For years, the attic fan market has competed on one number: CFM. Bigger fans, bigger claims. Solar Blaster has taken a different path since at least 2013. Instead of one high-volume fan, we use several lower-CFM solar units that boost the passive ventilation your roof already has. Here's why.
Code and warranties already point to balance
Building codes (IRC R806 and its equivalents) and most shingle manufacturers' warranties (GAF's ventilation bulletin, for example) call for balanced net free vent area. That's typically a 1:150 or 1:300 ratio, with at least as much intake at the soffits or eaves as exhaust up top. The goal is steady airflow from eave to ridge that washes the whole roof deck and carries off heat and moisture.
Our fans are sized and placed to work inside that design, not override it. They go in or near standard passive vent spots and run hardest when the sun is strongest.
What can go wrong with one big fan
Building scientists, including Joe Lstiburek and others tied to Building Science Corporation, Energy Vanguard, and the DOE's Building America program, have long warned about high-volume powered attic ventilators. The main problems:
- Negative pressure. A strong fan can pull cooled air out of your living space through ceiling gaps around recessed lights, attic hatches, and plumbing or wiring holes. That works your AC harder. A 1994 field study measured attic fans pulling 231 CFM of cooled air out of one house.
- Short-circuiting. The fan pulls from the nearest openings instead of drawing air across the full roof deck, which leaves dead zones and hot spots.
- Backdrafting and moisture. Depressurizing the attic can backdraft a water heater, furnace, or boiler, which can lead to carbon monoxide, and it can add humidity to the house.
- Inflated ratings. Advertised CFM is usually a free-air number. Once a fan is installed with flashing, screens, and limited intake, it can only move as much air as the intake allows.
- Robbing the ridge vent. A fan is only as good as the intake it gets. Put a high-volume fan next to a ridge vent and it pulls air from the ridge vent instead of drawing cool, low air in from the soffits. GAF warns that mixing a power fan with soffit and ridge vents can reverse the airflow. That same suction can pull in whatever is nearby, including smoke from a wood-burning chimney.
Solar power takes away the electric bill, but it doesn't fix the pressure problem if the fan is oversized and the system is out of balance.
Uniform flow is the goal
Moving air evenly through the whole attic and across the roof deck does a better job of scrubbing heat and moisture than one strong column of air at a single point. Our units are modest by design, about 35 CFM for standard fans and about 96 CFM for Mega models. Using several of them spreads that airflow out, so it works with the natural stack effect instead of fighting it.
Our units are also hybrids. When the fan isn't running, they still vent passively like a standard code vent. A big powered fan can't do both.
Dan Rheaume's decades as a roofer shaped this approach. Hot spots, short-circuiting, and moisture damage from oversized single fans are things roofers see in the field all the time.
Match the fan to the space
Bigger isn't automatically better. Size the fan to the space. In a 1,000-cubic-foot space with proper intake, Solar Blaster lists an air exchange of about 10.4 minutes for the Mega RoofBlaster and about 28 minutes for the standard RoofBlaster. Pick the one that fits your structure, not the one with the biggest number on the box. Even the Department of Energy's Building America Solution Center says that if you use an attic fan at all, use a low-CFM solar one, about 50 CFM per 1,000 square feet. It also notes that Georgia's energy code bans attic fans unless they're solar.
Being honest about the trade-offs
No ventilation approach is perfect for every house. In a hot climate, a properly sized high-CFM fan can pull more heat out of an attic that has plenty of intake and a well-sealed ceiling. Multiple small units also mean more roof penetrations to flash correctly. And independent, side-by-side testing of distributed low-CFM systems against high-CFM fans is still limited.
What matters most is the house itself: how airtight the ceiling is, how much intake you have, your climate, your insulation, and your roof. On most homes, those details affect results more than which fan brand you pick.
The bottom line
If your attic already has decent soffit and ridge or box vents, a balanced, multi-point approach is the safer, "first, do no harm" choice. It keeps the pressure balance your roof was designed for, avoids pulling cooled air out of your home, cools the whole deck evenly, and stays in line with code and your shingle warranty.
A bigger number is easier to market. Balance is what the physics rewards.
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.