Please explain radiant barrier comments
Quoting from “Radiant Barriers: A Solution in Search of a Problem”:
“…plywood or OSB with a radiant barrier on one side of the panel. These panels are installed OVER unconditioned attics, with the shiny side facing DOWN.. Radiant-barrier roof sheathing only makes sense in hot-climate homes that have HVAC equipment or ductwork installed in an unconditioned attic. Here’s the logic: the builder knows that the HVAC equipment and ductwork will get very hot in the summer…..So the builder installs radiant-barrier sheathing to keep the attic a little cooler. At best, it’s a halfway solution to a basic design problem…. That said, radiant-barrier roof sheathing is effective at lowering attic temperatures. Since it doesn’t cost much more than ordinary roof sheathing, it makes sense to install it on new hot-climate homes.”
The above does not seem to make sense, would you please explain? As I understand it, a foil sheet facing “DOWN” from the roof joists would reflect radiant heat back DOWN into the attic. This would make the heat generation by HVAC WORSE, not better. On the other hand, it would help retain heat in a cold climate during the winter by reflecting it back down into the house… a good thing.
IMHO, radiant barrier, assuming it works, is only good at keeping radiant heat OUT of a house in the first place, or IN a house in cold weather. By eliminating the air space on one side or the other, you can make it, in effect, One-Way. BUT understand that this is only useful in climates where it is mostly hot, or mostly cold. In the artic or tropic areas, great. In southern deserts where winters are mild, but most of the year hot-as-hell they should work also.
Would you agree, or am I missing something?
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Replies
John,
A radiant barrier has a low emissivity. The lower its emissivity, the less radiant heat it will emit to the adjacent air space.
When the roof sheathing gets hot, so does the aluminum foil or mylar on the underside. If the underside of the sheathing were painted black, the heat would quickly radiate downward. However, since it is shiny, is has low emissivity, and therefore the rate of downward thermal radiation is much less.
That's what emissivity means. Low-e surfaces, including radiant barriers, have low emissivity.
can't explain it any better than this ... by Allison Bailles ...http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/41522/Oooh-Shiny-Stuff-Radiant-Barrier-Fundamentals
John,
There's also an explanation in the article you cited, Radiant Barriers: A Solution in Search of a Problem.
In that article, I wrote, "By definition, a radiant barrier has a low emissivity (0.1 or less). Radiant barriers reduce radiant heat transfer across the space which they face. The lower a material’s emissivity, the more effective it is at reducing radiant heat transfer."
More explanation follows.
John,
You wrote, "This is only useful in climates where it is mostly hot, or mostly cold. In the arctic or tropic areas, great."
In fact, radiant barrier roof sheathing reduces the transfer of solar heat from the roof sheathing to the floor of the attic. This is only beneficial in a hot climate. In a cold climate, solar heat gain during the winter is beneficial, because it can lower heating costs -- so you don't want radiant barrier roof sheathing.
you are thinking of the radiant barrier like a mirror, which it is not. it doesn't reflect heat back away from it. it acts to reduce emissions.
think of an asphault blacktop road as highly emissive on a hot sunny day. the black top radiates the heat it takes in, so it feels very hot above it, whereas the less emissive ground/grass doesn't even though they are taking in the same amount of heat from the sun.
an air space is required otherwise the it doesn't matter much what the emissiveness of the material is as conduction through it takes over.
Thanks for the excellent articles from Paul, and Martin's comments. I have read them all very carefully, and I believe they prove my points.
Martin, you are focusing on emissivity concerning heat from above the roof. The "other side" is reflectivity from the reflective face of the shiny barrier under the roof. Under the roof it acts like one of those solar hotdog cookers we built as kids. It reflects radiation back down into the room. Just hold your hand a few inches from the shiny side of radiant sheathing and you will immediately feel the effect. For this reason, it seems to me that putting AC equipment under radiant sheathing is self defeating, an incredible waste of energy, and probably melts the shingles as well.
With Radiant barrier Under a HOT roof low emissivity would decrease heat flowing from the sun warmed roof. However for any heat entering the house thru windows, cooking etc reflectivity would keep that heat in the house.
Radiant Barrier Under a cold roof, emissivity would prevent what little sun there is from doing any warming, However, any warming thru windows and such, and from Heating Would be reflected back into the house.
Radiant barrier OVER a sun warmed roof, or Outside the walls, the reflectivity would reflect away heat. Consider white roofs as a perfect example of this. So it could be useful in a constantly hot climate. This is where radiant (even plain white paint) can help those of us in the desert.
