Sprayed radiant barrier vs. adding more
Sprayed radiant barrier vs. adding more cellulose insulation
Anyone have any feedback on the sprayed radiant barrier verse the aluminum foil sheet type
Dallas TX, temp 104 with high humidity
Our air conditioner cannot keep up runs all day not shutting off, thermostat set at 77 with temp reading 80-81
The foil would be stapled to the bottom of the roof trusses 3.5†air space
Sprayed would be applied to the bottom of the roofing deck
Foil faced Foam board?
The roof pitch is 8/12 I have a concern about the foil collecting dust decreasing its effectiveness, will have to reroof around 3 to 5 years
More cellulous insulation probably has an average of 6 to 8†now
I would be happy with 78 or so if the ac would cycle normally
What is the best course to take, wife is not happy, will be more unhappy when the electric bill comes
Ken
Replies
The foil is a little more efficient than the spray,but the spray is considered much easier to install and most people opt for the spray in an existing house.Either will likely help with your AC problem.
In your area R-30 insulation is considered adequate for attic.If you do not have this you should consider adding some.I would choose formaldahyde-free fiberglass blow-in or batts.I would NOT get cellulose blow-in.
Todd
Savvy,Would you please tell us why you prefer fiberglass to cellulose in this application? Thanks.Bill
Cellulose blow-in is essentially shredded newspaper and cardboard that has been chemically treated to be resistant to fire and pests. The problem is that it just breaks down, eventually becoming newspaper "dust".You end up breathing this dust, it does find its way from the attic into your living area. Also as it breaks down, it settles and your R-30 insulation over time becomes R-7 insulation. Additionally, it looses its fire and pest resistant qualities. Fiberglass costs a little more, but I think it is much better,as it does not break down and have the problems cellulose. I just finished my first 2 houses last year and I used cellulose. Most people do, its cheaper. But now I know a little more, and from now on, I will feel better about using the fiberglass.
Todd
Todd,Thanks for your reply. The concerns you outline are serious, and I can see why they are important to take into consideration.I don't know what your personal experiences with cellulose settling and 4:1 reduction in R-value have been, but would like to hear the specifics. The reason is that my own personal experiences with cellulose have been very positive.The home I built for myself in 1984 had R-50 cellulose blown into the attic. 14 years after that, I finished the attic and had to move huge amounts out of the way of the subfloor. There was no apparent breakdown of the material, nor of the insulating power. That home had no furnace in an 8000 degree-day climate, and was comfortable the whole time I lived there.When I moved to my current home in 1999 I tore out the fiberglass batts from the attic and put in R-30 cellulose. The change in comfort was very positive, with one lady even asking me if we had air conditioning as she visited us one warm day. That improvement continues to hold.A number of my customers have switched to cellulose attic loose-fill on my recommendation, and been very happy with the results. If the problems you mentioned do exist, I have a responsibility to understand them so I can better serve my customers.What, in your understanding, is the time frame in which cellulose loses 75% of its insulating power?Bill
Hi Bill,
I'm sorry, I did not mean to imply that cellulose is not good.I have heard from my installer that that the stuff breaks down and tends to settle over the years, losing some of its R value.I have been in my share of 50 year old attics and have seen the remains of old cellulose, which looks nothing like the fluffy new gray-white stuff. I don't have any hard scientific data about the time frame of the decay,but I would guess it becomes substantial around 40 to 50 yrs.
I believe that any blown in product has an advantage over batts in that it tends to fill nooks and crannies better than batts. There is a very small,but growing movement to remove cellulose.Some people are concerned about the chemically treated cellulose dust entering their living area.I have heard that a few insulation installers are also getting into the removal business.I don't know how valid the environmental concerns are,over time I'm sure we will learn more. In a nutshell I believe cellulose is a fair product,and it is still the standard.I just believe that,for a little more money the blown-in formaldahyde free fiberglass is worth it.Thats just my opinion(and you know what they say about about opinions).
