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A Frosty Heat Loss Puzzler

Ghosting Of Rafters Through Frost On The Roof

My house exhibits an interesting phenomenon on occasion.  A lot of people who understand building science sometimes jump to the wrong conclusion when they see it.  Hey, I did the same thing myself at first, too.  The puzzle lies in the frost pattern you see on the roof in the photo above.  And since it’s my house, I’ve seen it a few times.

Before I get into discussing the physics of frost here, though, let me point out that I don’t have a typical attic.  The insulation in my attic is on the underside of the roof deck.  It’s not on the attic floor.  I used open-cell spray polyurethane foam (SPF) to insulate the roof deck and bring the attic inside the thermal enclosure.  That makes a difference in what’s going on here.

Thermal bridging?

The pattern you see above is called ghosting.  You can see “ghosts” of the rafters because of a lighter frost pattern on the shingles directly above the rafters.  (A similar thing can happen on walls with ghosting of studs.)  But the puzzler here is about why that frost is thinner above the rafters.

This spray foam insulation under the roof deck doesn't cover the rafters
This spray foam insulation under the roof deck doesn’t cover the rafters, which leads to thermal bridging.

The conclusion people often jump to is a logical one.  It’s that there’s more heat loss from the attic through the wooden rafters than there is through space between the rafters.  The wooden rafters can act as a thermal bridge, allowing more heat to flow through that pathway.  The other pathway is through the insulation-filled cavity between the rafters.  Insulation has a higher thermal resistance (R-value) than wood, so the wood acts as a thermal bridge.

The photo above shows part of my insulated roofline as it looked when I bought the house in 2019.  The spray foam is between the rafters.  But you also can clearly see each rafter because the spray foam installer didn’t spray over them.  If this had been the state of my attic when I took the photo of the frost, thermal bridging would indeed be the answer.

Nope!  Thermal mass

Not long after I moved into the house, however, I had Woodman Insulation come in and insulate the attic properly with SES SucraSeal spray foam.  (Disclosure:  Woodman and SES provided their services and products for me at no cost.)   I had a lot more foam installed so that it completely covered the rafters.  There’s a good four inches or so over each rafter now and even more at the ridge.  The photo below shows what the attic looks like now.  There’s no hint of where the rafters are now.

Spray foam insulation installed with enough depth to cover the rafters completely
Spray foam insulation installed with enough depth to cover the rafters completely

What that means is that the wood rafters now have a thermal break.  The insulation over each rafter greatly reduces the heat loss through the wood.  And with as much insulation as I now have on the roof deck, there shouldn’t be much pathway difference at all between heat loss from inside the attic  to the outside.

So why the ghosting?  James Symanski, an engineer, had the perfect answer when I posted a similar photo on LinkedIn:

“Residual heat retained in those areas by the extra mass and [temperatures] not low enough for long enough to extract that extra heat.”

In other words, it’s not thermal bridging.  It’s thermal mass.  As the outdoor air cools off at night, the roof starts cooling.  The materials above the insulation-filled cavities cool off faster than the materials above the rafters.  The difference in cooling rates stems from the extra mass of wood in the rafters.  It takes longer to cool them down.  And that means they’re slower to be covered with frost.

The weather, of course, has a lot to do with the ghosting, too.  If we’d had freezing or near freezing temperatures for the previous 24 hours before the frost, you wouldn’t see the ghosting.  Or if we had snow after a cold spell, you wouldn’t see it.

Also interesting here is that the photo of frost on the roof occurred after a night when the outdoor temperature didn’t get any lower than 39 °F.  Night sky radiation can cool a roof from several degrees above freezing—seven in this case—down to freezing and below.

And now that you have a perfect understanding of this phenomenon, you can hold your own with the most sophisticated building science professionals!  Or at least look at roofs with a new appreciation for the kinds of heat loss that happen there.

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.

 

Related Articles

Why Doesn’t Frost Form in the Carport?

The Layers and Pathways of Heat Flow in Buildings

Frost on Indoor Walls – A Dramatic Insulation Failure

Can an Exhaust Fan Control Humidity in a Spray Foam Attic?

 

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This Post Has 30 Comments

  1. No fair. Ut was a trick question. – Although I should have thought through to know you probably would have gone the extra mile to do the job right and cover the rafters, I’ve never actually seen it done correctly. Bravo Zulu for a great example.

