Maintained by the BuildSolver engineering team · Updated
Calculator · Room addition
Room addition BTU calculator
Enter the size of the new bedroom, family room or bonus room, how it is insulated, the glass on each side and what sits under the floor. The calculator adds up heat loss and heat gain line by line and shows the heating and cooling load of the room in BTU/h.
- Enter the inside size, wall height and which walls face outside.
- Choose wall and ceiling insulation, enter the window area on each side and the glass type, then pick what is under the floor.
- Enter the climate zone, the winter and summer design temperatures for your location, the indoor settings and how many people use the room.
- The result shows the heating and cooling load in BTU/h with every line behind it. Add the spare capacity of the existing system if you know it for a rough comparison.
Example result for the default inputs
16 x 14 ft addition, 8 ft walls, a long side against the house, 3 exterior walls, zone 4A, 15°F winter and 92°F summer design.
Heating load
5,568 BTU/h
Cooling load
3,994 BTU/h
Sensible part of cooling
3,594 BTU/h
| Floor area | 224 sq ft |
|---|---|
| Exterior wall length | 44 ft |
| Windows and doors in exterior walls | 60 sq ft |
| Heating temperature difference | 55°F |
| Heating, walls net of openings | 1,235 BTU/h |
| Heating, windows | 1,056 BTU/h |
| Heating, entry door | 0 BTU/h |
| Heating, ceiling | 324 BTU/h |
| Heating, slab | 2,420 BTU/h |
| Heating, air leakage at 0.30 ACH | 532 BTU/h |
| Cooling temperature difference | 17°F |
| Cooling, walls net of openings | 401 BTU/h |
| Cooling, window conduction | 326 BTU/h |
| Cooling, sun through windows, E factor 1.00 on all glass | 2,040 BTU/h |
| Cooling, entry door | 0 BTU/h |
| Cooling, ceiling | 115 BTU/h |
| Cooling, slab | 152 BTU/h |
| Cooling, air leakage at 0.18 ACH | 99 BTU/h |
| Cooling, 2 people, sensible | 460 BTU/h |
| Cooling, people, latent | 400 BTU/h |
For estimation purposes only. Consult a licensed engineer for final designs.
Free without signup, 3 questions a day
How the addition load is calculated
Heating and cooling are added up surface by surface. On the heating side every wall, window, ceiling and floor loses heat in proportion to its area, its U-value and the gap to the winter design temperature, and air leakage adds its share. Cooling uses the same surfaces at the summer gap, then adds sun through the glass and the people in the room.
- Temperature differences
heating dT = indoor - winter design, cooling dT = summer design - indoorA difference at or below zero makes that side's surface and air leakage lines zero. The calculator never reports a negative load.- Exterior walls
Q = (exterior wall length x wall height - window and door area) x (1 / R) x dT x wall factorExterior wall length follows the walls you pick. A long side against the house removes one long side, a corner removes one long and one short side. The R-value is treated as the whole wall, the same way the BuildSolver load engine does it. A wall with no insulation is taken as R-4, this calculator's assumption. The wall factor is 1 for heating and, for cooling, 1.15 in zones 1 and 2, 1.05 in zones 3 to 5 and 1.0 in zones 6 to 8.- Windows and door
Q = area x U x dTWindow U by type is 1.04 single pane, 0.49 double clear, 0.32 double low-e and 0.22 triple. A solid door uses U 0.40. These are BuildSolver engine defaults, used here as assumptions.- Sun through windows
Q = total glass area x SHGC x zone solar factor x orientation factorSHGC is 0.86 single, 0.62 double clear, 0.40 double low-e and 0.35 triple. The zone solar factor runs from 55 to 100 BTU/h per sq ft. The orientation factor is 0.4 north, 0.7 south and 1.0 east or west, and the highest one among the sides you gave glass is applied to all of the glass, as the BuildSolver room-by-room engine does. Cooling only.- Ceiling
Q = floor area x (1 / R) x dT x ceiling factorThe ceiling factor is 1 for heating and, for cooling, 1.4 in zones 1 and 2, 1.15 in zones 3 to 5 and 1.0 in zones 6 to 8, engine defaults for sun on the roof. With a heated room above, the line is zero.- Floor
slab heating Q = F2 x exterior wall length x dT, slab cooling Q = 0.04 x floor area x dT, crawlspace Q = 0.30 x floor area x dTF2 is 1.0 with no edge insulation, 0.7 with R-5 and 0.5 with R-10. A floor over an unheated garage is counted like the vented crawlspace, with outdoor air below, which is this calculator's assumption. Over a heated room the floor adds nothing.- Air leakage
CFM = ACH x room volume / 60, Q = 1.08 x CFM x dTAir changes per hour are 0.20 tight, 0.30 average and 0.50 leaky for heating, and 0.10, 0.18 and 0.30 for cooling, from the BuildSolver engine. 1.08 is the sea-level sensible heat factor for air. Moisture in the leaking air is not counted.- People
sensible Q = people x 230, latent Q = people x 200BTU/h per person, BuildSolver engine defaults. The cooling load is the sensible total plus the people latent line.
