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Calculator · Sunroom
Sunroom BTU calculator
Enter the room size, the glass on each side, the roof and the design temperatures. The calculator adds up the heating and cooling loads surface by surface and shows how much of the cooling is solar gain through the glass.
- Enter the inside size and wall height, pick the wall or corner against the house and which way the long walls face.
- Enter the glass area on each side, the glass type, the roof, the floor and any solid door.
- Enter the winter and summer design temperatures, the climate zone, the indoor temperatures, people and shading.
- The result shows the heating and cooling loads in BTU/h, surface by surface, with the sun through the glass on its own line.
Example result for the default inputs
16 x 12 ft sunroom, 8 ft walls, house on the north, zone 4A. Heating 68°F inside with 18°F outside, cooling 75°F inside with 92°F outside.
Heating load
6,914 BTU/h
Cooling load
9,971 BTU/h
Sun through glass, part of cooling
7,344 BTU/h
| Floor area | 192 sq ft |
|---|---|
| Exterior wall length | 40 ft |
| Glass in walls and roof | 216 sq ft |
| Glass share of exterior walls and roof | 42% |
| Heating, temperature difference | 50°F |
| Heating, walls net of glass and door | 323 BTU/h |
| Heating, wall glass | 3,456 BTU/h |
| Heating, entry door | 400 BTU/h |
| Heating, roof | 320 BTU/h |
| Heating, floor | 2,000 BTU/h |
| Heating, air leakage at 0.30 ACH | 415 BTU/h |
| Cooling, temperature difference | 17°F |
| Cooling, walls net of glass and door | 115 BTU/h |
| Cooling, wall glass conduction | 1,175 BTU/h |
| Cooling, entry door | 136 BTU/h |
| Cooling, roof | 125 BTU/h |
| Cooling, floor | 131 BTU/h |
| Cooling, air leakage at 0.18 ACH | 85 BTU/h |
| Cooling, sun through wall glass, east factor 1.00 | 7,344 BTU/h |
| Cooling, sun through the glass roof | 0 BTU/h |
| Cooling, people sensible, 2 x 230 | 460 BTU/h |
| Cooling, people latent, 2 x 200 | 400 BTU/h |
For estimation purposes only. Consult a licensed engineer for final designs.
Free without signup, 3 questions a day
How the sunroom loads are calculated
The calculator works one room at a time. Conduction is U x A x dT for each surface, plus infiltration. In summer the solar gain through the glass is added on top, along with the people in the room. Heating and cooling are figured separately, each at its own design temperature.
- Temperature differences
heating dT = indoor - winter design, cooling dT = summer design - indoorA difference at or below zero makes that side's conduction and air leakage zero. The calculator never reports a negative load.- Solid walls
Q = (exterior wall length x wall height - glass - door) x (1 / R) x dT, times 1.15, 1.05 or 1.0 on coolingThe R-value is treated as the whole wall, the way the BuildSolver load engine does it. A wall with no insulation is taken as R-4, this calculator's assumption. The cooling multiplier is 1.15 in zones 1 and 2, 1.05 in zones 3 to 5 and 1.0 in zones 6 to 8. The wall against the house is left out.- Glass conduction and door
Q = area x U x dTGlass U by type is 1.04 single pane, 0.49 double clear, 0.32 double low-e and 0.22 triple. A solid entry door uses U 0.40. The BuildSolver engine sets these U-factors.- Roof
insulated Q = floor area x (1 / R) x dT, glass roof Q = floor area x glass U x dTOn cooling an insulated roof is multiplied by 1.4 in zones 1 and 2, 1.15 in zones 3 to 5 and 1.0 in zones 6 to 8. A glass roof uses the same U as the wall glass.- Floor
slab heating Q = F2 x exterior wall length x dT, slab cooling Q = 0.04 x floor area x dT, crawlspace Q = 0.3 x floor area x dTF2 is 1.0 with no edge insulation, 0.7 with R-5 and 0.5 with R-10. The slab and crawlspace factors are the engine's own.- Air leakage
CFM = ACH x room volume / 60, Q = 1.08 x CFM x dTHeating air changes are 0.20 tight, 0.30 average and 0.50 leaky. Cooling air changes are 0.10 tight, 0.18 average and 0.30 leaky, and both sets come from the BuildSolver engine. 1.08 is the standard-air sensible heat factor at sea level (60 min/h x 0.075 lb/cu ft x 0.24 BTU/lb-°F, ASHRAE Handbook of Fundamentals).- Sun through wall glass
Q = wall glass area x SHGC x zone sun factor x direction factor x shading factorSHGC is 0.86 single pane, 0.62 double clear, 0.40 double low-e and 0.35 triple. The zone sun factor runs from 100 BTU/h per sq ft in zones 1 and 2 down to 55 in zone 5C, with 85 in zone 4A. Direction factors are 1.0 for east or west, 0.7 for south and 0.4 for north. The engine supplies all three sets. Whichever glazed side has the highest factor sets it for all of the wall glass, the same rule the engine uses for one room.- Sun through a glass roof
Q = roof area x SHGC x 1.5 x zone sun factor x shading factorRoof glass faces the sky, so there is no direction factor. The engine takes it at 1.5 times the sun on east or west wall glass of the same type, which is about 2.1 times south glass and 3.75 times north glass.- People
sensible Q = people x 230, latent Q = people x 200BTU/h per person from the BuildSolver engine. The cooling load is the sensible total plus the people latent. Infiltration latent load is not included.
