Styrofoam ventilation chutes are the most common type of ventilation chute for attic ventilation between rafter spaces. The chutes provide a 1 inch air space for air to enter the attic and provide adequate ventilation to move moisture and, depending on the season, warm or cool air out of the attic. Attic ventilation is extremely important for the health of a home and its overall efficiency. Because of the poor quality and design of these chutes, there can be some major problems that can arise with moisture and efficiency.
Construction of The Edge Of A Roof
The eaves of the roof for typical residential construction are typically pitched. The truss/rafters sit on the top plate of the house walls on what is called it’s seat. The heal of the truss or rafter is the section that is in line with the exterior wall. This distance needs to be a minimum of 10 inches per residential building code to accommodate a minimum of R-38 insulation typical of fiberglass or cellulose insulation. This heal leaves a large gap to the soffit, which is great for ventilation, but is hardly ever treated properly in modern construction.
Styrofoam Chute Installation Issues
In order for the Styrofoam chutes to work for modern construction, the exterior sheeting needs to be ran up the exterior of the sidewall allowing a 1 inch gap for the chute to slide down into. The Styrofoam chutes are then slid into place and tacked to the bottom of the roof sheathing with staples. Because the chutes have tabs on the sides that sit flat against the bottom of the roof, they leave many gaps and are often installed improperly, not going into the soffit area far enough, or not taking up the full cavity space. The gaps can allow insulation to fill up the soffit in areas when blown, decreasing the amount of ventilation into the attic.
The biggest problem with these types of chutes is that they are often laid against the roof sheathing with a piece of fiberglass insulation stuffed along the edge to fill the gap between the top plate of the wall and the Styrofoam chute. This creates a problem with what is referred to as edge effect where the edge of the house losses significant heat due to improper insulation or lack of proper insulation R-value. This type of insulation also causes mold and mildew build up along the edge of the roof and can contribute to wood rotting in severe cases.
Discontinuing the use of Styrofoam vents and using formed cardboard proper vents attach by stapling the tabs to the rafter framing and the top plate. This significantly decreases wind washing, eliminates insulation accidentally getting into the soffit, and allows all of proper ventilation across the entire rafter cavity. These types of vents are more expensive, but actually will cost less for the total construction when plywood or OSB is figured in because it is unnecessary to run the exterior sheathing all the way up that far past the soffit line.
Styrofoam ventilation chutes are the most common and will provide minimum code attic ventilation in most cases. You need to be aware that this type of installation and product do create some interesting issues, however, you need to be prepared for some of these problems if they occur.