Attic insulation depth calculator

Target R, material R-per-inch, and optional existing attic R

An attic insulation depth plan is a one-line thermal budget in inch-pound units. You supply the target assembly R-value you want at the attic plane—commonly 38, 49, or 60 from IECC climate maps, utility rebates, or an auditor recommendation—and the R-value per inch of the product you intend to install. Optional material, name, product, or insulation strings are echoed for reports and do not change the arithmetic. The engine subtracts any existing_r you declare for batts or blown fill already in place, then divides remaining R by r_per_inch to obtain the installed depth in inches. When existing_r is omitted it defaults to zero, which models a bare deck or a full-replacement scope. Aliases keep forms flexible: target_r also accepts r_target, target_r_value, r_value, desired_r, and goal_r; r_per_inch accepts rpi, r_value_per_inch, r_inch, material_r_per_inch, and per_inch; existing_r accepts current_r, existing_r_value, base_r, have_r, and old_r. All numeric fields must be finite; target_r and r_per_inch must be strictly positive, existing_r must be non-negative and strictly less than the target so remaining R stays positive. Zero or negative r_per_inch would make depth undefined or non-physical and fails with invalid_input before any result is returned. Use manufacturer settled density and R-per-inch tables for blown products, because bag labels sometimes quote R at a stated depth rather than a pure per-inch constant—if you only know R at a listed depth, convert carefully before calling this endpoint.

Reading depth_inches, depth_feet, remaining_r, and the formula

Unit tags identify depth in inches and R in hour square foot degree Fahrenheit per Btu so UI copy stays honest about IP convention. Example: target R of forty-nine with cellulose at three point two R per inch and no existing insulation yields about fifteen point three one two five inches; the same target with thirteen R already present leaves thirty-six remaining R and about eleven point two five inches of added fill at the same product rate. Thermal bridging at joists, compressed batts, irregular joist depth, air sealing, and ventilation baffles are outside this product—fold those effects into the R values you pass if your energy model requires them. On success the response echoes target_r, r_per_inch, existing_r, and remaining_r with stable rounding controlled by precision (default four decimal places, aliases decimals and dp, range zero through fifteen). The primary planning fields are depth_inches and depth_feet, where feet equal inches divided by twelve for crews that mark rulers or tape in feet and inches. The method string is always target_r_over_r_per_inch and the formula field restates remaining_r equals target_r minus existing_r, depth_inches equals remaining_r over r_per_inch, and depth_feet equals depth_inches over twelve. Identical inputs always produce identical JSON because the path is pure arithmetic with no network, random seed, or wall clock. Pair this endpoint with home.insulation when you need the inverse problem of summing multi-layer thickness times R-per-inch into assembly R and U for a full wall or roof stack rather than a single attic fill depth.

API usage, pricing, retrofit workflows, and practical limits

Send a JSON body with required target_r and r_per_inch, plus optional existing_r, material, and precision. Use the prepaid API when a renovation app, energy audit tool, contractor tablet form, or property-management pipeline needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Prefer manufacturer data sheets for r_per_inch when products are specified, and keep existing_r conservative if you have not verified the true thickness and density of old fill with a ruler and bag chart.