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... The sole advantage within metal roof may be less thermally stressful to a building system is installation of furring beneath the metal permitting natural draft....
The don't generally put firring strips beneath metal roofs around here. We don't want a draft underneath the metal, due to hurricanes.
Location: Central Atlantic Region, though consults worldwide
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Quote:
Originally Posted by PCHI
Ok Im not the physics expert of the group, however, Im a home inspector. I travel in many attics throughout the summer and the heat can be brutal. The times Ive encountered the thermal backed plywood I have noticed reduced temps in the attic. So in that regard, it works. The open/closed cell debate is another topic but that also does a great job of reducing temps. You will need to compare a closed/sealed attic vs. a vented attic with the stuff. Also there is a debate on the characteristics of open vs. closed cell sprays. Additionally, in Florida, every 5 yrs you will probably be asked to renew the wind mitigation inspection report for insurance and a sprayed attic leaves no place to verify a good portion of the inspection report as the various reportable points are covered. All things to consider.
Great point PCHI. True, anything is better than nothing, without question. I would like to know what are your experience has been with foil backed plywood (Low E) roof decking? Better worse, the same compared to thin foam coatings?
As an aside: I hear you about the open vs closed cell... Polyiso reigns king for R per inch for roof apps however the open cell sucks if any moisture touches it. PolyIso has a fairly stable performance longevity curve.
Installing Low E buffered vented heat mirror products with a 1" min gap between roof dech and reflective surface should prove quite effective for inspection appraisal access. Heck, a simple fiber optic camera can visibly document deck conditions.
Oh BTW: Not to confuse a point commented on earlier - here is what heat barriers do in a hybrid energy formula:
Normally: BTU/hr * = SF area x **U x Temp difference (inside vs outside !)
So say: a 10 sf area x 0.0526 (1/R-19) x 80 (150-70 deg inside) = 42.08 btu(s)/hr or 4.21 btu/sf.
* amount of heat energy transported through a system in one hour.
** U = conductance, = 1/R (value)
! = HVAC contractors use ambient air / climate data to size heating and cooling machine sizes. If 100 def F is the upper limit cooling design temp ... What happens when the actual surface temperature in the direct sun is 150 deg F? That's right the cooling system is under designed for that instance in time and circumstance.
Ok same situation, except with a properly fitted vented heat mirror installed:
10 sf x 0.0526 x (150- (150-100) x .9) - 70 (interior temp) = 35 temp difference
Therefore Btu transport becomes = 18.41 or 1.84 per SF
From 4.21 btuhr/sf without venting barrier down to 1.84 with vented barrier. That's anall too common condition which accounts for a 56.3% btuhr throughput reduction without adding more R-value insulation.
@ 150 deg solar struck wall surface
@@ 100 degree ambient outdoor air temp
150-100 means 50 degrees above ambinet air temp is directly attributable to Solar converted thermal heat. After all, the 100 deg F outdoor air temp cannot alone be responsible for a 150 degree wall temp.
The .9 vent reflectance coefficient is a solid reduction multiplier without getting too technical.
In a basic sense, vented barrier systems added to R-insulations will only reduce the temperature difference - R-value does not change in the process however the btuhr potential is dramatically reduced. Its the Btuhr total resultant calculate do a total structure that contributes to a A/C system size. Without vented radiant barriers a large home may require 10 cooling tons where as with vented barriers a home might only need 4.5 tons. In my area, 1 cooling ton costs about $3,500. Saving 5.5 tons is equal to $19,250 equipment savings. That's vastly more than enough to pay for a quality vented barrier system. Smaller machines mean lower monthly cost, more space to run special duct or afford deeper control logics, smaller electrical service requirements and more. Certainly smaller HVAC also smaller loads for solar or wind systems if being off the grid.
Did you notice that R (or U) did not change however the btu/ hr dropped?
I have no idea about using foil backed plywood for a tile roof--
but in TX I know that using radiant barrier plywood decking for composition roofs and for side walls on stick built can be VERY effective at moderate cost upgrades--
especially now that more builders are using them
some builders use a reflective house wrap like Tyvek around the framing before bricking or putting up the exterior panels...
I have been in two new homes--facing same direction, same street next door or close to where one has it and one doesn't--(house hunting) and know that the attics in homes w/radiant roof decking are 15-20 degrees cooler..
In TX this is very important because many, many homes have the HVAC equipment in the attic space--
Most homes in TX don't have basements and don't use units that fit in closets as I have seen in condos...
Our FL house has the AC outside and the heater in the attached garage...
But as couple of you said--laws of physics can aid you--
in a two story house if people used the chimney effect of the staircase to help vent hot air during the hot months--by adding some ventilation aspects into that part of the roofline or wall--then it would immeasurably help the HVAC in those homes...
saw house designed for to be passive heat/cool -- was built in New Zealand I think by college students for world competition--
had a roof that was raised OVER the ceilings--so there was vent space between roof and ceilings--
in that climate it was effective cooling effect when you have so many more hot than cool days...
