Roof & Attic Insulation Calculator — m, R-value, cost
How to use this calculator
Enter the attic floor area (length x width, or a custom m² for irregular layouts), pick the insulation product and the target thermal resistance R (m²·K/W). If insulation is already in place, enter its depth in cm and type — its R is subtracted so you only buy the top-up. The diagram shows the existing layer plus the new layer on the joists.
Insulation types — blown fiberglass (λ=0.044 W/m·K), blown cellulose (λ=0.038), batt fiberglass (λ=0.040), batt mineral wool (λ=0.037).
R-value — thermal resistance in m²·K/W; required depth = R × λ. Lower λ means less depth for the same R.
Saved Calculations
| Time | Type | Area m² | Depth | R-Value | Qty | Cost |
|---|
Attic Insulation: Sizing for Target RSI
The attic floor is the cheapest place to add thermal resistance, so this estimator returns loose-fill bags or batt rolls plus a 5–10% waste allowance for joists, baffles and overlap trimming. Conductivity values follow EN 13162/13171: blown fiberglass λ=0.044, dense-pack cellulose λ=0.038, fiberglass batt λ=0.040 and mineral-wool batt λ=0.037 W/m·K. Required depth grows linearly with target R, and loose-fill bag coverage falls as depth increases. Pair this with the wall insulation calculator to size the whole envelope.
Formulas
Rneed = max(0, Rtarget − Rexisting); depth (m) = Rneed × λ. For blown products, bags = area × (1 + waste) ÷ (bag coverage at that depth); for batts, rolls = area × (1 + waste) ÷ roll coverage. Existing R = (existing depth in m) ÷ λ of the in-place material — layers are additive regardless of type.
Thermal Conductivity (λ) Values
Blown fiberglass λ=0.044, blown cellulose λ=0.038, batt fiberglass λ=0.040, batt mineral wool λ=0.037 W/m·K. Lower λ = better insulator = less depth for a given R. For a warm-roof deck instead of a cold attic, compare spray foam at the rafters and add a ceiling vapor barrier on the warm side in cold climates.
FAQ
Can I mix insulation types? Yes, R-values are additive — blown loose-fill works well over serviceable batts, so leave them in place and top up. Vapor barrier? In cold climates install a vapor retarder on the warm-in-winter side per EN 13788; in mild climates consult local code. For roof-to-wall continuity see the house wrap calculator, and the carpet calculator if you are finishing a conditioned attic floor. Why does bag coverage change? Loose-fill bags are rated by net volume; deeper installs cover less floor per bag, so always size by the target R.
On install day
Installed 2025 pricing across the EU runs roughly €10–18/m² for blown fiberglass at R-7, €13–24/m² for dense-pack cellulose, and €20–38/m² for mineral-wool batts — labour is typically half of that on a DIY-vs-pro split. The #1 estimator mistake on attic top-ups is forgetting to install baffles at every rafter bay before blowing — without them the new insulation chokes the soffit vents and the roof deck takes on moisture, and EN 1991-1-3 / national codes (e.g. NBN, DIN 4108) require a clear ventilation path. Spec the depth as final settled (cellulose drops ~15–20% in the first year) and mark the joists with depth pegs before the truck arrives so the installer hits the target R, not a guess.
