Oooh, Shiny Stuff! – Radiant Barrier Fundamentals
In the guest post I wrote for Bob Borson in his blog Life of an Architect, I gave an overview of the types of attic insulation. I didn’t discuss radiant barriers in the article, but it’s a hot topic, so to speak, and someone did bring it up in the questions. So, today’s as good a day as any to talk about shiny stuff!
I covered this topic a bit last year when I wrote about foil-faced bubble-wrap, a popular product for duct insulation and overblown R-value claims. The general category of radiant barriers is an area of great hype, so I’ll tell you what I know, explain the basic physics, and show you some great resources for more information.
OK, first of all, a radiant barrier is something that can keep your attic cooler. Unlike power attic ventilators, they actually go after the source rather than treating a symptom, but first, let’s look at the physics. Attics get hot because the Sun is beating down on them all day. The heat from the Sun comes in the form of electromagnetic radiation. When EM radiation hits a surface, it can do one of three things: (i) be reflected, (ii) be transmitted, or (iii) be absorbed. How much of each you get depends on the wavelength of the radiation and the properties of the material it hits.
The part we’re concerned with here is the sunlight that’s absorbed by your roof. Once the roof sucks up those rays, it gets hot. That heat at the absorbing surface (shingles on most residential roofs) then does what the second law of thermodynamics tells it to do – it looks for cooler places, so it starts conducting down through the roofing materials. When it finds the underside of the roof deck, it then can radiate down into the attic. Typical roof decking materials – plywood and OSB – are pretty good radiators, so everything it ‘sees’ in the attic starts getting hot, too.
You can see the heat transfer I just described in the diagram above:
- Radiation hits the roof.
- The radiant energy is absorbed by the roofing materials.
- The thermal energy conducts downward.
- Heat radiates from the roof deck to everthing in the attic.
So, the dominant form of heat gain in an attic is from radiation, which heats up all the solid materials it finds – framing, ductwork, insulation, all those boxes of Christmas decorations, and dead squirrels. That’s one reason power attic ventilators aren’t a good solution. They’re going after the air, but the air is hot only because all the solid materials are hot.
A better way to reduce the attic temperature is to go after the source – the radiation constantly blasting the attic from the bottom of the roof deck and rafters. Plywood and OSB are good radiators. In physics language, they have high emissivity, which means they’re good at emitting radiant energy.
That’s where radiant barriers come in. These are materials that have a low emissivity. When the underside of the roof deck has a radiant barrier installed, the heat still travels through the materials, but once it hits the radiant barrier, it’s come to a dead end. Little of that heat then radiates to the attic.
The radiant barrier gets hot. If you go up into the attic and touch it, you’ll see that the heat’s still getting there. But like that stainless steel playground slide, when you hold your hand near it without touching, it doesn’t feel like it’s hot. The magic of low emissivity! In case you’re wondering, the answer is yes, this is the same thing that makes ‘low-e’ windows so good. Also, if you’re wondering how hot the shingles get, the Florida Solar Energy Center has done research on that, which I wrote about last year.
OK, so if the radiant barrier doesn’t feel hot when you hold your hand an inch away but it can burn you when you touch it, what does that mean for installation? You got it! You have to have an air gap next to the radiant barrier. If you spray foam insulation right on the radiant barrier, you’ve wasted your money on it. Heat will conduct right through the radiant barrier because generally, materials with low emissivity have high conductivity.
There’s so much more to say about radiant barriers, but let me wrap up this article and give you a couple of resources where you can read more about how they work, how to install them, how much they might save, and why you probably don’t want to install one in Pennsylvania. The first is the Florida Solar Energy Center (FSEC). They have a bit of radiant heat gain in attics down there and know a thing or two about this topic. Here’s a good Question and Answer Primer that’s got a lot of good info. Oak Ridge National Laboratory, which does a lot of great building science research, has several good pages about radiant barriers, too.
I’ll come back to this topic and write more another time, but for now, let’s not forget what my friend Mike Barcik says is one of the best things about radiant barriers – People like shiny stuff!
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Good overview of what radiant
Good overview of what radiant barriers are, thanks. It reminds me of research on the subject that Bill Rose has written and presented about. His testing determined that in a house with no ducts in the attic and high quality air sealing and insulation on the ceiling, radiant barriers have no measurable effect on the interior of the building. So they can be viewed as useful primarily in situations where the building is otherwise deficient with either ducts in unconditioned attic (always a bad idea) and/or deficient ceiling insulation and air sealing. If we designed and built homes properly from the beginning, we wouldn’t have much need for things like radiant barriers.
