What does a radiant barrier really do?
jwinko
| Posted in Energy Efficiency and Durability on
It seems that a piece of tin foil or some paint on the underside of the roof is advertising hype. Has any one actually tested radiant barriers and published their results?
Thanks.
john
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Replies
John
There's 30 years of research reports on this. Start off going to the Florida Solar Energy Center's website. The essence of a radiant barrier is that it EMITS a very small percentage of the heat that it absorbed. This is why you saw foil wrapping on the Lunar Excursion Module that landed on the moon - the foil wrap limited the amount of heat that would escape the spacecraft. Net-net is that RBs reduce solar radiation heating of an attic by about 12% to 25% - roughly equal to adding R-19 of extra insulation to the rood system in the summer, about R-4 in the winter.
I ain't sayin' nuttin', because last time I screwed up. But, read here and see what Martin has to say. Search for other info here, too. I think this article mentions a NASA reference.
https://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-search-problem
The Canadian Mortgage & Housing Corporation (CMHC) did 4 test homes each with different underfloor insulation; extruded polystyrene, polyisocyanurate, bubble foil & a no insulation control. the bubble foil appeared as though there was no insulation present. There is a PDF with their findings floating around somewhere, search & i'm sure you'll find it. From what I gather a radiant barrier requires an air gap & a clean surface to work effectively. A new radiant barrier additive (Neopor) for styrofoam needs no air gap.
heres some trash talk on radiant barriers: http://www.healthyheating.com/Page%2055/Page_55_o_bldg_sys.htm
I think John Straube does a good job of explaining why the "spacecraft" analogy is not-so-relevant here:
https://www.greenbuildingadvisor.com/blogs/dept/building-science/how-heat-moves-through-homes-building-science-podcast
If you want Data...
Michael Blasnik did a large study of Homes in Houston with radiant barrier roof sheathing and reported a very modest annual cooling reduction of about 180 kWh.
The Houston Homes had HVAC in uncondtioned attics and Code minumum R-value at the ceiling.
I believe that HVAC does NOT belong in unconditioned attics ...
AND that we should be building homes with MUCH more than Code minimum R-values.
I can't imagine how a Radiant barrier roof sheathing could save energy during the heating season.
John,
Radiant barriers work -- they just cost more and are less effective than other methods of insulation. The only application that makes sense (sort of) is to install radiant barrier sheathing during new construction for a house with ductwork in a vented unconditioned attic. (Even better, of course, would be putting the ductwork inside the conditioned envelope of the house, where it belongs.)
In addition to the link provided by John Klingel -- Radiant Barriers: A Solution in Search of a Problem -- and the link to the John Straube podcast provided by John Brooks -- you might also want to read a couple of articles about so-called "insulating" paint:
‘Insulating’ Paint Merchants Dupe Gullible Homeowners
An ‘Insulating’ Paint Salesman Is Tripped Up By His Own Product
(Thanks, John Klingel and John Brooks, for providing the links.)
Radiant barriers have limited applications. The Florida Solar Energy Study shows that as you increase insulation the benefit of a radiant barrier decreases. If you are building and can use sheathing with the barrier applied then it may be worth the investment. If you are looking at a retrofit then is probably isnt worth it. If you home has an attic with an R value of less than R20 then you would get a good deal of benefit. But if you house is that under insulated then it makes sense to air seal and insulate. As the R value approaches R30 the benefit is much smaller. Get the R value to 38 or higher than there is almost no value to an RB. Better off spending that money on something that will save you more money.
Thank you all for your answers. My daughter and I ran a simple experiment as we wanted to know if white paint would serve the same purpose as foil? We built 2 model doll houses with detachable roofs, one was regular osb and the other was tech shield. We put them both under a 100 watt light to see what the radiant barrier did. The results were the radiant barrier did as much as the ordinary osb.
So thank you all again...our experiment says we have all been duped into spending money on something that doesn't work.
Happy new year
john
Radiant barrier on roof sheeting causes more heat in hot climates by trapping it inside the attic for it too be effective it needs to be placed on top of the insulation above the ceiling then it keeps the heat from transfering as quickly to the insulation. Radiant barrier on or below the roof sheeting makes zero sense unless you have complete soffit vents and a roof ridge vent installed the full length of the roof so the air can move between the space between the sheeting and rafters. Then it will keep the attic cooler but again far more effective to put on top of insulation. The issues with dust are not as much of an issue now with using the reflective film vs the aluminum. the film is a ton more reflective and if you want go clean the dust off once a year.
Greg Brown... Rediculous, no soup for you. The dust is a factor. Who the H*** is going to dust their insulation system annually in their attic. LOL
Enjoy your weekend though, get out a play a bit on this beautiful big blue ball.