Wind-Driven Rain Calculator

Calculate rain penetration risk by wind speed, roof pitch, and overhang — determines underlayment and flashing upgrades

Quick Estimate

Rainfall Intensity?

in/hr

Wind Speed During Rain?

mph

Roof Pitch?

:12

Eave Overhang Depth?

Driving Rain Index

100.0

Pitch Protection Factor0.75

Overhang Effectiveness75

Penetration Risk Level

Moderate

Underlayment Recommendation

Synthetic underlayment recommended (30 lb equivalent)

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Need more accuracy?

Extended mode adds scenarios, charts, and detailed breakdowns

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How to Use the Wind-Driven Rain Calculator

Proper roof insulation pays for itself through reduced heating and cooling bills. The Wind-Driven Rain Calculator helps you find the right R-value for your climate zone and calculate exactly how much insulation you need.

Find your climate zone — the DOE map divides the US into zones 1–8.
Look up the recommended total attic R-value for your zone (R-30 to R-60 for most US homes).
Enter your attic floor area and current insulation depth/type.
The calculator tells you how much additional insulation to add.
Divide by coverage per bag (printed on the bag) to determine how many bags to buy.

Tip: Start with the Quick calculator for a fast estimate. Switch to Extended mode for scenario comparison and charts, or Professional mode for complete parameter control.

Formula & Methodology

The Wind-Driven Rain Calculator uses the following established formulas:

Total R-Value = Σ(R-value of each layer) Heat Loss (BTU/hr) = Area × ΔT / R-Total Annual Heat Loss (BTU) = Heat Loss × Hours per Heating Season Required Depth = Target R-Value / R-value per inch of material

R-value is additive: two R-19 batts stacked give R-38. Different materials have different R-values per inch.

Frequently Asked Questions

What R-value does my attic need?

The US DOE recommends R-38 to R-60 for attics in most climate zones. Zone 1 (hot climates like Miami) needs R-30, while Zone 7 (very cold like Fairbanks) needs R-60. Check EnergyStar.gov for your specific zone.

What is the best attic insulation type?

Blown-in cellulose and fiberglass are the most cost-effective for existing attics. Spray foam (closed-cell) offers the highest R-value per inch and also air-seals, but costs more. Batts work well in new construction with clear joist bays.

How much can I save by adding attic insulation?

The EPA estimates that proper air sealing and insulation saves an average of 15 % on total heating and cooling costs — about $200–$400 per year for a typical US home. Payback periods are often under 5 years.

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Sources & References

The formulas, code requirements, and reference data used in this calculator are sourced from the following authoritative organizations and publications. Local codes vary — always verify with your jurisdiction's building department.

International Residential Code (IRC) 2024 — Chapter 9: Roof Assemblies — ICC
NRCA Roofing Manual (Steep-Slope & Low-Slope) — National Roofing Contractors Association
Asphalt Shingle Installation Manual — ARMA (Asphalt Roofing Manufacturers Association)
IECC 2024 — Energy Efficiency Requirements — ICC
DOE Building America Solution Center — U.S. Department of Energy
ASHRAE 90.1 / 90.2 — Energy Standards — ASHRAE

Disclaimer: Calculator results are estimates intended for planning and educational purposes only. They do not replace professional engineering analysis, building permit review, or contractor estimates. Always consult a licensed roofing contractor or structural engineer for final specifications.