Good stuff as always, Allison
Good stuff as always, Allison. I’d be curious to see a cost-saving/performance comparison of a house with a radiant barrier installed in the attic vs a house with a foamed attic, those houses being the a Southern climate of course.
Carl: You
Carl: You nailed it! Radiant barriers are unnecessary if we seal and insulate the attic floor adequately AND keep duct work and air handlers out of the attic.
Matt: I’m not aware of such a study right now. The FSEC link above talks about a study of the effect of a radiant barrier on homes that were retrofitted.
Allison, As always the
Allison, As always the basics must be reviewed. Thanks for that. 2 questions: How large an air gap? What about a gap on the framing?
There are huge performance
There are huge performance differences between radiant barrier as well… like the painted, spray-on, blow-in, foils or blanket radiant barriers vs the TechShield kind.
bubble wrap/foil insulation
bubble wrap/foil insulation made quite a splash in the radiant heating industry about 7 years ago – making claims that it would perform to an R-10 level at least. When pressed some of the manufacturers would tell you that it had a real R-value of about 1. It has since gone by the wayside.
Robert Bean has an excellent article on this topic:
http://bit.ly/pVfzHD
John N.:
John N.: As long as you don’t have a direct conductive path between the radiant barrier and another material, it’s enough. Because things move and shift, I’d want at least a 1″ gap. It doesn’t need to be on both sides, though. I’m not sure exactly what you mean about a “gap on the framing.”
Armando: Absolutely. You have to make sure the material has an emissivity below 0.1 if you want a true radiant barrier. That’s generally some kind of aluminum foil facing. Spray-on coatings have higher emissivities that may reduce heat gain but not enough to be cost-effective.
John B.: I wrote about foil-faced bubble wrap last year and gave it a brief mention above. The article by Robert Bean that you mentioned is nice because it has links to government advisories.
Some current research
Some current research resources:
Dick, K., Fedirchuk, K., Comparison of Energy Consumption for a Wood Frame Building using Batt insulation and a Foil Backed EPS Foam Board, University of Manitoba, 13th Canadian Conference on Building Science and Technology, Winnipeg, May 10-13, 2011
Saber, H.H., Maref, W., Swinton, M.C., Numerical Investigation of Thermal Response of Basement Wall Systems with Low Emissivity Material and Furred Airspace, 13th Canadian Conference on Building Science and Technology, Winnipeg, Canada, May 10 –13, 2011
If you fasten the RB to the
If you fasten the RB to the rafters, with foam above; you have two points of contact. The cavity. The gap there is created with a recessed shaving of the foam. You also have contact between the RB and the rafters (framing).
Do you live with this contact? Do you do something to decrease the conductivity of the contact?
Since most attics have ducts
Since most attics have ducts and are not that well sealed, I have been recommending these barriers for many years. Follow up finds a consistent 30-35 degree reduction in temperature of the attic components. Stapled on the bottom of the rafters works great. John N’s concern about the barrier being in contact with the framing is a reasonable concern. However, no need to worry. As you stated Allison, though it may be hot to the touch it radiates little. In teaching this concept to my classes and explaining why thermostatically controlled attic fans are not only useless but also often have a negative effect I remind my students that they will get just as sunburned on the beach when the wind is blowing as when it is calm. You cannot remove radiant heat with air flow! As for cost benefit, the cost of installing the radiant barrier is minimal when compared with the cost of foaming and bringing the attic into the building enclosure.
Installing exterior insulation on top of the sheathing when building or re-roofing is another story and fodder for another blog.
Bean:
Bean: Thanks for the references. I’ll have to check those out.
John N.: If you’re putting spray foam in the cavities, you don’t need a radiant barrier below. And what’s on the roof side of the radiant barrier doesn’t matter as long as it’s not touching anything below. The heat comes at the radiant barrier from above and then doesn’t get radiated below. Am I understanding your comments/questions correctly?
John R.:
John R.: All good points! I also like to mention in classes that wind or fans won’t keep you from getting sunburned.
Allison & Carl, I
Allison & Carl, I ‘get’ the point that radiant barriers provide a greater benefit in homes with deficient attics or ducts in the attic. But I’m not sure I understand (or agree with) the point that there’s no measurable benefit in homes with tight, well insulated ceilings and no attic ducts.
IIRC, Rose’s research wasn’t based on a hot climate. As you (Allison) pointed out, there’s an obvious trade-off in cold climates.
Whether or not the benefit is worth the cost is another matter. But with products like TechShield that only add a few hundred dollars, I would recommend a radiant barrier as standard fare for new construction in markets like Phoenix, Brownsville and Orlando, regardless of how well the home is built.
