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Air Flow Through an Attic

Attics can be designed to be either vented or unvented. Ventilation size and location is determined by many factors such as insulation component used, trade winds engineered into the design and types of vents being utilized.

Most homes are designed to be vented.

Which of the following is the primary purpose of attic ventilation?

  • Cooling the attic, the cost of cooling the living space in the house is reduced.
  • Ventilating the attic allows the house to “breathe” which contributes towards a healthy house?
  • Proper ventilation helps remove moisture in the attic.

If you chose C, then you are absolutely correct. Water and moisture are natural enemies to the structure of the house. Moisture can cause wood rot on the roof sheathing (or deck), rafters, collar ties, purlins, and ceiling joists. Mold and fungus can appear where there is a source of moisture. In northern climates, ice damming on the roof occurs if the attic does not have adequate ventilation. The cost of repair can run into thousands and tens of thousands of dollars.

Proper attic ventilation will provide the following benefits to the home owner:

  • Extends the shingle life, thus extending the life of the roof.
  • Reduces heating and cooling bills.
  • Prevents roof rot.
  • Prevents ice dams in winter.

Air Flow Through an Attic

In hot climates, attic ventilation prevents super heating the attic which can reduce the life of the shingles on the roof.

So what exactly is attic ventilation?

Attic ventilation is nothing more than allowing a constant movement of air in and out of the attic. This movement of air helps move moisture and air that is inside the attic to the outside.

The diagram below illustrates various ways an attic can be vented.

Air Flow Through an Attic

The best attic design is using soffits and ridge vents. Soffits are cutouts or holes in the boards that run horizontally alongside the external walls of the house where the roof meets the external wall. Outside air is drawn through the holes into the attic.

Good attic ventilation depends on

  1. Good attic design
  2. Unobstructed flow of air entering in and out of the attic.

One of the common issues that occurs with insulation is that it is installed in such a way as to block the air flow from the soffit vents.

Insulation Installed Blocking Air Flow From Soffit Vents

Pictured above is a soffit vent which draw outside air into the attic.

Ridge vents are installed along the ridge of the roof and allows air to exit the attic.

Are turbine vents or powered vents required?

In most homes, if proper soffit and ridge venting is installed, there is not a need for a can vent, power vent, or a gable vent.

There may be some instances where additional ventilation using can vents, power vents, or gable vents are required. All of these are relatively inexpensive options, especially compared with costly repairs that result with improper ventilation.

What is the proper ratio of ventilation per square foot of roof?

Ventilation is based on attic floor size instead of roof size. The proper ratio is between 1/300 to 1 ½ inches per square foot of the total attic floor area. An attic floor area of 1,000 square feet would require between 480 sq. inches and 1500 square inches. The more ventilation, the better.

Should I add turbine vents or powered vents?

If the house has proper ridge and soffit vents, then probably not. In fact, adding powered vents could cause problems. Powered vents will suck air out of the attic. If there is insufficient air flow from the soffit vents, then it will begin to actually pull conditioned air from the living space of the house. Conditioned air will seep through ceiling light fixtures, air vents, and other vents that protrude through the attic to the roof. The result is increased cost of cooling and heating.

How can I improve attic ventilation beyond adding vents?

Sometimes moisture can enter the attic from the condition air living space. Here are a few tips to help prevent this from occurring.

  1. Make sure the cracks between the interior walls and ceilings are caulked. You may have to remove crown molding and reinstall it to do this.
  2. Caulk around ceiling air vents.
  3. Ceiling exhaust vents in the bath rooms and utility rooms are also a source. Remove the cover and use foam insulation to seal the vent. Then replace cover.
  4. In the attic, use foam insulation to seal around vent stacks, can lights, light fixtures, and exhaust vents.
  5. Go through the house with a candle. It will sometimes indicate where air is being drawn towards the ceiling.
  6. Look for spider webs which are often located where there is a draft.