Living a Radiant Life – Understanding Radiant Heat Transfer in Buildings

Radiant Energy. Something that is so widely misunderstood, and yet has such a tremendous impact on our daily comfort. I have been doing some home study and experimentation in the field of radiant heat transfer in the context of a typical home’s heating and cooling needs and have stumbled across some truly revolutionizing concepts that rarely before have been considered for mainstream practice. Now, these ideas expressed in this post may still be theoretical in nature, but we hope to qualify them in time and in practice so we can utilize the physical laws of nature to our advantage.

First of all, I have to say that the core item that started the snowball effect of my thinking came from a simple object in my son’s camping first aid kit: the space blanket. As first sight, I thought to myself, how could this paper-thin foil “blanket” (if you could call it that) possibly keep you warm. Upon further experimentation, it turns out the thing actually works remarkably well. Giving credit where credit is due, the folks at NASA really know what they’re doing here. Now, it is important to note that while the space blanket will effectively reflect the body’s radiation back to the body, those other modes of heat transfer (conduction and convection) are still at work, so if you are lying on a snowbank with a space blanket on, you will still arrive at the doorstep of hypothermia.

But enough about backcountry survival tips. The idea went from the simple space blanket very quickly to the idea of a space blanket for the home. I have posted before about heat loss through convection and conduction, and even how thermal mass plays into that equation. But I have spent relatively little time to date explaining the effects of radiant heat transfer for buildings. As it turns out, it is quite astonishing, and may really challenge conventional construction practices as we move forward into the 21st century trying to design zero carbon footprint homes and zero net energy buildings.

The basics are this: All objects radiate heat energy from themselves in all directions all the time through the mode of electromagnetic waves of energy. The hotter the object, the more it will want to radiate, however there is a very important material property that also describes how much the object “wants” to radiate. That property is called emissivity. Now, without going into the mathematics of Stefan’s Law or it’s derivative proofs, suffice it to say that it exists and explains why we feel thermal comfort or discomfort in different environments.

The basic outcome of the physics behind emissivity is this: If you stand near a warm object that has a reasonably high emissivity, then you, in turn will feel its warmth regardless of the air temperature that separates you and the object. Now, obviously, conduction and convection are still players in your comfort, however, if you are absorbing that heat from radiation, the surrounding air temperature can be significantly colder than what you would expect the appropriate comfort range to be. Anyone who has gone spring skiing in the rockies can attest to this fact as their sunburned bodies are exposed to the high altitude spring sun and feeling quite comfortable, while the surrounding air temperatures are still in the 40s and they’re standing in a giant snowfield at an even 32 degrees. By the way, that snow field does a pretty good job at reflecting the sun’s radiant energy to that sunburned body as well (so don’t forget sunscreen under the chin).

So, getting back to our space blanket, the core idea is a simple one: If you can keep your body from releasing all of it’s radiant energy to its surroundings, then you will be considerably more comfortable without having to add additional heat. Well, until we are all brandishing space blanket clothing (which will be my next business venture to be sure), we need to figure out how to get the buildings that we occupy to perform the same function.

If we can provide a building skin that can reflect as much of the radiant heat back into the space during the cold winter months, and release it to the outside during the warm summer months, we will have solved the radiant problem of buildings. Now, I would be perfectly satisfied with a permanently reflectant interior shell and an open window when the weather requires, but that is partly because I live in the mountains of Colorado and I prefer fresh air cooling in a climate that will allow for it. Someone in south Florida has different environmental factors, but we would simply apply all of these physics in reverse for that climate.

Consider the temperatures of the objects in your home. If the mode of heating is gas forced air, then you are probably uncomfortable most of the winter. The reason for this is simple. It takes a tremendous amount of warm air (or a significantly hotter amount of air) to heat a solid object. Here is a simple relationship that I calculated this morning (and I would be happy for anyone else to corroborate this simple analysis). I call it “The Spaghetti Pot Example” and it goes like this: In order to heat a spaghetti pot of water (1 cubic foot for the scientists in the room) by 1 degree, it is equivalent to heating the entire volume of three bedrooms of air (each 12’x12’x8′ in dimension) by one degree. The implications here are really quite staggering. This, in a nutshell explains why radiant heating in any mode is going to be more effective than heating air to provide thermal comfort. Whether it’s the floor that you heat, or radiators, or even a Franklin Stove, the heated objects are easier to heat and can carry much more heat per unit volume than air can.