Radiant is a very complicated subject, one of the reasons there is no standard for it to compare to R values. But it should be studied, not dismissed. Radiant heat will NOT go away, it must be dealt with. I agree that unproven claims should exposed, but let's not totally dismiss the subject.
My current best evaluation is that for temperate climates radiant barriers pros and cons are about a wash unless you can put them up and down like storm windows. However in polar, warm desert, or tropical areas I think that good use might be made of them if we can do the research.
John,
If you feel that your point has been proven, then there is little likelihood that anything I say will change your mind.
There are two properties, emissivity (emittance) and reflectivity (reflectance) that affect how much radiant energy is absorbed by a material and how much radiant energy is emitted by a material. Engineers have developed the solar reflectance index (SRI) to rate roofing products for solar reflectance.
If you are worried about heat flow from your roof sheathing towards your attic floor, there are two things you can do: (1) Choose roofing with a high SRI; this will help reflect some of the solar radiation striking the roof. (2) Install radiant barrier roof sheathing; this will reduce the rate of radiant heat flow from the underside of the roof sheathing to the attic floor.
Concerning your question: When sunlight shines on roofing, the quantity of radiant heat transferred from the sun to the roofing materials (and, by conduction, to the sheathing below) can be considerable. That's because the sun is a strong source of radiation.
There usually isn't anywhere near the same amount of thermal radiation from the attic floor upwards toward the underside of the roof sheathing, so radiation upwards towards the roof is not a significant factor. That's because people don't keep a sun in their attic.
What people in hot climates worry about is the radiant heat flowing downward from the underside of their roof sheathing towards the attic floor. A radiant barrier can reduce that radiant heat flow.
Radiant barrier roof sheathing isn't useful in a cold climate.
Gosh, Martin, thanks for the slap in the face. I carefully phrased my comment with "believe" rather than "certain" to indicate that I could be wrong. I will continue to research how to best approach my hvac situation, but perhaps I should look elsewhere for supportive help in understanding a complex and still relatively new field.
John,
I'm sorry for the ungracious tone of my last answer. I hope some of the information I provided was helpful.
The basic concepts of radiant heat transfer are not a "relatively new field." This branch of physics has been well established for decades.
Isn't a low-e surface such as that found on a radiant barrier going to keep heat on whichever side of the surface is hotter? In summer when the roof assembly is likely to be hotter than the interior surfaces of the attic, the surface of the radiant barrier will prevent the heat coming from the roof from radiating down. In the winter, when the roof assembly is likely colder than the surfaces in the attic, the low-e surface DOES act like a mirror, reflecting the radiant energy coming from the warmer surfaces.
Unless I'm mistaken, this is what high solar gain low-e windows do! The keep long wave infrared radiant energy coming from warm interior surfaces in in the winter and in summer reflect heat coming from hot exterior surfaces such as an asphalt parking lot. Assuming the low-e coating is on surface #3, they keep the warmer interior pane of glass from radiating heat across the air space where it would be absorbed by the cooler exterior pane.
A radiant barrier on the underside of roof sheathing works the same way as a low-e coating applied to a single pane of glass. It keeps heat on whichever side is hotter, which in the summer is the roof assembly and in the winter is the interior of the attic.
Bill,
You've got it right. If you include a radiant barrier in a wall assembly or roof assembly -- or if you include a low-e surface in an insulated glazing unit -- the effect of the radiant barrier is to raise the R-value of the entire assembly (as long as the radiant barrier or low-e surface is facing an air space).
radiant barriers have 2 properties. Reflecting radiant heat and emitting radiant heat. a highly reflective barrier has a low emisstivity. Something like a roof is low on the reflective rating and high on emisstivity. That is it asbosrbs radiant heat and gives off radiant heat. It gives off its radiant heat to the attic floor. Trees will block radiant heat and factors in to the benefit of a barrier.
The reflective barrier on the bottom of the sheating prevents the emitting of radiant heat. The roof now must give off its heat to the top/exterior or by heating the air which flows out the roof vents. It keeps the heat from reaching the attic floor.
There are 3 ways to install a barrier in an attic. Hanging it from the rafters, labor and cost intensive. Laying it on the attic floor. ORNL says that this installation is degraded by dust accumulation. It is also labor intensive. The third method is by having it applied to the sheating which result in a small material up charge.
