• THERE SHOULD BE AT LEAST 1 SQ. FT. OF UNRESTRICTED ATTIC VENTILATION FOR EVERY 150 SQ. FT. OF CEILING AREA IF THERE IS NO VAPOR BARRIER.
• THERE SHOULD BE AT LEAST 1 SQ. FT. OF NET FREE VENTILATION AREA (NFVA) FOR EVERY 300 SQ. FT. OF CEILING AREA IF THERE IS A VAPOR BARRIER.

BRENDA AT LOMANCO (800) 643-5596 CONFIRMS THE FOLLOWING:

1. WITH A STATIC SYSTEM (EXAMPLE - SOFFIT / CORNICE VENTS AND CONTINUOUS RIDGE VENT) THE NFVA SHOULD BE DIVIDED EQUALLY BETWEEN MAKE-UP AND EXHAUST AIR.

2. 80 SQ. INCHES OF NFVA / 100 CFM OF POWER VENTILATION IS A GENEROUS THUMB RULE FOR MAKE-UP AIR PROVISION WITH A POWER EXHAUST UNIT. ALL POWER VENTILATOR MANUFACTURER’S SPECIFY HOW MUCH NFVA IS REQUIRED WITH THEIR UNITS AND THIS DEPENDS ON THE SIZE OF THE UNIT.

SHINGLE ROOF SYSTEMS - MUST HAVE VENTILATION BELOW DECKING TO VENT AND CONTROL MOISTURE IN ATTIC & TO INSURE MANUFACTURERS WARRANTY ON SHINGLES WILL NOT BE VOIDED. 

CRAWLSPACE VENTILATION CONCERNS - THIS LINKS LEADS TO A GREAT ARTICLE ABOUT CRAWLSPACE VENTILATION CONCERNS.  CRAWLSPACE MYTHS