For estimation and sales-phase use. A simplified heating and cooling estimate for one space. It is not a Manual J load calculation. Below-grade coefficients are code defaults applied as an assumption for any location.

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Calculator · Basement

Basement BTU calculator

Enter the basement size, how tall the concrete wall is, the wall insulation and your design temperatures. The calculator adds up the heating and cooling loads surface by surface and shows how much of the heat leaves through the walls and floor in the ground.

  1. Enter the finished area, the wall height, the concrete wall height and which walls face the ground.
  2. Choose the basement wall insulation, the insulation above the mud sill, the windows and what is over the ceiling.
  3. Enter the winter and summer design temperatures, the indoor temperatures, the climate zone and the usual number of people.
  4. The result shows the heating and cooling loads in BTU/h, with the below-grade walls and floor broken out from the rest.

One finished basement, heating and cooling at design. Walls to unfinished space are left out, and moisture is not calculated.

Inside dimension of the finished area.

Inside dimension of the finished area.

Basement floor to the underside of the floor above. The part above the concrete wall uses the insulation you pick for the wall above the mud sill.

The Seattle table lists only 2, 3.5 and 7 ft and assumes 6 in of that concrete sticks out above grade, so 7 ft means about 6.5 ft in the ground. Pick the closest. Shallower than 2 ft is a slab on grade, which this page does not model.

The four rows of the table this page uses. Pick the nearest one below what you have.

Walls against the ground. Long means the longer of length and width. Walls to an unfinished part of the basement are left out.

The strip of wall between the top of the concrete and the floor above. With a framed wall inside, it is usually the same batts.

All basement windows, glass and frame.

Frame included. It has to fit in the wall above grade, which is the wall above the mud sill plus the 6 in of concrete the table puts above grade. Windows down in a window well are not modeled.

Only changes the cooling sun line. Several sides uses the sunniest exposure for all the glass.

Most basements sit under the living space, and that ceiling adds nothing. An unheated space above is taken at the outdoor design temperature, winter and summer.

Use the 99% heating design temperature for your location, from the ASHRAE Handbook - Fundamentals, Chapter 14 climatic design data. Look it up on the HVAC design temperatures page at /hvac-design-temperatures.

Use the 1% cooling design dry-bulb for your location, from the same design temperatures page.

Sets the sun on the glass and the wall factor on the cooling side. The design temperatures page lists the zone for your location.

Usual number of people in the space on a hot afternoon.

Example result for the default inputs

40 x 30 ft basement, 8 ft walls, 7 ft concrete wall, 140 ft of exterior wall. Heating 70°F with 5°F outside, cooling 75°F with 90°F outside.

Heating load at design

12,798 BTU/h

Below grade, part of the heating load

8,154 BTU/h

Cooling load at design

2,473 BTU/h

Calculation details
Floor area1,200 sq ft
Exterior wall length140 ft
Concrete wall area, slab to mud sill, net of windows975 sq ft
Wall area above the mud sill, net of windows129 sq ft
Wall height above grade, for windows1.5 ft
Heating temperature difference65°F
Heating, concrete walls at U 0.0513,231 BTU/h
Heating, basement floor at F 0.5414,923 BTU/h
Heating, walls above the mud sill764 BTU/h
Heating, windows510 BTU/h
Heating, ceiling0 BTU/h
Heating, air leakage at 0.30 ACH, 48 CFM3,370 BTU/h
Cooling temperature difference15°F
Cooling, concrete walls and floor0 BTU/h
Cooling, walls above the mud sill185 BTU/h
Cooling, windows, conduction118 BTU/h
Cooling, sun through windows843 BTU/h
Cooling, ceiling0 BTU/h
Cooling, air leakage at 0.18 ACH, sensible467 BTU/h
Cooling, 2 people, sensible460 BTU/h
Cooling, people, latent400 BTU/h

For estimation purposes only. Consult a licensed engineer for final designs.

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How the basement load is calculated

A basement loses heat through three surfaces. The concrete wall loses through the concrete and the soil, the floor loses at its edge, and the wall above the mud sill behaves like any other exterior wall. Summer is the easy side. The ground below grade pulls heat out of the room.

