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Radiant Barrier?
Old 05-23-2008, 09:05 AM   #1
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Radiant Barrier?

Anybody done this?

Radiant Barrier Fact Sheet

I had to replace a leaking pipe in the attic recently and was looking at insulation and thought to myself "Hey, the temperatures are rising, why not spend some quality time sweating like crazy in the attic?""I could use some more insulation here." And if I'm going to be up there, I might as well decide now if I want to do a radiant barrier, because the insulating job will most likely cure me of wanting to go up in the attic for at least a decade.
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Old 05-23-2008, 09:28 AM   #2
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I think Nords has done this; maybe he'll be along after the morning surf. Maybe CFB too?

Radiant barrier is third on my list. I have ridge venting, of the older, mesh type, which seems to be "clogged". Case in point, on a day when the outdoor temperature was in the 60s, my attic was 130, from the direct sun. So I tried a "smoke test", holding a lit cigarette next to a soffit vent. The smoke just blew away. With such an extreme temperature gradient, you'd think there'd be some pretty good suckage. So, without better ventilation, the radiant barrier wouldn't really accomplish anything, except maybe frying my shingles and roof decking.

Anyway, (1) is replace ridge venting with a more open design, (2) is blow in more insulation, then, (3) radiant barrier.
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Old 05-23-2008, 09:56 AM   #3
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I knew a guy who installed a radiant barrier across the underside of the roof rafters. So the shiny surface faced the roof and the open space between rafters. It was important to leave a big enough gap down near the wall top plate, and up at the ridge board, so air that was heated by conduction could flow up and out. He said it made a big difference in attic temperature. He was a fellow Engineer with lots of hands-on abilities, so I trust his results.

He did have a concern that over the years the radiant barrier's reflectivity would be adversely affected by dust that would eventually start to coat the barrier. He moved a couple years later, so he never found out about long-term effects.

Around here, there are companies hawking spray-on reflective barriers. They spray silver-colored stuff on the underside of the roof. This is a simulation of the foil-faced OSB roof decking that is used in a lot of new houses now. In actuality, those are NOT reflective barriers, a reflective barrier needs an air space to reflect radiant energy back for it to work. The spray-on and foil-faced OSB facing down are actually an emissivity barrier. A shiny silver surface is a poor infrared emitter, so with one, the roof's heat radiates less infrared energy down into the attic. An of course, if it radiates less energy, then its (the roofs) temperature must rise.

Silver-color is low emissivity, high reflectance.
Flat black is high emissivity, low reflectance.

Which is why a good heat sink for semiconductors is black anodized aluminum, rather than just mill-finish aluminum.

I don't have any direct knowledge of a proper before-and-after test with the sprayed-on silver to say whether it works well enough.
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Old 05-23-2008, 09:58 AM   #4
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In the DOE report they underline the bit about shiny surface. There is a reason for that. In the attic once the dust settles on the shiny surface, you just have a barrier. with some thermal transfer function.

To work, radiant barrier reflects heat back into the attic. Without good ventilation, now there is a fine oven. With dust on it the barrier now reaches nearly ambient, then becomes a good radiator. In both directions. Around 5 pm or so.

Good luck.
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Old 05-23-2008, 10:09 AM   #5
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My attic, without reflective or emissive barriers, easily gets up to and holds Easy Bake Oven temperatures (without a light bulb too - I'm saving energy ).

Dust is a real problem here, another reason not to move to Texas. I have to clean out all the soffit intake screens, otherwise there would be no airflow. And the airflow just brings in the finest dust.
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Old 05-23-2008, 10:16 AM   #6
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So put in radiant barrier, don't clean out vents, get rid of oven.
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Old 05-23-2008, 12:54 PM   #7
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Right, because of the dust issue, if applied to the bottom of the trusses /joists supporting the roof then the shiny side should be down (so less dust gets on it, and what you have is an "emissivity barrier" rather than reflective barrier. Technically, either way it is a "radiant barrier" since you are blocking the flow of radiant heat.

Factors:
- Rule of thumb: These barriers only make sense in hot climes
- Roof construction is important. Installing the film to the underside of the joists is realtively easy in a rafter-type roof system, but can be a PITA in a truss-type roof system (The cross-braces in the trusses hinder movement n the attic as you do the installation, and they cause more "working around" with the foil material.
- Headroom in the attic/longevity: As a practical matter, will people using the attic inadvertently tear the film down? When I'm working in or attic (low-pitched roof, not much room) I'm constantly in the spaces between the joists (and, yes, it stings wen those roofing nails scratch your back). So, the foil would be a problem for me.
- Longevity of roof materials: Any blockage of radiation into the attic will drive up the temp of the roof deck somewhat, and higher temps of asphalt shingles will reduce their life somewhat. Some sources claim there's no problem with this.