  2. I’m actually so used to the rafter thermal mass thing I thought you were talking about the overhang lacking frost. Which I was also thinking might be the thermal mass of the brick facade and maybe a little convection keeping it warmer longer. What do you think?

  3. Thankyou for this particular article. Reminds me to keep my thoughts to myself until I’ve fully examined ALL the potential causes in situations like this. Easy to jump the gun as we acquire more overall experience. Joe

    1. I had the same question. My thought is that the eaves are not insulated at all. Therefore any heat that leaks upward from the heated house below is able to keep the eaves warm enough to not frost over, unlike the insulated roof portion over the attic which remains cool and tracks more closely the outdoor temp. Another possible explanation might be that if there is little air flow in the eaves portion, that similar to the main section of the roof, heat is stored in the eaves framing and air space throughout the day, enough to prevent frost, just like the rafter pattern of the article.

  4. All makes sense. The thing that was most interesting to me was the comment about the “night sky radiation effect”. Lots of research going on now to be able to utilize this “effect” with surface coatings that would enhance radiation in order to cool buildings (reducing the need for produced energy). It would seem logical that technology could also be employed to REDUCE the effect, by retarding this energy transfer to outer space.

  5. Excellent, Allison, per usual. I initially went with the obvious — thermal bridging due to the lower R-value of the roof rafters vis-a-vis the insulation between the rafters. Delighted to learn that explanation did not hold, given the (absolutely wonderful way) in which the underside of your roof was insulated. You score double points here in my opinion: 1) Demonstrating how frost patterns on roofs can be misinterpreted, and 2) Demonstrating the value of an insulation contractor who knows how to spray foam correctly.

  6. Allison,
    This is a great article and it appears you were up to your trick or misleading questions…

    That was great analysis of the thermal mass of the wood as it does absorb and release the heat at a different rate than the insulated cavity. But what’s causing the frost? Isn’t frost frozen water vapor. Where is the water vapor coming from? Isn’t it condensing from within the air and dropping out onto all of the surfaces? Can’t it be condensing up higher in the sky and falling down to earth? It would take more energy to re-evaporate the already condensed moisture, wouldn’t it?

    This could still cause the ghosting effect as you describe and is demonstrated on your roof. Because the moisture vapor would contact the warmer surfaces and allow it to evaporate but the colder surfaces could accumulate this and show as frost.

    Thanks for keeping us thinking! These are great questions to ponder that may help us someday down the road!

    1. The frost is from water vapor in the air that is freezing when it comes in contact with any surface whose temperature is below freezing and below the dewpoint of the air above it.

        1. Bob: I deal a lot with evaporation and condensation (liquid/vapor phase changes) but I never knew there was a word for the process that is the opposite of sublimation. Thank you for enlightening me on this subject. Every day where I learn something new is a great day.

          1. Actually, I didn’t know that word for that concept either, but Dr. Google came through. Portland often has the right combination of temperature and humidity to create frost and an early morning walk can be a lesson in heat content and transfer by conduction, convection and radiation- a heat map in shades of white.

            I’ve become fascinated by radiative transfer. A surface pointed at a cloudless night sky is radiating heat to outer space and outer space isn’t radiating much back- it’s about –450°F, while a cool night may be 40°F, a difference of nearly 500°F. Simplistically, the heat transfer is proportional to the 4th power of the temperature difference. https://www.sciencedirect.com/topics/engineering/thermal-radiation

            By comparing frost formation on comparable surfaces, you can judge what part of a surface is aimed at the night sky, versus, say, a tree or a building.

          2. Bob,

            I think you have oversimplified radiation exchange with the sky. The atmosphere is not transparent to radiation, especially long-wave radiation as is emitted from cold roofs. The atmosphere simultaneous absorbs and emits radiation with the amounts depending on the specific wavelengths, i.e., the “greenhouse effect”. Cloud cover greatly reduces the sky transparency. Heat convection due to wind can easily overwhelm this radiation effect. These are the reasons that you only see this roof frosting issue on clear and calm nights.

          3. I’ve definitely simplified- comes under the heading of ““All models are wrong, but some are useful”, courtesy of George E. P. Box.

          4. Bob,
            I pointed out this issue about the complexity of sky radiation because there are several university and national lab researchers who are developing roof and wall coatings with selective radiation properties that they claim can radiate directly to outer space and provide free cooling. They seem to know enough about radiation to be dangerous, but not enough about other forms of heat transfer to be practical.

