Attic Ventilation

Temperature and moisture control are the two major reasons you must control attic ventilation.

 

The correct ventilation of the house’s attic is very important for two primary reasons.

  1. During the summer months the excess heat in the attic will lead to excessive air conditioning bills; and
  2. The roofing material will prematurely age therefore drastically shortening the life of the roof.

I have seen twenty year roofs worn out in five years.

Also, moisture produced within the home may move into the attic if ceiling vapor barriers are not correctly installed and used. If this moisture is not exhausted from the attic, it could condense and cause construction materials to deteriorate and contribute to the growth of mold.

To vent, or not to vent, is not the question. The question is how much attic ventilation do you need.

Not enough attic ventilation is bad as the interior of the attic can easily hit in excess of 200 degrees.

Too much attic ventilation can cause excessive energy bills as the attic is drawing cool air from inside the house. Also, moisture accumulation during the winter months can condense and cause ice problems.

Quite often this area of the house can use some improvement to make it perform more efficiently. The object of this article is to help make your home more energy efficient and save you some money.

Basic Definitions Before We Continue
 

Roof Pitch: This is the term used to describe the angle or steepness of a sloped roof. It is stated as a factor of rise over run. The rise is the height [vertically] and varies from 4/12 or 18.43°, to 8/12 or 33.69°, to 12/12 or 45°, to 16/12 or 53.13° roofs.

The run is the horizontal span with a constant of 12 inches.Pitches under 4/12 are considered Low-Slope Roofs and they have very strict specifications. Roof pitches above 4/12 may be considered steep and are subject to additional charges and safety requirements.

Cubic Feet Per minute or CFM: The amount of air flow that occurs in one minute, e.g. 100 CFM, would equal 100 cubic feet of air per minute. Fans are commonly rated by CFM.

Soffit Vents: A soffit vent is the area under an eve. This is the area where the roof overhangs the side walls of the home – typically 12 inches. Looking up from the ground, you should see some open spaces covered with screen, slots or perforations that will allow air to enter the attic space.

Most newer homes today use vinyl soffits. These could be 100% perforated or every third row perforated. The air flow through this material will be approximately 4.78 square inches.

IF you happen to have the more expensive “three row perforated”, the air flow is still only 14.34 square inches per square foot.

You may have an older home where the builder used 1×4 wooden strips with a 4” space of screening between the boards. This is still only 43 square inches per foot.

LPV Vents or Low Profile Roof Vents: These are sometimes called turtle vents.. They usually offer 50 square inches of net free vent area. They are completely enclosed with louvers on three sides to help prevent birds or insects from building nests. They offer a slanted top to keep rain out and come in metal and plastic versions.

Ridge Vents: These are special vents where in the top ridge of the roof is cut out and covered over with this special material.

It can be plastic or metal. The theory is that this ventilates the highest part of the roof and is more conducive to efficient heat transfer. Each manufacturer has their own specifications. Talk with your roofer about your application.

Roof Turbines: I’m not a big fan of these. We had one in Florida that seized during a rain storm. In all fairness, I should add it really was a squall where the rain and the wind come from different directions about 60 MPH. We incurred several thousand dollars in damages. Of course, it was not covered by insurance.

Rafter Vents: These are plastic or foam baffles that do not allow the insulation the press against the bottom of the roof. They allow enough room for air to flow from the soffits all the way to the top of the roof.

They also do not allow “blown in insulation” to flow over the vent holes in the soffits IF they are properly installed.

Power Vents: There are many pros and cons for using power for attic ventilation. IF you decide to use this method, make sure you have a thermostat to control the installed fan. Remember, you are using electricity to save electricity. They have their place and specific applications will require a power fan, just not all applications apply.


To calculate the correct amount of air it would take to keep your home within acceptable temperatures, the air in your attic should be changed every two minutes. Acceptable is considered 10 degrees above the ambient outside temperature.

The temperature usually is much higher. The most common method is to provide soffit vents and roof vents sized to the appropriate air flow you are looking for. This method is using the convection of heated air to keep rising. Convection is the hotter the air, the faster it rises unless blocked. If there is sufficient area for intake and output this method works just fine.

How much vent area do you need? Common rules of thumb apply here.

  • IF the attic floor has a vapor barrier (not common with blown in insulation – more common with fiberglass bats), the standard is 1:300. One square foot of attic ventilation for every 300 square feet of attic.
  • IF the attic floor does not have a vapor barrier, the rule is 1:150.

Lets say your house has a vapor barrier and the house attic dimension is 1200 SF. You will need 4 SF .of attic ventilation. That is 4 SF of air intake and 4 SF of air outlet. IF your house does not have the vapor barrier you will need 8 SF. of attic ventilation.

The soffit ventilation is relatively easy to install if your house has eaves. There are many devices that can be installed easily. Soffit/under eave vents are the easiest. Cut a hole smaller than the opening and screw the vent in place

Installing a roof vent can be as difficult as you want to make it. Some people are squeamish about cutting a hole in the roof. If you are not comfortable, hire a pro to do it.

The money you spend will be quickly returned in energy savings.

Dave Altman
 

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