Allison,
Allison,
Great point on the wind not affecting sunburns! I’ll have to use that in my explanations to people about radiant heat. I also often use the car analogy – car parked in the sun versus the shade, to explain surface temps versus air temps.
David B.:
David B.: You’re probably right. I was a bit too quick to agree with Carl on that point. I think in a place like Phoenix, I’d want a radiant AND a fully sealed and insulated ceiling. In fact, since I generally like the belts-and-suspenders approach, I’d do it even here in Georgia. Thanks for calling me on that. I do like radiant barriers and think they definitely have their place in some homes.
Carly: A car in sun vs. shade is another good example of radiant heat gain. Thanks for the reminder!
David – I don’t have my notes
David – I don’t have my notes from Bill Rose’s presentations, but I do remember that he did research in both hot and cold climates. One in Illinois and I believe the other was in Arizona, but i could be wrong about the location. I suppose that since things in construction tend to get screwed up that adding a radiant barrier to an attic with not ducts might offset potential gaps in the building envelope at the ceiling, but I still think that it is a fix for what is at its core a poor design and execution. If I had to choose, I would probably spend the money improving the ceiling insulation and air sealing before I install a radiant barrier in both a new and an existing attic.
Allison – Why the sudden backpedaling? Stand by your guns, man!
Carl: You
Carl: You’re right. I shouldn’t change my mind just to agree with the most recent commenter, but I just want everyone to get along. ;~)
No, actually, I’m sticking with David on this one. How much of an effect a radiant barrier has on the conditioned space below will depend on the climate and the quality of the building envelope. In a hot climate with code-minimum insulation, I think there would be a measurable effect. It might not be cost-effective to add the radiant barrier in some cases, but if you lower the ΔT, that reduces the BTUs/hr of heat gain, too.
Maybe in Arizona … NOT in
Maybe in Arizona … NOT in Decatur……Allison, I think Carl has the best recipe for Decatur.
Just say no to HVAC in the attic …create An Airtight Lid at the attic floor and spend your money upsizing the R-value instead of on Shiny Stuff.
I think the year round performance will be better.
Being from California most
Being from California most homes are built with slab on grade floors. Therefore the duct system and air handler are in the attic space. Radiant barrier should be the #1 install during new construction and at retrofit. Your picture showing radiant barrier sheathing installed is a good one from the barrier standpoint. The duct installation and loose fill insulation is another problem. The ducts should be installed as low as possible and the lengths need to be shorter. Covering all ducts with blown in cellulose after air sealing would be the perfect install.
I was told that combining
I was told that combining radiant barrier with spray foam did not add much additional value. From a cost/benefit perspective, it was an either/or. Is this true?
I used Techshield on my home here in the Atlanta market and I am a raving fan.
John B.: I
John B.: I’m not going to be so quick to say no to a radiant barrier in Decatur. I’d do some calculations first and what they tell me. Of course, I’d absolutely never put ducts in an unconditioned attic!
Steve H.: Yes, if the ducts are already in the attic or the builder/design team won’t move them before construction, a radiant barrier can be a great thing in a cooling climate. You also bring up a great point that ducts should be kept as low as possible in the attic.
Bill H.: How much value you get in combining spray foam insulation and a radiant barrier depends on two things: (i) whether it’s done correctly or not (the RB needs an air gap on one side), and (ii) the details of the particular installation – climate, amount of insulation… There’s not a single yes or no answer to this question that covers every case. Using a product like Techshield, with the RB adhered to the underside of the roof decking, is a great improvement over an attic with no RB and much less expensive than upgrading to an roofline insulated with spray foam.
John B.: I
John B.: I left out a word. That second sentence should read, “I’d do some calculations first and see what they tell me.”
Allison, I am not saying that
Allison, I am not saying that a radiant barrier is bad for Decatur… I just think upgrading the insulation would be better for both heating and cooling seasons.
David B.,
what do you think about Decatur?
Radiant Barrier vs upgrade insulation?
I am new to REM/Design so I may be missing something….and perhaps REM/Design is not a good tool for this … but out of curiosity…
If I model a 1600sf one-story home in Atlanta with R-30 blown ceiling .. there is a $5/yr reduction in cooling cost with a radiant barrier….and no change in heating cost (which seems very odd)
If instead of a radiant barrier … I increase to R-38 blown ceiling ….the cooling cost goes down $5 AND the heating costs go down $12….so according to REM/Design more insulation is better than shiny stuff.
Isn’t it odd that REM/Design is not showing any change at all during the heating season with a radiant barrier?
John, I agree those numbers
John, I agree those numbers don’t look correct, but specific questions like that are best directed to the technical gurus at Architectural Energy.
