Night Sky Radiation

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Night Sky Radiation

On a clear, windless night, roofing and window glass can get colder than the outdoor air temperature

Posted on May 26 2017 by Martin Holladay

Let’s say you walk into an unheated room. The air temperature is only 50°F. There is a wood stove in the room, but the wood stove is unlit.

Your body is at about 98°F. Since your body is warmer than absolute zero, it is radiating heat in all directions. (All objects that aren’t at absolute zero emit infrared radiation). The wood stove is cool — it’s at 50°F. Still, the wood stove is also radiating heat in all directions.

Since you are warmer than the wood stove, you are radiating more heat to the wood stove than the wood stove is radiating to you. If you stand there long enough, the wood stove will get a little warmer, and you will get a little colder.

If someone lights a fire in the wood stove, and the temperature of the stove rises to 300°F, what happens? You are still radiating heat to the wood stove. And the wood stove is still radiating heat in your direction. But since radiant heat is flowing at a faster rate from the stove to you than from you to the stove, you are going to end up feeling warmer.

The phenomenon I’m describing is explained this way in a document posted on the National Weather Service web site: “Every warm object loses energy by radiating electromagnetic energy (e.g., infrared photons). If it receives an equal amount of energy from other objects, it is in radiative equilibrium; if it receives less from other objects, it loses energy and cools down.”

An online article by Mike Luciuk notes, “The outgoing radiated power of [an average human] would be about 1,039 watts. In other words, a human body radiates power close to that of a toaster, with a peak wavelength of about 9.5 µm. However, in normal temperatures of 72°F, a room radiates about 897 watts back to the body. The net radiation loss at this skin temperature would be about 142 W. This is why a naked person feels chilly at room temperature.”

Heat is radiating from earth to outer space

We all know that radiation from the sun warms the earth. We feel the results of this phenomenon when we sunbathe at the beach. Fewer of us realize that the earth also radiates heat into space. These two types of radiation are more or less in balance. In theory, because of this balance, the temperature of the earth remains fairly stable. (I know, I know… But let’s not talk about it.)

Tree leaves can limit night sky radiation. So can clouds and atmospheric moisture, so the effects of the phenomenon are most pronounced when the weather is clear and dry. That’s why visitors to deserts often notice the phenomenon (observing, for example, that "once the sun goes down, everything cools off suddenly").

If the weather is windy, objects are warmed by convection, so nights without any wind allow objects to cool off quickly. Objects in contact with a thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. (like the earth) stay warm by conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., so the objects with air on both sides (like metal roofing or leaves) cool the quickest.

Under the right weather conditions — still, dry nights without any clouds — a warm object on earth can get cold fast — colder, in fact, than the temperature of the outdoor air. Why? Because outer space is cold. The sky has an effective temperature of only 3.9°C.

This phenomenon explains why, on a morning when your outdoor thermometer reads 34°F, you may still need to scrape ice from your car’s windshield.

Count Rumford was a keen observer

Count Rumford (1753-1814), the inventor of the Rumford fireplace, spent a lot of time thinking about radiant heat. He was aware of night sky radiation, although he explained the phenomenon differently than we do now. While we now understand that heat radiates from warm objects to outer space on clear nights, Count Rumford thought that objects were cooled at night by “frigorific rays” coming from the sky.

In an 1804 paper published by the Philosophical Transactions of the Royal Society, Count Rumford wrote, “The excessive cold which is known to reign, in all seasons, on the tops of very high mountains and in the higher regions of the atmosphere, and the frosts at night which so frequently take place on the surface of the plains below in very clear and still weather in spring and autumn, seem to indicate that frigorific rays arrive continually at the surface of the earth from every part of the heavens.” This was a keen observation, and not far from the truth. He was right about the rays; it just turns out that the rays are headed in the opposite direction than Rumford assumed.

Count Rumford continued, “The inhabitants of certain hot countries who sleep at night on the tops of their houses, in order to be more cool and comfortable, do wisely in choosing that situation to pass their hours of rest.”

