Extra insulation and/or radiant barrier, for an attic

joel_bcJuly 7, 2006

I want to make my house cooler in summer afternoons and if possible a little warmer in the winter. I've pursued the question via a thread asking about extra attic insulation, and the idea of using a radiant barrier was brought up.

My house's axis is east-west, with the faces of the steep gable roof (metal-clad) being toward the north and toward the south. The walls are about R22 (fiberglass insulation plus thick wood siding) and about R20 in the second-floor ceiling (vermiculite between the ceiling joists in an unused attic space). The attic space is well ventilated. The house is fairly energy efficient - neighbors tell us we're using less winter heating fuel than they often are, even with smaller homes.

I'd like the house to stay cooler in mid-summer weather, and if the strategy can make the house retain heat better in winter, so much the better.

Originally, I thought I'd add another R20 (6-inch) layer of fiberglass insulation above the ceiling joists in the attic. I could do this, or I could use a radiant barrier behind the southside roof rafters. I could also use blown-in cellulose above the ceiling joists, but I'm not sure about the cost of the cellulose and particularly getting the equipment and personnel here to do the application (our land is semi-remote).

I suppose I could use both a radiant barrier as described above, plus more ceiling insulation. I haven't costed that out yet.

Seems to me a radiant barrier facing outward to repel heat back out of the south-facing roof will not have much effect to increase winter heat-holding efficiency.

I invite your thoughts about a cost-effective strategy in the case described. Thanks.


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We covered our entire roof over the rafters with reflective aluminum radiant barrier. Cost us $500 for 5,000 sqft. Our house is in Virginia and no one heard of this technique to reduce cooling costs. The house is finished framing but it is truly amazing how cool the house stays with a thin radiant barrier. The framers couldn't believe it. The roofers were perplexed. It is well worth the investment. You will immediately feel the difference without adding an inch of ceiling insulation.

Note - check the warranty of your roof shingle. We went with Certainteed shingle. If our roof was NOT vented then the warranty would be invalid. Without venting the shingles would all melt - all the heat is reflected back to the shingles from the radiant barrier.

    Bookmark   July 9, 2006 at 10:50PM
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rychnc, you wrote: "We covered our entire roof over the rafters with reflective aluminum radiant barrier." I'm not sure just what you mean when you say "over the rafters" - do you mean just under the shingles, or on the opposite side of the rafters from the "top" side on which the roofing shingles are attached?

I'm glad to hear of your success with the barrier, though!

My roofing (which is metal) is already in place, and was just put up (as a replacement for cedar shingles) about one year ago. I cannot put the radiant barrier just under the roofing, but would have to put it along the opposite edges of the 2x8 rafters (in other words, to the inside of the rafters), shiny-side out in order to reflect the heat back out through the roof.

And I doubt there would be any point to putting it on the north-facing roof side. I have a gable roof, and only half of it faces the sun at all.



    Bookmark   July 9, 2006 at 11:14PM
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Let me explain what we did first. After the framers set the roof trusses they rolled out the radiant barrier on top of the trusses prior to adding the roof plywood. When they unrolled the radiant barrier over the rafters (trusses) they made sure to create a 1" air space between the plywood and the radiant barrier. This air space is necessary for improved reflection of the heat wave.

In your case since your roof is already up you will need to staple the radiant barrier on the bottom (not the top like I did) of the trusses. This is very labor intensive but will work the same. DO NOT lay the barrier flat on top of your insulation. Over time dust will settle on the aluminum and reduce the reflective properties.

Here is an excellent educational link:


    Bookmark   July 10, 2006 at 9:00PM
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Don't know it this pertains or not; but if Reflectix qualifies as a radiant barrier for this use; I'll point out that it's available with staple tabs . . . in various standard widths . . EXACTLY for stapling to joists etc. Believe I bought it through Real Goods on the web. This would make it VERY easy to staple to the underside of the rafters; allowing for air circulation to boot in a retrofit application. It's available in pretty good sized rolls too . . I used the stuff as a radiant barrier underneath my radiant in-floor heating system. I'd also make sure I have plenty of attic ventilation too; to help the reflected heat get out of the attic . . . metal roof or otherwise.



    Bookmark   July 11, 2006 at 6:00AM
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Thanks, guys.


    Bookmark   July 11, 2006 at 10:30AM
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joel... two things. Radiant barriers are never placed "facing out." They must face the air space in a vented attic in order to work. I know that sounds counter-intuitive, but them's the facts.

