Radiant heating in ceilings

What is the best way to insulate radiant heating in a ceiling which is below an unfinished attic?

With a radiant barrier.

Hmmmm…ya think?

Radiant barrier is a very good answer, but I was looking for suggestions beyond radiant barriers (BTW I knew that!) meaning, since the house is in a cold climate having just a radiant barrier and the minimum amount of insulation (R40) isn’t good enough because of the amount of heat being lost into the roof. So on top of the radiant barrier what would be the best way to insulate it, Fiberglass and or loose-fill Cellulose Fibre, etc… and perhaps a vapor barrier well sealed on top of all that? Now to figure out how much ventilation space is needed should I use a higher ratio than the normal 1:300 ( because of all the heat) . The type and amount of roof vents will change, perhaps an electromechanical roof vent might be needed. What do you suggest now

I posted this in another thread last month:

*"This house with electric radiant ceiling heat may benefit from the addition of more cellulose depending on the electric rates for the area.

Radiant heat panels operate at 95-100 deg F which raises the Delta T (temperature difference from indoors to outdoors) which drives heat to the exterior. We usually use 68-72 deg F as the indoor temp when calculating heat loss from a building when determining HVAC heating equipment size and individual room heat needs at design temps. The Delta T is now raised by about 30 degrees!!

An economic analyses of heat $$ saved by the extra insulation versus the cost of the extra insulation should be done before adding any insulation. If the payback is within 6-10 years, its a pretty good deal.**"*

I would recommend at least R50 and possibly R60*.*

(1) No need for a radiant barrier. It’s metal so when it touches the ceiling/radiant heat panels it loses its reflectivity and becomes a good conductor of heat.

(2) No vapour barrier on top of insulation!! But…a good AIR barrier at the ceiling level.

(3) No need to change the ventilation from normal!!! If you’ve airsealed well and insulated to the maximum economically favourable R value, very, very little heat will be lost. BTW, heat contains no moisture, so don’t worry about it. If you haven’t airsealed well, then warm air from the house containing moisture from family activities will leak upward, potentially causing a problem. You don’t want to lose warm heated air that you paid for…airseal, airseal airseal.

Don’t install it touching the heating panels…

As with ANY insulation, touching causes conduction.

(1) for a relective barrier to perform well, it is reccommended for at least 3/4" air space. How are you going to install a flexible radiant tin foil barrier so that it will support the weight of R50-60 insulation…And…you have now created a continuous air space under the insulation…How are you going to seal the outer edges and all the joints of the barrier to create a dead air space. IMHO, this creates another type of “attic bypass”!!

BTW, the added R value for reflective foil in this orientation and upward radiation/warm air movement from the heating cables/pipes/ESWA foils is approximately 1.6/1.8…not very much R value gain for a lot of cost!! Use the cheaper insulations, cellulose and/or fiberglass, appropriately and get the best bang for the buck. Remember the “Law of Diminishing Returns”!!!

(2) Foil “insulation” needs an air space to be effective and not be a good conductor.

All conventional insulations usually touch one (the ceiling beneath) or two (the enclosing wall panels) house surfaces…this is accounted for in their rated R values. Your insinuation that this contact diminishes their R value is “smoke and mirrors” and a MOOT point!!

This is how it goes:
In a wall cavity, by installing insulation, the heat transfer by radiation across the cavity is reduced to essentially zero and the convective loop heated air movement is reduced to essentially zero but…the conductive heat transfer component across the cavity actually rises due to the fiber to fiber contact / foam continuity. The net effect is to give a very large heat transfer reduction by the first two mechanisms so that in even the cheapest of insulations, fiberglass and cellulose, with lots of fiber to fiber contact/conduction, you get net R20 in a 5.5" wall cavity!!

Sir, the R-value is not pertinent in radiant barriers. It is not there to produce resistance ® against heat energy transfer, it is there to reflect emitted heat energy.

We should not confuse how these components function.
Total heat transfer (or loss) is the sum of heat transfer through conduction, convection, radiation. In designing the structure’s envelope we must take into account the source of heat energy and its mode of transfer. What source is creating the greatest load?

The reflective barrier will block radiant energy but is not a good insulator.
A good insulator does not conduct well but it does not reflect radiant energy. It absorbs it and conducts it.

If your greatest source of heat energy is radiant, we must block the radiant energy.
If the greatest source is conduction, we must increase resistance.

As you pointed out, Radiant barriers don’t hold up because they emit when they get dirty and conduct when they come in contact with building components. I am not promoting their use in every case.

As for the airspace created, convection only occurs when there is a temperature differential and a significant vertical rise. There is not much convection going on in a three-quarter inch space because there isn’t much temperature differential between the top and bottom (not to be confused with a three-quarter inch space in a wall cavity).