Actually painting the underside of the roof deck itself with a highly reflective paint might be a good answer. It won't be as shiny as the film, but it would help reduce emissivity to some degree. I'd want to assure that whatever was put up there had good water vapor transmission qualities (approx 10 perm or greater) to let the inevitable water vapor in the roofing decking dry out to the inside as well as the outside.

Hunch: If there are no other factors, you'd probably find that using the money for the radiant barrier to buy a few inches of blow-in cellulose insulation in your attic would do more to keep your house warmer in winter and cooler in warmer in winter.

And, I agree with HFWR's observations on ventilation-- it helps cut the AC costs and (more importantly) significantly reduces the chances of water damage to the structure of the attic/roof.
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Old 05-23-2008, 02:33 PM   #8
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Anyway, (1) is replace ridge venting with a more open design, (2) is blow in more insulation, then, (3) radiant barrier.
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Hunch: If there are no other factors, you'd probably find that using the money for the radiant barrier to buy a few inches of blow-in cellulose insulation in your attic would do more to keep your house warmer in winter and cooler in warmer in winter.

And, I agree with HFWR's observations on ventilation-- it helps cut the AC costs and (more importantly) significantly reduces the chances of water damage to the structure of the attic/roof.
This thread is the first I've ever even heard about radiant barriers.....hmmmm, where the heck have I been?!

But I can thoroughly relate to the added insulation and ridge vents!! Back in the late 70's I shinnied up into our low-slung attic and added 6" of additional fiberglass batting......the house stayed much cooler in the hot weather, and the A/C didn't have to work so hard.....and it stayed much warmer in the cold weather, and the furnace didn't run so much! I don't know the exact figures, but I know that the savings on A/C & Heating more than paid for insulation in that first year!

When I had the house re-roofed a couple of years ago, I had them install a ridge vent the whole length of the house. We already had gable-end vents, and soffit vents, but the ridge-vent has increased the air-flow up there a great deal!

We used to have 4 window air conditioners running to keep the house cool. Now we have 2, lower btu units. If it gets into the mid-80's and/or is muggy, we turn the one on that's centrally located in the living room, and it keeps the entire house cool (~1250 sq.ft.). Otherwise we usually just use the whole-house fan with the north-facing windows open.....or leave the fan off and keep the house closed up during the hottest part of the day. The 2nd window A/C is in my bedroom, and since I can't have the window open with that in it, I run it either just on 'fan' or 'low' a/c.......and then only at night.

In any case, we keep the indoor temp around 68-70° year-round, and neither the A/C's nor the furnace get much of a workout.

Similarly, my 400 sq.ft. workshop/hobby room has similar insulation and venting, and I heat it (only when I'm out there) with two 250 watt infrared bulbs mounted in 'chicken coop' light fixtures (ceramic bases with aluminum shades), one hung from the ceiling toward each end of the room. They keep the room at about 55° in the winter. Once in a while I'll run my ceramic heater on 'low' for a little while to help get up to temp quicker, but then I turn it off again.

Fixtures similar to this:
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Old 05-23-2008, 11:56 PM   #9
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I did the spray on barrier and added some inches of insulation at the same time last winter, so it is difficult to isolate which is the causative factor, however my results so far, are a noticeable difference in the power bills (about $100 per month lower), during the winter. Now that we are having our mildly warm temperatures (95-100), my DW actually set the thermostat up 3 degrees, as she was complaining of it being cold, with it set at the old normal temperature. It feels quite comfortable, with a ceiling fan or two circulating the air. I'll have to wait till I get a bill for these warmer months to see the impact there, but I expect a similar if not better performance. Oh and it is much cheaper for spray on installed, rather then the sheets or tack on films, by a professional installer, and I'm not about to get up there and do it myself. I fell through the ceiling once, and that's enough.
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Old 05-24-2008, 12:05 AM   #10
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I have read that the paint on barrier is not worth the time, trouble and money....

But insulation is good...


Has anyone just done the 'paint' kind and had proof it worked? Just curious is what I read was true...
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Old 05-24-2008, 10:01 AM   #11
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In my rather non-scientific way of thinking, the barrier would ideally be on the outside, on the roof decking.

