Attic ventilation Baffles

February 17, 2021
What are Soffit Baffles?
In a house with vented cathedral ceilings, the baffles should begin above the ventilated soffit and extend all the way to the ridge.

Manufactured ventilation baffles

Most builders install manufactured ventilation baffles. The most popular baffles are made from polypropylene, vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate)., expanded polystyrene, or cardboard.

The ideal ventilation baffle:

  • Is durable and stiff — stiff enough to resist the pressure of dense-packed insulation;
  • Extends the entire width of the rafter bay;
  • Is relatively easy to air seal; and
  • Creates a ventilation channel that is at least 2 inches deep. (While building codes require that ventilation channels be at least 1 inch deep, most experts advise that 2-inch-deep channels perform better than 1-inch-deep channels.)

As far as I know, the only commercially available baffle that fulfills all of these criteria is the SmartBaffle. Another product — the AccuVent — comes close, but the depth of the ventilation channels created by the AccuVent is only 1 3/4 inch instead of 2 inches.

Most manufactured baffles made of expanded polystyrene or cardboard aren't sturdy enough to resist the pressure of dense-packed insulation.

Site-built baffles

The best-performing ventilation baffles are site-built. It’s possible to make site-built baffles that are sturdier, more airtight, and more vapor-permeable than any manufactured baffles.

[Photo credit: Graham Fisher]

In most cases, you want to create a ventilation channel that is between 1 inch and 2 inches deep. A 1-inch-deep channel barely meets code, while a 2-inch-deep channel allows for more air flow and probably performs better. If you’re working in a tight area, however, you may prefer to build 1-inch-deep or 1 1/2-inch-deep channels to leave more room for insulation.

The first step to assembling site-built baffles is to install “sticks” in the upper corners of each rafter bay. These “sticks” are the stops that establish the depth of the ventilation channel. For 1-inch-deep channels, most builders rip 1"x1" pieces out of softwood lumber. These sticks are then tacked into place with a few finishing nails.

Needless to say, the sticks don’t have to be continuous; short lengths separated by gaps will also work.

For deeper channels, it’s possible to rip sticks that measure 1"x2". It’s also possible to cut rectangular pieces of rigid foam to any desired dimension, and to use those pieces as stops.

If you are planning to use foil-faced rigid foam as your baffle material, you can score the foil facing on one side to create a hinge, and fold the foam over so that you create integral stops that are oriented 90 degrees to the baffle.

A video shows Mike Guertin demonstrating this method: How to Ventilate Rafter Bays When Adding Insulation. One disadvantage of Mike Guertin’s approach: unless shims are added to the folded-over stops, the channels are only as deep as the thickness of the rigid foam.

What material should use use to create the baffles?

The most common materials used to create ventilation baffles are thin plywood, rigid foam, fiberboard, and stiff cardboard.

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