Most roof specs name the shingle, the underlayment, the drip edge and the ridge vent, then say nothing about the small piece of plastic or board that sits between the attic insulation and the underside of the roof deck at the eave. That piece is the baffle. It keeps blown or batt insulation from sliding into the soffit opening and choking the air that is supposed to move from the soffit up through the attic and out the ridge. If the spec does not name it, the framer or the insulation crew may skip it, and nobody will see the gap until the attic runs hot in July or the deck shows staining in February.
Why the eave is where the air gets blocked
A typical St. Louis house has soffit vents along the eave and a ridge vent or box vents higher up. Air enters low, crosses the underside of the deck, and leaves high. That only works when each rafter bay has a clear channel from the soffit into the attic. Loose-fill insulation is the usual culprit. It is blown in to a target depth, and it piles up against the first foot of attic floor at the eaves, often right where the soffit vent opens. The vent is still on the outside of the house, but the air behind it has nowhere to go.
The residential code's attic ventilation section assumes the intake is open. A balanced system of soffit and ridge venting counts only when both the intake and the exhaust are unobstructed. A soffit vent with a wall of insulation in front of it is not an open intake, and you cannot tell from the ground whether the channel exists. That is why the spec should require it in writing.
What the spec should name
A roof spec that takes baffles seriously says a few things in plain words.
- Baffles are installed at every rafter bay at the eave, not only where the crew remembers.
- The baffle type is named, whether rigid plastic, foam board or a rafter vent chute, and the product is written down rather than described as a generic guard.
- Each baffle holds a clear channel from the soffit opening into the attic, with insulation kept back from the vent.
- The count of baffles per side of the house is written out, so the number can be checked against the rafter count.
- Photos of the baffles are taken before insulation is blown or laid back over them.
Rigid plastic baffles hold their shape and are the common choice. Foam board baffles are cut to fit and can crack if a crew bends them while working around the rafters. Either can work. The point is that the material is named and that someone has looked at each bay. Baffles are not a shingle line item, but they sit under the shingle system, and they decide whether the ventilation the roof depends on is real.
You can check part of this yourself. If there is an attic hatch, look at the eaves with a flashlight before the roof work starts. Note whether insulation is sitting against the soffit openings and whether any baffles are already there. A phone photo taken from the hatch, dated and kept with your roof file, gives you a starting point. It also tells the roofer that you know what the eave should look like.
Where baffles come up on a roof job
Baffles usually come up when the roof is being replaced or when the attic is open for other work. If the roof is coming off and the insulation is already in place, a crew can often reach the eaves from the attic side. If the ridge vent is being replaced, the spec should also say whether the soffit intake is being inspected at the same time. A roofer who replaces shingles over blocked soffits has not fixed the attic, even if the new surface looks right from the street.
The attic matters in a St. Louis winter. Warm, moist air from the house rises into the attic. When the attic and the deck are cold, that moisture can condense on the underside of the sheathing and freeze. Blocked soffits make it worse because the moist air stays put. Over several freeze and thaw cycles, the deck can show staining, and the nail tips can frost over. Good baffles will not stop all frost, but they keep air moving so the deck can dry between cold nights and warm afternoons.
What it costs
Baffles are a small line on a roof bid. They are usually priced per rafter bay or as a lump sum for the eave run. The cost is not usually the reason they get skipped. The reason is that they disappear once insulation goes back in, so nobody sees them again. A bid that says ventilation without a baffle line may still include them, but the homeowner should ask. If the baffles are priced separately, compare that line across bids the same way you compare the underlayment or the drip edge. A bid that leaves them out may look cheaper only because the attic work is missing from it.
If insulation has to be pulled back to reach the eaves, that labor is real and belongs in the bid. It should not appear later as a change order after the crew is already in the attic and the insulation is on the floor. Ask how the insulation will be handled, whether it is pulled back and reinstalled or topped up, and who pays for the extra depth if the insulation is replaced.
What to ask
- Which baffle product will be installed, by name, and how many per rafter bay?
- Will the crew take photos of every bay at the eave before the insulation goes back?
- If insulation has to be pulled, is that labor in the bid or in a change order?
- Who checks the soffit intake at the final walk, and will they show you an open bay?
- Does the written scope from Keys list the baffles and the attic photos as deliverables?
The Keys office can answer these at (314) 220-2333 before you sign. Ask in writing, so the answer goes into the scope rather than staying in a phone call.