Does anyone have experience with metal roofing? I've been thinking that, when hail finally gets around to destroying my shingles, it might be a good, albeit expensive, replacement for shingles. Watching too much This Old House maybe...

Fifty year warrantly, more reflective than shingles, and they already have a clip system for mounting solar panels.
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Old 05-24-2008, 10:05 AM   #12
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Fifty year warrantly, more reflective than shingles, and they already have a clip system for mounting solar panels.
On my next house, I'll be doing a metal roof for those reasons plus a metal roof is more conducive to a rain water capture system.
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Old 05-24-2008, 10:13 AM   #13
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I havent done any radiant barrier stuff although I probably could have in the current siding job I'm doing by picking a foam layer with a radiant backer. Would have jacked up the cost of the foam by about 20%. I've heard so much contradictory stuff about it that I decided to just add the r-4 foam layer to the existing wall insulation which I'm guessing is r-9 or r-11. We're also improving the attic ventilation a bit by installing larger, less restrictive soffit vents. I have three huge eyebrow vents on top, so exit air isnt much of a problem.

I looked at the paints and thought about that a while. Looks like they have something in the area of 30-60% of the reflectivity of the foil.

Not sure how effective it is when sprayed over 1000lbs of dust and spider webs.
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Old 05-24-2008, 04:41 PM   #14
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On my next house, I'll be doing a metal roof for those reasons plus a metal roof is more conducive to a rain water capture system.
Way way back when.... I was in a house with a metal roof.... the guy said it cut his air conditioning cost to only 25% of what it was...

When I had my siding replaced with Hardi plank.... I had them use the foil foam insulation..... seems to work well.... my utility usage dropped... but with the higher rates.... my costs have not gone down a whole lot... but I am saving enough that it is paying for itself...
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Old 05-25-2008, 11:56 AM   #15
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Okay Gang,

Thanks for all the responses, but I felt the need to go seek hard data, and here is what Leonidas Laboratories came up with.

A good field study using three houses in Texas can be found here:

Welcome To Home Energy Magazine Online

And a detailed executive summary from a study in Florida is at this link:

http://www.eagleshield.com/articles/...arrbreport.pdf

And a brief summary of a field study by Oak Ridge Natl labs done on houses in (I think) Tulsa.

Energy Citations Database (ECD) - - Document #5462630

Conclusions: Radiant barriers do reduce heat flux in attics, but they are more suited for climates with very hot summers and mild winters. In fact, if you have a real winter in your climate I don’t think you should have a RB. But places like the Gulf Coast, where summers are hell and winter a joke, rock-on with a RB.

Reduction is more significant in attics with less insulation, but even with a lot of insulation there will still be as much as 25% heat flux reduction. So, if you have a lot of insulation there will still be benefits, especially so if you have the central ac and ductwork in the attic.

Airflow through the attic improves the effectiveness of a RB. It doesn’t have to be a lot of airflow, but air does need to flow or the effectiveness will be significantly decreased.

Dust can reduce effectiveness. However, I couldn’t find a field study measuring that. A Texas A&M master’s thesis from the early 1990’s, that I found online, seems to be what everyone is referring to. But the experiments in that case only studied how different amounts of dust hindered effectiveness, not how dust accumulated or how long it took for those amounts of dust to accumulate on a RB in a real attic. Most people agree that the effects of dust can be minimized by not installing the RB horizontally over the insulation, and using a two-sided RB will prevent most, if not all loss of effectiveness due to dust.

Living in a climate where it is hot enough that Hell sends the overflow here, and with RB at 13 cents (or less) a square foot, and free installation (two teenage sons) I think a radiant barrier will work for me.
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Old 05-25-2008, 04:37 PM   #16
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I had to replace a leaking pipe in the attic recently...
Ruh-roh, sounds like a scary story there.

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I think Nords has done this; maybe he'll be along after the morning surf. Maybe CFB too?
Let me summarize the most important things to remember about this project:
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..."why not spend some quality time sweating like crazy in the attic?"
... because the insulating job will most likely cure me of wanting to go up in the attic for at least a decade.
Keep in mind that I used to get paid extra to lie on my back sweating profusely in dark, tight, hot spaces-- we used to call it "officer berthing".

It involves settling your heels on one rafter, your butt cheeks on another, your scapulae on a third, and then pounding the foil overhead about 6" away from your nose (or reaching way back behind your head) with a hammer tacker while hoping you don't wake up the carpenter ants. Don't get stuff in your eyes. Oh, and try not to fall through the drywall. Try not to let your puddles of sweat eat through it, either.

