roof and attic ventilation

Attic and roof ventilation is crucial in cooling strategies during hot weather conditions. Attic temperatures of 130ºF (54ºC) and more are common in summer, causing high cooling bills. Attic fans and attic ventilation using roof vents are a good solution to the attic overheating problems.

Climate, attic designs and ventilation
The content of this page applies to climates with significant differences in summer-winter temperatures. It doesn’t consider the unvented attics approach, or the approach to hot-tropical climates...

Attic ventilation & Insulation

It may seem odd to consider insulation and simultaneously the attic cooling through roof vents. But these combination works in most climates, and supports an energy-efficient approach. Insulation will avoid ice-dams (in cold climates) and can cool and remove moisture from the attic in summer.

The main issue, concerning attic insulation, involves its type and installation - and not its usefulness. In cold and temperate-mild climates, insulation involves the attic/ceiling and materials like fiberglass (see: Attic Insulation) while in hot climates the insulation involves reflective materials and mainly the roof (see: Reflective insulation).

Attic ventilation & Ice damming

In climates with cold-freezing winters, allowing natural ventilation will help to keep the attic cold, reducing the potential for ice damming.

On the other hand, high levels of ceiling and attic insulation will keep the attic cold and more protected from moisture (in winter) since it blocks the entry of heat and moist air from the living space (or even crawlspaces).

Attic ventilation & Insulation & Cooling

A properly sealed and insulated ceiling is crucial to get a cooler home, since the insulation will resist heat transfer into the living space. That’s the usually solution in mild-temperate and cold climates with hot summers.

The solution, in hotter climates, can involve reflective insulation associated to the roof (with no ceiling insulation) and natural ventilation for the whole-house. Attics, in this case, may simply be elevated and open structures, benefiting from natural ventilation as other parts of the house, or more than other parts...).

Note: When adding and installing insulation, do not block the flow of air at the eaves and soffit vents. Rafter vents and soffit vents are crucial to airflow and natural ventilation.

Attic ventilation and moisture

Attic ventilation can also be important to minimize or eliminate moisture problems, though many experts consider that as irrelevant in well sealed and insulated ceilings.

Moisture problems in the attic have their source in moist air carried into attics from below: humid air (the cause of mildew) rises to attics from living and crawling spaces. And to fight it, the best protection is dry basements and crawlspaces, and well insulated and airtight ceiling, ducts, etc. – and not attic ventilation.

Attic ventilation can help minimize moisture problems in the attic. But it will not solve it, truly.

See: Attic Insulation

Attic ventilation and fans

A good attic ventilation will avoid overheating in hot weather conditions. Besides, since attic ventilation reduces temperature swings, venting it through vents will also prevent excessive cooling in winter times, and help alleviate the ice damming problem (besides extending the life of a roof and the installed insulation).

To diminish attic temperatures you may also use attic fans: a cheap and convenient solution in many situations, though not a structural one.

See: Attic fans

Cooling the attic with vents

Attic ventilation with roof vents if very effective in reducing temperature swings, particularly attic excess heating in hot weather.

The effectiveness of this ventilation system demands, however, a properly designed system, with low and high vents along the roof. An improperly distributed web of roof vents will cause loss of effectiveness.

Area of vents and baffles

In cold and temperate climate, the vent’s surface is often calculated at 1/300 of the attic area, and is referred to as NFA or “Net Free Area”, but that value depends also on local climate conditions (and building codes).

In hotter climates, without low temperatures all over the year, larger vent surfaces are advantageous.

This roof vents should be properly distributed. A simple rule of thumb estimates 50% of the ventilation in an attic coming from low on the roof (soffits?) and 50% coming from high on the roof (the ridge?). The soffit vents should be distributed evenly, and include the corners.

Vents should involve low air flow resistance, and some basic judgments over how does the the air blows (through the vents or over them), how easily air flows through the vent filters, or on baffles and their role...