Is a radiant barrier the new round earth?

Teemok

I know advertising has taken advantage of ignorance. That doesn't mean a radiant barrier is fictitious. Are any of you using them? How? Or are they ,in your opinion, not worth the hassle and risk of getting it wrong?

hot_rod

Do you mean a reflective layer under the tube? Or bubble foil?

Jamie Hall

The only possible answer -- short of a small book -- is "it depends".

EdTheHeaterMan

hot_rod said:

Do you mean a reflective layer under the tube? Or bubble foil?

Or could he mean the death rays form outer space?


Big Brother is watching

Teemok

I mean the real reflection of radiant heat energy ... period! I'm laughing.

Teemok

Jamie Hall, Yes! but it's not that complicated. Reflective surface and an air gap. I see foam on foil all the time. PHD's hate being reminded of basic physics.

hot_rod

Two problems with the reflective fiberglass. it has lots of printing on the foil so maybe 50% actually reflects. It also gets dusty under the subfloor and no longer reflects the radiation
So unless you keep it clean it is not doing much other than r value

Jamie Hall

Teemok said:

Jamie Hall, Yes! but it's not that complicated. Reflective surface and an air gap. I see foam on foil all the time. PHD's hate being reminded of basic physics.

Don't they Well, in that case, the reflective surface is effective always assuming, as @hot_rod said, it really is reflective and is kept clean. But -- it is only effective for radiant heat, and for almost all of our normal everyday heating applications that is actually a rather small fraction of the heat output. Almost all the actual heat output of a radiant floor, for example, is actually conductive and, at the floor/room interface, convective.

ethicalpaul

I'm just curious--who are the PhD's we're talking about here?

hot_rod

A friend used to sell reflective foul for under the roofs  of poultry buildings in Arkansas. It did help lower the hot summertime temperatures

Probably for the first day, then the dust reduced the effectiveness 

Teemok

ethicalpaul said:

I'm just curious--who are the PhD's we're talking about here?

Good question Mostly architects who spec barrier roof sheathing then have it sprayed with foamed. Owner customers who are convinced a layer of foil with bubble rap on both sides is effective. These days there's a good chance your barista could have a PhD. Infrared emission and reflection are not topics people deal with or consider regularly. Lots of very smart people find the concept of radiant heat a little mysterious. The formulas for radiant heat transfer are not commonly considered much less used.

Dust, how much energy there is to potentially reflect, the added labor of install and the cost of material. Is it worth it? It depends, seems to be a good no calculation answer.

FKS is a foil faced fiberglass that is commonly spec'd for HVAC duct insulation. It must be worth it even with the dirt that collects on it. Or maybe it's just a vapor barrier that looks cool.

The lower emissivity of a radiant barrier sheet facing down would stay cleaner for longer. I've been in homes while they are un-insulated durring building with radiant barrier roof sheathing. The difference in feeling between it and the barn/garage without barrier next to it is dramatic in mid day hot sun. Putting a barrier facing up under 2x sub-floor might be covered with dust fast.

The loss of fiberglass volume given up to create the air gap must be made up for by the barrier system performance. Is the bottom of the assembly sealed from air currents? So many factors. Not worth the hassle and or the risk of getting it wrong seems to be the common consensus.

Obviously the difference in temperature of the floor assembly system and the underside conditions might change whether it is worth it or not. The times I've made the effort were not calculations. The choice was based on a conversations with the owner builder and their desire to make every effort for efficiency cost be damned. The floors worked very well by feel but most well done systems do. No further info was gathered about it. I wonder what positive difference it made if any. I have a customer that would let me pull a few joist bays in his living room with wood floors to put up a down facing foil bubble and take some in joist and glass measurements and thermal camera shots.

EdTheHeaterMan

These days there's a good chance your barista could have a PhD.

Holy Crap... You mean to say that you need a PhD to work at Starbucks now?

Or maybe it's just a vapor barrier that looks cool.

And who here does not want a cool looking vapor barrier?

GGross

EdTheHeaterMan said:

These days there's a good chance your barista could have a PhD.

Holy Crap... You mean to say that you need a PhD to work at Starbucks now?

Or maybe it's just a vapor barrier that looks cool.

And who here does not want a cool looking vapor barrier?

