Installing Rafter Vents in an Attic Before Insulating for an Office

Before adding insulation to an attic being turned into an office, one step comes first: a clear channel for air from the soffit vents up to the ridge vent. A homeowner demonstrates installing rafter vents in rafter bays that are 16 in. on center, using Pro Vent products.

Why the vents are needed

His roof breathes through soffit vents and a ridge vent, where a foam filter under the cap shingles keeps out insects. Had he left the space unconditioned, he says, he would not need rafter vents. Because he is insulating, adding heat and drywalling, packing the bays with insulation would make the soffit and ridge vents useless. The channel lets the roof decking breathe; he notes that overheating in summer could damage shingles, and that moisture sitting between insulation and the roof deck could cause rot or potentially mold. He also lists ice dam prevention and energy savings.

Blocking the bottom opening

At the bottom of each bay is a large opening that should be blocked so that all soffit air goes through the vent. He has previously tucked in a sacrificial piece of insulation, but notes moisture can come through fiberglass and suggests mineral wool does not mind moisture. Other options he mentions are accordion-style rafter vents, 1 in. foam sealed with fire-stop foam or caulk, and folded cardboard. He instead bends the first vent into place as an air dam, setting it back far enough that insulation can sit on top of the wall and not lose R value. He wonders whether the inspector will accept it.

Installing and overlapping

He does this before the strapping and before the frame inspection, so the inspector can see it. Roofing nails protrude through the deck; he staples where they are clear, pokes the vent over some, and cuts long ones with snips, a cut-off wheel or a reciprocating saw. He uses 1/4 in. staples and prefers an electric stapler to a hammer tacker.

On overlap, some people leave a gap so moisture can escape and some underlap so it drains toward the soffit. He lays each vent on top of the previous one so they lock in for a tight seal, and advises checking the manufacturer's instructions.

Stopping point and odd bays

With blown-in fiberglass he needs about 14 to 16 in. between the vent top and the insulation; his vents reach about 26 in., which he says leaves room even for more than R49, which he gives as the current code figure. The rafters are 2×8, so he chose blown-in insulation to fill the gaps. For smaller bays he folds the vent and staples it to the sides; for larger ones he suggests foam or caulk on the sides. Ceiling joists force cuts in some bays.

Dormers

The two dormers need airflow from outside. He drills a pilot hole first so he does not go through the roof, then 1 1/4 in. holes high up on the other side, and cuts vents in half (4 ft long) for those bays. A fog machine smoke test is used to see whether fog comes out the top. About 70 vents remained at that point.