Radiant Barrier (Dallas, Spring: new home, buying, contractor)

[Rane84]

I got the RB on top of my insulation. For one reason, the product is a little cheaper. I heard the same thing about dust, but the studies I looked at suggest that, although dust does effect a radiant barrier, its effect is minimal. I just Googled it in "Google Scholar". Even a radiant barrier with a "heavy dust load" was only 8% less efficient than a clean radiant barrier. I don't think my attic will accumulate enough dust to make a big enough difference. After all, the vents will help circulate air.

If my math is right, that means that instead of my current 25% savings, if heavy dust were to gather, I would be looking at 11.66% savings. That's still really good.

"Results showed that the lighter dust loading increased total house cooling loads, compared to a clean HRB (Horizontal Radiant Barrier), by 2.3%.^The heavier dust loading increased house cooling loads by 8.4%, again compared to a clean HRB.^However, horizontal radiant barriers with these dust loadings still decreased house cooling loads by 7% when compared to the same house with no radiant barrier."

Energy Citations Database (ECD) - - Document #5162138

[LOK222]

So you just un rolled it and set it on top of the insulation?
do you have pics? it sounds like it would be pretty easy!

Originally Posted by Rane84

I got the RB on top of my insulation. For one reason, the product is a little cheaper. I heard the same thing about dust, but the studies I looked at suggest that, although dust does effect a radiant barrier, its effect is minimal. I just Googled it in "Google Scholar". Even a radiant barrier with a "heavy dust load" was only 8% less efficient than a clean radiant barrier. I don't think my attic will accumulate enough dust to make a big enough difference. After all, the vents will help circulate air.

If my math is right, that means that instead of my current 25% savings, if heavy dust were to gather, I would be looking at 11.66% savings. That's still really good.

"Results showed that the lighter dust loading increased total house cooling loads, compared to a clean HRB (Horizontal Radiant Barrier), by 2.3%.^The heavier dust loading increased house cooling loads by 8.4%, again compared to a clean HRB.^However, horizontal radiant barriers with these dust loadings still decreased house cooling loads by 7% when compared to the same house with no radiant barrier."

Energy Citations Database (ECD) - - Document #5162138

[skodicious]

bricor,

I definitely agree that foil type radiant barrier works like a charm. They usually install just under the roof--it's a good a idea to leave a pocket of space between the wood and foil barrier. There is a huge change in energy efficiency that comes as a result of the foil.

[Lakewooder]

Does anybody have an idea of the cost of roof decking without vs. with radiant barrier?

[GayleTX]

We used the spray-on radiant barrier and we could tell a difference this summer. We were convinced to do it by a friend who works in subdivison construction.....he said you could tell just by walking in the spec houses whether or not one had radiant barrier - the ones that had it were much cooler than the ones that did not.

[Plans Examiner]

It has been my observation that the placement of radiant barrer against the roof deck shortens the shingle life. It will cook the oils out of the shingles.
I had a laydown perforated mylar blanket coated w/97% pure aluminum installed. Insulation was updated where needed to minimum R30, duct work (radiant barrer) checked for tight seals. Had house built with ridge and soffit vent system.
With plate beginning at 9' and running to 12' and wife that keeps the thermostat at 70-72 range our bills have been very reasonable.

[EarthFirst!]

Originally Posted by KantLockeMeIn

...The one concern about stapling it to the rafters is that you have to really analyze your airflow in the attic. If you have soffit and ridge vents, you're safe.

[I am answering to a stale post dated July 2007.]

Yes, you are correct. Just be sure that you have a balance of static vent free area on your total roof vent system. A great percentage of homes have sorely inadequate vent area to the attic. You should calculate 1 square foot of static vent net free area for every 300 square feet of attic floor space so an adequate amount of intake and exhaust vents can be installed.

Most homes are not properly ventilated when the roof was originally installed. Merely cutting a few soffit holes and adding a couple of whirly-bird turbine vents to the roof ridge is not proper building science.

You can have a bazillion soffit intake vents, but if there is little or no gable/ ridge/ turbine vents to expel the stale hot air, your attic is destined to grow overheated. This free flow of attic air is needed, regardless if a radiant barrier is used or not.

