.2 to R-3.8 per inch
- Cellulose (blown): R-3.2 to R-3.8 per inch
- Rockwool: R-4.0 per inch
- Spray foam (open-cell): R-3.7 per inch
- Spray foam (closed-cell): R-6.5 per inch
Important Tips:
- Never compress insulation – it reduces R-value
- Seal air leaks before adding insulation
- Proper ventilation is critical in attics
- Use vapor barriers in cold climates
- Wear protective equipment when installing
- Consider energy audit to identify priorities
What this insulation calculator does
- Attic/Ceiling: area, current and target R-value and material to the thickness to add and a rough bag or batt count.
- Walls: wall length and height with stud spacing to the number of stud cavities and 8 ft batts, plus a rough count of spray foam bags.
- Blown-In: area, current and target R-value for cellulose, fiberglass or rock wool to the depth to blow in and a rough bag count.
- Cost Estimator: material, labour, vapor barrier and old insulation removal.
How it is calculated
R-value is a measure of resistance to heat flow, and a higher number insulates better. The extra R-value you need is the target minus the current value, and the thickness is that divided by the R-value per inch of the material.
| Material in this calculator | R-value per inch used |
|---|---|
| Fiberglass | 3.2 |
| Cellulose | 3.7 |
| Rock wool (blown-in tab) | 4.0 |
| Spray foam | 6.5 |
Worked example: moving a 40 ft × 30 ft attic (1,200 sq ft) from R-11 to R-38 needs R-27 more, which is about 8.4 inches of fiberglass or 7.3 inches of cellulose. Blowing cellulose into 1,000 sq ft from R-19 to R-49 needs R-30 more, about 8.1 inches deep. A 1,000 sq ft job at $1.50 per sq ft for material and $1.00 for labour, plus $100 for a vapor barrier and $200 for removal, comes to $2,800.
Things to keep in mind
R-value per inch differs by product and by form. For example, fiberglass batts are often listed at roughly R-2.9 to R-3.8 per inch, and closed-cell spray foam at about R-6 to R-7 per inch, while open-cell foam is lower, around R-3.7. The spray foam value here is the closed-cell figure, so open-cell foam would need more thickness. The bag and batt counts rest on assumed coverage figures and are only rough, so use the coverage on the product label. The prices in the cost dropdown are example figures, not current prices. For general information on insulation types and R-value, see the U.S. Department of Energy guide to home insulation. The target R-value must be higher than the current one. These are planning estimates only. Measure your own project carefully, check product labels and local building codes, and confirm quantities with your supplier or a qualified contractor before you buy.
Frequently asked questions
What R-value do I need?
It depends on your climate zone, the part of the building and local codes. The calculator lists common targets, and your local building authority can confirm the requirement.
Why does blown-in insulation need extra depth?
Loose-fill insulation can settle over time, so installers often add some extra depth to reach the target after settling.
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