Estimate Insulation Area, Boards, R-Value & Cost
Use this Concrete Insulation Calculator to estimate rigid foam insulation for under-slab concrete, slab edges and foundation walls. Calculate square footage, board quantity, perimeter coverage, material cost and approximate insulation thickness for a target R-value using product-specific R-value per inch.
Estimate under-slab board quantity, slab-edge insulation, foundation-wall insulation, or the approximate insulation thickness needed for a target R-value.
Estimate rigid foam board quantity and cost for insulation installed beneath a concrete slab.
Material estimate only. Verify insulation type, compressive strength, thickness, R-value, moisture resistance, vapor-control details and installation requirements against the project documents and product technical data.
Calculate perimeter length, vertical edge area, board quantity and optional cost.
Estimate board quantity for a straight or combined foundation-wall insulation area.
Estimate theoretical insulation thickness and layer count from a target R-value and product R-value per inch.
Use manufacturer-rated value for the selected product
Thermal planning estimate only. Code-required insulation levels, thermal bridges, effective assembly R-value, edge conditions, moisture control and product suitability must be verified for the specific building and climate.
A Concrete Insulation Calculator helps convert slab, edge or foundation dimensions into insulation material quantities. Under a slab, the basic calculation is surface area divided by the coverage of one rigid foam board. Around a slab edge, perimeter is multiplied by the insulated edge height to produce square feet. For a foundation wall, total wall length is multiplied by insulated height and any openings are subtracted.
The R-value planner performs a different calculation. It divides a target R-value by the manufacturer-rated R-value per inch to estimate theoretical insulation thickness. It then compares that thickness with the board thickness entered by the user and rounds up to the next whole layer. This is a planning calculation only because real assemblies may include thermal bridges, joints, fasteners, concrete edges and other details that affect overall thermal performance.
Board count is rounded up to whole sheets.
Edge height is converted from inches to feet.
Use the product's current rated R-value rather than assuming every foam type performs the same.
Estimate rigid foam board quantity beneath slabs, pads and heated floors.
Calculate vertical insulation area around the perimeter of a slab.
Estimate wall coverage after subtracting doors, windows or other openings.
Compare target R-value, product performance, board thickness and layer count.
Concrete is durable and massive, but it is not a high-R-value insulation material. Heat can flow through a slab, slab edge or foundation wall when there is a temperature difference between indoors, outdoors and the ground. Insulation can become part of the building's thermal enclosure by reducing heat flow through those concrete elements.
The location and importance of insulation depend on the building design. Heated slabs, conditioned basements, slab-on-grade homes, cold-climate buildings, refrigerated spaces and high-performance construction can all use rigid insulation in different ways. Perimeter or slab-edge insulation can be particularly important because concrete edges can create a direct path between conditioned and exterior environments.
The calculator does not decide whether insulation is required. Applicable energy codes, project drawings, climate zone, building use and design documents determine the thermal requirements.
Several rigid foam products are used in building construction, including expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanurate and specialized high-density or below-grade products. Their thermal resistance, compressive strength, moisture behavior, facing, long-term performance and allowed applications can differ substantially.
That is why the calculator leaves R-value per inch editable. A single fixed number would make the tool less accurate across different products. Use the current technical data sheet for the product being considered, especially when the insulation will be placed under a load-bearing slab or in contact with soil.
For a simple rectangular slab, multiply length by width to find the surface area. A 30 ft × 20 ft slab contains 600 square feet. A 4 ft × 8 ft rigid foam board covers 32 square feet before cuts. The exact-area calculation is therefore 600 ÷ 32 = 18.75 boards, which must be rounded up to 19 whole boards before any cut or waste allowance is added.
With a 10% planning allowance, 600 square feet becomes 660 square feet, and 660 ÷ 32 = 20.625 boards. The purchase estimate rounds up to 21 boards. A more complex slab may require additional cutting around plumbing, columns, thickened edges, sumps or irregular perimeter conditions.
Find the finished insulation footprint rather than the excavation size.
Confirm board size, thickness, R-value and required compressive properties.
