What R-Value Do You Actually Need?
R-value measures resistance to heat flow — higher numbers mean better insulating power. The right target depends on what you're insulating and your climate zone, not just "more is better."
- Exterior walls (2x4 framing): R-13 is the standard fit for a 3.5" wall cavity.
- Exterior walls (2x6 framing): R-19 to R-21 fits a 5.5" cavity for colder climates.
- Attics: R-30 is common in moderate climates; R-38 to R-49 is recommended for northern states with harsh winters.
Blow-In (Loose-Fill) vs. Batt Insulation
Both reach similar R-values, but they suit different jobs:
- Batt/Roll Insulation: Best for open, accessible wall cavities and attic floors with standard joist spacing. Easier for one person to install without special equipment.
- Blow-In Cellulose or Fiberglass: Best for irregular attic spaces, around obstructions, or topping up existing attic insulation without removing it. Requires a rented or borrowed blower machine, but fills gaps that rigid batts can't reach.
If you're working with blown-in material, switch the "Target R-Value" dropdown above to "Custom / Blown-in" and enter the bag coverage rate printed on your specific product's packaging — coverage per bag varies by manufacturer and target depth.
Frequently Asked Questions
How do I calculate the R-value I'm getting from blown-in insulation?
Check the coverage chart printed on the bag — it lists net coverage area in square feet for several target R-values and depths (e.g., a bag might cover 50 sq ft at R-30/12" depth, or 35 sq ft at R-38/16" depth). Use the depth, not just the bag count, to confirm you've hit your target R-value.
Can I install batt insulation over old blown-in insulation, or vice versa?
Yes, you can layer blown-in insulation over existing batts (a common attic top-up method), but don't compress batts under a heavy layer of blown-in material — compression reduces the batt's effective R-value.
Does insulation need a vapor barrier?
In most climates, the paper or foil facing on batt insulation acts as the vapor barrier and should face the heated/interior side of the room. Blown-in insulation typically has no integrated vapor barrier, so check your local code for whether a separate poly sheet is required in your climate zone.
How do I use a ProCat insulation calculator?
ProCat is Owens Corning's blown-in fiberglass system. Each bag of ProCat covers a different area depending on target R-value and depth — the coverage chart is printed on every bag. Switch the dropdown above to "Custom / Blown-in" and enter the sq ft per bag from your specific product's chart. Typical ProCat coverage: ~42 sq ft per bag at R-30, ~29 sq ft at R-38.
How do I calculate cellulose blown-in insulation?
Cellulose blown-in coverage varies by brand and target depth. Check the coverage chart on your bag and enter that number in the "Custom" field above. A common cellulose product covers about 40 sq ft per bag at R-30 (approximately 8" depth). Always add 10–15% extra bags since blown-in can settle over time.
How much insulation do I need for a garage ceiling?
A detached garage ceiling typically uses R-19 (for a 2×6 rafter cavity) or R-30 if you want to treat it like an attic. Measure your garage ceiling length × width, add 10% waste, then divide by the coverage per roll or bag for your chosen R-value. The calculator above handles this — enter your garage dimensions and select your R-value.