Temperature difference
heating dT = indoor - winter design, cooling dT = summer design - indoorA difference at or below zero makes that side's wall, window, ceiling and air lines zero. The calculator never reports a negative load.
Concrete walls
Q = (exterior wall length x concrete wall height - window area below the sill) x U x heating dTU comes from the default U-factors and F-factors in the Seattle Energy Code, Appendix A, Table A104.1, which this calculator applies as an assumption for any location. The rows are 2, 3.5 and 7 ft of 8 inch concrete, measured from the slab to the top of the mud sill with 6 inches above grade. The code publishes them as defaults for code calculations. Multiplying them by the full indoor minus outdoor design difference, the same one used for the wall above the sill, is this calculator's assumption, and ground temperature and soil type are not modeled.
Basement floor
Q = F x exterior wall length x heating dTF is per foot of slab perimeter and comes from the same table row as the wall U. The code is explicit that the two only work as a pair, so the calculator never mixes a U from one row with an F from another.
Walls above the mud sill
Q = (exterior wall length x (wall height - concrete wall height) - window area above the sill) x (1 / R) x dTThe R-value you pick is treated as the whole wall, as the BuildSolver load engine does. No insulation is taken as R-4, this calculator's assumption. On the cooling side the wall line is multiplied by the engine's climate zone wall factor.
Windows
Q = area x U x dT, window height <= (wall height - concrete wall height) + 0.5 ftWindow U by type is 1.04 single pane, 0.49 double clear, 0.32 double low-e and 0.22 triple. These are BuildSolver engine defaults, used here as assumptions. A window taller than the wall above grade is refused. The wall height above grade used for this check adds the 6 inches of concrete the table puts above grade to the wall above the mud sill. When the tallest window reaches below the mud sill, that share of the window area comes out of the concrete wall and the rest comes out of the wall above the sill. All the glass is treated as the height of the tallest window, a BuildSolver assumption.
Sun through windows
Q = window area x SHGC x zone solar factor x exposure factorCooling only. SHGC is 0.86 single, 0.62 double clear, 0.40 low-e and 0.35 triple. The exposure factor is 0.4 north, 0.7 south and 1.0 east or west. The solar factor runs from 55 to 100 by climate zone. All are BuildSolver engine defaults, used as assumptions.
Ceiling
Q = floor area x (1 / R) x dT, or 0 with a heated floor aboveWith an unheated space above, this calculator assumes that space sits at the outdoor design temperature in winter and in summer, which keeps the ceiling line on the high side. The cooling line gets no roof sun factor because a room sits over the ceiling.
Air leakage
CFM = ACH x basement volume / 60, Q = 1.08 x CFM x dTHeating uses 0.20 tight, 0.30 average and 0.50 leaky air changes per hour, and cooling uses 0.10, 0.18 and 0.30. These are BuildSolver engine assumptions, set after comparing them with completed Manual J reports. Moisture carried in by that air is not counted.
People
Q = people x 230 BTU/h sensible + people x 200 BTU/h latentCooling only, BuildSolver engine defaults. The cooling total adds the people latent line to the sensible lines.
Cooling below grade
Q = 0 for the concrete walls and the basement floorThis calculator assumes the ground is cooler than the room in summer and sets the below-grade wall and floor cooling lines to zero.

Worked examples for five basements

Every row is the same 40 x 30 ft basement with 8 ft walls, 16 sq ft of windows and a heated floor above, at 70°F with 5°F outside and 75°F with 90°F outside in zone 5A. The first row is the default on the form. The others change the wall insulation and the concrete wall height.

Heating load, below-grade share and cooling load for five example basements
BasementHeatingBelow gradeCooling
7 ft concrete wall, R-11 interior, R-11 above the sill12,798 BTU/h8,154 BTU/h2,473 BTU/h
7 ft concrete wall, bare concrete, nothing above the sill21,614 BTU/h15,633 BTU/h2,797 BTU/h
7 ft concrete wall, R-10 exterior foam, R-10 above the sill12,671 BTU/h7,951 BTU/h2,491 BTU/h
3.5 ft concrete wall, R-11 interior, R-11 above the sill14,906 BTU/h7,398 BTU/h3,166 BTU/h
3.5 ft concrete wall, bare concrete, nothing above the sill27,129 BTU/h13,272 BTU/h4,705 BTU/h

In the default basement the below-grade walls and floor carry 64% of the heating load. Stripping the insulation multiplies the heating load by 1.7. The floor line moves the other way, from 3,913 BTU/h bare to 4,923 BTU/h insulated, because the table pairs R-11 interior insulation with a higher floor F-factor. With the same insulation, a basement with a 3.5 ft concrete wall needs 16% more heat than one with a 7 ft concrete wall, since more of its wall sits above the mud sill and faces outdoor air. Cooling in the default is 19% of the heating load.

