Does roof underlayment with a radiant barrier actually work?
Why the air gap matters
Foil works by reflecting radiant heat and by being a poor emitter of it. When foil is sandwiched tight against wood and shingles, heat simply conducts through it like any other thin sheet. Reflective underlayments make sense under metal roofing installed on battens, where there is an air space, but under asphalt shingles nailed tight to the deck the foil has nothing to reflect across.
Options that do help in Georgia
- Radiant barrier sheathing. OSB with a foil face laminated on one side, installed with the foil facing down into the attic. It is easiest to add when decking is being replaced or on new construction.
- Foil under the rafters. Perforated radiant barrier stapled to the bottoms or sides of rafters, leaving an air space, on an existing roof.
- Ventilation. Balanced intake at the soffits and exhaust at the ridge carries off the heat that the deck absorbs. A radiant barrier is not a substitute for it.
Where the payoff is biggest
The U.S. Department of Energy notes radiant barriers are most effective in hot, sunny climates, and the benefit is largest when ductwork or an air handler sits in the attic, as it does in many Georgia homes. Do not lay foil directly on top of attic insulation, where dust kills its reflectivity and it can trap moisture.
Remember what underlayment is actually for: it is the backup water barrier, so choose it for water protection, traction and warranty compliance. How attic heat ages shingles is covered in attic heat and shingles, the airflow side in roof ventilation basics, and underlayment choices in synthetic underlayment vs felt.