Multiply slab length by width or divide irregular areas into sections.
Apply a realistic project-specific cutting and waste allowance.
Check joints, edge conditions, penetrations and project details before purchase.
Slab-edge insulation is a perimeter calculation rather than a full slab-area calculation. For a 30 ft × 20 ft slab, perimeter is 2 × (30 + 20) = 100 feet. If the insulated vertical edge is 12 inches high, the vertical insulation area is 100 square feet because 12 inches equals one foot.
If the edge height is 18 inches, the same 100-foot perimeter creates 150 square feet of vertical insulation area. This demonstrates why perimeter and edge height both matter. Openings, transitions, steps and changes in slab elevation should be measured separately.
Foundation-wall insulation uses wall area rather than slab area. Multiply total insulated wall length by the vertical height of insulation. If 100 linear feet of foundation wall is insulated to a height of 8 feet, gross area is 800 square feet. Subtract windows, doors or other areas that will not receive insulation.
Exterior below-grade walls also require attention to waterproofing, drainage, protection boards, backfill and product suitability. Interior systems can involve different fire, moisture and finishing requirements. The calculator is limited to material area and board quantity; it does not design the wall assembly.
R-value measures resistance to heat flow. Higher R-value means greater thermal resistance. Rigid insulation products often publish a nominal or rated R-value per inch, but the exact value depends on material type, density, temperature, aging and product standard.
The R-value planner uses a simple theoretical relationship: target R-value divided by R-value per inch. For example, if a product is rated at R-5 per inch and the design target is R-10, the theoretical thickness is 2 inches. If only 1.5-inch boards are available, two layers would provide 3 inches total nominal thickness and exceed that theoretical target.
Rigid insulation is commonly sold in sheet formats such as 4 ft × 8 ft, but product lines also include other widths, lengths, scored panels and specialty shapes. Some insulation systems use tongue-and-groove edges or shiplap details that affect effective coverage. For that reason, board length and width are editable inputs rather than fixed values.
When exact coverage differs from the nominal sheet dimensions, enter the effective coverage recommended by the manufacturer. If joints are staggered in multiple layers, the number of boards may also be influenced by layout rather than area alone.
The calculator separates these locations because each one uses a different geometry and material takeoff.
When rigid insulation is installed below a slab, it becomes part of the load path between the concrete and the supporting base. Product compressive strength therefore matters in addition to R-value. Residential floors, warehouses, garages, vehicle areas and equipment pads can impose very different loads. The project designer or specification should identify a suitable insulation product where structural loading is important.
The insulation layer also interacts with base preparation, vapor control, sub-slab utilities and slab construction. Poorly supported boards, open joints or damaged insulation can affect placement. Follow the project details for sequencing, seams and vapor-retarder location.
Concrete and soil can be part of a moisture-management system as well as a thermal system. Below-grade insulation may be exposed to soil moisture, drainage water or water vapor. Some foam products are specifically intended for ground contact or below-grade service; others have restrictions.
A slab may also include a vapor retarder beneath the concrete or insulation depending on the assembly design. The correct location and detailing are project-specific. This calculator does not select a vapor retarder or waterproofing system.
Required slab-edge, under-slab or foundation insulation can vary with climate zone, building occupancy, heating system and the energy code adopted by the local jurisdiction. Heated slabs may have different requirements from unheated slabs. Basement walls can be treated differently from slab-on-grade construction.
For general building-envelope guidance, the U.S. Department of Energy provides consumer and technical resources on insulation and building efficiency. Always compare general guidance with the locally adopted code and project documents.
The calculator's first three modes can multiply the estimated board count by an optional price per board. This provides a simple material subtotal. It does not include delivery, adhesives, fasteners, tapes, sealants, vapor-control materials, protection board, excavation, labor, disposal, taxes or contractor markup.
Board pricing can vary by thickness, density, facing and regional availability. Use a current supplier price for the actual insulation product rather than a generic cost-per-square-foot assumption.
Common questions about under-slab insulation, rigid foam boards, slab edges, foundation walls and R-value planning.