  7. Hello,

    I’m currently going through a VERY complex building science issue. Insulation was added to my attic that is plugging up the soffits and some baffles are even crushed. Shortly afterwards, carpeting in every single room became filled with sandy like material and did not stop until every surface was filled or covered or stained with this strange substance. Filtration lines are seen all throughout carpeting and are the most filled with this substance. The state health department has concluded some of this substance in Insulation from the attic. Upon vacuming it appears gritty, with black debris similar to what I call “attic debris”. Shiney, glowing matter reflects constantly from all areas and the very odd shape of this particulate matter is the biggest mystery of all. Let’s see who can diagnose this one as no one can around my area. FYI: multiple appliances are backdrafting, or pulling air in as my home has become like a vacume. This substance comes from a water vapor of sorts that has completely overtaken my entire home. I could certainly use some help with this one. Would like to share photos, I don’t see a way to do so.

    1. Not sure about the substances you are describing, not my area. HOWEVER – the comments about appliance back-drafting and the house in a vacuum condition is right in my wheelhouse. Get this looked after STAT!

      It appears as if you may be in imminent danger.

      Back-drafting combustion appliances are just plain dangerous. You need to find a Firm that is expert in investigating and identifying the causes of negative pressure, and can recommend remedial action. This is not a DYI project. You have a serious and possibly life threatening condition.

      1. I fully agree with you. Which is why the frustration I have due to this serious situation and no one taking it seriously but myself. I have quite literally spoken to hundreds and hundreds of individuals and companies about this issue as it has been going on for two years now. It took a full year just to diagnose, which after I spent nearly all of my savings on contractors, HVAC technicians, air quality experts, ect, I am now left with an empty checkbook and still a toxic environment for myself and my children. Last night I used a thermal camera I rented from my public library in a back bedroom which is the worst of the rooms. I could literally see this stuff in the air full of particulates and soaking into the carpeting while using that near the floor. Warm and cold spots were seen changing throughout my 2 minutes of video I took with this thermal camera. Debris filled air hitting the walls and falling to the floor was noticed, which explains why the line that looks wet along the edges of carpeting in every single room is present. That line that appears damp upon looking at it is not damp but very full of this material that is quite like sand, yet is seen from close up photos as having a glow about it.

        I’m really needing some help with this and where I can go to find that help. I have reached out to the EPA who gave me air quality monitors to use. The PM numbers were in the hundreds, which I also documented with numerous photos and uploaded graph results for 30 days. That’s all they are doing to help me. The health department proved there was insulation present with in carpeting with a 30 page report. Yet upon sending this certified mail to those who added this insulation to my attic, I received no replies. I have filed complaints, on both a state and federal level as this is a state funded program. My representatives know me well but believe The Department of Administration quality assurance gentleman who came here to asses and called me crazy, then going back to the Senator I complained to and telling him everything is fine here. I literally have done everything possible and here I sit, still at square one with an even more toxic home as this becomes worse as the days go by. Subfloors appear moisture ridden and are buckling, my two year old furnace heat exchanger is rusted and full of debris, and everything in my home is just the same. Trails of this stains flooring, counter tops, there’s nothing here that has not been touched by this. Including ourselves and our lungs and bodies. I’m desperate for help and desperate for the fear I have every day of the worst case scenario happening. Lawyers and law firms are difficult to obtain for this issue as no one wants to take on the state. The state who won’t touch it due to believing The Department of Administration over a random homeowner and mother of two.

        1. To be clear – I was only referring to your comments about back-drafting combustion appliances and your house in a negative pressure state. This type of condition is all too common and should be easily diagnosed by a competent building envelope specialist. It is a hazardous condition and should be dealt with immediately.

          The other problems you mentioned are unknown to me but perhaps others on this blog have an idea.

          1. I fully understand that this is hazardous, which is why the desperation I currently have, and have had. It is beyond frustrating upon me not being able to find someone who knows what is happening here. I have been through nearly all of the local HVAC technicians, plumbers, ect. The building scientist was a retired gentleman who did a blower door test which came back at a staggering number of 400. I can’t hardly believe that, yet it also makes sense as to why my home has become like a vacume and sucking air in at every crack and crevice. It is definitely seen in the carpeting as trails of this sediment is coming from the edges of the walls and outward. Return vents are covered in a lint like dust, a thick layer of it. I almost wonder since my clothes dryer is also backdrafting if that might be the culprit?
            The most frustrating of all is calling my states Energy Services, who installed this furnace, insulation, water heater, ect, and they tell me they will not come to my home since there were “Modifications” done to the furnace which disqualifies from any future service. These “Modifications” consist of changing the furnace filter from an electronic air cleaner to a paper filter with a housing to keep it in place. Something The Department of Administration had done upon their assessment here, which is those who oversee this program. This goes to show just how manipulative and so very unfair this all is for my family. We are quite literally stuck between boulders and mountains. With no way out and no one else to help us. I don’t know what to do. I am located in central Wisconsin. I need help. Please someone help us.

