Solar heat gain, heat loss, and exterior temperature
This is something that as passed a few times in my background brain processes,
How does SHG on exterior cladding/glass influence heatloss ??
If glazings specs 50%VT , a part of the remaining 50% must be converted to heat?
If exterior cladding on a building is black, and sun is shinning on the south face,
it must affect the heatloss greatly for the shined on walls ??
As an example, let’s assume a building up here cold north.
20c inside 0c outside.
Sun shines on a large flat south wall that was cladded with a black painted steel sheets finish.
The steel gets heated up to 20c, and it was installed flat , directly on the sheathing
( i know i know ..as an simple example )
Let’s also assume a near mass less wall ( wall made out of EPS )
Logically, if i run my calculator to do heatloss on this wall,
it will get to 0 energy loss, because there is no delta t …am i wrong ?
( theoretically please )
Then what about the glazings on windows.
During sunny winter days, the glazing gets much hotter than without sun.
So it must also affect the heat loss ??
I am sure some of you have the answer to that, or already though similarly.
Please share the knowledge !!!
🙂
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Replies
I may also have had too many Salt and Vinegar chips and my brain is acting out of an acetic acid trip.
:P
Jin,
The short answer is that you are right. When the sun is shining on the south wall of your house during the winter, there is less heat loss per square foot of wall on your south side than on your north side.
But on clear nights the surface temperatures of the roof and sometimes the siding will drop BELOW the ambient air temperature.
That's why it's silly to go to the n-th degree on second and third-order characteristics on any of this. Yes, it makes a difference, but in most cold climate conditions it isn't enough of a difference to matter during the seasons that really matter.
The solar heating effect is still an important factor from a moisture purging point of view though. South facing roofs and sides can get rid of a lot more moisture in winter than the north side due to the solar warming episodes. Mold and rot conditions are more prevalent on the north side when the assembly stack-up is a bit marginal for moisture control.
Dana: yes
BUT!
cold is inevitable, whereas SHG is controllable with cladding color etc..
the question now would be, just how much of an effect on heat loss it has.
Let me put this in perspective,
my factory building, which i should be working on next spring if all goes well,
will feature a 100ft long by 20ft high wall that will be facing solar south.
I need to install finish anyhow, it would be fun if i could reduce heat loss during winter
by choosing a dark cladding material .
But just how much would it save ??
Would it be possible to come up with a formula to approximate savings on that kind of situation ?
I plan on using a similar strategy to "preheat" intake air for the multiple mini splits that will be used for heating on this same building.
Also , as some canadian product already desmontrated, it can be used to "preheat" intake air for air exchange to help somewhat with the large loads experienced in commercial settings from exhaust requiremens.
Again, how does this phenomen affects windows heat loss during day time ?
Delta T in heat loss formula plays a very important role as you all know .
Jin,
Heat loss through your building's thermal envelope varies depending on the time of day and the weather. Of course there is more heat loss on cold days than on warm days.
Moreover, the solar effect you are discussing occurs on sunny days but not on cloudy days.
In general, we size our heating equipment for worst-case conditions -- close to the coldest day of the year -- and heat load calculations always assume nighttime conditions, for two reasons: (1) because that's when temperatures are coldest (early morning) and (2) because houses need more heat when the sun isn't shining.
So the effect you are discussing is irrelevant when it comes to sizing heating equipment.
That said, we burn less heating fuel during winters with lots of sunny days (all other factors being equal) than we do during winters with lots of cloudy days. That's good. But no one ever said that every winter is the same.
Dark materials with good solar absorption are also highly emissive in the infra-red spectrum=- it can run somewhat colder at night than some other less-emissive/absorptive finishes. The peak surface temps in direct sun helps the drying more than the cooler nighttime temps increase moisture adsorption, so all things being equal there is a modest net improvement form a moisture point of view, but negligible or even negative performance gains from a purely thermal point of view.
Windows have much bigger effects on solar gain than surface finishes. This is because the glass is transparent to a large fraction of the solar spectrum- the energy makes it all the way inside the thermal envelope and warms up the materials inside. Windows (particularly those with low- E coating) are much less transparent to the deep infra-red, which is most of what the materials inside your house are radiating. This makes the window something of a thermal-diode- the energy comes in as high-frequency radiation, but the low-frequency radiation emitted from the stuff in your house gets reflected back in.