Worked examples for one addition, four ways
The first row is the default on the form. Each of the other rows changes one thing about the same 16 x 14 ft room attached along a long side, zone 4A, 15°F winter and 92°F summer design: the floor, the glass, or the insulation and air sealing.
| Addition | Heating | Cooling |
|---|---|---|
| 16 x 14 addition on a slab, long side against the house, R-13 walls, R-38 ceiling, 60 sq ft low-e glass | 5,568 BTU/h | 3,994 BTU/h |
| Same room over a vented crawlspace or an unheated garage | 6,844 BTU/h | 4,984 BTU/h |
| Same room on a slab with clear double-pane glass | 6,129 BTU/h | 5,289 BTU/h |
| Same room with R-21 walls, R-49 ceiling, R-10 slab edge, tight | 3,637 BTU/h | 3,771 BTU/h |
In the default room, sun through the low-e glass is 51% of the cooling load, more than any other line. Swapping it for clear double-pane glass raises cooling by 32% while heating moves 10%. Moving the room onto a vented crawlspace or over an unheated garage, counted the same way here, adds 1,276 BTU/h of heating over the uninsulated slab. The better walls, ceiling, slab edge and air sealing in the last row cut heating by 35% and cooling by only 6%, because the sun through the glass does not change.
Standards and data sources
- BuildSolver load engine defaults - window U and SHGC, door U, slab F2 and floor U, air change rates, zone solar and wall and ceiling cooling factors, orientation factors and people gains. The engine code names ACCA Manual J 8th edition and ASHRAE Fundamentals tables as their origin, and several values were recalibrated against published load reports. This page uses them as assumptions, not as quoted ACCA figures.
- Outdoor design temperatures and climate zone - entered by you. The 99% heating and 1% cooling design temperatures and the IECC climate 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 room. It is not a Manual J load calculation and not a room-by-room calculation of the house with the addition in it.
- It does not judge the existing furnace, air conditioner or heat pump against the room, and it does not size a mini split or any other unit. The rough check only puts the addition load next to the spare capacity you entered.
- Moisture in outside air leaking into the room is not counted, only the latent heat from people. In humid climates the real cooling load runs higher than the number here.
- The highest orientation factor among the sides with glass is applied to all of the glass. A room with most of its glass facing north and a little facing west is counted as if all of it faced west.
- No credit for shades, overhangs or trees, no lights or equipment, and no duct losses or gains. Ducts run through an attic or a garage can add a lot.
- A room over an unheated garage is counted as if outdoor air were under the floor, with the engine's vented crawlspace floor U. An insulated floor over a garage that stays above outdoor temperature loses less than that.
- Nominal R-values are used as the whole wall or ceiling, with no allowance for studs or insulation gaps. Knee walls and sloped ceilings in a bonus room are taken at the floor area of the ceiling.
Common room addition load mistakes
- Using a BTU per square foot rule for the whole room. A rule of thumb ignores how much glass faces west, what is under the floor and how many walls face outside. Two additions with the same floor area can land far apart.
- Tapping into the nearest duct and hoping. Adding a supply run to a system that was already sized for the house takes air away from the other rooms. Check spare capacity and airflow before you cut in a new branch.
- Forgetting the floor over a garage. A bonus room over a garage has a cold floor on the heating side and often a hot one in summer. Air seal and insulate that floor with the same care as the exterior walls, and pick the floor option on this page so it shows up in the load.
- Counting glass without its direction. West and east glass take the afternoon and morning sun head on. On a low-e window that is the single biggest cooling line in many additions.
- Picking a unit straight from the load. The load is the heat the room gains or loses at design. Equipment capacity changes with outdoor temperature, so choose it from the manufacturer's performance data at your design conditions.
Common questions about room addition loads
How many BTU does a room addition need?
It depends on the glass, the insulation, the floor and the climate. In the default example on this page, a 16 x 14 ft addition on a slab, attached along a long side with three exterior walls, R-13 walls, an R-38 ceiling and 60 sq ft of low-e glass needs about 5,600 BTU/h of heat at 15°F outside and about 4,000 BTU/h of cooling at 92°F outside.
How do I compare the addition with my existing system?
Enter the spare heating and cooling capacity if you already know it. The page shows it next to the addition load, with the load as a percent of that spare, as a rough check. It does not work out the spare capacity and does not judge the system. For that you need the load of the whole house with the room added, the output of the equipment at design and the airflow that reaches the room. The free whole-house Manual J in chat or on the Manual J calculator page gives the house load. A room-by-room Manual J with the new room and Manual S equipment selection are in the Pro and Team plans.
How do I enter a bonus room over a garage?
Pick the floor over an unheated garage. This calculator has no separate garage floor, so it counts that floor like a floor over a vented crawlspace with outdoor air below. For a well insulated floor over a garage that stays warmer than outside, the heating number runs on the high side.
Why does the cooling number jump when I add west windows?
Sun through glass is counted with an orientation factor of 1.0 for east or west, 0.7 for south and 0.4 for north, and the highest factor among the sides with glass is applied to all of the glass. Adding even a small west window moves all of it to the west factor.
What outdoor temperatures should I use?
Use the 99% winter heating design temperature and the 1% summer cooling design temperature for your location, not record lows or highs. The HVAC design temperatures page on BuildSolver lists them with the climate zone.
Does this tell me what size mini split to buy?
No. It gives the heating and cooling load of the room in BTU/h. Pick equipment from the manufacturer's performance data at your design temperatures.
Is this a Manual J calculation?
No. It is a simplified heating and cooling estimate for one room, using U times area times temperature difference plus air leakage, sun through glass and people. A room-by-room Manual J for the whole house is a separate BuildSolver Pro and Team feature.
Used on the same job
Run the whole-house load in chat
The chat has no single-room tool, so the addition number comes from this page. Describe the house in chat and the assistant runs the free whole-house Manual J heating and cooling load, and the same load is free on the Manual J calculator page. A room-by-room Manual J of the whole house with the new room, and equipment selection under Manual S, are in the Pro and Team plans.
Open the chat