Higher levels of insulation reduces the benefit of a barrier. Therefore if you have an attic with R40 or higher of insulation the benefit of a barrier is small. It comes to the point that it is not cost effective when installed on the rafter or floor .
A radiant barrier is most effective when the insualtion levels are very low. The first step a homeowner should take it to beef up the insualtion and to air seal. If this is done then the benefit is small. Radiant barrier salesmen will show you utility bills of homes that show a reduction. But they wont tell you how well or should I say porrly these homes were insulated.
Robert,
Thank you. I agree with your points.
Thanks, Robert. I followed references from Paul and found some websites which provide explanations of radiant, especially in warm climates. This one from Florida was especially helpful. http://www.fsec.ucf.edu/en/publications/html/FSEC-EN-15/ It addresses radiant specifically in hot climates. Another one is from Oak Ridge Nat Labs http://www.ornl.gov/sci/ees/etsd/btric/RadiantBarrier/
My tentative conclusions are specific to, and limited to a desert, or semi-desert area where avoiding radiant heat inflow in warm weather is much more important than the benefits of radiant inflow in cold weather. That is: annual cost of cooling is much greater than heating. 1) Best to Reflect radiant from the outside surfaces of the house using light colors 2) some radiant paint additives may help, but more research is needed in this area. 3) Radiant on the inside can help, but there must be an air gap 'below" the barrier, and the reflective surface must not be compromised. 4) Radiant cannot replace, only augment, good sealing and high R value insulation. The high R value will also reduce the cost of whatever heating may be occasionally needed.
So, any comments?
Thanks for all the discussions. We are trying to decide if we should install a radiant barrier under a standard thickness, new stucco installation. We have R-13 walls with plywood sheathing. We are in a hot, dry Cailifornia climate, and would only install the barrier on west and south walls in direct, baking sun. I would install it myself, so the cost would be limited to the materials, which is about $0.50 per square foot, with the foil tape. Here are the material's properties;
(Single-sided scrimmed aluminum foil laminated
to polyethylene foam perforated (¼” nominal thickness)
WIDTH: 4’, 5’, or 6’ with overlapping flange
LENGTH: 125', 100', 84'
Core Resistance: R-VALUE R-1.03
System R-Value: R-4*
PERM RATING: ASTM E-96 7 or 40 grams/(day-m2)
The product goes under the double layer of "D" grade papered stucco lath. Does anyone have any experience with this? We can't add much thickness to the outside surface of the walls, as we have window trim set for the stucco thickness. I have also spent a lot of time sealing air gaps with high quality caulking and 3M's 8067 flashing tape.
Thanks for any comments, Alan
Alan,
A radiant barrier is worthless unless it faces an air space. If you put it between the plywood and the paper-backed lath, it will act as a conductor, not an insulator.
More information here: Radiant Barriers: A Solution in Search of a Problem.
Thanks Martin, for the quick response. I guess the only benefit would be the closed cell foam, but this would be an expensive way to do this, for nominal added value. I'll read the article. I am going to look at alternative ways of getting better than R-13 in our 2x4 wall thickness. Maybe using foil faced rigid insulation set with a small air gap behind the sheathing, instead of batt insulation?
One 'real life' example. I put electrical radiant heat in my bathroom floors. Since I want all of the heat going "up" into the bathroom floor, and not into the basement, I needed to insulate between the floor joists. I took FBF (foil-bubble-foil about $180 for 750 sq ft) cut strips taped and stapled one layer between the joists about an inch below the floor leaving about a 1" gap. Then I put in another layer about halfway down the 2x6 joists, and finally a wide layer across the bottom of the joists. Thermal image clearly shows that almost no heat is getting thru. I'll let the bean counters figure out R values and such. Cost less than $100 in materials and my tests show it works. HOWEVER, to achieve a similar result with your stucco wall, you would have to put in spacers between the existing outside wall, radiant barrier layer(s) with at least 1" of air space, and build a new outside wall for to hold your stucco. Not a simple, or inexpensive option. You could replace the insulation in your existing wall with spaced layer(s) of radiant FBF. Again, a major project. IF you live in Palm Springs, or similar place where radiant heat greatly outweighs considerations of cold, consider it. Certainly consider white color and trees which keep the radiant OFF your walls in the first place. Same applies to your roof which is a much greater concern. The criticism of radiant "R" values is correct in that by itself it does not have much R Value. But in well sealed layers, with air spaces between, you can achieve both high R and reject the radiant as well. Just as you can get high R values with layers of clear plastic, but little radiant rejection.