In almost all cases, a window with a U-factor of ~U0.35 or lower (any low-E double-pane) will take in more energy than it loses in winter, even in Quebec, and even on the north side of the house, on both sunny days and cloudy (just not at night.) But windows are still the lossiest surface area on your house, and if you make the window area too big and don't have sufficient thermal mass in the house to manage it, the house overheats during the day then gets too cold at night (a common problem with many PassiveHouse designs in cold/very-cold climates, depending a bit too much on the sun.)
Sifu Dana and Sifu Martin ...
I do not wish to be impolite here,
but are you talking to me now or just adding general information for all users ???
Dana : please explain
"""""""""
Dark materials with good solar absorption are also highly emissive in the infra-red spectrum=- it can run somewhat colder at night than some other less-emissive/absorptive finishes. """"""""""""
Is this true for ALL dark materials ???
what governs emissivity of IR?
and why does it affect the heat loss of the wall assembly during night time ?
I would like to learn why you believe a darker material could have a negative gain effect on heat loss of wall assembly .
Than about the windows,
i was more referring to the heating up of the glazing itself during direct solar hours .
If the glazing gains several degree( spreaded through the glass and air space ) , doesn't it reduce the heat loss going out of it ??
how is VT rated on windows ? is it rated for visible spectrum alone ?
how do we know how much of the higher energy is absorbed or reflected by the glazing assembly?
And i have yet to find confirmation about SHG from north windows ,
i know we discussed this matter in the past, and you pointed out some numbers
but it was very much dependent on the reflection of surroundings and the U value of the glazing.
From what i could gather a few months ago, here in Quebec most windows from north east to north west orientation ..and even true west facing depending on surroundings, are net loosers even during daytime, down to ridiculous U value ( something like 0.5U RSI ) and you know problem is that SHG usually goes down as the U value goes up and it misses its rendez vous there.
Passivhaus that are overheating usually do not have the correct heating setpoint, tow low mass
and inappropriately tuned sunshades, thus overheat because they start at 21c from night heat,
and have way too much SHG during the high gain months of late FEB to APRIL , where sunshades need to start blocking some mid day light.
Most houses do not have high enough mass to buffer and smooth everything out neway.
Martin: i am not trying to determine this data for sizing heating equipment,
but rather to understand how we could calculate the gains,
and evaluate potential , if there is any
but it needs to be calculated to some extent before anything else
Dana : http://en.wikipedia.org/wiki/Emissivity
this is what you are referring to?
I have never read anything about emissivity in my whole life ..now i did
form the solar tube thermal collector photo description on wiki :
______________________________________
Solar water heating system based on evacuated glass tube collectors. Sunlight is absorbed inside each tube by a selective surface. The surface absorbs sunlight nearly completely, but has a low thermal emissivity so that it loses very little heat. Ordinary black surfaces also absorb sunlight efficiently, but they emit thermal radiation copiously.
__________________________________________
So paint and black painted surfaces absorb but emit.
So for this to work and have any kind of positif effect,
we would need a surface with similar properties as those use
in these solar thermal collectors .
high absorption but low emissivity
i'll look into this,
but anyhow, thanks for pointing this out!
This is what i like about this site, learn so many things everyday!!! :)
Jin,
Also some information on "Black Chrome" coatings used on some fluid solar collectors:
http://en.wikipedia.org/wiki/Selective_surface
Yes, selective coatings do exist, but they don't come in buckets of house paint. :-)
These can work pretty well, given that the blackbody radiation of the sun is at about 5800 Kelvin, whereas the blackbody radiation of your house is less than 300 Kelvin. From a cost/benefit point of view applied to an unglazed collector like the side of a house, energy would have to be priced orders of magnitude higher than it currently is.
Selective coatings that have fairly high reflectivity for a 5800K spectrum, yet reasonably high emissivity for 300K radiation exist, and have cost./benefit rationale for roof coatings in cooling dominated climates (or low-slope roofs even in colder climates), but the rationale for walls or even high-slope roofs is pretty weak. Search the web for "cool roof coatings ".
And i thought i would be able to use spray cans to cover up 2K sqft of wall with it!!! :(
ahah
Going through list of emissivity, it comes out funny to learn that white paint is just a shy close to black paint , concrete and alsphat, bricks and glass.
That said, it means there are not really any difference from a standard gauge thickness steel sheet finished in white or black as far as loss due to emissivity during night time.