Why does night sky radiation matter?

Night sky radiation explains a few phenomena that occur in buildings. It explains, for example, why condensation often develops on the underside of metal roofing. (Metal roofing with an air space below it can cool very quickly, since it isn’t warmed by conduction from below.) When metal roofing is cold — on clear nights, it can easily be colder than the outdoor air temperature — moisture in the air can condense on both sides of the roofing. The outdoor moisture is just dew; it ends up dripping from the eaves like rain. The moisture on the underside of the roofing, however, may be problematic, especially if it drips onto a ceiling.

Night sky radiation also explains why triple-glazed windows sometimes get condensation or frost on the outside of the window. This happens during certain mornings after a clear night. The phenomenon is most common on windows facing downhill on homes perched on a steep hill: these are the windows that are most likely to “see” the sky.

Finally, night sky radiation explains why roof sheathing in vented unconditioned attics often gets moldy in the Pacific Northwest. An article on the topic was authored by five employees of RDH Building Engineering in Vancouver, BC: Graham Finch, Robert Lepage, Lorne Ricketts, James Higgins, and Marcus Dell. The authors wrote, “Condensation due to night sky cooling occurs when the roof sheathing temperature falls below the ambient dew point temperature as a result of net radiative heat loss to the much cooler night sky. This condensation (and potentially frost) on the underside of the sheathing is absorbed and increases the sheathing moisture content. A sufficiently elevated sheathing moisture content may create conditions suitable to support mold growth. … The findings of this study clearly demonstrate that sloped wood frame attic roof assemblies experience wetting in the Pacific Northwest despite the elimination of typical moisture sources. Worse, this research indicates that the typical preventative approach of attic ventilation only exacerbates the problem. For the test roofs, this wetting — combined with seasonal fluctuation in the sheathing moisture content created by changes in ambient conditions — led to sustained periods in which the sheathing moisture content was sufficient to both allow for the initiation of fungal growth and sustain it.”

Radiation to space happens during the day, too

Roofing temperature depends on several factors: the amount of solar radiation striking the roofing; the reflectivity of the roofing (a measure of how much of the solar radiation striking the roofing is reflected); and the emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. of the roofing (a measure of the ability of the roofing to emit radiation). A so-called “cool roof” has a high reflectivity and a high emissivity; because of these characteristics, a cool roof (usually white) doesn’t get as hot as a conventional roof (especially, a black roof).

A roof radiates heat into outer space, even at noon. When the weather is clear, this radiation can be significant. Usually, however — especially if the roofing is dark — the radiation from the sun keeps the roofing warm. However, in the case of a so-called cool roof, the high reflectivity and high emissivity of the roofing keep the roofing quite cool, especially on clear days. What’s happening is that the roofing is radiating heat to outer space, even during the middle of the day.

In 2006, there was a cluster of roof-assembly failures in Arizona caused by radiative cooling. Most of the failed assemblies were low-slope (flat) roofs insulated with fiberglass batts. The key element of the failures: the roofing was white. (On a sunny day, white roofing can be 50 to 60 F° cooler than black roofing.)

I wrote an article about the failures, “In Arizona, White Roofing Causes Wet Insulation,” for the June 2006 issue of Energy Design Update. My article quoted building scientist William Rose. “‘In December, January, and February, the fiberglass was wringing wet,’ said Rose. ‘The OSB was 5 to 7 degrees colder than the outdoor air — colder than the indoors or the outdoors. In this climate, radiant effects become really important. There is nothing standing in the way of the roof radiating out to space. You have a whole lot of heat loss from the roof surface, day and night. With this white roofing, 80 percent of the heat that hits the roof is reflected. The sun can’t keep up with the heat losses to the sky. What you’ve created is a sky-powered cooling coil, and the fiberglass insulation is like a dirty condensate pan. The roof sheathing gets so cold that it is sucking wetness out of dry air.’”

