Why Attics Are So Hot

Illustration of hot air flow in homes
Illustration of hot air flow in homes
Hot Air in the Attic

Just about every home in Texas has some type of attic ventilation. The reason for attic venting is easy to understand. The sun beats down upon the roof and heats the air in the attic. Methods have changed over the years, but the International Residential Code and most roofing manufacturers recommend balanced venting of 1 sq.ft of venting for every 300 sq.ft of attic space. The venting can be divided evenly with 0.5 sq.ft of intake venting to 0.5 sq.ft of exhaust venting.  Most modern roof venting in the San Antonio area appears to be a combination of ridge venting to exhaust warm air, and soffit venting, which is venting under the eaves, to draw in the outside air. Both are needed. If one port is expelling air, there needs to be another port that allows air in. Ideally, the port expelling air is positioned high on the roof and the port allowing air in is positioned low on the roof and the eaves are usually the lowest point. Other types of venting common to San Antonio include gable vents, roof louvers and wind turbines.

The idea behind passive attic venting is that as air in the attic warms, it is forced upward by cooler air entering from below and exits near the top of the roof. Convection currents are formed. In a perfect world, a light breeze of about 5 mph blowing across the ridge vents with cool air from the shaded side of the home entering the soffit vents will keep the attic at the optimum temperature. However, we don’t live in a perfect world. Typically what happens is that the sun beats down upon the roof. Air in the attic heats and expands everywhere. It expands out both the roof vents and the soffit vents. It transfers heat through the insulation into the ceiling of the home and then into the air in the home. Air conditioning ductwork is often located in the attic, putting additional workload on the air conditioner to cool the air in the ducts. Everything in the attic is heated, including anything that is stored up there.  What most Texas homeowners need is an efficient way to cool the attic.  That’s where attic ventilation systems come in.

We’ll talk about those in part 2 on Friday.

G’day

Tony

1 thought on “Why Attics Are So Hot”

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