On a tall hip roof with a 12:12 roof pitch (very steep), the number of static (unpowered) roof vents can be scarily high in number to properly exhaust the required volume of air. Electric roof vents are an option to cut down on the number of roof vents needed.

Originally Posted by Plans Examiner

It has been my observation that the placement of radiant barrer against the roof deck shortens the shingle life. It will cook the oils out of the shingles...

It seems that the consensus among testing laboratories of roofing membranes and shingle manufacturers is adverse shingle life is not observed while using a radiant barrier decking.

Radiant Barrier Frequently Asked Questions | McCoy's Building Supply
Radiant Barrier Effect on Shingles
Green Building Sustainable Building Sourcebook - Energy: Radiant Barriers

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[loves2read]

EarthFirst--agree that shingles deteriorate due to radiant barrier decking is wives' tale--and adequate ventilation is as necessary as the air you breath--easier to get wind turbines with larger diameter shafts vs smaller or electric fans--greater volumn of air moved for significant difference in price point efficiency...
also need to ensure that the soffit vents are actually OPEN and free-flowing vs just nailed/stapled/punched into the soffit on installation with nothing actually removed to allow effective air flow...there should be a plastic tray-type thing that prevents the insulation in the attic floor from blocking the soffit vents...

[johnrex62]

Originally Posted by Lakewooder

Does anybody have an idea of the cost of roof decking without vs. with radiant barrier?

I dont recall how much difference there was when I built in 2000, but today the retail difference is $2.61 per sheet at Lowes. For my house (approx 2500 sq ft with a hip roof) that would be an additional $410 for the shielded roof decking over standard OSB decking. Back in 2000 it seems that the price difference was less, but I dont recall by how much.

[NRGpro]

Originally Posted by m00se

Has anyone had experience or opinion on radiant barrier and its effectiveness. I've seen discussion here about the cost of energy but have not seen anything regarding radiant barrier. I'm about to have this installed in a new build and I'm wondering if this is a wise investment. Thanks.

Fact: There is more misinformation on the 'net about radiant barrier and the related physics associated with it than there is reality.

I've just read on here "two promising radiant barrier applications are reflective roof decking and stapling it up under the deck". Both are the worst thing you can do a home in a hot climate.

Why? Follow the heat, follow the logic and physics will explain the rest:
<li>Radiant heat strikes the roof from the sun which is immediately absorbed and conducted through the roof deck.
<li>Wood, with an emissivity of .95, freely radiates this heat into the attic.
<p>
Solution #1: Use reflective roof decking (benefits, detriments)
Benefit: Lowering the emissivity from .95 obviously lessens the amount of radiation entering the attic. <i>But does it lessen the heat???</i> There is no free lunch in energy or physics and for every action there is a reaction. If you agree that a roof deck placed in the sun freely emits radiant heat at .95 without a reflective surface, and agree that a roof deck with aluminum on one side does not, where does the heat go if it cannot radiate into the attic? Logic and physics dictate it must go back out through through the roof. If you agree with this, read on. If not, you won't agree with the following.
Lowering the emissivity of a surface exposed to radiation causes elevated temperatures. A roof deck with an aluminum surface below it is going to be much hotter than a roof deck without out and here's where it gets interesting:<br>
<li>If the roof deck is hotter with a lower emissivity than that of a raw wood surface, is this going to increase the convective action in the attic? Of course it will.
<li>If the attic temperature goes up will this increase the insulation temperature? Physics dictates it will.
<li>So all day long this increased convection superheats the attic insulation and the sun goes down, what is the hot insulation now facing? A radiant barrier. <br>
This installation is one way to create a passive solar oven in any home. An Arizona utility company leased for identical homes for a period of one year and put three different types of radiant barrier in three of the homes. One home had no radiant barrier. Two of the homes actually used <i>more</i> energy than a home without a radiant barrier. <a href="http://www.savenrg.com/apsgrph.gif"> Click here </a>to see the graph.
But don't take my word for it, find two pieces of plywood and staple tin foil to the back of one and leave the other one raw and set them in the sun, reflective side away from the sun. See which one gets hot. Now mulitply this effect by several thousand square feet.
If you need more proof, take your toolbox outside and open it up so the sun hits the chrome ratchet handle and sockets. Then return an hour later an try picking them up. It's basic physics.