  8. I learned early on in my spray foam business that rafters have to be covered as you have shown. What do you think happens in the Southeastern US when 2″ or 3″ of closed cell foam is sprayed in the rafters of a 2 x 6, 2 x 8, or 2 x 10? The thermal gains in the summer are huge.

  9. Thermal mass capacitance will have an effect. However, regardless of adding insulation beneath the rafters, as long as the same insulation as used below and between the rafters, and the overall depth is the same, there will still be a thermal resistance differential that can look like thermal bridging. The thermal resistance differential between the roughly R-1.2/inch wood rafter and the roughly R-3.6/inch open-cell spray foam insulation is unavoidable in this case, and like you said, in some weather conditions it will show.

    The eaves did not frost because of exposure to the “not lower than 39 F” ambient air under the eave roof sheathing.

    1. Armin,

      I think you’re right. The whole roof radiated to the night sky, but the eaves were warmed from below by the ambient air, while the main roof was too well insulated to be warmed from below.

  10. I get the idea of the spray foam. But it look like you have an normal roofing singles.

    Ie. Tar and rock/ asphalt roofing singles.

    My understanding is you’ve made yourself a hot roof. You will probably notice in a few years youre singles “bleeding” it kinda looks like black water stans. Its your roof cooking from the under side as its no longer able to be cooled by air moving through youre attic. In short most cellings are insulated. not the roof deck to alow air flow accross the back side to cool the singles through the gable/ridge vents
    Thats my understanding from a roofers point of view.

  11. I thought you were going to talk about why air sealing and insulating the lid is always better than hot decking a roof.

  12. It may even be more complicated than proposed. The fact that the author used open cell foam (not bad in-and-of-itself…less likely to shrink than closed cell which WOULD have caused the thermal bypass/bridging) but the open cell foam is usually more permeable the water vapor transmission from within the attic. Unless moisture levels and vapor drive are extremely high, this too would probably not result significant problems, but it could allow for somewhat greater moisture content within the rafters, thus increasing the thermal mass of the offending rafters resulting in the ghosting observed.
    There are rarely simple, one-cause actions at work in building science. Understanding the first principles all these phenomena is critical we should realize that they are all in play at any time. To assign any observed issue to just conduction, radiation, convection, thermal mass, emissivity, permeability…etc is usually oversimplifying. That why forums like this are so helpful. Let the conversation continue and we may someday fully figure it out.

  13. What a passionate page of responses… 😊 Michelle, burn your house down and start over. If you have a full house covered like that in crap, it’s your duct work…. Duct work through furnaces are the only thing in your house that can deposit crap “around” the house… doubt the insulation, which is generally a dry substance, is causing all the moisture problems such as buckling floors. Stop spending money on assessing the problem, but spend your money on fixing the problem. You could pay me to come over and “criticize” your house if you’d like as well.

    As for the thermal mass/transfer. Yeah, there’s thermal mass there, where the hell does the attic ventilate to since the spray foam insulation has been put in? Probably why the eves are melting due to the soffit being the best location for the heat to exit.

    Another culprit that can cause this ghosting is the simple fact the roofing decking (plywood) slightly sags inbetween the framing. You can put H-clips in the wood to lessen the sagging of the wood… but there are probably tiny valleys created between rafters as well. So the framing is the highest point on the roof, and the 16-24 inches in between slightly sags lower.

    Thermal bridging and mass still occurs, but there are probably other reasons as well, as generally things are many things and not one.

    1. Scott: There’s no need for venting in a conditioned attic. You do need to control humidity here in my mixed-humid climate, though, and you can see how I do that in this article.

      There’s nothing to melt on the eaves because condensation never formed there. See the comment by Armin Rudd.

      The roof deck is 1×6 lumber, not plywood, in this 1961 house. If there’s any sagging, it’s practically undetectable.

      Yes, thermal bridging does still exist, but my hypothesis is that thermal mass is the dominant effect.

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