Reguards,
Todd
Thanks for the reply. I have never seen a cellulose installation older than about 30 years. Could the product have changed since then?The current treatment for fire retardance and vermin repellence is to soak the paper in a borate solution, then dry it again. Since the borates are an inorganic salt, they ought to last indefinitely in a dry place.Newsprint, being a low-quality paper, may well self-destruct over time. I have found 100 year old newspapers in wall cavities and they tend to be fragile exhibits.I have also found that insulation contractors dislike blowing cellulose because fiberglass is easier for them to work with. I have never seen the attic insulation infiltrate the home's living space except when someone crawls up there. Same with fiberglass.I agree about blown-in outperforming batts. Finally, my own experience is that cellulose is vastly superior to any fiberglass at rejecting thermal gain from hot attics. My conviction is that cellulose is opaque to the full range of infrared emissions from roof decks, while fiberglass is transparent in at least some of that band. I can feel the rays on my bald head when I walk into a fiberglass-insulated house in summer. That is my personal detection system.Bill
Hey Bill,
Perhaps the quality newspaper has gotten better(though, not necessarily the quality of the news reporting).Or the borate treatment is better.I do not know. Concerning the efficiency of cellulose vs. fiberglass I defer to your highly attuned personal detection system, as my own personal detection system is flawed by some insulation remaining on my head.
Cheers
Todd
"Chemically treated" cellulose is treated with a borate compound that serves both as a flame retardant and a bug retardant. As chemicals go, it's about as innocuous as they get.The fluffy appearance of cellulose changes over time primarily because of the dust that settles on top of it in most environments.They've maybe improved it recently, but used to be that chopped fiberglass would settle a lot more than cellulose. Cellulose does settle maybe 10-20% over time, but that's factored into the numbers for R value.Fiberglass, if not covered somehow to prevent air infiltration, is relatively ineffective when high temperature differentials are present. Maybe not a big deal down south, but it is up here where the temp differential inside vs outside may be 100F.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
> Cellulose blow-in is essentially shredded newspaper and cardboard
> that has been chemically treated to be resistant to fire and pests.
> The problem is that it just breaks down, eventually becoming newspaper
> "dust".You end up breathing this dust, it does find its way from the
> attic into your living area. Also as it breaks down, it settles and
> your R-30 insulation over time becomes R-7 insulation. Additionally,
> it looses its fire and pest resistant qualities.Must be something that happens down south -- have never seen such problems with cellulose up here.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I prefer chopped FG but his concerns are probably exaggerations about the cellulose. I have seen it burn three times though so I don't trust the retardent 100%
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Several items to consider.
1) The summer design conditions for Dallas TX is 100 degrees outside and typical indoor conditions are 75 deg. At 104, if your AC cycles, its oversized, and would short-cycle too much during milder times.
2) Your AC, if properly sized, will maintain a 25 degree difference with 100 degrees outside, maybe 20 when its 105. Based on whjat you posted, the AC appears to be sized just about perfectly. While it may not cool to 75 on a 105 day, if it runs all day, the relative humidity in the space is probably 50% or less, which is very good.
3) Radiant barriers are mostly misunderstood and poorly utilized. Of the two you mention, the staple up foil is the better choice. There is no effective "spray-on" radiant barrier. Insulating the ceiling under the hot attic space is the best improvement to make.
Tim,
I have to disagree with you about the spray on barrier. I have installed Sherwin Williams E Barrier on several projects and it effectively lower attic temp by at least 10 degrees in the summer. Most folks here in Texas have all their duct work in the attic and maybe r30 insulation. That ten degrees is significant when trying to cool the house. Could you point me toward the data showing regarding the spray on barrier?
Bruce
My point is that for a barrier to be able to reflect radiant energy, it has to have a surface that is not in contact with the heat "source". The sprays may function as an insulator, bit NOT a radiant barrier.
The barrier must not be in contact on both sides. Does not matter much, when you run the numbers, whether it's the hot side or the cold side.The fundamental laws of thermodynamics demand that any surface that absorbs heat also radiate heat equally well, so a reflective surface will not radiate very well.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
"The fundamental laws of thermodynamics demand that any surface that absorbs heat also radiate heat equally well..."
Your statement is not accurate (close, though), but I know where your trying to go with this.
Heat can be transfered from an object to its surroundings in one of three ways; conduction, convection and/or radiation.
The First Law of Thermodynamics (there are two, BTW) simply states that the net energy transferred to a system, less the energy stored within the system (assuming no nulcear fusion or fission is taking place) is always equal to zero. This is the law of conservation of energy. The Second Law of Thermodynamics has to do do with the fact that the world is entropine and that all real transfers of energy on this planet, are irreversivble, i.e., no perpetual motion machines.
The concept that coating the underside of a hot surface with a substance of low emissivity would theoretically limit the radiant heat transfer from the surface is a good idea, IF it weren't for that First Law. When soalr energy is incident on the roof surface, a given amount of energy is absorbed. The roof will store some in the form of an elevated temperature and will transfer ALL of the rest. (For brevity and simplicity, we'll consider only a steady state, steady flux system). In a well ventilated attic, conduction and convection remove a large portion of the heat. The same occurs on the exterior of the roof on a limited basis. Only so much heat can be transfered to the surrounding air by natural convection and conduction. The remainder of the energy is transfered via radiation.