Keep in mind that REM is based on HERS, which is very different from an hourly simulation engine. So it’s not surprising that certain improvements, especially those involving radiant heat transfer, are weakly modeled. That being said, REM is probably better than most other non-DOE2 modeling programs for general comparisons.
If a Radiant Barrier is great
If a Radiant Barrier is great in the cooling climate and so so to of not much value in a heating climate; why do those numbers not make sense?
No cost reduction is the heating I would expect. The $5 per year for cooling might be low, but I don’t think REM is the best to model something like this.
JohnN, I think JohnB was
JohnN, I think JohnB was looking for at least a small *increase* in heating cost. That’s why radiant barriers make the most sense in hot climates.
I generally only specify radiant barrier if CDD > 1500 and CDD > HDD. More aggressive with ducts in attic. I’m not aware of any modeling tool that can correctly model this trade-off.
JohnB, in your Atlanta model, how much was cooling $ with R30 and no radiant barrier? Also, referring to Component Consumption Summary Report for same configuration, what’s the ceiling load and total load (MMBtu/yr)?
David B, since I am a REM
David B, since I am a REM/Design novice … I forwarded my Atlanta BLG file to you.
And yes… I was expecting to see the heating cost go up …my thinking about CDD and HDD is very similar to yours…
So I am still thinking that Atlanta/Decatur would be better off to upgrade the insulation and skip the shiny stuff….Allison what software are you planning to use for your “calculation”?
I’ve got a little data to
I’ve got a little data to share from an analysis I did of thousands of Energy Star homes in Houston. These homes all had R-30 attics with tight R-6 or 8 ducts. Many homes used radiant barrier roof sheathing and many did not. The analysis found that the radiant barrier sheathing reduced annual cooling use by about 180 kWh, — about 3% of cooling use.
My own simple modeling indicates savings of 140-200 kWh so I’d say that’s pretty good agreement.
In older homes with worse ducts, lower insulation levels, and lower AC SEER, you could expect considerably larger savings from a radiant barrier. But in new homes — even with ducts in the attic — the savings are fairly modest.
In all situations, it sure makes it a lot more pleasant for anyone that has to go up into the attic.
John B.:
John B.: After seeing what Michael Blasnik said above, I may not be doing any calculations, but I was just going to start with some simple UAΔT calculations for homes with and without RBs.
Michael B.: I’ve read a little about your study but only about the overall energy savings of ENERGY STAR homes compared to other new homes. I’d’ve thought the savings from radiant barriers in Houston would’ve been a little better than that, but if those are the data, those are the data. Thanks for letting us know about your work there.
Allison: You should feel
Allison: You should feel free to do your own UAdT calcs — the tricky part is the dT. People often forget that the sun doesn’t shine at night and thermal mass effects are hard to figure, so the dT values are the key and back-of-the-envelope guesses almost always over-estimate the dT without the radiant barrier.
The Houston study found lots of interesting things — the comparisons between Energy Star and non-Energy Star homes was perhaps the least interesting part of it. Still, it’s just one study and isn’t the last word on anything — including radiant barriers. But it does provide some evidence of the likely size of the impact in these homes and climate. That evidence is only really convincing to me because the results are consistent with my own calculations 😉
Michael B., what do your
Michael B., what do your calculations tell you for a city like Atlanta … do you factor in an additional cost for heating?Atlanta is flip-flopped from Houston
Houston 1600 HDD 2700 CDD
Atlanta 2990 HDD 1667 CDD
John:
John:
When I run the same model using Atlanta I find cooling savings of about 90 kWh/yr and heating use increase of about 4 therms. So that’s maybe a $5/yr net benefit. Again this is for a new Energy Star level home. ymmv
Thanks Michael…your
Thanks Michael…your estimate of benefit for Atlanta is not-so-different from REM/Design
So instead of spending $200+/- for Radiant Barrier = +/- $5/year benefit
What if you upgrade the insulation to R-38 instead … what is the benefit/yr?
Can I trouble you for another estimate?
Are you using your own spreadsheet or a certain software?
John: if I change the attic
John: if I change the attic from R-30 to R-38 I see savings of about 40 kWh/yr in cooling and 16 therms/yr in heating — about $20. Again, these estimates are all based on my taking a generic Energy Star home from Houston (slab, SEER14, low SHGC windows. tight well insulated ducts, etc) and moving it to Atlanta.
I did the calculations using my own simple spreadsheet algorithms (which aren’t really so simple) and they include some assumption that the insulation isn’t perfect in any scenario.