My article continued, “Warm objects, including roofs, radiate heat continuously; unless the weather is cloudy, a roof sees nothing but infinite space above — cold, empty outer space. ‘With a clear sky, there’s radiation loss to the sky, and it happens 24 hours a day,’ says Rose. ‘A clear sky is always really cold.’ In sunny weather, a dark roof will absorb solar heat; however, highly reflective roofing — the type often installed in hot climates — doesn’t absorb much heat. ‘At the house we’re monitoring, it appears that perhaps solar heating can’t even compensate for the daytime losses,’ said Rose.”

The failures in Arizona lead to the conclusion (confirmed by subsequent research) that white roofing can result in sheathing that is damp — more damp than sheathing under black roofing.

One more reason to specify continuous exterior insulation

The Arizona builders soon realized that insulating low-slope roofs with fiberglass batts is risky — especially when white roofing is specified. Two possible solutions were proposed: either paint the roofing black, or install an adequately thick layer of rigid foam above the roof sheathing.

Night sky radiation often leads to cold roof sheathing. The Arizona failures, like the problems associated with moldy sheathing in Seattle attics, remind builders that warm sheathing is dry sheathing — and that the easiest way to keep sheathing warm is with a continuous layer of exterior insulation.

These cases also highlight the important role that sunlight plays in keeping our roof assemblies safe. When an insulated roof assembly with dark roofing is heated by the sun, the sheathing heats up quickly, “blasting” the moisture out of the assembly (assuming, of course, that the interior layers are vapor-permeable). The beneficial effects of this phenomenon are greater on south slopes than north slopes; greater on unshaded roofs than on roofs with overhanging trees; and greater on roofs with black roofing than white roofing.

Anyone contemplating an unvented roof assembly insulated with cellulose (a type of roof assembly promoted by 475 High-Performance Building Supply) should keep these facts in mind, since an assembly that works on a south slope might not work on a north slope, and an assembly that works with black roofing might not work with white roofing.

Martin Holladay’s previous blog: “Natural Gas Pipelines Are Leaking.”

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Image Credits:

  1. Image #1: Aaswath P. Raman
  2. Image #2: Graham Finch - RDH Building Engineering

May 26, 2017 9:49 AM ET

by Robert Opaluch

Wondering what building practices would you recommend, such as:
-- Cool roof helps reduce AC costs and reduce summertime overheating
-- For ventilated attics in humid climates, avoid OSB sheathing (more subject to moisture damage) and cellulose insulation (absorbs moisture, may contribute to attic humidity and mold?), ...
-- For unvented roofs, use exterior rigid insulation and ...

May 26, 2017 9:52 AM ET

Response to Robert Opaluch
by Martin Holladay

I'm not sure what problem you are trying to address.

Are you talking about a real building? If so, tell us your climate zone, and tell us more about the configuration of the roofs and the preferred location for the insulation layer.

May 26, 2017 11:21 AM ET

Timely Article
by Joe Dwyer

We are very close to replacing old asphalt shingles with a standing seam metal roof. Could one of the building experts here answer my question. Thanks in advance.

Should insulation be applied underneath a SS metal roof (above sheathing) in a mixed humid climate (S. IN)?

My plan is high temp peel and stick underlayment over 1/2" plywood deck, followed by 24 g SS metal roof attached with hidden brackets. Color choice is copper penny (45% reflectivity), gabled roof has 5/12 pitch and faces E-W. E facing is full sun and west side is heavily shaded. E side will have solar PV panels attached after metal is installed. Full soffit venting along eaves (24" wide vent) and full ridge vent. R70 insulation in sealed home attic, but no insulation in attached, unheated garage that will also get metal roof. Work will be done by licensed roofing contractors.

Any better ideas?

May 26, 2017 11:29 AM ET

Edited May 26, 2017 11:39 AM ET.

by Malcolm Taylor


May 26, 2017 11:34 AM ET

Response to Joe Dwyer (Comment #3)
by Martin Holladay

I agree with Malcolm. You have a vented attic, and you don't live in the Pacific Northwest. There is absolutely no reason to believe that your roof sheathing will be damp or moldy.