Standards and data sources

  • Seattle Energy Code 2018, Appendix A, Section A104 Below-Grade Walls and Slabs, Table A104.1 - default U-factors and F-factors for basements that this calculator applies as an assumption for any location. They are not IECC, ASHRAE or ACCA values. Section A104.2 says "The wall is assumed to extend from the slab upward to the top of the mud sill for the distance specified in Table A104.1, with 6 inches of concrete wall extending above grade." The code also says "Below-grade wall U-factors are only valid when used with the accompanying below-grade slab F factor, and vice versa." and "Basements shallower than two feet should use on-grade slab coefficients."
  • BuildSolver load engine defaults - window U-values, SHGC, solar and exposure factors, climate zone wall factors, air change rates and people gains. The engine code names ACCA Manual J 8th edition and ASHRAE Fundamentals tables as their origin. Several values were adjusted by BuildSolver after comparing against completed Manual J reports. This page uses them as assumptions.
  • Outdoor design temperatures and climate zone - entered by you. The 99% winter and 1% summer design temperatures are the climatic design data of the ASHRAE Handbook - Fundamentals, Chapter 14. The values and the zone for your location are on the BuildSolver HVAC design temperatures page.

Limitations of this calculator

  • This is a simplified heating and cooling estimate for one space. It is not a Manual J load calculation and not a room-by-room calculation of the house.
  • The concrete wall coefficients are code defaults for 8 inch concrete with 6 inches above grade, used here for any location. Soil type, water table, frost depth and ground temperature are not modeled.
  • One concrete wall height applies to every exterior wall. A walkout wall that is fully out of the ground on one side, and a door straight outside, are not modeled.
  • The table offers only 2, 3.5 and 7 ft concrete walls and four insulation rows. The calculator does not interpolate between them. A basement with more than 6 inches of concrete showing above grade loses more through that exposed concrete than the table assumes.
  • Windows are taken out of the wall above grade, split at the mud sill as if all the glass were as tall as the tallest window. Windows set down in window wells below grade are not modeled.
  • Nominal R-values are used for the whole wall above the mud sill, with no allowance for studs, rim joist gaps or air films.
  • Walls to an unfinished part of the basement, heat from the furnace or water heater, lights and appliances are not counted.
  • Moisture is not part of this estimate. The cooling total includes latent heat from people only. Moisture from air leakage or damp walls is left out.
  • No duct losses or gains and no altitude correction.
  • The results are raw BTU/h. This page does not pick a tonnage, a heat pump or a furnace size.

Common basement load mistakes

  1. Sizing a basement by square feet like an upstairs room. A basement with most of its wall in the ground and a heated floor above has no attic, little glass and almost no sun. A per-square-foot rule built for above-grade rooms can badly oversize the cooling side.
  2. Mixing a wall U from one source with a floor F from another. The Seattle table pairs every wall U with its own floor F. Insulating the wall from inside pushes more heat out through the slab, which is why the R-11 and R-19 interior rows carry a higher F than the bare concrete row at every height.
  3. Forgetting the wall above the concrete. With a 7 ft concrete wall under 8 ft of total height, only 1 ft of wall sits above the mud sill, but it faces outdoor air. With a 3.5 ft concrete wall that strip grows to 4.5 ft, and it can lose more than the concrete wall below it.
  4. Using an average winter temperature. An average January temperature sits well above the design night the system has to carry. Use the 99% heating design temperature for the location.
  5. Treating a dehumidifier problem as a cooling problem. A basement that feels clammy in July usually needs a dehumidifier. A big air conditioner on a small sensible load short-cycles and leaves the humidity behind.

Common questions about basement heating and cooling

How many BTU does a finished basement need?

It depends on the concrete wall, the insulation and the climate. In the default example on this page, a 40 x 30 ft basement with 8 ft walls, a 7 ft concrete wall, R-11 interior framing, 16 sq ft of windows and a heated floor above needs about 12,800 BTU/h of heating at 70°F with 5°F outside, of which about 8,200 BTU/h goes through the concrete walls and floor. Cooling at 75°F with 90°F outside is about 2,500 BTU/h.

Why is basement cooling so much smaller than heating?

