How Many Soffit Vents Do I Need?

Attic ventilation is something most of us hardly spare a thought for, until we notice mould developing in the living areas downstairs. A well-ventilated attic can prevent this happening. If you call a specialist for advice, ask them how ‘many soffit vents do I need and why’. You’ll soon learn how well they know their business.

How Do Soffit Vents Prevent Mould Developing

Mould grows almost anywhere that is damp, and the air is reasonably warm and still. You don’t need to be a ventilation engineer to understand how attic ventilation can discourage mould. However, this is not the kind of ‘interior decorators’ job we would like to spend our discretionary money on.

And so the inevitable question arises, ‘how many soffit vents do we really need?’ Perhaps you suspect there is a bare minimum number you must have, on the basis you could always add a few more at a later date. This article explains how to work this minimum out, but first we need to spend a few moments on attic ventilation theory.

The Theory Behind Attic Ventilation That Works

The general idea is to introduce a catchment area low down on the roof, that draws fresh air into the attic from outside. This moves the air already in the roof towards the attic ridge, from where it escapes through vents up there too. This creates a natural airflow through the attic that removes the warm damp air.

The vents in the soffits that admit the fresh air, have a number of outlets the occupants can adjust to regulate, or even stop the flow in winter. However, we seldom recommend closing them at Valiant Exteriors. This is because heat build up from the HVAC system can melt any snow on the roof, and cause ice dams to form above the eaves.

How Many Soffit Vents Would I Need on My Roof?

The number of soffit vents depends on the amount of attic air you need to shift. Roof heights vary, although as a general rule of thumb you need  to calculate the square feet directly above the ceiling:

  • Multiply the length by the width to obtain the square feet at this point.
  • Divide the result by 150 to determine the total number of vents you need.
  • Half of these vents should go in the soffits. The balance belong near the ridge.

You may need to add a few more vents to allow for the characteristics of your roof. However, based on our extensive experience at Valiant Exteriors, the 150-square-foot factor should be sufficient.

More Information

Eavestroughs Are Not Responsible for Ice Dams

Attic Ventilation Must Be Comprehensive

Image of Attic Ventilation System