Conduction of heat energy will also only occur if there is a temperature differential. How much of a temperature differential do you have between a ceiling joist and the attic installation where the radiant barrier is located? The rate of conduction is insignificant in this case in comparison with the radiated source.

A thermal barrier simply reduces the load on the installation installed in the attic. If used, they should be applied to complement one another. You cannot depend on a radiant barrier to insulate the attic and you cannot depend on attic insulation to prevent thermal conductance (there just isn’t enough space in the attic eaves for this much insulation).

So if you had a choice of one “insulation” to use over a radiant ceiling, would you use the foil or conventional insulations?

IMHO, the best use of $$$$$$ is to install R50-60 directly on the radiant ceiling!!

The only place it seems that radiant barriers have any potential beneficial effect in most housing is in poorly insulated attics to reduce radiant heat transfer from the roof sheathing to the attic floor area and hence down into the house. According to the Florida Solar Energy Center, this can be financially questionable at that , depending on the cost of the barrier installation. One study that can be found on their website, gives an almost 30 year payback!! BTW, increasing attic ventilation for cooling savings had a 113 year payback.


Never mind…

I had a hard time with “emissivity” as well.



There is a lot of misconception and misinterpretation of radiant heat transfer.

This low-e paint reduces emissivity.
They are using thermal imaging to show that the painted surfaces are cooler than on painted surfaces.

This is not the case. Both surfaces are the same “temperature”!
What has changed is the emissivity or its ability to radiate heat energy (I know you don’t like using the redundant form of the term heat energy, however heat energy differentiates between other forms of energy as mechanical energy etc.). Thermal cameras measure radiant emissivity and unless corrected for emissivity it is only an “apparent temperature” .

If we correct the thermal scans for emissivity, we will find that both painted surfaces are the same temperature. Therefore heat transfer through convection is not affected. Heat transfer through conduction would possibly increase because of the low emissive properties of the paint (though not applicable in this situation as nothing touches the ceiling or exterior walls except the air).

The paint does in fact reduce radiation from the building components, but what is the percentage of the savings? That is what is disputed as to what is most energy and cost efficient. Radiation from a building is not that significant. Reducing heat transfer through the wall, thus lowering the temperature of the exterior wall produces the greatest savings in this application. However, we are talking about radiant heaters in the ceiling here.

I am not promoting the use of one application over another. I’m not here to select one over the other. If you have a radiating source that you wish to block or redirect, the use of a radiant barrier is applicable. I’m not discussing how cost-effective the installation of any radiant barrier is. Simply that it will stop radiant heat loss into the attic if it is redirected back into the house.

ASHRAE gives an R-value to everything on earth! It doesn’t mean it’s a good insulator or should be used for insulation. Even a sheet rock screw has an R-value! R values are used to determine U-values (which is the inverse sum of the R-values comprising the building envelope). I remind you that R-value is not the only value applied materials.

Another subject that was brought up on this board was painting radiators a different color to improve their performance.

A radiator is a radiator of heat energy.
If you touch the radiator, it becomes a conductor.
If you put your hand above the radiator you feel it being a convector.

If you paint the radiator with a shiny silver low emissive paint (which is commonly done to make them look pretty), you reduce the emissivity of the device and reduce its capacity to heat and provide comfort within the room through radiation.

If you paint it with a high emissive paint, it will increase its emittance of radiant energy (lowering the temperature of the radiator) but the radiator will continue to convect (at a slightly lower rate).

Baseboard heaters are not radiators as they are often called, they are convectors. However, they do radiate! Their design is to improve convection and their primary function is to convect even though they radiate.

My point is that all three methods of heat transfer can occur simultaneously. It is a matter of at what percentage is the most dominant or designed method.

If you only look at one form of heat transfer, you can formulate an argument supporting a particular type of installation to be used.

If you have a wood burning stove (which primarily heats through radiation) 6 inches away from a sheet rock wall, should we install R-90 installation? I can’t think of any cost-effective housing material of that value which fits in a 6 inch space. Even if it did, we operate the stove 24 hours a day and eventually the installation will reach equilibrium and become as hot as the wood stove due to conduction.

If you install a shiny metal plate between the stove and the wall, even though the metal plate may get almost as hot as the wood stove (due to convection) it does not radiate because of its emissivity. It will effectively “insulate” the sheet rock wall on the opposite side from the wood stove. This metal plate is effective even if it is 1/1000" thick.

I agree that simply adding more insulation above a radiant ceiling is probably the most cost-effective/efficient way to go, however I do not want to distort the concept of radiant insulators.

