Building Science vs. B.S.: Radiant barriers – the perfect insulation?


An attic radiant barrier application

Before I dive into another couple thousand words on attic insulation, I thought we’d take a break and talk about an energy saving miracle product!

What if there was an insulation product that was light, cheap, easy to install, and gave an incredible R-value?

If you believe the advertising, then radiant-barrier bubble wrap insulation is this ‘miracle’ insulation. In that advert, they claim an R-value of 15.67 for a quarter inch thick piece of aluminum coated bubble wrap! Wow, that’s over R-60 per inch! All our insulation problems are solved!

In fact, if you do a Google search on ‘radiant barrier’ you’ll find lots of ads for radiant barriers, all making spectacular claims. Unfortunately, it’s difficult to find independent  information. Fortunately, some sites  do try to do some ‘myth busting’. For example, RIMA International has their page on insulation myths. They are a trade organization for reflective insulation, so you’d think they’d be making the claims, but they’re actually helping to debunk the myths. Kudos to RIMA!

There’s also the Radiant Barrier Fact Sheet, put out by the ORNL (Oak Ridge National Laboratory). This is the most complete reference source I’ve found, so if you’re really interested, head over there to learn all the technical details. But keep reading here if you want the plain English version…

The B.S. claims about radiant barriers

  • It gives you the same insulation as R-15 of conventional insulation products
  • It insulates under a concrete slab
  • You don’t need any other insulation
  • It can reduce your air conditioning bills 97%
  • NASA uses radiant barriers to keep space ships cool so it’s good enough for your house

The science of radiant barriers

  • Radiant barriers can be very effective at reflecting radiant heat but do little to stop conductive heat. Radiant heat is like the heat from the sun or other very hot sources. Think about standing in the shade on a hot summer day. The shade blocks the radiant heat. But if it’s 95 and humid, you’re still going to be miserable hot even in the shade. Because of this, radiant barriers DO work well when it is sunny and can reduce attic temperatures substantially when installed under the rafters.
  • Radiant barriers are worthless as insulation under a slab. Unfortunately, many builders have been conned into believing that radiant barriers are useful as insulation with a slab with in-floor heating. As soon as a radiant barrier comes in contact with another material, it loses its ability to reflect heat. Radiant barriers only work if there is an air gap of about 2″ on the shiny side. As a side note – any builder who does this is ripping their clients off for the life of the house. Without proper insulation under a slab, a substantial fraction of the heat in the slab is going down into the earth.
  • The claim that you don’t need other insulation with a radiant barrier is also utter B.S. The radiant barrier only reflects radiant heat. Because much of heat transfer is conductive not radiant, you still need conventional insulation in most locations, especially walls. Trust me. My 1957 home had only radiant barrier foil in the walls when I moved in and we used to use about 1500 gallons of oil to heat it. You do not want to do this!
  • Radiant barriers loses some of their effectiveness when they get old and dusty. In fact, someone wrote a thesis on this, and for you, dear readers, I read through 92 pages to get to the conclusion: the dustier the barrier, the less effective it is. Duh.
  • What about air conditioning bills? I refer you to some graphs on the ORNL page. They even have a radiant barrier savings calculator.  As an example, the calculator shows that if you live in Miami, have a 2000 sf (square foot) attic, good insulation, and insulated ducts in the attic, the radiant barrier will save you $60/year. That’s not bad but it’s not huge either. What about if you live in Baltimore? That’s a $20/year savings. Woohoo! I think I’ll go out and buy pizza with those savings!
  • NASA uses radiant barriers? Well, yes, in fact it does. But outer space is nothing like your attic! Space is a vacuum. The only way heat is transferred in space is by radiant transmission. So if you block radiant heat flow, you block heat gain. So if you see anybody trying to compare space and Earth-bound usage of a radiant barrier, you know they’re ignorant and full of B.S.

Radiant barriers are usually vapor barriers

A very important characteristic of a radiant barrier that isn’t often publicized is that the aluminum coating is a near perfect vapor barrier. When applied to a quarter inch piece of plastic bubble wrap, there’s no way water is getting through. Because of this, you have to be extremely careful of where you install a radiant barrier. Do you really want a perfect vapor barrier in that location?

I’ll give you an example. Suppose you live near Philadelphia, like I do. It gets cold here during the winter and you don’t want to trap moisture in the wall. So if you go and install a radiant barrier on the outer wall, behind the siding, you’re going to trap all that moisture in your walls, likely rotting them out over time.

The same thing could happen if you install a radiant barrier on the inside of the wall, just behind your drywall, and you live in a hot-humid climate like Florida. In that case, the wall will be cool from air conditioning and the moisture is coming in from the outside. Plus, you probably have fiberglass in the walls. So the moisture from outside would go through the fiberglass and come in contact with the cool radiant barrier, condense to liquid water and possibly lead to a big mess.

