How does foil facing on attic side of roof sheathing reduce attic heat gain?
icestationzebra
| Posted in Energy Efficiency and Durability on
I thought that in order for foil surfaces to reflect heat, they need an air space 3/4″ – 1″ and that the foil must face the source of heat (in this case – outside)
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Replies
Aaron,
Aluminum foil is a low-e surface. It has a low emissivity compared to plywood or OSB. The plywood or OSB roof sheathing gets hot during the day when the sun shines on the roofing; if the surface facing the attic air has low emissivity -- to put it plainly, if it's shiny -- then the object will radiate less heat than it would if it weren't shiny. Less heat is radiated from the underside of the roof sheathing -- the hot surface -- to the floor of the attic (usually, the insulation) -- the not-quite-so-hot surface -- when the underside of the roof sheathing is shiny.
For more information on this topic, see Radiant Barriers: A Solution in Search of a Problem.
reflectivity vs emissivity are reciprocals. If reflective surface has a reflectivity of .95 is emissitivity will be .05. Thus is will give off very little radiant heat.
The opposite is a dark surface that would have a low reflective surface and would give off much of its heat via radiant heat.
Shiny side down forces the roof to give up its heat by convection or to radiate it up. As we know convection is not the main method of heat transfer from the roof to the house.
What about when the radiant barrier is facing outward, such as foil faced polyiso on the roof deck with furring strips on top of it ? Does it still work despite being backwards ?
Nick,
The air space de-couples the hot roof sheathing from the upper surface of the polyiso, limiting conduction, so that helps. Moreover, the shiny aluminum foil increases the R-value of the air space a bit -- from R-1 or R-2 to R-2.5 or R-3. So the air space helps, and the foil helps too.
There are usually cheaper ways to add R-value to an assembly than to build an air space with foil facing the air space. But if you have to have an air space for other reasons, the foil gives you a little boost in R-value.
The 3/4"-1" spacing is really the minimum space at which any appreciable performance is gained when there is only one low-E surface, and it's less that the benefit you get at 4", due to the local convective heat transfer. But the performance difference between a 4" gap and a 10 foot gap is surprisingly small.
With two low-E surfaces facing one another the spacing can be a bit tighter, and you can stack them to achieve any thermal performance you like, but the practical difficulties of going high performance on multi-layered foil assemblies with out introducing thermal bridging that dominates the heat transfer are not trivial.