Ludwig Boltzmann's (PhD) reaction when he finds out he is only qualified to work at Starbucks and not to be taken seriously on matters of radiation despite defining as a scientific law, radiant heat transfer

Teemok

It's not really surprising that many of these credentialed people aren't working in their fields of study. The positions are not as plentiful and available as was advertised 40 years ago. 2018 data is old. I think the trend continues.

hot_rod

Snoop around Robert Beans site, healthyheating.com. He has spent his career following some of these insulation/ barrier claims.
He claims there are still cases at the FTC regarding claims on bubble and foil insulation R values.

https://www.healthyheating.com/Page 55/Page_55_o_bldg_sys.htm

ethicalpaul

Thanks @Teemok! I will stay away from the topic of the value of higher education but I will say that whoever installed the panelling in every room in my house in the 70s did put a foil layer (no bubble wrap) under every single bit of it.

I'm pretty sure he didn't have a PhD though.

mattmia2

Teemok said:

FKS is a foil faced fiberglass that is commonly spec'd for HVAC duct insulation. It must be worth it even with the dirt that collects on it. Or maybe it's just a vapor barrier that looks cool.

Probably has more to do with flammability than radiant heat.

Teemok

It's FSK foil scrim kraft I got it wrong above. Guessing it would burn just fine but might be better than just paper. A layer of dust doesn't render a barrier useless it just reduces it's effectiveness. It's not all about reflection. Silver doesn't emit radiant energy well so it has the benefit of reducing one of the ways heat flows. Code requires r8 FG with silver jacketing for a reason. The formula for radiant heat transfer has an emisivity factor in it. 1 is for a pure blackbody and 0 for shiny mirror. It makes a difference.

L. Boltzmann Is not the kind of PhD to get his math and physics wrong. You'd be hard pressed to find comparable minds today even with billions more people to choose from. Looks like he has logarithmic slide rules in his head. The capability of those pioneers is staggering.

@ethicalpaul Is there an air gap on one side of it or is it in contact with the panel, fiberglass or ?

Teemok

In a paper studying radiant floor heat transfer they claim their test floors have a radiant heat flux of 57Watts/sqm with a room air temperature of 60F while the convective heat flux is no more than 5.2Watts/sqm.

As the air temperature in the room rises output drops. In a 72F room radiant flux drops to 30Watts/sqm while the convection heat flux drops to 2.5watts/sqm.




We call them radiant floors not convective floors.

I don't pretend to understand the work done in this study but I think I read the charts close enough to glean that a majority of heat delivered by a radiant floor is in the infrared form vs convection.

https://iopscience.iop.org/article/10.1088/1757-899X/881/1/012075/pdf

hot_rod

And conductive, when you or your stuff touches the radiant panel

Teemok

Ecowarm claims: "In radiant floor heating approximately 58% of the energy output is radiant and 42% convective. They offer no source for these numbers." 68% Rad. and 32% Con. Is more like what the studies bellow indicates. I know the surface and finish materials might change the ratios.

From another study on radiant floors that shows there's just under double the energy flux in the radiant form than in convection @ floor to surroundings delta of 8 degrees(K). (A) is radiant flux and (B) is convection flux



https://dergipark.org.tr/tr/download/article-file/1307394

Teemok

hot_rod said:

And conductive, when you or your stuff touches the radiant panel

That would make the stuff an extension of the heating system right? It will just radiate, convect and conduct as well. Couch chair and table leg contact is not a lot of surface area for conduction.

hot_rod

The defination we used in nthe Radiant Panel Association days was.
Radiant Panel; A heat emitter that emits 50% or more of it's heat output as thermal radiation, and operates at surface temperatures below 300F.

Getting back to reflectivity, the foil back Dow products I used and various bubble foils were not very shiny. More like obscure glass.
I know when you point a mirror into the sun you feel that strong energy bounce immediately.
No so much with the insulation reflective surfaces.

With plate radiant we push the batt insulation right against the aluminum heat transfer plate. Or spray foam against it. Conduction is the strongest transfer so tube to plate and plate to floor contact is a make or break.
I think the r-value trumps, Trumps?, the small questionable reflective value of having it suspended below.

EdTheHeaterMan

Does the student loan forgiveness plan use my tax dollars to pay for those PhD degrees also?

Since @hot_rod started to talk politics (Bringing up TRUMP).
And many of my liberal friends don't come to Bridge Club anymore... They won't call trump when it comes to that part of the game. Really holds up the game

hot_rod

EdTheHeaterMan said:

Does the student loan forgiveness plan use my tax dollars to pay for those PhD degrees also?

Since @hot_rod started to talk politics (Bringing up TRUMP).
And many of my liberal friends don't come to Bridge Club anymore... They won't call trump when it comes to that part of the game. Really holds up the game

A typo

EdTheHeaterMan

Since this discussion is not going away, I must post my first reaction from earlier this week

Is a radiant barrier the new round earth?