Summary

  • If you have a hot attic and want to cool it, by all means, you can install a radiant barrier under your rafters and it will help.
  • If you want to insulate a concrete slab, a radiant barrier will do no good.
  • Radiant barriers must be used in combination with “real” insulation in order to really insulate your house.
  • Remember that most radiant barriers are also almost complete vapor barriers, so don’t install them in places that you don’t want a vapor barrier or you could end up with a rotten floor/wall/etc.

16 thoughts on “Building Science vs. B.S.: Radiant barriers – the perfect insulation?

  1. Pingback: Wall Vs Barrier

  2. I have a section of basement wall (concrete) along a narrow stairway. I have only 1.5 inches for insulation directly against the concrete. I can either use .5 or 1 in of foam faced with foil radiant barrier facing the heated space, or 1.5 inch of only foam. In this specific case would a radiant possibly be better than the extra .5 in of foam? It seems foam wise I can get about r6 or r9. cost isn’t an issue with such a small area. Just trying to maximize insulation as the stairs are only 32.5 in wide after insulating. I appreciate the clarity against the miracle radiant barrier sales pitches.

    Anything I do is better than bare concrete, only a minimal amount of foam on the exterior of the wall. Issue is to keep in the heat as I am in Southern NH. the rest of the wall will have 3 or 4 inches of foam only.

    • Part of it depends on how much heat there is in the basement. If your heating system is down there, the radiant barrier might be slightly better because it’s most effective when you have a heat source that’s hot. That’s more radiant energy to be reflected back into the room.
      However, since the staircase is there, it would block that radiant benefit anyway so most likely you’ll do best from more insulation thickness.

      • Thank you much! I’ll just fill with foam. Stairs are somewhat open but any heat is from a pellet stove circulating warm air in the room. There isn’t a hot heat source shining directly onto the wall.

  3. Just purchased a 1991 house and the basement outer wall insulation was not properly covered with vapour barrier over the last 20+yrs. Because of this the batt insulation is slightly moist from condensation and sticking to the concrete (I live in Northern Alberta). I am planning on replacing the fiberglass with EPS foam that I can get custom cut to fit the 2×4 frame. I would do spray foam except that it will be approximately 4x the cost of simply doing the EPS and using spray foam to seal the edges and the .5″ gap between the concrete wall and the bottom plate 2×4. My question is: should I have the EPS cut to 4″ to fill as much of the cavity as possible? or have them cut to 3.5″ and add a radiant barrier on inner side of the EPS foam (facing the 0.5″ air gap between the drywall and EPS). Cutting to 3.5″ saves about as much in foam as the foil would cost…

    • I would personally skip the radiant barrier in this application for a couple of reasons but most important is the chance of it trapping moisture in there which could rot your wall framing. Also, is the 2×4 framing right up against the outer wall? If so, you really have to be careful about moisture getting trapped in there. You’re probably good with the EPS in there but ideally there’d be a little gap between the 2×4 and the outer wall. Even slipping some tyvek behind it could help protect the wood a bit by slowing down moisture movement.

      • Yes, there is about a 1/2″ gap between the 2×4 and the outer wall. I was planning on filling this gap with spray foam between the wood and outer concrete wall, and then putting the EPS foam in the big framed opening.. would you advise against this?

      • That sounds good. Anything to reduce moisture flow into the wood will help. Since you have a good gap, you may want to feed some Tyvek back against the wall. It doesn’t have to be tight. You don’t really need to foam the gap. That probably won’t make much of a difference

  4. We have an uninsulated A-Frame. We’re re-roofing and are considering putting radiant barrier with an air space under metal roof, as it is next to impossible to insulate. Will this help? Are there any other options that wouldn’t require an arm, a leg, and our first born?

    • I’m sorry for the delay, I’m afraid you’ve already re-roofed. In case you haven’t….
      Have you considered a couple of layers of polyiso foam board directly under the roof? That could give you some real insulation under the roof, as well as provide a moisture barrier, both from above and below.
      The radiant barrier can definitely help. I used this technique in my garage attic and it’s still hot but much more comfortable than it would have been without it.

  5. I stapled to the bottom of the rafters (1/2 4×8 sheats with foil facing down) About 19 inches down from the peek (ridge line roof vent) and with full vented soffits at the bottom. Did a reading of 10 to 15 degrees less off the surface of the foil compared to the exposed OSB under the roof. Didn’t do it for the insulation, but to vent cooler air up under the roof surface. My thinking was to lower the heat bonce down against the top of the ceiling insulation.

    My quick view of radiant-barrier bubble wrap insulation is this ‘miracle’ insulation is at:
    http://haveit.com/green/2011/2/23/solartek-save-47-and-the-free-meal.html

  6. Pretty dang good! I’ve been wishing for some good data on this, and here it is. Now I know where to send folks for answers who keep asking me about this “miracle cure”. The higher the petroleum prices, the deeper the B.S. and “greenwash”. Thanks again, Ted.

    • Thanks Larry,

      It’s the truth. People are desperate for quick fixes and there are lot of people out there trying to take advantage. I need to keep getting articles up. The truth shall set you free…

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