"What size solar attic fan do I need?" is one of the most common questions we get, and the honest answer is that it depends on the space. Not on the biggest number on the box. A fan that's right for a 10x12 backyard shed is a different job than one for a 40-foot shipping container or a 1,500-square-foot attic. The good news is that the math is simple, and you can do it with a tape measure and a calculator.
Step 1: Find the volume of your space
Fans move air in cubic feet per minute (CFM), so start with cubic feet:
- Sheds and containers: length x width x average inside height. A 10x12 shed with an average inside height of 8 feet is 960 cubic feet. A space measuring 19 x 7.5 x 7.5 feet inside, close to a 20-foot container, is about 1,069 cubic feet.
- Gable attics: the attic is a triangle in cross-section, so use about half the peak height as your average. A 1,500-square-foot attic with a 6-foot peak averages 3 feet, or about 4,500 cubic feet.
Measure the inside, not the outside, and round up if you're not sure.
Step 2: Divide by the fan's CFM
Here's the formula: volume / CFM = minutes per air exchange. That's how long it takes the fan to move as much air as the space holds.
Our own product pages use a 10x10x10 room (1,000 cubic feet) as the yardstick. The Solar Mega RoofBlaster is rated at 96 CFM and lists one air exchange every 10.4 minutes "with proper intake ventilation." The standard Solar RoofBlaster is rated at 35.2 CFM and lists one every 28.4 minutes, also "with proper intake ventilation."
Apply that to the examples above:
- 10x12 shed (960 cu ft): about 27 minutes with a RoofBlaster, or about 10 minutes with a Mega RoofBlaster.
- Container-size space (1,069 cu ft): about 30 minutes with a RoofBlaster, or about 11 minutes with a Mega RoofBlaster.
- 4,500 cu ft attic: about 47 minutes with one Mega RoofBlaster, about 23 minutes with two, and about 16 minutes with three.
On paper, fan ratings add up. Two fans spread across a roof also pull from more than one spot, which helps move air through the whole space instead of one corner.
Step 3: Size the intake before you add the fan
This is the step most people skip, and it's the one that matters most. Notice the phrase on our product pages: "with proper intake ventilation." A fan can only push out air that has a way to get in.
The building code doesn't size attic ventilation by fan CFM. It sizes it by vent opening. IRC R806.2 says "The minimum net free ventilating area shall be 1/150 of the area of the vented space." It allows 1/300 when the vents are balanced, with part of the vent area up high and the rest "located in the bottom one-third of the attic space." For that 1,500-square-foot attic, that works out to:
- 1/150: 10 square feet (1,440 square inches) of net free vent area.
- 1/300 with a balanced layout: 5 square feet (720 square inches).
The roofing consultants at IIBEC put the intake side plainly: "Intake area should always be equal to or more than exhaust area, or intake air may be pulled from the interior of the building/residence through openings in the ceiling (e.g., can lights, attic access doors)." Building scientist Joe Lstiburek goes further in his rules for venting roofs: "Put more vents down low than up high."
For sheds and containers, the same idea applies. Put intake vents low, like our 2.5" intake vents, and the fan up high. That's the cool-air-in-low, hot-air-out-high pattern we covered in What a Hot Air Balloon Teaches Us About Attic Ventilation.
Why bigger is not better
It's tempting to buy the biggest fan you can find, but a high-CFM fan that outruns its intake has to get air from somewhere. In an attic, that can mean ridge vents, gable vents, gaps around a chimney, or the house itself. The Department of Energy's Building America Solution Center explains that attic fans "create a negative pressure zone inside the attic. This negative pressure draws conditioned air from the living space into the attic through gaps in the ceiling."
That's why Solar Blaster fans are built to assist passive vents, not overpower them. A right-sized fan paired with enough low intake keeps air moving gently across the whole space. We go deeper on that in Why Solar Blaster Bets on Balance, Not Brute Force.
A quick sizing checklist
- Measure the inside volume of the space in cubic feet.
- Divide by the fan's CFM to get minutes per air exchange.
- Add intake vents low, at least as much open area as your exhaust.
- In attics, check your vent area against IRC R806.2 and your local code.
- Spread out several smaller fans on big or long spaces rather than relying on one large fan.
Being honest about the limits
This math is a starting point, not a guarantee. A few things to keep in mind:
- CFM ratings are fan specs. Real airflow depends on how much intake you have, how restrictive your vents are, and wind. Our exchange times assume proper intake ventilation.
- Solar fans run on sunshine. They move the most air on bright days and slow or stop at night and under heavy clouds.
- There's no magic exchange rate. The IRC sets attic vent area, not a fan speed, so the "right" number of minutes depends on your space, your climate and what you store in it.
- A fan isn't a fix for leaks or wet ground. If water is coming in through the roof or up from bare soil, deal with that first. Our container rain post covers moisture sources in detail.
If you're unsure, give us the measurements and we'll run the numbers with you.
The bottom line
Measure the space, divide by the CFM, and make sure the air has a way in down low. A right-sized fan with balanced intake does more good than the biggest fan you can buy.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.