Worked examples for five sunrooms
The first row is the default on the form. Rows two and three change one thing in that room. The last two rows put all of the wall glass on one long wall under a glass roof, first facing south, then turned to face west.
| Sunroom | Heating | Cooling | Sun through glass |
|---|---|---|---|
| 16 x 12 room, house on the north, low-e glass 96 sq ft south and 60 sq ft on each end, R-30 roof | 6,914 BTU/h | 9,971 BTU/h | 7,344 BTU/h |
| Same room, low-e glass roof in place of R-30 | 9,666 BTU/h | 20,682 BTU/h | 17,136 BTU/h |
| Same room, double clear glass in place of low-e | 8,750 BTU/h | 14,634 BTU/h | 11,383 BTU/h |
| Glass on the south long wall only, 96 sq ft, low-e glass roof | 8,207 BTU/h | 15,135 BTU/h | 12,077 BTU/h |
| Same room turned, house on the east, the 96 sq ft glass wall faces west | 8,207 BTU/h | 16,114 BTU/h | 13,056 BTU/h |
The default room is 42% glass across its outside walls and roof, above the 40 percent in the IRC sunroom definition. Swapping its R-30 roof for low-e glass multiplies the cooling load by 2.1, and the sun on the roof alone is 47% of that total. Going from low-e to clear double glass adds 47% to cooling and 27% to heating. Turning the long glass wall from south to west adds 43% to the sun through that wall and 6% to the whole cooling load, with no change to the heating.
Standards and data sources
- National Sunroom Association, Sunroom Datasheet 101, Frequently Asked Questions About Sunrooms - the International Residential Code definition it quotes, a one-story structure attached to a dwelling with a glazing area in excess of 40 percent of the gross area of the structure's exterior walls and roof. It also explains that Category I, II and III rooms have no heating or cooling system, and that Category IV rooms have their own heating or cooling with the house doors and windows left in place.
- 2021 IECC sections R402.2.12 and R402.3.5, sunroom and heated garage insulation and fenestration - code minimums for a sunroom with thermal isolation. Ceiling insulation R-19 in climate zones 0 through 4 and R-24 in zones 5 through 8, walls R-13 in all zones, and in zones 2 through 8 a maximum fenestration U-factor of 0.45 and a maximum skylight U-factor of 0.70. The link opens the Denver adoption on UpCodes, which leaves both sections unamended. State editions can change these rules, so check the one your jurisdiction adopted. None of these numbers enter the math.
- BuildSolver load engine defaults - glass U-factors and SHGC, sun factors by climate zone, direction factors, cooling wall and roof multipliers, slab and crawlspace factors, air change rates and people gains. The engine code names ACCA Manual J 8th edition tables as their origin, and several values were recalibrated against published load reports. This page uses them as assumptions and does not present them as quoted ACCA figures.
- Design temperatures and climate zone - entered by you. The BuildSolver HVAC design temperatures page returns the winter and summer design temperatures and the IECC climate zone for a ZIP.
Limitations of this calculator
- This is a simplified heat loss and heat gain estimate for one space. It is not a Manual J load calculation and not a room-by-room calculation of the house.