Secondly, radiant barriers stop radiated heat gain. Doesn't have to be sun shining on the roof. Is it as hot outside of your house on both sides in the summer? It is here and it is at my brother's home in Vancouver.

    Bookmark   September 1, 2006 at 4:07PM
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What is "REFLECTIX"? Is it expensive? You say it is stapled between joists? I don't have any 'winter" problems, don't really have any "winter" to worry about in Houston.We do,however,have heat and humidity,11 months out of the year! I slashed my electric bill by blowing 38" of insulation into my attic(don't laugh) and adding two solar powered attic fans on my roof. Had ridge vents and gable vents and added continous soffitt vents on the entire west wall.My bill hasn't exceeded $207.00 in mid-summer Houston. I am always interested in MORE ways to cut costs (though I'm not really complaining). Solar fans were a tremendous help for me.

    Bookmark   September 13, 2006 at 9:46PM
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Reflect ix, is known mostly for its double bubble foil insulation. They have single sided too and some other niceties.

Sort of a double sided radiant barrier with bubble pack separating each side so there is a small air gap between the foils. Pretty darn nice stuff for radiant barrier but a bit expensive for most people.
I have used some here and there where appropriate .. makes nice 2 liter coke bottle coolers too ;) And make some of those alum foil protetive hats to keep your head warm in the winter.

Lowe's lumber here has small selection of it.
4' x 25 foot roll about $50
15"? 16" oc 25 foot staple tab rolls about $20

You can get 1000 sq ft of double sided commercial radiant barrier for under $150 ... but instead of bubble pack separating the foils it has reinforced polypropylene .

excuse misspellings.

Here is a link that might be useful: http://www.reflectixinc.com/script/products/buildingmaterials.asp

    Bookmark   September 14, 2006 at 3:59PM
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Ok I got it now..reflectix is a foil bubble foil..
is this the stuff they claim has R-14.5 per inch??

just fyi..do you know that the lesser cost radiant barriers
work just as effectively?

as it is only one side of the RB that works, as dust
builds up & reduces reflectivity (50% in 5 years)on the
foil facing the roof decking..
it is the foil surface facing in to the attic space
that reduces the heat gain.

I have been having rolls of RB (single sided foil
with kraft paper & nylon scrim) for several years.
This year I finally installed it..house is
30 degrees cooler in the summer (La. summer may - oct)
and comfort of house is greatly improved.
and hey I lost 10 lbs in the 2 days it took me to
install...win win imo!

Lowe's has a solar shield RB like the one I used
for .10 per sq ft.

oh and the state tells me..R-11 for correctly installed
RB in attic application.

OP unless the RB is installed on all attic area
you will be defeating the purpose of the install.
heat will simply enter the areas with no RB &
your efforts will be wasted.

while RB work very well in cooling climates
the benefits of heating climates are much lesser.

best of luck

    Bookmark   November 13, 2006 at 7:17PM
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Seem to recall reading there is some current revised data out there showing added value for radiant foil both sides and the cost adder is minimal for the diy'r. The thicker foils go in fast too so there is some smile creating ease once you use it compaired to the thin stuff.

Don't know for certain but think exposed foil & paper would get a fire code violation if its not enclosed behind a fire rated cover here.

We use craft backed foil on the exterior of concrete block walls behind stucco mostly. Double layer too with a flapper paper to fold out an added air gap between them.

    Bookmark   November 14, 2006 at 10:47PM
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Gang....check out Dullys house take the "tour" he has radiant Barrier in his atic .....along with a lot of other stuff.

PS it's a hoot

    Bookmark   October 2, 2007 at 5:01AM
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hello all,As Im reading here tonight deceiding and which way my one sided bubble foil insulation should go. Foil to face roof or face attic. The stuff i have is 25"x100" rolls with one side of foil then air bubbles. I was going to staple the foil side facing the roof on the 2x6s.
Next question. What does this do for moisture in the attic? do I have to add more vent? Im new to this so go eazy thanks Fespo

    Bookmark   December 31, 2007 at 9:36PM
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Someone said that if you don't do the *entire* attic it's all for naught. I don't see how this can be true. I would bet that one would benefit from any coverage with the foil.

    Bookmark   January 1, 2008 at 8:50PM
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Anywhere you put radiant barrier it does good things. Put over a bedroom, that room was suddenly amazingly cooler than the others.

    Bookmark   January 6, 2008 at 4:59PM
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how about using just mylar? i would think it would be cheaper?

Here is a link that might be useful: mylar

    Bookmark   January 6, 2008 at 8:37PM
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Hope this isn't silly, but would aluminum foil work as well?