If you do a good job, then the attic temps will drop from 130 degrees to the high 80s. (Yes, I brought a thermometer.) It's a very cheap (yet very arduous) way to reduce your home's heat load. You may wish to install an attic floor or "the ultimate attic storage" rafter system before you do this at your house. I am not aware of anyone who seeks paid employment at these tasks.

Do not install radiant foil in climates with winter snow. The foil will make it easier for the roof to melt the snow, which will freeze the gutters and form ice dams. Some homes have anti-ice-dam roofs but you may not wish to test this feature.

Dust may be an overblown (heh!) issue because we're almost at five years with the same temps. We've also put the radiant foil in a storage shed, most of our south-facing walls, and our west-facing (wood) garage door. This weekend we're finishing up the last of the walls.

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Radiant barrier is third on my list. I have ridge venting, of the older, mesh type, which seems to be "clogged". Case in point, on a day when the outdoor temperature was in the 60s, my attic was 130, from the direct sun. So I tried a "smoke test", holding a lit cigarette next to a soffit vent. The smoke just blew away. With such an extreme temperature gradient, you'd think there'd be some pretty good suckage. So, without better ventilation, the radiant barrier wouldn't really accomplish anything, except maybe frying my shingles and roof decking.
Anyway, (1) is replace ridge venting with a more open design, (2) is blow in more insulation, then, (3) radiant barrier.
Some homes just need to have the bird holes and exhaust grids cleaned out or rescreened; ambient breezes may take care of the rest. Of course it's a lot easier to do this work during a roof replacment, especially if the sheathing comes off.

We highly recommend 800+cfm solar-powered attic exhaust fans. They're totally silent but they make a big temperature difference. We even put one above a stairwell next to a cathedral ceiling-- cut a hole in the ceiling, ducted it through the attic to the roof, and put the fan on the roof. As soon as the roof hole was cut you could feel the heat whooshing out of the stairwell, which temps dropped from nearly 100 degrees to 80.

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In my rather non-scientific way of thinking, the barrier would ideally be on the outside, on the roof decking.
Does anyone have experience with metal roofing? I've been thinking that, when hail finally gets around to destroying my shingles, it might be a good, albeit expensive, replacement for shingles. Watching too much This Old House maybe...
Fifty year warrantly, more reflective than shingles, and they already have a clip system for mounting solar panels.
Ridge vents are popular here, as are foam roof-insulation panels and standing-seam aluminum roofs. (With only slate lasting longer.) We watched a lot of aluminum blow away during hurricanes Iniki & Ewa, though, so they should be storm-rated and perhaps have storm anchors.

Ovonics even makes a photovoltaic roof tile. It's hideously expensive but very fashionable.
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Old 05-25-2008, 04:44 PM   #17
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Do we really want to know how you learned this?
The attic is not a place where I normally hang out, think about, or really care about. I figured others would be of the same mind and wonder what the hell I was doing up there.
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Old 05-25-2008, 05:59 PM   #18
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On my next house, I'll be doing a metal roof for those reasons plus a metal roof is more conducive to a rain water capture system.
While cedar shingles might absorb a bit of rain, why is metal better to capture water?


My brother has metal roofing. Since their bedroom is on the top floor, he says it gives you a new perspective on the 'soothing effect of a gentle rain upon the roof'. In our part of the world, attics usually have at least R60 worth of cellulose or fibreglass insulation, and that doesn't do much to muffle the sound. We won't mention hail other than it doesn't destroy a metal roof.
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Old 05-25-2008, 06:15 PM   #19
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My ridge venting is a maybe 1.5" sheet of what looks like a scrubbing pad, with shingles nailed over it. The newer kind I've seen at Lowes is a metal cap, with vent holes under an overhang. No bug filtering; I suppose you add that (or not).

The roof over the LR/K/DR is about ten feet at the peak, so I would have to install temp flooring to provide footing for a ladder. Any permanent flooring would have to be raised above the ceiling joists, otherwise you couldn't adequately insulate...

I plan to add insulation; maybe 12" of blown-in cellulose. Also exploring new windows to replace the el cheapo supremo, 23yo builder grade ones I have now.
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Old 05-25-2008, 08:02 PM   #20
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While cedar shingles might absorb a bit of rain, why is metal better to capture water?
Well, cedar would be ok but out of place next to photovaltic cells. Standard shingles blend well, but the last thing you want in your cisterns is water that's just run over gravel and tar.
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