Radiant heat transfer is proportional to the 4th power of the absolute temperatures of the surfaces. If the emmissivity of a surface is reduced, the heat transfer is reduced at a lower temperature. The "system" continues to absorb energy and heats up until the temperature rises to the point where the radiant energy transfers is again in equilibrium with the other processes as described above. This is unchangeable physics. The only real effect of such a "barrier" is to raise the overall temperature of the roof, and very likely to shorten the life (and this is a big one, most l;ikey VOID the warranty of) the composite shingles.
Radiant barrier coatings on the underside of roof decks are high tech bamboozles.
The reason that the foil-coated bubble wrap or kraft paper is (or at least can be) a more effective barrier to radiant heat transfer, is that it raises the temperature in the rafter cavity which can be readily removed by natural convection with proper venting in place.
You are forgetting one way that energy is disipated.Re-radiated out the high emissive shingles..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
You're right. As the temperature of the roof structure rises, it will radiate more energy from the exterior surface as well as the interior surface.
Shingles are "high emmisive"?
There are more than two laws of thermodynamics. The first two are the best known (1: You can't win. 2: You can't even break even.) but I was referring primarily to the laws of thermal radiation, Kirchhoff's law in particular.You're description of the effects of a radiant barrier misses a couple of points:1) By reducing emissivity, the rate at which heat is transferred from roof to attic floor is reduced. This lowers the temperature of the attic floor, in steady state, and thus lowers the rate at which heat is transferred into the house.2) Similarly, the temperature of the roof is increased. This raises the temperature of the roof exterior, increasing both convective and radiation heat transfer from the roof to the surrounding environment. This in turn limits the heat rise of the roof.The heat's got to go somewhere. The radiant barrier causes a larger percentage of the heat to be "rejected" and passed off to the environment, vs entering the house.Now it's legitimate to ask how significant this effect is, and whether the radiant barrier is worth the expense/effort, but, based on the physics, the effect exists.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
There are two Laws of Thermodynmics, as such, but you got them wrong, even in the simplest terms. The first is colloquially known as "you can't get something for nothing" and the second you actually got right.
You need to learn the "Law of Holes". Simply stated, when you're in one, stop digging.
"...of thermal radiation, Kirchhoff's law in particular.."
Kirchoff's laws pertain to currents (and lotages) in electric circuits.
So while previously, I gave the benefit of the doubt and thought you may have been a little rusty on TD. I misjudged.
The law that applies is the Stefan-Boltzmann Law. IF you were familiar with this law (I have to assume based on the inaccuracy of your statements, that you are not), we could discuss this further in technical detail. I think in this case, however, it would be fruitless.
Both of your "points", by the way, are excellent examples of a Formal Logical Fallacy (affirming the consequent). It doesn't work with words or with energy.
"Kirchoff's laws pertain to currents (and lotages) in electric circuits."Not when it is Kirchoff's law of thermal radiation.http://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiation.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
What were you saying about holes?
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Reflecting is only one way to control radiant energy.The other way is to have a low emissive surface facing the interior.Look at the Florida Solar Institute. They have some school scieince experiments shows how low emissive surfaces serve the same function as high reflective surfaces..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Low emissive vs. high reflectance.
Its kinda like keeping water out, vs easily dryable. The point is arguable from either perspective. Keep the energy out or limit its transfer.
I prefer the former, because I don't understand how the latter can be acheived (without the benefit of a large thermal mass that actually can store a large amount of incident energy). My understanding of this process is outlined above.
Light colored, reflective shingles make more difference than any thing on the underside of the deck.
Else, good ventilation and good isulation are the only effective "barriers".
FSEC reconizes paint. But the last time I searched it they have much in the way of details.I found this.http://www.fsec.ucf.edu/en/publications/html/fsec-en-15/"What kinds of radiant barrier materials are available?
...Although it is not by definition a radiant barrier, there is a low-emissivity paint available that can be applied directly to the underside of the roof decking.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Tim, You sure know your stuff...I just heard the same advice from a local (Dallas) hvac guy who I really respect. He said radiant barriers are good...but the spray on was usually not a cost effective solution for retro-fit.
Edited 8/17/2007 12:33 pm by homedesign
Thank you for all your responses
After reading and more reading
I am going to add more insulation, cellulose to bring my insulation up to or greater than R-30
Then the next thing to do would be to tighten up my HVAC system
i hope those two things will be the most effective things to do
Again thank you