There are two places to put a radiant barrier in an existing attic: stapled to the underside of the rafters, or laid flat over the insulation on the attic floor. They cost about the same. They do not perform the same over time, and the reason is dust.
Side by side
| Staple-up (under rafters) | Deck-applied (over floor insulation) | |
|---|---|---|
| Initial performance | Very good | Very good |
| Performance after 10 years | Largely retained | Degrades as dust settles on the foil |
| Labour | Harder, overhead work | Easier, faster |
| Material needed | More: rafter area exceeds floor area | Less: floor area plus overlap |
| Moisture risk | Low with perforated product | Higher: can trap vapour in insulation |
| Access to the attic afterwards | Unchanged | Foil gets walked on and torn |
| Best for | Most attics, especially with ducts up there | Very low-clearance attics where staple-up is impractical |
The dust problem
A radiant barrier works because its surface has very low emittance, around 0.03 to 0.05. Dust does not. As dust settles on an upward-facing foil surface, the effective emittance of that surface rises toward the emittance of dust, and the barrier progressively stops being a barrier. Attics are dusty places.
Staple-up sidesteps this because the reflective surface faces the roof deck at an angle and does not collect settled dust the way a horizontal sheet does. This single factor is why staple-up is the default recommendation despite being harder work.
The moisture problem
Deck-applied foil sits directly on top of your insulation. If it is solid rather than perforated, water vapour rising from the house cannot pass through, and it condenses in the insulation underneath. Wet insulation loses R-value and stays wet.
If you lay foil over insulation, it must be perforated. That is not optional. See perforated vs. non-perforated.
The material cost difference
Staple-up needs more foil, because the sloped roof plane is larger than the floor below it. For a 1,500 sq ft attic on a 6/12 roof that is 1,677 sq ft of rafter area against about 1,500 sq ft of floor. At $0.15/sq ft the difference is roughly $26.35, which in practice often disappears because both round up to the same number of rolls.
Which to choose
Staple-up unless you physically cannot. It holds its performance, it keeps the foil away from your insulation, and it keeps the attic usable. The case for deck-applied is a low-clearance attic where you cannot get a tacker to the rafters, and there it is a reasonable compromise made knowingly.
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Step-by-step method in the DIY installation guide, background in the complete radiant barrier guide.