The number of heating/cooling days is important factor in deciding what should be the focus of design...
Location: Central Atlantic Region, though consults worldwide
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Great post loves2read.
Haven't much time so I'll blast through this.
Attn Post readers: Do not get lost in whether or not radiant barriers (RBs) work. They absolutely do. Every major sector of the economy relies on heavily on RBs (actually heat mirror tech) to deliver benefit to consumers. Your oven has it, the firewall of your car, space research cannot function without it, laboratory processes need finite thermal control and cannot achieve it otherwise economically... the list goes on.
RBs for use within opaque building systems should be certified 97%+. A read one guy's opinion saying infrared (IR) will not go through "that" material. He was correct. However, through ignorance, he was unaware the material absorbed the IR then converted it into sensible heat (i.e a rise in actual temp). After absorption and conversion this heat gain is both conducted and RE-Radiated inward in or outward in winter.
RE-Radiation is omnidirectional. So, if a floor wall, ceiling, or roof absorbed IR - it will go in all directions. Informed math suggests approximately 46-49% IR re-radiated will negatively impact the interior space. Do not forget there is thermal convertible energy in some visible light, as well as UV, not just IR. RBs cover the spectrum, mostly dealing with micron to nano wavelengths. Most of the heat energy is contained in the 700-1300 IR range. This is electromagnetic energy, radio wave like frequency. To abate this form of energy one must install RBs so as to achieve a 1:100 air to mass relationship.
IR absorbed heat converted to sensible increases surface temperature. Suppose a sun-struck wall is 140 deg F. However the outdoor air temps is only 90 deg F. The surface temp increase of 50 degrees is directly attributable to solar energy.
Now if your going to use RB, it is HIGHLY advisable to exploit LOW E properties. Emissivity, as in Low E, is a material property rating a material's ability to release its stored thermal energy. Low E means a poor ability to RE-RADIATE!
Going back to the omnidirectional thing: Full radiative emissions omni-directionally is no longer possible dues to a Low E surface. Meaning if omnidirectional of 100% potential of the 46-49 percent trying to negatively impact interior space is reduced to 1.38 to 1.47% due to an effective LOW E material. What any RB can do is minimize surface temperature directly involved with re-radiation. So, if properly installed the temp difference with properly installed RBs will reduce actual exterior surface temp to outdoor air temp. Low E kick in from there if property configured.
Just because a RB is installed do not think your bases are covered. You'd likely wrong in most all cases. RBs reflective surface and the LOW E material must have a space, but how much? I've stated this before: Think of IR energy as visible light reflecting off a mirror. Without direct contact your see all encumbered reflection. Press your thumb against the mirror. The mirror is not longer reflecting about the space your thumb occupies. What's worse, many reflective barriers are made of highly conductive materials, like aluminum. In the case of the thumb, its longer reflecting, instead it is conducting.
Now, go back to the 140 deg wall. Absorbed/converted heat is conducted through the R-Value insulation. A higher thermal difference (inside wall temp vs outside wall temp) WILL INCREASE/ACCELERATE BTU GAIN in summer. Now R-Value is compromise as a basis of time is considered. People who install RBs professionally still get it wrong- Sorry fellas. Yes they do a great job at installing RB in attic spaces. However, every RB install must still vent surplus heat. WHY?
Within a closed cavity the air temp will rise due to the prevention of re-radiate heat transmission. Heated air also heats the RB and the surface of the insulation. Once again, one returns back to the same conduction problem.
I wish I could continue with the subject but have to go. Another time perhaps. Feel free to DM me if interested.
Lastly, I am shocked at ordinary radiant barrier pricing. There are few ultra high performance barriers out there around $0.10 per sf, they exist though. I get them frequently for most of my project. There is no excuse NOT to use effective heat mirror and Low E barriers in in every project. Only uninformed or special agenda Big Energy would advise otherwise.
Oh, forgot to mention, When venting be mindful that one reason RB and LOW E fail to perform over the long run is corrosion, dust, and pollution - In my knowledge there is only one brand and one configuration able to remedy those issues.
Location: Central Atlantic Region, though consults worldwide
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Forgot to mention: A full radiant barrier attic installation which extend out to the soffit one may never know one had a leak. Prbably would not have to worry about rot either becasue the wood and resins would be forces to dry due to increase venting and heat. Don;t get me wrong a leak is a leak however a good Radiant barrier without paper substrate will certain offer another level protection. Certainly some time some might notice a stain in the soffit... I'd rather fix an exterior underneath issue tha replace attic insulation and sheetrock.