Building codes require the installation of roofing underlayment (asphalt felt or a synthetic roofing underlayment) between the roof sheathing and the roofing, but there is no need to install peel-and-stick underlayment under standing-seam metal roofing.

May 26, 2017 12:06 PM ET

Martin or Malcolm
by Joe Dwyer

Thanks for the quick response.

The idea with using peel and stick underlayment instead of synthetic is two fold. First, the roof will be walked on after roof install (solar, maintenance) and I didn't want any fastener or washer denting or damaging the underside of the metal roof. Second, in the event of worse case storm where the metal is blow off, then peel and stick will more likely stay down and offer water protection, whereas synthetic won't. Peel and stick is only about $300 upcharge to synthetic underlayment, at least for the Corning hi temp. brand designed for underneath metal roofs.


May 26, 2017 12:12 PM ET

How does this apply to RVs?
by Mark Walker

I had a travel trailer with a white, flat roof that rotted apart even with repairs done to the roof. PacNW, zone 4c, 2 - 3" of fiberglass in the ceiling (maybe).
Can what you described here happen to RVs?
Would keeping the trailer in a carport help?

May 26, 2017 12:32 PM ET

Response to Joe Dwyer (Comment #6)
by Martin Holladay

A peel-and-stick underlayment will work, as long as you choose a product designed to withstand the high temperatures under metal roofing -- for example, something like Grace Ice & Water Shield HT.

May 26, 2017 12:35 PM ET

Response to Mark Walker (Comment #7)
by Martin Holladay

I'm sorry, but I can't diagnose the causes of your roof rot problems based on the limited information you have provided.

If there is sheathing rot under a flat roof, a roof leak would certainly be the first thing to investigate. But it's also possible that condensation played a part.

May 26, 2017 1:32 PM ET

Good read.
by Gavin Farrell

Excellent article! Thanks as always. Building science isn't rocket science, but I'll be darned if it isn't at least as complicated...

May 26, 2017 2:43 PM ET

Dwyer issue
by Antonio Oliver


I'm in a similar spot as Mr. Dwyer, wanting to install a standing seam metal roof (somewhat similar climate-northern part of zone 4 in VA). Like him I was pondering whether a peel and stick membrane may offer extra insurance against moisture while reading this article. One major difference, I'm considering an unvented roof, possibly with foam on the underside of the sheathing. I suppose closed cell foam would be a no-no with Grace Ice & Water Shield. But it seems, Martin, that you are making the case against an air space between the metal roofing and sheathing regardless. Is that correct? Because I was initially thinking an air space between roofing material and sheathing was a good idea. Now I'm somewhat puzzled.

May 26, 2017 2:52 PM ET

Edited May 26, 2017 2:53 PM ET.

Response to Antonio Oliver
by Martin Holladay

I'm not sure why you concluded that I am "making the case against an air space between the metal roofing and sheathing." Such an air space is usually a good idea.

If your roof assembly has such an air space, you can expect condensation on the underside of the metal roofing under some weather conditions. That's why our building codes require, and building scientists recommend, that you have a layer of roofing underlayment between the roof sheathing and the air space.

The condensation drips on the underlayment, and usually evaporates before it causes any problems.

May 26, 2017 4:20 PM ET

Closed cell foam under the roof deck @ Antonio Oliver # 11
by Dana Dorsett

"I suppose closed cell foam would be a no-no with Grace Ice & Water Shield."

That's a common misconception.

As a general rule most roof assemblies don't dry toward the exterior. A typical #30 felt + asphalt shingle layup runs about 0.1 perms, on the verge of being a true vapor barrier. Taking it down to 0.05 perms or less doesn't make much of a difference.

The low vapor permeance of closed cell foam applied to the under side of the roof deck inhibits drying toward the interior, but it also protects the roof deck from moisture drives from the interior that occur whenever the roof deck's temperature is below the dew point of the interior conditioned space air. As long as the closed cell foam is applied when the roof deck's moisture content is low enough, it's not a problem.

