Please explain how foil-faced roof sheathing works
rdfeibleman
| Posted in Energy Efficiency and Durability on
Some say that foil-faced roof sheathing can reduce the temperature in an attic. I’m unclear how this works. Considering two types of attic designs:
1 – The attic is ventilated, and the insulation is laid on top of the ceiling drywall.
2 – The attic is not ventilated, and the insulation is attached tight to the roof sheathing.
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Replies
Robert,
When you install radiant-barrier roof sheathing (foil-faced roof sheathing), the foil won't provide any thermal benefit unless the foil faces an air space.
In Case #1, the sheathing should be installed with the foil facing down, toward the attic. The foil has a low emittance -- in other words, it is low-e. That means that even when the sheathing is hot, the amount of radiant heat transmitted by the sheathing to the insulation below will be less than if conventional sheathing were used.
In Case #2, there is no benefit, because insulation was installed tight to the sheathing.
Quick followup here. In a cooling climate what is the possible role of reflective foil in a wall assembly. I've seen and heard of people using foil faced foam facing outward towards a rain screen (ie inside->outside foam->foil->air gap->siding.
Given that we're talking low e then that arrangement is only going to reduce heat loss, and not affect heat gain at all. One would need something like (inside to outside) sheathing, air-gap, foil, foam, siding to affect heat gain (or alternatively foil on the back of the siding, before the air-gap). Is that ever done?
Similarly this means that foil laid on an attic floor impacts only heat loss, not heat gain (and so companies selling reflective 'chips' in cooling climates are even more problematic).
Do I have this right?
James,
When foil faces an air space, the air space plus the foil facing the air space works together to create an assembly with a measurable R-value. In the case of a 3/4-inch deep air space with foil on one side, the assembly has an R-value of about R-2.3. The R-value of the assembly works in both directions; it keeps heat out and it keeps heat in.
Thanks Martin,
This sent me to Wikipedia :) In essence my misunderstanding relates to seeing the word emissivity and assuming that it is a measure only of what might be emitted (ie outgoing heat) and not of absorption (i.e. incoming heat). Digging around a little I find:
http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation
which very clearly states,
"a corollary of Kirchhoff's law is that for an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity"
and even better, for those experimentally minded:
"Prior to Kirchhoff's studies, it was known that for total heat radiation, the ratio of emissive power to absorptive ratio was the same for all bodies emitting and absorbing thermal radiation in thermodynamic equilibrium. This means that a good absorber is a good emitter. Naturally, a good reflector is a poor absorber."
Also fascinatingly the page explains how some paints are described with different values for absorption and emmisivity (The calculations are weighted at different wavelengths, in-coming solar radiation vs outgoing room temperature radiation). I believe this is the same effect being exploited in low-e but high SHGC windows (Yup: http://www.efficientwindows.org/glossary.cfm#e )
The upshot is foil+air works the same both ways, as you say. On reflection (hah), that does make more sense. So the often seen advice "turn the foil in sheathing downwards" is really not about the direction but really saying "turn the foil airwards".
Thanks for the answer, Martin, much appreciated.
I am unsure i understand the reason why the foil doesn't work if it is direclty facing a material other than air ??
It would mean also that the insulation side of the Al foil is transparent to radiation ?
i thought that the aluminum sheets worked both ways reflection only radiation on its wavelength pattern ..
Please Martin also explain how a 0,75" of air space + foil gives a R2.3 of insulative value ??
Jin,
Q. "I am unsure i understand the reason why the foil doesn't work if it is direclty facing a material other than air?"
A. Aluminum foil is a conductor, not an insulator. If it doesn't face an air space, its R-value is essentially zero. However, a 3/4-inch air space has a measurable R-value (about R-1). The R-value of that 3/4-inch air space is improved when one side of the air space is lined with a low-e material. The lower the emissivity of this material, the better the R-value of the assembly. If one side of the air space is faced with aluminum foil, the R-value of the resulting air space is about R-2.3. For more information on radiant barriers, see Radiant Barriers: A Solution in Search of a Problem.
