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.
- 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.