Closed cell foam is one of the least-green insulation products out there, primarily due to the industry-standard HFC245fa blowing agent use (there are a few exceptions), but also because of the high polymer per R ratio. As such it's greener to use the minimum amount that ensures a moisture safe stackup.

In zone 4A only ~30% of the total R needs to be low permeance foam on the exterior to limit the risk of moisture build up in higher permeance insulation materials applied directly to the underside of the foam, and that is built into the IRC prescriptives for unvented roof. The IRC requires a minimum of R15 out of a code minimum R49 (which 30.6% of the total) to be low permeance on the exterior of fiber insulation or open cell foam. So you can get to a code minimum R49 with 3" of closed cell polyurethane (R18-R21) and R28-R30 of fiber directly below and in contact with the closed cell foam. That's definitely greener, and usually cheaper than doing it with 7-8" of closed cell foam too.

At 3" most closed cell foam will run about 0.3-0.4 perms, which is sufficiently vapor open in a zone 4A climate that any seasonal accumulations of moisture can dry in a reasonable amount of time toward the interior, but sufficiently vapor tight that the peak moisture levels in the sheathing over a winter are well controlled to a safe level.

HFC blown closed cell foam can only be safely installed in 2" lifts, with a cooling period between lifts for fire safety as it cures. Though some contractors will take the fire risk and try to cheat that, the higher temperatures during cure becomes a quality issue as well. The newer much lower impact HFO blown foams can be blown in depths of 4" or more, and tend to be a bit higher-R (~R7/inch). Even though it's usually a bit more expensive to go with HFO blown foam, for a 3-4" installation it might come in about the same, since it can be done in one pass, not two, and thus no cooling period is required.

May 26, 2017 7:57 PM ET

Thanks Dana and Martin
by Antonio Oliver

As usual, you guys have advice backed up by solid reasoning.

Martin, I suppose I thought you were recommending avoiding the air space between metal roofing and the roof deck because it's that air space that allows moist air to get between the roofing and the deck in the first place. Thus, I reasoned that eliminating that air space eliminates (or at least greatly reduces) the chance of of water condensing onto the underside of the metal roofing. Does that make sense? BTW, I see a lot of metal roofing installed in my area with no spacing between the roofing and the roof deck. But thanks for the clarification.

And, Dana, it is not intuitive to think that sandwiching plywood between metal and rubber/metal foil would not be a bad idea. Thanks for your explanation.

May 27, 2017 10:54 AM ET

Here measured some drying to
by Jon R

Here measured some drying to the exterior advantage to permeable underlayment.

"...the outer moisture content in the vapour impermeable assembly
is consistently wetter than the permeable assembly" (figure 20).

My guess is that ventilated Pacific NW attics would benefit from smart, pressure balanced ventilation. Ie, stop moving air through the attic under condensing conditions. But maybe simply painting the sheathing underside (Table 4) is more cost effective.

May 27, 2017 11:25 AM ET

Edited May 27, 2017 11:27 AM ET.

by Malcolm Taylor

My own experience with mouldy sheathing here in the PNW is that in ventilated attics with good air-sealing between it and the conditioned space below, although you do get some surface discolouration, it isn't a concern. You see about the same level on the underside of un-heated outbuildings, or structures, like carports, open to the elements. Where the problem becomes acute is typically confined to poorly air-sealed attics and cathedral roofs. All of which is a long way of saying the study seems to get it right.

Two things that seem to influence the mold growth are the orientation of the roofs and the micro-climate of the site. Shady forested lots understandably see more.

Traditionally most roofs here were either metal on purlins, or shakes installed on skip sheathing. Both strategies seem to eliminate the problem entirely. The other thing that proved remarkably effective was the absence of any air-sealing at all. Very old structures, like our local community hall, leak such vast amounts of warm air in all directions that they experience no damage. Of course the energy penalty is tremendous.