The Earth isn't Flat? Well great. Caesar's Ghost

Teemok

@hot_rod that's a good point about the quality of the barrier. Scrim foil is dull. R-value is king and contact eliminates radiant emission. Problem is fiberglass being fiberglass and installers being those guys, a perfect contact doesn't exist. My current thinking is a very shinny double sided barrier film tacked up close, in contact with tube and some floor. This application might reduce dust on top. It eliminates conduction contact and reduces radiant emissions dramatically. Bouncing energy back up has questionable long term full effect due to dust. But it will still reflect some at low cost. Limiting emission is the real approach. The fiberglass is not pushed up creating the air gap while insulating the joist bottoms taking advantage of the R1/inch of the top of the joists. The underside of the sub-floor, aluminum plates and fiberglass have these emisivity ratings .82, .77, and .75 respectively. I would think reducing the total energy reaching the top of the fiberglass reduces the energy it transfers at the sides and bottom. No R value is compromised but the energy gradient is reduced, so total flux is reduced. Is it worth it? We can guess highly educated or not guesses but tests would be better. Have you seen any like this?

Teemok

@EdTheHeaterMan Questioning if the earth is really roundish and if radiant barriers are real or if science in general has any validity are in vogue. There are people who think birds are fake. LOL I was asking if we have reached the point of understanding that radiant barriers are as questionable as the spherical nature of our singular spaceship home.

hot_rod

Teemok said:

@hot_rod that's a good point about the quality of the barrier. Scrim foil is dull. R-value is king and contact eliminates radiant emission. Problem is fiberglass being fiberglass and installers being those guys, a perfect contact doesn't exist. My current thinking is a very shinny double sided barrier film tacked up close, in contact with tube and some floor. This application might reduce dust on top. It eliminates conduction contact and reduces radiant emissions dramatically. Bouncing energy back up has questionable long term full effect due to dust. But it will still reflect some at low cost. Limiting emission is the real approach. The fiberglass is not pushed up creating the air gap while insulating the joist bottoms taking advantage of the R1/inch of the top of the joists. The underside of the sub-floor, aluminum plates and fiberglass have these emisivity ratings .82, .77, and .75 respectively. I would think reducing the total energy reaching the top of the fiberglass reduces the energy it transfers at the sides and bottom. No R value is compromised but the energy gradient is reduced, so total flux is reduced. Is it worth it? We can guess highly educated or not guesses but tests would be better. Have you seen any like this?

If the aluminum surface of the foil touches the tube, doesn't it become a heat transfer plate conducting some energy into the foil? I'd leave a bit of space if you go with any reflective layer.

If there is some testing it would be in that huge database the healthyheating.com has. Looks like most every country and hydronics affiliate weighed in on the reflective and bubble issue. Probably a weeks worth of reading. Much of the engineering data would be a lot of number crunching based opinions.

Teemok

The foil can be in contact in spots. The majority is not. Yes it would conduct at small areas but to where? To the edges where it's attached to wood? The film can heat up all it wants and it still reflects energy back up where ever it's not in contact (while clean) The underside has very low emissiviy. The energy has a hard time getting out as radiant energy with a .005 emissivity value. Obviously some will get out but its far less than .82 from plywood or .77 from dull aluminum. Fiberglass, BTW has a .75 emissivity value. There's no conduction to the fiberglass from foil. The main means of heat transfer to the FG might be convection set up by the foil, FG and wood temperature differences. R19 is the indicator of resistance to the flow of energy. I think the difference of energy between top and bottom determines the rate of heat transferred.

Nordic cultures figured out that conduction/ thermal bridging was the biggest problem in building efficient wood framed structures. They use practices to eliminate it at added cost because it's the best practice.

From what I remember the bubble foil guys lied about multiple things. Under slab products were the worst. All had inflated R values that were Trump finance documents worthy I don't know if testing/debunking of those products speaks to the question, especially if their shiny surfaces were not as good as the best barriers.

PC7060

I used RMax panels on roof with 3/4” air gap below roof sheathing.  Underside surface finished with 12”-15” open cell foam insulations.  

Reflective layer was a bonus, ability for roof deck to breath was the main goal. 

Teemok

Foil facing down on the underside of the roof sheathing would have been the move. If you're in to preventing heat transfer down. That's a lot of foam. A good well vented cap.

Teemok

To close: Barriers have been used to rip people off and their good use is not universal and cookie cutter easy. It seems wrong to toss the principle out wholesale because lots of products and applications have questionable value. The physics are real. What that might mean in a well built underfloor radiant assembly is what I'm after. Variations of construction, materials and applications complicates. Cost/benefit is still an open question to me. Until I know for sure, I'll treat it like a pre-Wright bros. flying apparatus. Skeptical but interested in success.