Radiant barrier

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

A radiant barrier is a reflective material used in buildings to cut heat transfer that comes from radiant heat. It’s usually a very thin sheet of aluminum foil that reflects heat back toward its source, helping to keep spaces cooler in hot weather. Because heat can also move by conduction and convection, radiant barriers are often paired with traditional insulation to slow those modes of heat transfer as well.

How it works
Radiant barriers work best when there is an air space next to the reflective surface. The barrier’s surface has a low emissivity (it emits little radiant heat) and a high reflectivity (it reflects most radiant heat). The result is less heat moving from a hot roof into the attic and down into living spaces. The key is to keep the reflective surface facing an open air gap, not in contact with other materials.

Common materials and forms
The most common radiant barrier is a thin aluminum foil, sometimes with a protective coating. It can be one- or two-sided, and may be attached to insulation, rigid foam boards, OSB, or used as a laminated layer in roof decking. Radiant barriers can also be perforated to allow moisture to pass and to serve as part of a vapor barrier in some installations. In windows, metalized films can be used to reduce radiant heat while still letting light through.

Applications and installation
- Attics: The most common use is as a facing for attics, placed under roof rafters so heat radiates away from living spaces. For metal or tile roofs, the barrier is often installed with battens to create an air gap, and it’s usually best laid shiny side down so the air space stays open.
- Roof decks and new construction: Some radiant barriers come pre-laminated to roof decking or OSB, saving labor.
- Existing attics: It can be stapled to the underside of rafters, ensuring attic vents stay open to prevent moisture buildup.
- On the attic floor: A breathable, perforated barrier can be laid over insulation, but it must have adequate air space and be installed to avoid blocking ventilation.
- Exterior walls: A vented “skin” of radiant barrier material can be used around the outside of a wall with furring and vents to allow convection.

What to expect and where it helps
- Effectiveness: Radiant barriers are most helpful in hot, sunny climates, especially when ducts run through the attic. They can reduce cooling loads and may allow a smaller air conditioner. Savings vary by climate and home design; typical studies show modest cooling-cost reductions in hot areas, with less benefit in cooler climates.
- Weather and dust: The reflective surface should stay relatively free of dust to maintain performance. Dust or heavy debris can reduce reflectivity, though the impact varies.
- Climate considerations: In very cold climates, adding more traditional insulation is usually more cost-effective than a radiant barrier. Some energy agencies advise against relying on radiant barriers alone in cold regions.

Historical notes
Radiant barriers have roots dating back to studies on reflection and air spaces in the 19th and early 20th centuries, with practical development in the 1920s and widespread use in homes and even space missions. They gained popularity in the 1970s and are recognized for their role in reducing radiant heat transfer.

Practical tips
- Install with an air gap: Keep the reflective surface facing an open space to maximize effectiveness.
- Avoid excessive contact: Do not press the foil flat against roof sheathing where possible; use battens or similar spacers.
- Dust management: Periodically check and clean to maintain reflectivity; perforated versions help with moisture and dust concerns.
- Ventilation: Ensure attic vents remain open to prevent moisture buildup when using radiant barriers in attics.

In short, a radiant barrier is a lightweight, reflective layer that deflects radiant heat when placed with an air gap. It’s most beneficial in hot climates and when installed properly with adequate ventilation and air space, often paired with other insulation to minimize heat transfer in a home.