- Glass on each side is checked against that wall alone, sized from the length, the width and which way the long walls face. The solid door is not tied to a side, so it is only checked against all of the outside walls together.
- The highest direction factor among the glazed sides is applied to all of the wall glass. A room with glass on three sides always picks up the east or west factor, which keeps the cooling number on the high side.
- A glass roof uses the same U-factor as the wall glass, with no adjustment for slope.
- Infiltration latent load is not included, only the people latent. In a humid climate the real cooling load and the moisture to remove will run higher.
- Heat that moves through the wall shared with the house is not counted. That fits a room closed off from the house. A room left open to the house shares its load with the house system.
- There is no allowance for duct losses, altitude, lighting or appliance gains in the room.
- Nominal R-values are used as the whole wall or roof, with no allowance for framing, air films or aluminum frames.
- The loads are raw BTU/h. Equipment is picked from the manufacturer's performance data at your design conditions, which is outside this page.
Common sunroom sizing mistakes
- Sizing a sunroom by square footage. Per square foot rules come from ordinary rooms with a few windows. A sunroom carries far more glass than an ordinary room, so its load follows the glass area, the direction it faces and the roof far more than the floor area.
- Using the same glass factor for every direction. West glass takes the late afternoon sun at the hottest part of the day. This calculator gives it a factor of 1.0 against 0.7 for south, so the same glass turned west carries more cooling.
- Forgetting the roof. A glass or polycarbonate roof sees sun for most of the day. On a small room the roof can outweigh all of the wall glass, so check the roof line in the result before you look at anything else.
- Counting a glass door as a solid door. A sliding patio door is glass. Add its area to the glass for that side and keep the solid door field for a real opaque door.
- Sizing only for summer. In a cold climate a sunroom can need more heat than cooling. Run both sides with the winter and summer design temperatures before you settle on a heat source.
Common questions about sunroom BTUs
How many BTUs does a sunroom need?
It depends mostly on the glass and the roof. In the default example on this page, a 16 x 12 ft room on the south side of the house with 216 sq ft of low-e glass on its three outside walls and an R-30 roof in zone 4A needs about 6,900 BTU/h of heat at 18°F outside and about 10,000 BTU/h of cooling at 92°F. About 7,300 BTU/h of that cooling is sun through the glass.
Does a glass roof make a big difference?
Yes. The engine takes sun on roof glass at 1.5 times the sun on east or west glass of the same type, about 2.1 times south glass and 3.75 times north glass, and the roof covers the whole floor. In the worked examples, swapping the R-30 roof for low-e glass roughly doubles the cooling load.
Which way should sunroom glass face?
For cooling, south or north glass adds less than east or west glass. This calculator uses a direction factor of 1.0 for east or west, 0.7 for south and 0.4 for north. For winter sun, south glass collects the most, but the heating load here does not credit sun, so it stays on the safe side.
What is the difference between a three-season and a four-season room?
Sunroom Datasheet 101 from the National Sunroom Association sorts rooms by category. Rooms in Categories I, II and III have no heating or cooling system. A Category IV room has its own heating or cooling with the house doors and windows left in place. This calculator is for a room you plan to heat, cool or both.
What design temperatures and climate zone should I enter?
Use the 99% winter and 1% summer design temperatures from ACCA Manual J Table 1A or the ASHRAE Handbook of Fundamentals climatic data. Record extremes are too severe for sizing. The HVAC design temperatures page on BuildSolver returns both and the IECC climate zone for a ZIP.
Why can the heating load be bigger than the cooling load?
Heating is figured at the full winter temperature difference, often 50°F or more, and gets no credit for sun. Cooling in a mild summer climate works on a smaller difference, so unless there is a lot of east, west or roof glass, heat can be the bigger number.
Is this a Manual J calculation?
No. It is a simplified heat loss and heat gain estimate for one space. A room-by-room Manual J for the whole house with the new room, and equipment selection from manufacturer data under Manual S, are in the BuildSolver Pro and Team plans.
Used on the same job
Run the whole-house load in chat
The chat has no sunroom or single-space tool, so the sunroom numbers come from this page. Manual S equipment selection and a room-by-room Manual J with the new room are in the Pro and Team plans. 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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