Skip to main content
Shops & Outbuildings Free Pro Features Available

Concrete Slab Heat Loss Calculator - Edge Loss, Floor Loss & Insulation Savings for Shops

Calculate BTU loss through uninsulated shop and garage slabs with insulation upgrade payback

Free concrete slab heat loss calculator for shop, garage, pole barn, and outbuilding floors. Most shop owners know they lose heat through walls and the ceiling, but the concrete slab is a hidden energy thief that accounts for 10-25% of total heating load in slab-on-grade buildings. Heat transfers through the concrete in two ways: edge loss around the slab perimeter where the concrete is exposed to outdoor air, and downward loss through the slab to the ground below. Edge loss dominates in most shops because the exposed perimeter runs at or near outdoor temperature. Enter your slab dimensions, edge exposure condition, soil type, perimeter insulation details, and indoor/outdoor temperature difference to get BTU/hour losses for both paths. The calculator also models the impact of adding perimeter insulation - 2 inches of XPS foam board from the slab edge down 24 inches can reduce edge losses by 50-70%, often paying for itself within 2-3 heating seasons. Results include comparison with wall and ceiling losses so you can prioritize your insulation investment.

Pro Tip: Edge insulation is only effective if it is continuous and protected. A 2-inch XPS board installed vertically against the slab edge and extending 24 inches below grade cuts edge loss in half. But if the above-grade portion isn't covered with metal flashing or cement board, UV and lawnmowers will destroy it within a few years. Below grade, use extruded polystyrene (XPS), not expanded (EPS) - EPS absorbs moisture and loses R-value underground.

Size a heater that accounts for slab losses

Shop Heater BTU Calculator →

Calculate overhead door infiltration losses

Overhead Door Infiltration Calculator →

Estimate concrete volume for slab pours

Concrete Volume Calculator →
Start Using This Tool See How It Works

PREVIEW All Pro features are currently free for a limited time. No license key required.

Concrete Slab Heat Loss Calculator
Pop Out

How It Works

  1. Enter Slab Dimensions

    Input slab length, width, and thickness. Standard shop slabs are 4 inches for light use and 6 inches for vehicle traffic. The calculator computes both perimeter length and total slab area.

  2. Describe Edge Exposure

    Select how your slab edge is exposed: fully exposed above grade, partially buried, or insulated. For shops with grade beams or frost walls, the edge condition differs from a simple floating slab.

  3. Set Soil and Temperature Conditions

    Choose soil type (sand/gravel, clay, or rock) and enter indoor and outdoor design temperatures. Soil type affects the deep-ground loss rate. Sandy soil conducts more heat than clay.

  4. Add Insulation Details

    If you have existing perimeter insulation, enter the R-value and depth. If you have none, the calculator shows current losses and the savings from adding it at various R-values.

  5. Compare Loss Components

    Review edge loss, downward loss, and total slab loss in BTU/hr. See how slab loss compares to typical wall and ceiling losses for the same building, so you can decide where insulation investment has the best return.

Built For

  • Shop builders deciding whether to insulate the slab perimeter during construction
  • Shop owners diagnosing cold floors and high heating bills
  • Concrete contractors specifying vapor barriers and insulation for heated shops
  • Energy auditors quantifying slab losses in commercial and agricultural buildings
  • Radiant floor heating designers calculating slab edge losses

Frequently Asked Questions

For an uninsulated 40×60 shop slab at 60°F indoor and 0°F outdoor, edge losses run about 8,000-15,000 BTU/hr depending on exposure, and downward losses add another 3,000-6,000 BTU/hr. Total slab loss of 11,000-21,000 BTU/hr is typically 15-20% of the total building heat load. Edge loss is roughly 2-3 times greater than downward loss in most climates.
Perimeter insulation (2-inch XPS along the slab edge to 24 inches below grade) is almost always worth it for heated shops. Material cost is typically $1-2 per linear foot of perimeter. For a 40×60 shop in a cold climate, the savings can be $200-500 per heating season, giving a payback of 1-3 years. Full under-slab insulation is more expensive and has longer payback - typically justified only for radiant floor heating.
Concrete has an R-value of about 0.08 per inch, so a 4-inch slab is R-0.32 and a 6-inch slab is R-0.48. That is essentially zero insulation value. The slab doesn't insulate - it conducts. The ground beneath the slab provides moderate resistance to downward heat flow, but the exposed slab edges have direct contact with outdoor air temperatures.
For most heated shops, perimeter insulation gives the best return because edge losses are 2-3 times greater per unit area than downward losses. Place 2-inch XPS vertically against the slab edge, extending from the top of the slab to 24 inches below grade. Full under-slab insulation (1-2 inches of XPS under the entire slab) is worthwhile if you plan radiant floor heat or need a very warm, dry floor for automotive work.
Disclaimer: This calculator provides estimates based on ASHRAE slab heat loss methods. Actual losses depend on soil conditions, water table depth, slab construction details, and local climate. For radiant floor heating design, consult an HVAC engineer who can perform detailed heat loss analysis including slab-on-grade edge loss coefficients for your specific climate zone.

Learn More

Shops & Outbuildings

Why Your Shop Is Always Cold

The three places your shop loses heat: overhead doors, uninsulated slabs, and air infiltration. How to figure out where your BTUs are going and which fix pays for itself first.

Read Guide →

Related Tools

Shops & Outbuildings Live

Shop Heater BTU Sizing Calculator

Calculate the exact BTU output your shop or garage heater needs. Factors in wall R-values, ceiling insulation, slab edge loss, overhead door infiltration, and air changes per hour to size propane, natural gas, and electric heaters correctly.

Launch Tool →
Shops & Outbuildings Live

Overhead Door Infiltration Loss Calculator

Calculate heat loss through overhead doors in shops, garages, and warehouses. Compares open-door vs closed-door losses, seal condition impact, and annual cost of infiltration with payback on door seals and high-speed doors.

Launch Tool →
Shops & Outbuildings Live

Long-Run Voltage Drop Calculator

Calculate voltage drop for long wire runs to detached shops, barns, garages, and outbuildings. Compares copper vs aluminum, shows motor starting voltage impact, and recommends the right wire size for your distance and load.

Launch Tool →