Most of the wall is in the ground, the floor above is conditioned and basement windows are small. This calculator counts nothing for the concrete walls and floor in summer, so the cooling load comes from the wall above the mud sill, the windows, air leakage and people.

Where do the below-grade U-factors come from?

They are the default U-factors and F-factors from the Seattle Energy Code 2018, Appendix A, Table A104.1, for 8 inch concrete walls 2, 3.5 and 7 ft tall. This calculator applies them as an assumption for any location. They are not IECC, ASHRAE or ACCA values. The code notes that "Below-grade wall U-factors are only valid when used with the accompanying below-grade slab F factor, and vice versa."

What does the 7 ft in the table measure?

It is the height of the concrete wall from the slab to the top of the mud sill. Section A104.2 says "The wall is assumed to extend from the slab upward to the top of the mud sill for the distance specified in Table A104.1, with 6 inches of concrete wall extending above grade." So a 7 ft row is about 6.5 ft in the ground, and the wall above the mud sill is counted as a regular wall.

Why does the floor loss go up when I insulate the walls from inside?

Because the table pairs each wall U with its own floor F. In the R-11 and R-19 interior rows the F-factor is higher than for bare concrete at every height, so more of the heat is booked to the slab. The total below-grade loss still drops a lot.

My concrete wall is under 2 ft tall. Can I use this?

Not as a basement. The Seattle code says "Basements shallower than two feet should use on-grade slab coefficients." This page does not model a slab on grade, so the result would not be right for that space.

What temperatures and climate zone should I enter?

Use the 99% winter heating design temperature and the 1% summer cooling design dry-bulb for your location, plus its IECC climate zone. Both design temperatures come from the ASHRAE Handbook - Fundamentals, Chapter 14 climatic design data. The HVAC design temperatures page on BuildSolver lists them by location.

Does this tell me what size mini split or furnace to buy?

No. It gives heating and cooling loads in BTU/h. Equipment selection (Manual S) and a room-by-room Manual J for the whole house are in the BuildSolver Pro and Team plans.

Is this a Manual J calculation?

No. It is a simplified heating and cooling estimate for one space, using U times area times temperature difference, the Seattle below-grade defaults, air leakage, sun through the glass and people.

Below-grade coefficients this calculator uses

These are the default U-factors and F-factors from the Seattle Energy Code, Appendix A, Table A104.1, which this calculator applies as an assumption for any location. Wall U is in BTU/h per sq ft of wall per °F. Floor F is in BTU/h per foot of slab perimeter per °F. The 2, 3.5 and 7 ft columns are concrete wall heights measured from the slab to the top of the mud sill. Section A104.2 says "The wall is assumed to extend from the slab upward to the top of the mud sill for the distance specified in Table A104.1, with 6 inches of concrete wall extending above grade." The code also says "Below-grade wall U-factors are only valid when used with the accompanying below-grade slab F factor, and vice versa." and "Basements shallower than two feet should use on-grade slab coefficients." and "Heat-loss calculations for wall areas above-grade should use above-grade wall U-factors, beginning at the mudsill."

Below-grade wall U-factor and slab F-factor by concrete wall height and insulation
Insulation2 ft wall, U and F3.5 ft wall, U and F7 ft wall, U and F
UninsulatedU 0.331, F 0.58U 0.271, F 0.51U 0.185, F 0.43
R-11 InteriorU 0.063, F 0.67U 0.058, F 0.61U 0.051, F 0.541
R-19 InteriorU 0.042, F 0.68U 0.041, F 0.62U 0.036, F 0.54
R-10 ExteriorU 0.089, F 0.56U 0.075, F 0.52U 0.058, F 0.47

The code publishes these numbers as defaults for code calculations. This calculator multiplies them by the full difference between the indoor and the winter design temperature, the same difference it uses for the wall above the mud sill. That is its own assumption. This calculator does not model ground temperature, so the below-grade lines are likely on the high side. Source table on up.codes.

Moisture and dehumidification

A basement load in BTU/h says nothing about how much water the air holds. This page counts latent heat from people only. For pints per day, use the dehumidifier size calculator for the same space.

Used on the same job

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Equipment selection (Manual S) and room-by-room Manual J are in the Pro and Team plans.

The chat has no basement or single-space tool, so the basement numbers come from this page. Describe the house in chat and the assistant runs the free whole-house Manual J heating and cooling load for the same job.

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