It seems I may be getting off the subject of radiation, but this is actually the crux of the entire radiant space blanket discussion. It is this: We already have most of the heat energy necessary within our homes (and our bodies) to keep us quite comfortable in the cold winter months, if only we can efficiently contain it and manipulate it. By the Spaghetti Pot Example above, it can actually be inferred that you can open your doors in the winter and let all of the heated air out of the home as long as the heat is contained within the objects of the room, because the air carries a significantly smaller amount of heat within the home than the heated objects and building materials themselves. I do not encourage you to do this because it is still terribly wasteful, however, it is far more wasteful if your heating mode is gas forced air because there are fewer heated objects in the home to supplant that lost heat (only those heated by the warm air). In the gas forced air scenario, you’re throwing all of your heating dollars out the window…literally, because doors and windows account for the largest piece of the energy loss pie within any building. There is something to be learned here.

What is that lesson? Heat your building with a radiant mode of heat that will actually heat the dense objects and building materials within the building. Use thermal mass within the building. Control radiation loss at the shell of the building. Also, I don’t want to diminish the high value of controlling conduction and convection losses in that shell as well, but the point of this post is really about radiant heat control.

So, how can we make a thermal blanket around the shell of a building? That question will require a lot more research and development, but I can tell you that in the next building I design, I will consider finishes for the interior of the shell that are reflective in the infrared spectrum (IR). Building grade foils and IR reflective (aluminum based) paints like this are already in use with some radiant heating systems under the floors and I propose to use these relatively low-cost and easy to procure materials around the interior skin of the building, whether the exterior walls are cavity with insulation, SIPs, ICFs, or any other building type. With emissivities around .03 (very reflective), this skin can effectively reflect the radiant heat back into the home, saving a significant amount of radiant heat loss, while also directing heat directly back to the occupants (with emissivities around .9 – very absorptive). Low-e windows that have been available for some time also take this very phenomena into account and we are seeing significant enhancements to building performance by using windows that reflect the radiant heat energy back into the home. At the same time, it is vitally important to consider highly absorptive finishes for interior elements that have thermal mass or are a part of the heating delivery system for the building. Floors, interior walls, built-in objects and even furniture. In some cases, the exterior walls may also play a role in that thermal mass as well. Every building construction and configuration is different and will require its own analysis. The point here is that these are must-have technologies for every building we build from here on and should become standard details for our design.

As we continue to learn more about the science behind these concepts, and commit ourselves to looking forward into 21st century design and construction (rather than repeating the errors of our past out of convenience or ignorance), we will be able to develop truly revolutionary new details with relatively simple and cost-effective solutions that will take sustainable and energy efficient design into a completely new stratosphere. I encourage everyone to test and challenge these principles and ideas in order to further innovation, as I too am committed to this search for knowledge in order to drastically improve every building that we design.


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6 thoughts on “Living a Radiant Life – Understanding Radiant Heat Transfer in Buildings”

  1. Thanks Jim for your comment, it is great to hear about actual field experience with these less common strategies and I am glad to hear about your success.

    As far as your other question, destratification is a common strategy here in the dry mountain climate of Colorado where vaulted ceilings are the norm. While there may be high tech approaches to doing this, the most common and cost effective approach is the simple, but effective placement of one or more ceiling fans in the vaulted room. The net effect of circulating the air is as you describe, and prevents any one location to react to a concentrated heat load.

    Another approach (in homes with a forced air furnace and/or A/C system) is to have the air in the space destratified throughout the entire home. This would be accomplished by simply running the blower with the furnace or A/C off. In a dry climate, this is often all that is necessary in the warm season to maintain a comfortable interior environment.

  2. I have just come across Dean Dalvit’s November 9, 2008 “Living a Radiant Life – Understanding Radiant Heat Transfer in Buildings” posting in January 2010!!

