Question: Many homebuilders install either cardboard or foam baffles in the attic to keep insulation away from the soffits. Why is it important to keep insulation away from the soffits?
Answer: Unless you’re ready to seal the attic to make it part of the home’s conditioned space, an attic needs balanced, natural ventilation. Proper air flow is crucial to extend the life of a roof and help alleviate ice damming in the winter. Good attic ventilation also helps reduce heat buildup in the summer. By holding back the insulation, baffles allow air to freely enter the attic space and travel upward and out through exhaust vents.
To properly ventilate an attic, 50% of the ventilation should come from low on the roof (generally the soffits) and 50% from high on the roof line (generally the ridge). A vent’s capacity to allow air flow is referred to as “Net Free Area” (NFA). NFA is described in terms of open square inches (in²). Local codes dictate the amount of required NFA, but the typical amount is 1/300 of the attic area. Here’s the equation to calculate the minimum NFA:
For example, a 900 ft² area would require 432 in² of ventilation, 216 in² from low on the roof and 216 in² from high on the roof.
If this balance is disturbed, the attic won’t ventilate properly. Think about what happens when you try to ventilate a space by opening a window. If you open just one window, not much happens. If you open a second window across the room, you get a breeze. This same principal applies to the attic. If there isn’t enough NFA at the low vents or the soffits are blocked by insulation, air won’t move properly through the space. Even adding more vents higher in the attic wouldn’t balance the air flow, because the roof won’t vent any more air than is introduced by the low vents.
Cardboard and foam baffles
There are two types of baffles commonly installed today: cardboard and foam. Cardboard baffles are similar in appearance to pizza boxes, and foam baffles look a lot like egg cartons.
Cardboard baffles are inexpensive and easy to cut to fit a space. They’re typically face nailed to the top plate and at the sides to the trusses. However, cardboard baffles prevent insulation from covering the top plate. Insulation needs to cover the top plate, especially in cold climates, where ice damming is common. Cardboard is also a material that absorbs moisture when it’s present, acting as a food source for mold.
Foam baffles are straight, formed pieces of foam that are nailed to the roof deck. Because these baffles are straight, another piece has to be installed as an end dam to stop insulation from flowing out into the venting space. While less likely to absorb moisture when it’s present, foam is brittle and susceptible to breaking where it’s attached to the roof deck.
A new type of baffle has recently been introduced to the market called AccuVent. Manufactured from 100% recycled PVC, this baffle combines the best features from the other two types. Like a cardboard baffle, AccuVent is face nailed to the top plate; however, because it’s flexible, it can be rolled back over the top plate before it’s nailed to the roof deck. This pocket allows insulation to be installed over the top plate. And unlike cardboard, AccuVent also doesn’t absorb moisture, helping to deter the growth of mold.
Making changes to the design of homes is inevitable as we strive to work the risk out of our business. Almost 100 years ago, John Muir wrote, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." So, as we find better ways to build homes, we always have to keep in mind how these changes affect other parts of the home.
- Arn Burdick
Building Performance Specialist, IBACOS Homebuilding Quality Consulting