Cet article a déjà été traduit en français: Membranes réfléchissantes: une solution en quête d'un problème.
Q. "Would it also mean that the insulation side of the Al foil is transparent to radiation?"
A. Radiation is the transfer of heat through air or a vacuum. If the aluminum foil is sandwiched between other materials, there is no air available for it to radiate to. If the aluminum foil is at a higher temperature than the adjacent materials, the heat will flow from the aluminum foil to the adjacent materials by conduction, not radiation.
Q. "I thought that the aluminum sheets worked both ways. Reflection only radiation on its wavelength pattern."
A. I'm not sure what you mean here.
Q. "Please Martin also explain how a 0,75" of air space + foil gives a R2.3 of insulative value?"
A. Again, I'm not sure I understand your question. Every air space has a measurable R-value. These values can be determined by measurements made in a laboratory. The results have been published in many books, including ASHRAE Fundamentals. You can look these values up.
You have to think of the think methods of heat transfer. Convection, conduction and radiant.
Radiant transfer is through an air space. So without an airspace there is not radiant heat transfer. So a foil barrier in contact on both sides does nothing to stop radiant heat.
There are two parts to a barrier reflectivity and emisstivity. The two add up to 1. The foil barrier on the bottom of the roof will limit the roof from giving off radiant heat in a downward direction. It can still conduct or convect.
Also radiant transfer is greater with the temp differences are greater. So a very hot surface and a cold surface will exchange a much greater amount of energy than a warm and a cool surface.
This wiki page cleaded my undestanding of the principle ...
http://en.wikipedia.org/wiki/Radiant_barrier
Is there still radiation through materials ? do we not care because the % is too low ?
How would you guys explain/rate the performance of foil faced insulation used on the interior side of a wall in cold climate? ? (under gypsum with the spacing of furring strips i guess .. )
I've seen it fairly common here in Quebec that "higher quality" house builds will use "airfoil" type of layer before installing the gypse on the interior side of walls using it as reflective and vapor barrier
( usually poly bubble paper coated with alum on 1 side )
Martin : what would you think about using a reflective layer facing inside of the wall assembly under the osb/ply sheating on a PERSIST/REMOTE wall type ??
gypsum // framing //
should've affect the conduction temperature of the peel stick membrane much since stciked on the osb ?
just a thought .. :)
Martin : ok now only i read your article named: Radiant Barriers: A Solution in Search of a Problem.
As i mentionned, it is fairly comon pratice to use that kind of bubble wrap coated paper under walls here in Quebec .. doesn't seem like it would do much ?
I seriously thought that it would be radiating alot of heat absorbed by the gypsum boards,
but then u mention that the temperature differential limits it's effective use ??
Would this mean that using a radiant barrier inside a room or wall cavity in which the exterior walls or panels have the same or very close temperature than the air within the heated space, would recude the effectivity of the product because there are not much radiation to reflect??
interesting discussion :)
love your QA forum martin :)
Jin, And Whomever, Radiation Barriers Are More Effective The The Higher The Delta T.They Also Need To Be Dust free Best Use, In Space Where Temps Vary 500 Degrees Hot To Cold Side And Assemblies Need To Be Light And Thin.
Pardon My Cell Has Lost Its Mind, Adding Caps.
Jin,
In Quebec and other cold climates, there really aren't any reasons to use a radiant barrier. Your wall assemblies and ceiling assemblies need to be well insulated. If you have adequate insulation, you don't need a radiant barrier -- because there won't be much of a delta-T between the interior surface of your wall and the interior air. Nor will there be much of a delta-T between the exterior surface of your wall and the exterior air. If there isn't a delta-T, there isn't much radiation.
Ok guys thanks for clearing that up ..
So any position within the "hot" enveloppe has approx the same temp and will not work then.
Is there any real use for radiant other than reflecting heat off roofs for residential use ?
Martin: http://www.isolofoam.com check out the isolofoil i believe
2" type 2-3 EPS with a alum front end ..
i believe they've been installing that in probably 1 on 2 house basement in the last few years ..
this product is almost double price than regular EPS :p