But the black painted steel sheet will get hotter during sun exposition,
and could potentially reduce the heatloss through that ??
I still fail to see how it would end up negative.
I've seen pictures of selective solar coating applied to concrete.
Will ask about it.
I have very low interest about cooling climates sorry ! ( time limited )
It's more useful to use selective coatings for solar gain if it's on the interior side of the building envelope than on the exterior, but even there it's not as if a large fraction of the radiant energy that makes into the house via the window can radiate out (especially if there are low-E coatings.)
Found a somewhat interesting paper on the subject...
it is geared toward roofing, but should have similar conclusion if worked on walls.
http://web.ornl.gov/sci/buildings/2012/1998%20B7%20papers/003_Akbari.pdf
The only thing this paper points out for super insulated heating dominated buildings,
is to expect diminishing returns with higher insulation, as always.
It seems this is the conclusion to many discussions around here ...
Once you get close to PH insulation levels, all ROI get pushed back soo far that all efforts
are rendered useless.
I contacted a single selective surface coating manufacturer to see what was possible,
their product greatly enhances absorption of solar heat but also raises emissivity,
so it needs to be used on a very low emissivity material to be usefull.
Aluminium could do it.
Application needs to be in the 25um thickness
and a gallon of coating is worth less than 100$ .
Not as expensive as one would've thought.
But, it needs to be protected by glazings.
So it got me to think,
i wonder how much it would cost per SQ FT
to arrange some kind of "boxes" with alum extrusions as contours and a thin coated
aluminium foil as backing, and a single glass pane in front.
Would get pretty hot during day time, and the low emissivity and the air space would reduce heat loss during cold nights.
just a thought ... i like to think about products :)
Could be fun to test with a mock up this winter !!!
I'll ask if they can send a small sample of coating.
If i build 2 boxes out of insulation, place temperature probe in each
and use the coated aluminium on south side of one of those, it would make a nice testing rig,
simple and cheap. :)
Emissivity ...
something i am not understanding here ..
After reading on emissivity and how it affects building components etc..
I had this "auto-thought" about why aren't we using low-e building component in walls to reflect back
IR toward inside.
Found a few papers and study proving the ineffectiveness of this scheme in cold climate.
What am i not getting?
From the lists i've found, even EPS insulation has emissivity of near 0.5 ,
and everything else we use falls in between .8 and .9 .
So why aren't we using let's say a thing aluminum foil with low-e characteristic,
directly inside of the house ?
I read in one of the papers that we loose up to 70% of heat by radiation.
I know i know, was already discussed here a few times ( radian barrier article by Martin and follow up discussions )
The only explanation i can think of is that we try to use a radiant barrier within a wall system , built in an small air space.
It does reflect let's say the Gypsum radiation, but the gypsum gets it back and still emits it in the air space , so basically it is doing nothing as it ends up heating the surface of the gypsum and thus heats the air in the space so convection/conduction take the place as it would've been if the wall system was full.
Is it what is happening ?
If we would be to line up interior walls of a test room with very low-e aluminum foil,
would it slow down much of the thermal movement ?
i mean, there would be almost none radiation passing through, and air is not a very good conductor.
ok i'm tired ...
sorry to bother you all with my simple dimple questioning
:) peace!
Jin,
Here's the easiest explanation: If your wall is uninsulated, the gypsum wallboard will tend to be cold during the winter. If you are standing in the room with no shirt on, your skin will be radiating heat to the cold drywall. Covering the gypsum wallboard with aluminum foil would have a benefit in that case, because of the big delta-T between the drywall and the people in the room.
If the wall is insulated to code minimum requirements, however, the gypsum wallboard will be at room temperature during the winter. It won't be cold. Your bare skin won't be radiating as much heat to the wallboard. There is no delta-T between the drywall and the interior air. So the aluminum foil won't have much of a useful effect.
Radiant barriers and low-e layers make sense in poorly insulated assemblies, where surfaces see a big delta-T. If the assembly is well insulated, the low-e surface doesn't contribute much.
Delta T ...
i knew i was skimpin on some basic thing.
Thanks for bringing me back on course sifu Martin :)
That is why selective Low-e works ok good in windows, because the delta-T is higher
and glass is invisible to some wavelengths ??
So the gradient of temperature in insulated assemblies lessens the possible radiation ,
so a radiant barrier/reflector has not much to reflect.