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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Adding insulation without keeping the soffits clear cooks your roof deck. This gives the ventilation area your attic needs and the baffle count to protect it.
With these inputs:
| Result | Value |
|---|---|
| Net free vent area required | 1,152 sq in |
| Intake (soffit) — 50% | 576 sq in |
| Exhaust (ridge) — 50% | 576 sq in |
| Ridge vent length | 32 ft |
| Baffles required | 60 baffles |
| Material cost | $432 |
Net free area = attic floor area ÷ 150 (IRC R806.2); ÷ 300 only under the exception — balanced vents (40–50% exhaust within 3 ft of the ridge) plus, in climate zones 6–8, a Class I/II vapor retarder on the warm side. A vapor retarder alone does not qualify. · Split 50% intake at the soffits, 50% exhaust at the ridge · Baffles = soffit length in inches ÷ rafter spacing
Yes, anywhere insulation meets the soffit. Without baffles the loose fill blocks the intake vents and the whole ventilation path stops working.
Unbalanced ventilation is the real problem — more exhaust than intake pulls conditioned air up through ceiling leaks. Keep intake at least equal to exhaust.
Because insulation blown to the eaves blocks the soffit vents, and a vented attic with blocked intake cannot work. Baffles hold a channel open from the soffit up past the insulation so air can still enter. Skipping them is how an insulation upgrade produces ice dams and a damp attic the following winter.
One per rafter bay that has a soffit vent below it, along every vented eave. They are not something to space out — a bay without a baffle is a bay where the intake is blocked, regardless of how well the neighbouring bays breathe.
Not in the same way. A sealed unvented roofline assembly deliberately has no soffit-to-ridge airflow, so there is nothing to keep open. This is a different roof design rather than a variation on the same one, and mixing the two — foam at the roofline plus vented soffits — gives you the disadvantages of both.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.