    Bookmark   January 17, 2008 at 10:22PM
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Pure aluminum has a desirable charaturistic in that not only does it reflect well but maybe more importantly it doesnt radiate heat well at all. So it stops heat transfer. That's why they want an air gap. Where aluminum alloy's will radiate heat.

Radiant barrier is made of near pure aluminum. Something greater than 99% frequently advertised.

Polyester mylar... well it may reflect light and capture body heat in the envelope when you wrap your body with it.

Aluminum foil is something near 22% aluminum.. the rest is ?shrug?, something else.

    Bookmark   January 18, 2008 at 11:53PM
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I believe mikie is sadly misinformed on the composition of aluminum foil. I got the following off the reynolds web site.

Why is aluminum foil sometimes called tin foil?
In 1919, the U.S. Foil Company, parent of Reynolds Metals Company was founded in Louisville, Kentucky to produce lead and tin foil. Then in 1926, the company entered the aluminum business, rolling aluminum foil for packaging. Today, Reynolds Wrap is made from 8111 alloy aluminum, at the thickest gauge specifications available in the marketplace. Reynolds Wrap® Aluminum Foil is 98.5% aluminum. The balance is primarily iron and silicon. These are added to give the strength and puncture resistance obtained only in the alloy used in Reynolds Wrap® Aluminum Foil.

So mikie needs to check his facts before he posts them.

    Bookmark   January 23, 2008 at 7:30PM
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sorry Rjohn... i took that aluminum percentage off somebody's website. I was sadly misinformed just like you said.

Somebody Else's website says that standard ;
AA8111 spec is 97.2%*typical* alum .
and some other 'stuff'

reflectivity is 88 & 80%,, shiny and dull sides, fresh off the press.

Thank you for correcting me.

Now I wonder what is the emissivity of alum foil and how fast does it corrode in walls and attics ?

    Bookmark   January 24, 2008 at 1:09AM
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Wouldn't use straight alum foil in an attic as a radient barrier. All the samples i have gotten my hands on are alum. bonded with another material to strengthen it. I would think straight alum. foil would be too fragile.

    Bookmark   January 24, 2008 at 4:14PM
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I live in St Petersburg FL and my attic used to be like an oven. I used the cheap barrier long ago and it helped a little. I replaced it with a bubble wrap radiant barrier stapled to my roof rafters. That gave it a R value of about 12 and I installed two solar attic vents on the house to help remove excessive heat within this barrier. I attached 2x4 on the peak area of the roof in the attic and made it so all the hot air rises into this space. Then added an electric fan blowing outside air into the attic and the peak area. I added another fan on the other side of the house to exhaust the air out. My attic is cooler than ever. This makes my a/c unit that is in the attic to run less and keeps the house cooler. One more thing add another layer of insulation on top of anything that is there alreay. have it run the oposite way of what is up there now. Do you have an attic set of stairs or a hole with a lid. I installed a hatch cover with hinges and added a layer of insulation on top of it. This way when I leave the attic the hatch is closed and keeps out any hot air into the house and cold air in the winter.

    Bookmark   February 7, 2008 at 11:26PM
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Did the same thing to my house in Virginia in 2007, Cost me 1500 for the products & went through 2 electric staplers, 10,000 5/16 Staples & the time it took to lay this out over a 3700 ranch style home.

The result in 2008 is my electric bills in the summer are dwn by 30% at least even though their has been an increase in the electric rates & they charge a fuel surcharge now.

For anyone to say in these time that not using a radiant barrier is NOT a good idea doesn't know much about saving money!!!

Radiant GUard of Texas is the 'originator' of this product.

Here is a link that might be useful: Radiant Guard

    Bookmark   July 31, 2008 at 10:18PM
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I live in Florida and have both. I added another lay of insulation in the attic and the radient barrier is stapled to the under side of the rafters. All the excessive heat is channeled into the peak and goes out through two solar vents and two electrical gable fans if needed. I used a bubble wrap barrier instead of just the old paper barrier. I got an R 12 for the rafters and made a big difference My house has insulation in the walls (foam) and under the floow fiberglass. My a/c never comes on during the day and the house is cool

    Bookmark   August 5, 2008 at 9:22AM
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Built our new home with the foil faced plywood roof decking...2000 sq ft house....$135 electric bill last month...keep house at 77 degrees.

    Bookmark   August 9, 2008 at 6:37AM
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The foil faced plywood does work well as a radiant barrier.
I wished I had known this and could have had them do the detached garage also.

    Bookmark   August 11, 2008 at 11:12AM
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