Since our code here mandates the use of panel goods for roof sheathing, that leaves us with the unpalatable option of applying a coating to the underside of the sheathing - or perhaps a manufacturer will begin marketing a product with a factory applied barrier. Until then simple roofs with adequate ventilation seem to be the best option.

May 27, 2017 12:29 PM ET

There is LP TechShield (foil
by Jon R

There is LP TechShield (foil covered OSB), although it may not be available there.

May 27, 2017 5:10 PM ET

by Malcolm Taylor

Thanks. Somethng like that might make the difference, although it is starting from a more vulnerable point using OSB as its base than plywood.

May 27, 2017 6:19 PM ET

Edited May 27, 2017 6:22 PM ET.

Blue-Green people?
by Bill Dietze

Martin, I can’t help myself.
You state the following above: “In other words, a human body radiates power close to that of a toaster, with a peak wavelength of about 0.5 µm.” But the peak in radiated power from the human body occurs at a wavelength of about 10 µm. (the article you site states 9.5 µm) The peak in the emission of a toaster element is about 2 µm. It’s the sun’s radiation that peaks at 0.5 µm (or so). Here’s a pretty good plot of the radiated power vs, wavelength for the sun, a toaster element (ish) and a warm room (or a cold person):

May 28, 2017 5:40 AM ET

Response to Bill Dietze
by Martin Holladay

I am indebted to your sharp eyes and knowledge of wavelengths. Thank very much for catching the typo; I have corrected it.

May 31, 2017 12:40 AM ET

3 Celsius not 3 Kelvin
by Kathryn Oseen-Senda

Another nitpick.

The radiative night sky temperature is (by your reference) 3.9 Celsius, not 3 Kelvin. The microwave background radiation of the universe is about 3 Kelvin, but our atmosphere is much warmer than that. The roof is thus in radiative exchange with the earth's night sky, not with deep space. Otherwise we'd be losing heat a lot faster.

May 31, 2017 4:20 AM ET

Response to Kathryn Oseen-Senda
by Martin Holladay

I'm grateful for your correction. The error was due to my misunderstanding; I have corrected it. Thanks.

Jun 1, 2017 2:00 PM ET

Night Sky Radiation
by Frontier Energy

Harold R. Hay, inventor of the original "Sky Therm" roof pond cooling system and of the process for pressure treating wood once commented that British officers stationed in India would place trays of water on the north sides of buildings overnight so they could have ice for their gin and tonics.

We are currently monitoring the undersides of California roof decks in "high performance" vented attics that have fiberglass insulation applied to both the underside of the deck and the ceiling to see if the dew point temperature is approached or if condensation occurs.

Jun 1, 2017 2:07 PM ET

Response to Frontier Energy
by Martin Holladay

If British officers in India were making ice outdoors at night, they must have been stationed at a high elevation in the Himalayas. The air temperature has to be close to freezing to make ice that way.

Jun 1, 2017 2:33 PM ET

Either high, or very dry
by Dana Dorsett

With very clear dry air with a dew point below freezing the air temp can be quite a bit above freezing and still be able to create ice at night. I've seen overnight skim-icing and some frost at only moderate elevation while camping in AZ, when it had hit 90F the prior afternoon, but that may not be very common. I'm not sure if the not-so-elevated deserts of Rajasthan or Gujarat are quite that dry on a regular basis either, but they're pretty dessicated. The gin & tonic ice tray story sounds a bit apocryphal, but may have grown from overnight skim layer icing observations such as that.

Jun 2, 2017 12:59 PM ET

"frigorific rays" is my new
by Leigha Dickens

"frigorific rays" is my new favorite phrase.

Jun 2, 2017 1:13 PM ET

Response to Leigha Dickens
by Martin Holladay

I've spent many nights outdoors when camping without a tent, and I've shivered in my sleeping bag, suffering the effects of those darned frigorific rays.

Jun 2, 2017 3:25 PM ET

by Malcolm Taylor

If I decide to change careers and become a blues musician I think I'll call myself Figorific Ray.

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