Right on queue for this
Right on queue for this thread, RESNET signs MOU with RIMA, the shiny stuff trade association…
http://bit.ly/mTex5S
RIMA membership includes makers of foil faced bubble wrap. I visited the Relflectix website prepared for bear and was pleasantly surprised to see an extensive section “About Reflective Technology” that seems to go out of its way to provide factual information and dispel some of the myths about reflective insulation. Too bad the distributors aren’t always on the same page.
Enjoyed everyones comments,
Enjoyed everyones comments, we often spray foam over radiant barrier in the Houston Market. Temp go down from 155-125 on a 100 degree day to 77-85. when we are quoting against radiant barrier in the after market. we are often about the same price, some times $500.00 less or $500.00 more. We alsway reduce the attic temp down two between 77-85 dgrees on a 100 degree day. Radian barreir is a good band aid. Foam insualtion does what people hoped the radiant barrier would do. It does work, just not near as good, and the paint is a joke!
recnetly had a customer say
recnetly had a customer say that he was quoted $3500.00 for radiant barrier and two fans with a guaranantee to drop temps in attic by 10 degrees. we foamed it for $500.00 more and he is already cold in his two story home. Foam is the only soulution for a two story hot upstairs.
I am a homeowner getting roof
I am a homeowner getting roof estimates. I live in the Detroit, MI area. I live in a two story and we have a problem with a hot upstairs. We need a solution to cool the upstairs. The roofers are talking about checking the soffits, eaves, and baffles for the best ventilation in. They say our roof vents are not placed correctly on our hip roof. About 9 yrs ago we had insulation blown into our attic, but not 100 percent sure it is R38. I hope it is. After reading multiple posts I believe the radiant barrier or more insulation would be the answer in addition to proper ventilation. What do you think? None of the roofing companies have mentioned it.
Stephanie:
Stephanie: A radiant barrier doesn’t make sense in Detroit. You’re better off spending your money on air sealing and insulating the ceiling. Have you had a blower door test done? Here are some articles that might help:
How to Choose a Company to Do a Home Energy Audit
Hidden Air Leakage Sites in Your Attic
Mind the Gap – Air Leakage at the Top Plates
How to Sheathe an Attic Kneewall — And How Not to
@Stephanie, I agree with
@Stephanie, I agree with Allison’s advice (re: radiant barrier not making sense in Detroit) with the following caveat… IF your roof sheathing must be replaced anyway during re-roofing, TechShield or similar (OSB with radiant barrier factory bonded to the wood) costs less than $0.10 per square foot extra. Field-installed radiant barriers cost many times that due to labor and rarely if ever worth the cost. In any case, a radiant barrier might help reduce your attic heat load, but you’d be better off spending your money to fix the underlying problem(s).
In addition to what Allison recommended (be sure to read those articles), improperly designed HVAC is often a major contributor to upstairs cooling issues. In most cases, comfort issues are the result of poor duct design and/or installation quality, not inadequate capacity.
The most common problems include leaky ducts and undersized returns. Also, a single zone AC/heat system (e.g., one heat/cool thermostat for both floors) cannot maintain comfort on two levels without some way of redirecting some of the supply air between seasons.
It may be possible to add a balancing damper to direct more air upstairs in summer, and less in the winter (requires an adjustment every spring and fall). Installing a balancing damper to an existing system usually requires significant ducts mods, especially if duct system is already undersized (another common problem).
You mentioned roof/attic venting… when you added insulation, the installer may not have installed baffles to keep insulation from spilling into the soffits, thus blocking ventilation air flow.
When solving problems like this, it’s important to look at the house as a system. Unfortunately, most contractors focus on a single component — if I only have a hammer, then every problem looks like a nail. In reality, most comfort issues are the result of multiple contributing factors.
@Stephanie, when re-roofing,
@Stephanie, when re-roofing, there’s one more important thing to consider. Attic heat gain largely depends on the roofing materials. For example, dark roofs obviously absorb more heat than white roofs.
White and light color metal and tile roofs offer the best performance on pitched roofs (other than ‘flat’ roofs), but cost much more than shingles. And white shingles aren’t very practical since they get dirty very quickly.
Research has led to a new class of shingles that reflect heat better than than conventional shingles without having to be white. Check out the Cool Roofs Rating Council website for more information and a rated products directory. Certainteed currently offers the highest rated “cool roof” shingle — the Landmark Solaris Platinum (comes in three medium colors).
Energy Star has a Certified Roof program (products with initial reflectance > 0.25 and aged reflectance > 0.15), but its product search page isn’t very useful since it doesn’t have a filter for roofing type.
Finally, here’s a calculator for the Department of Energy for estimating the benefit of cool roofing products: Roof Savings Calculator.