Location: Central Atlantic Region, though consults worldwide
266 posts, read 450,103 times
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Quote:
Originally Posted by d4g4m
Ceilings get R-39 insulation ]about 12"] I wouldn't waste money on foil for the very small improvement. Spend the money on the walls. R value for block walls is less than 4. Have 3/4" foam board installed between the dry wall strips. Or spend the money on the windows that have R- value lass than 2
d4g4m,
Thought your post was interesting so I did a quick calc.
The effective average system R for your ceiling described is R-35.9 (assume 24" o.c., average R will be less if otherwise). The Btu/hr throughput load with a 170 deg F roof is 2.8 Btu per square foot, AT BEST.
With Radiant Barrier:
The effective system R for the ceiling described remains R-35.9 . However, the btu load for the same 170 deg F roof is 1.13 Btu/hr/sf, ~50% nominal reduction at peak load. Adding a Low E surface you could reduce the Btu/hr to 0.83, or by as much as 70%.
Economics would then dictate whether or not your small project could deliver a R.O.I for the owner.
Example: If the addition is calling for its own HVAC but the existing system does a very fine job at heating and cooling the existing structure, then: the existing system may be over-sized to begin with. Mathematically speaking, it is quite doable to zone the original system with a branch duct for instance without adding capacity (a new system) to the entire project.
The key to any radiant barrier or Low E is understanding the applied physics of the material for the environment to which it is applied. Do not just install it for the sake of doing so. Research it.
Both FPL and Florida Solar Energy have done a wonderful series of research one the radiant barrier matter. However, they have not performed Low E studies for opaque building systems -- as they have for windows. Florida? Without low E windows? That's crazy few would even think of building in Fla without low e windows!
So why would one consider the remaining 93% of the building surface area be Low E?
(Why? its kind of a dirty little secret known to higher policy of trade. Its good to save energy but, energy taxes are more important than saving too much, at least not until energy prices rise so high that they get the continued equal allotment of consumption taxes.) Not against taxes have you, just biased, special agenda research.)
Of course, it could also be the subject is too far beyond others comprehension to address the issue these days, the future perhaps.
Radiant barriers reflect IR heat back to source, Low E minimizes effects of re-radiated heat inward. These are two very different properties each having special needs. Do not assume a 97% reflective means 3% emissivity. The opposite of reflective is absorption, not emissivity. Different properties, different function - Each must be dealt with differently.
Location: Central Atlantic Region, though consults worldwide
266 posts, read 450,103 times
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Quote:
Originally Posted by beninfl
I'm pretty sure I'm the physics expert here.
beninfl,
That's Great! I very much enjoy working with you guys.
Years ago, I had several of the physicists involved in the Manhattan project as customers. Sadly, they all have died off since. Nevertheless, Harold Taylor was an amazing man.
I am not a theoretical physicist for sure. I tip my cap to you!
Radiant barriers is a complicated subject for people to grasp. Am I wrong trying with explaining basics in reasonable layman terms that they understand?
That's Great! I very much enjoy working with you guys.
Years ago, I had several of the physicists involved in the Manhattan project as customers. Sadly, they all have died off since. Nevertheless, Harold Taylor was an amazing man.
I am not a theoretical physicist for sure. I tip my cap to you!
Radiant barriers is a complicated subject for people to grasp. Am I wrong trying with explaining basics in reasonable layman terms that they understand?
Great post!
I was an egg then. My parents werent even teenagers yet!
Any way to convey science to the public that is easy to understand is the right way. Usually that means no math.
Location: Central Atlantic Region, though consults worldwide
266 posts, read 450,103 times
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beninifl,
Thank you. Sage advice.
Sometimes I think this stuff to be so easy. I don't always know how to best illuminate relationships of cause and [measurable] effect because variables are variable. I write for someone in the south and an Alaskan chimes in suggesting that can't be correct.
Seeing something in formula at least helps those who do know formula to gain perspective. Frankly, the latter are favored to whom I write as they are more likely to better assimilate and represent this stuff in their respective world.
I am reminded of a sketch with Chevy Chase playing a futuristic role on a game show or operating as a White House press agent -- can't recall. Anyway, a question posed to him was: There were three men that... Chevy's character interrupted, chiming in . "I was told there would be no math." So, point taken.
In the mid 90s, Hazel O'Leary's held the Trinity Anniversary Symposium at Forestall in DC. Harold kept me busy> year on his various projects. He asked me to drive he and another of the honored physicist to the event. On the way there these guys talked only of quantum entanglement. Fascinating stuff! In a vague, I could almost picture how energized particles on opposite ends of the universe are bonded, despite vast space between. Science, since then, has come along way.
Curious, do you do any US Dept of Energy work? I know quite a few specialist there, just wondering if there is a 6 deg of separation here?
Thanks again, I'll see to doing better.
Have a great weekend!
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My parents werent even teenagers yet!