    When purchasing a home in Georgia in 1994 I discovered that it had radiant energy reflective mylar with trapped/dead air as “insulation” in the walls instead of the traditional fiberglas batt insulation. From front to back there were four double sided reflective mylar surfaces plus the two side mylar surfaces against the studs that created three dead/trapped air spaces in the wall cavities. My wife and I loved everything else about this home and we decided to take a chance that this reflective “insulation” would work as well as fiberglas batt; so, we bought the home. Over the 13 years we lived there we loved the reflective radiant ewnergy barrier as we discovered that our heating and cooling expenses were lower than our neighbors – even those neighbors with smaller homes. In the winter i smiled as I placed my hand on the exterior wall painted gyp-board surface and felt the heat reflected back instead of the cold slowly conducting into the home. In the summer the outside heat seemed to simply not penetrate (conduct) into the home — even long into the evenings when the heat of the day would have finally “arrived” with normal insulation — so our conditioned air stayed cooler – longer than it would have with conventional insulation.

    This same multi-surfaced mylar reflective “insulation” was installed in the rafters of our attic — which kept the heat of our roofing from penetrating / conducting into the attic. The result was that our attic was at least 10 – 15 degrees cooler than our neighbors in the summer; so, our relatively cool attic greatly reduced the amount of heat that would have normally penetrated the blown-in insulation covering the attic floor into our living spaces below during the evenings.

    Now I’d like to shift gears to a new / related subject. We all know how air will stratify in spaces with high ceilings with the hot air near the ceiling and cooler air at floor level. In the summer time the hot air at the ceiling seems to radiant heat from that very volume of hot air itself and any objects (i.e., the ceiling materials and any connected structural trusses/joists/etc.) causing heat to be felt at the floor level despite the relatively cooler air temperatures.

    QUESTION: Would it make sense to efficiently move the stratified hot air at the ceiling in a “jet column’ to the floor near the center of the space and induce the air to gently circulate back up along the walls in a slow moving torus to de-stratify the air such that air tempertures are relatively constant from floor to ceiling – thus eliminating the source of radiant heat energy that was formerly at the ceiling?

    Jim McCollum

  3. Thanks Steve for your input. You are absolutely correct in both the timelessness of physics as well as the required gap for radiation not to be overcome by conduction. And of course, it all goes out the window with convection (pun intended).

    My primary concern is the example of a room with an object of a temperature higher than that of the walls of the room. In this case, the airspace separating the object and the walls acts as our air gap and therefore, if the walls have high reflectivity, they would reflect the radiative energy from the object back into the room.

    If the object is in direct contact with the wall, then you are absolutely correct – there is no radiant transfer as the mode of heat transfer then becomes conduction. This is also the case with the air itself – it is changing temperature by conduction with the surface of the wall as well as convection as it circulates. This is why I think that both insulation in an airtight wall as well as an interior surface that is reflective (or absorptive depending on climate and how you want the room to respond to internal temperature) is the best way to control all three modes of heat transfer.

    Conversely, on the outside of a home, the surrounding air, atmosphere and space is our gap between the home and the sun. So, if absorptive exteriors are used, the home will collect that radiant heat while reflective exteriors would reflect it away. Depending on climate, the exteriors would play a role in the heating or cooling of the home.

  4. I’m a little late to the discussion, but physics is timeless, right?

    You touched on it a little, but perhaps not explicitly enough. In order for a reflective surface to reflect radiant heat back, you need an air gap between the radiant source and the reflective surface. (Technically it doesn’t have to be an air gap. Any void – an air gap or vacuum gap – will do, but it’s kind of hard to maintain a vacuum in a building structure.)

    Without that gap you’ll have conductive, not radiant, heat transfer and thus the reflective material will provide no benefit in terms of reducing heat loss.

  5. Probably the simplest way to increase absorption is dark colors. Having it furnished will add some mass that can hold heat.

    We might be able to help you out but I’ll need a lot more information about the house design. Feel free to email me.

  6. Very interesting…we are near completion of a passive solar home in the high desert (6500′) of south central utah…where the mountains meet the desert. This is our first winter using the radiant floor heat (open system- using a Polaris high efficiency water heater) We are shocked at our fuel usage…6 gals/day to heat the floor and very minimal water usage. We are unfurnished, vaulted ceiling…cellular shades on order…
    Where can I get info on “highly absorbitive finishes, etc.”

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