What's the difference between R and RSI?

RSI is metric (m²K/W); US R-value = RSI × 5.678.

How much attic insulation do I need?

US DOE recommends R-38 to R-60 (RSI 6.7–10.6) in most zones.

Can I layer insulation?

Yes — run the second layer perpendicular to cut thermal bridges.

How many inches of attic insulation?

About 12–20 in of blown fiberglass for R-38 to R-60.

Blown-in fills gaps better in attics; batts are easier between open joists.

Insulate the attic floor or the roof deck?

The floor for vented attics; the roof deck for conditioned attics.

Ceiling insulation calculator

Area, target thermal resistance and insulation conductivity: you get the thickness needed and the rolls or bags to buy.

Purchase quantity

63 m²

Net measurement: 60 m² · Waste 5 %

Insulation rolls or packs11 roll
Thickness needed18.5 cm
Resulting U-value0.2 W/m²K

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How it's calculated (visible formula)

thickness = R × λ; rolls = ceil(area × (1 + waste) ÷ roll coverage); U ≈ 1 ÷ R

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure the attic or ceiling area.
Pick the target R-value for your climate zone.
Look up the insulation's conductivity (λ) or R per inch.
Thickness = RSI × λ (or target R ÷ R per inch).
Divide area with waste by coverage per roll or bag.

Worked example: 650 sq ft attic, RSI 5 (≈ R-28), λ 0.037

5 × 0.037 = 0.185 m → 7.3 in
60 m² × 1.05 ÷ 6 m² per roll

→ 11 rolls and about 7.3 in thick (U ≈ 0.20).

Common mistakes

Compressing batts — they lose R-value.
Covering recessed lights not rated for contact.
Forgetting the vapor retarder on the warm side.

Pro tips

Air-seal first (wire penetrations, hatches).
Keep soffit vents clear with baffles.
Install the vapor retarder on the warm side.
Wear gloves, goggles and a mask handling fiberglass.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

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FAQs

What's the difference between R and RSI?: RSI is metric (m²K/W); US R-value = RSI × 5.678.
How much attic insulation do I need?: US DOE recommends R-38 to R-60 (RSI 6.7–10.6) in most zones.
Can I layer insulation?: Yes — run the second layer perpendicular to cut thermal bridges.
How many inches of attic insulation?: About 12–20 in of blown fiberglass for R-38 to R-60.
Batts or blown-in?: Blown-in fills gaps better in attics; batts are easier between open joists.
Insulate the attic floor or the roof deck?: The floor for vented attics; the roof deck for conditioned attics.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.