Here’s part of an article by John Straube (Phd. Building Scientist and Principal at Building Science Corp with Joe Lstiburek, Phd, P.Eng):
(my coloured emphasis)

How Heat Moves Through HomesSpace-age radiant barriers, like ceramic paints, work great in the vacuum of outer space, but here on Earth, heat moves by conduction, convection, and radiation, simultaneously, all the time.

Radiation likes empty space

Then we have radiation. These vibrating molecules create waves in space-time, which we call electromagnetic radiation. At the temperatures that we’re talking about, they’re infrared radiation.

If you were to make these molecules move fast enough, they would eventually glow red-hot; you’d be able to see them. They’d go from infrared to actual red. And if you keep heating them up, they’d get white hot. And if you kept heating them up, they would actually start giving off ultraviolet radiation — and then a nuclear explosion, you’d get gamma radiation, they’re so hot.

But most of our building applications aren’t worried about that. We’re worried about infrared radiation. That’s what infrared cameras look for: They look for the emission of radiation given off by the temperatures that the molecules have. Now, for radiation to be important, it really likes to transfer through — not solids; it likes to go through voids. It doesn’t even like to have gases in the way. That’s why the sun is able to transfer its energy from 93 million miles away to the planet Earth; because basically there’s nothing between us and the sun other than vacuum, except for the last 30 miles or so. Not even — it’s really only about 5 miles of air between us and the sun. So, radiation is quite effective. About 90% of the radiation given off by the sun hits the planet’s surface. We’re trying to change that, of course.

Now, what you need is a gap. If you have aluminum foil, which does not emit radiation very well, it does not change the heat flow across the building assembly unless there’s an air gap. So, you need a gap.

The Thermos bottles — they’re shiny glass on the inside, and that’s so there’s an air gap, and the shininess deals with the radiation. If you filled that void up with foam, the R-value would go down, not up, because you would eliminate any radiation benefit of the shiny metal. So, you have to have the gap; a slightly bigger gap would be good.

Now, within the pores of insulation like fiberglass or rockwool, there are so many voids that radiation actually does play a role in jumping from fiber to fiber inside that product. And in a fiberglass batt, about 40% of the heat transfer at common temperatures is due to radiation. Foam, it’s about 30%. Now, the reason that matters is that as the temperature changes, the contribution of radiation changes.

We’ve all probably been around a fire on a cold night; watch the campfire burn and you can feel the heat radiating to your face. That’s because it’s hot. It really makes a difference whether that fire is hot or cold whether you feel the radiation on your face or not. As you get down to building-related temperatures — 100 degrees, 50 degrees — radiation gets less and less important.

But at high temperatures, radiation is important and it’s a major transfer mechanism; at low temperatures, it doesn’t play as big a role. So, radiant barriers are very good for high temperatures — say, the roof in a sunny climate. They’re less important for cold conditions — say, the underside of a crawlspace; they don’t play as big a role. But in every case they need an air gap.

NASA’s radiant barriers are useless when you pour concrete over them

With radiant floor heat, it is actually kind of misnamed. There is radiation transfer, but actually most of it is by convection. So, convection matters, radiation matters, but more importantly, when I think of radiant floor heat * snake-oil salesmen who sell these radiant barriers underneath radiant slabs.** Radiant-radiant, right? They should go together; they’re both named radiant.**

But of course, when you pour concrete on the aluminum foil, there’s no air gap, is there? So, there’s no R-value benefit. The R-value of a piece of aluminum foil underneath a chunk of concrete on top of soil is around 0 — somewhere between 0 and bupkus. However, they don’t test them that way, do they? They test them in horizontal apparatuses with an air gap above and an air gap below with ridiculous temperature differences across them. And then they get, like, R-8. But it’s hard to suspend that slab 4 inches above the radiant foil in most of my radiant slab designs.

So, what they’ve done to address that is they put the little bubble wraps — the radiant foil bubble wraps, and so on — and they have, well, tiny bubbles (Don Ho sang about that until his recent death). These tiny bubbles in between the aluminum foil do help the R-value, and you can get as much as R-1 on some of the bigger bubble products.

Now, the cost of that R-1 bubble wrap per R is about 3 times the cost of buying extruded polystyrene foam,** but you can market this stuff as “space age.” Well, NASA uses it.** OK, let’s think about this. I’m in outer space. There is no air. So, what are the heat transfer mechanisms? Well, there’s no convection; there’s no air. All I’ve got is conduction and radiation — so, if I’m not touching it, of course there’s only radiation. NASA uses radiant barriers because radiation is the only way they can transfer heat from them to other spatial bodies. It’s the only mechanism that works.

But as long as you’re building your buildings on Earth, in an air-filled environment, there are other mechanisms that are actually more important. But the NASA technology and the “ceramic balls” —** it’s all just bull****.**

But somehow they manage to sell this stuff by playing on people’s ignorance. They’re not sure about how all this works.*