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.