Heat loss through sun tunnels?
Ledebuhr1
| Posted in Energy, Heating & Insulation on
How much heat loss goes through thoes sun tunnels? We are in Se Michigan. Also, how much light is lost because of the tube length? We have several near by homes that have them in the Kitchen, and they must have a 10ft long tunnel to reach the roof top.
Replies
When you find the answer, know that I have one new still in the sealed box 14'' flextube Sun Tunnel-low profile. Could not use on a very recent job and I do not want to stash it up in the attic of the shop.
Suntunnel-TGF 14'' Acrylic Dome.
Located just outside Toledo.
thanks.
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Edited 11/8/2007 7:26 pm ET by calvin
Didn't FH do an article that answered this?
I have look for articles but have not found any. I wonder if anyone has taken a look at a house with a sun tunnel with a thermal imaging camera in the winter? The manufactures all say no heat loss, but i dont buy it. Its almost like cutting a hole in your celing and opening it up to the rooftop.
The ones I've seen have a lens cover at ceiling height, why would so much heat escape? Mike
Trust in God, but row away from the rocks.
That lense has a R-value of prabally 1. A ceiling here has a R-value of 25-30. Heat follows the path of least resistance.
I installed one recently in my house with a 10' tunnel. I used the rigid pipe 14" energy star version. Say that 10 times really fast. I love it. I believe the lense is double pane. The rigid tube one is the only one that is energy star rated. I am assuming that with the energy star rating that it has a better rating than R-1. I asked my Velux rep and he said not to insulate the tube.John
J.R. Lazaro Builders, Inc.
Indianapolis, In.
I simply insulate the tunnel with bats of insulation running the length of the tube (I use the comfortherm--it tapes together nicely with housewrap tape). You could also make a tunnel chase out of foamboard and PL300 the joints.I live in Minnesota and we put a good number of these in bathrooms where condensation can be an issue.They are nice...and insulating them is pretty easy.
There's a BIG difference between a hole and a bubble.Some heat might convect through there, but it's probably negligible.If it were just a hole, the air would rush through and carry your heat out with it.But it is sealed at both ends. It is essentially a bubble.Instead of a hole through the attic, as you put it, you have a ten foot dead air space. That piece of plastic at the bottom, and the dome on top may not have very much R value themselves. But ten feet of dead air has some SERIOUS R value...I seriously doubt very much heat escapes though one of those.
Get your head out of the box.
You may be right, but if that is so,why do we even insulate attics? Why not just use the large amount of air space as an insulator? I know here in the winter thoes attics are really cold. The heat could excape much easier through the plastic lense than the insulation. Inst this why we have IC/Airtight can lights?
The keyword there being airtight.Attics are not airtight. Attics are not a bubble. Ait comes in, ait goes out, pretty redily.Insulation helps stop that air movement.If you could make your attic absolutely airtight, it would sit there as a dead air space bubble, with all the R-value of that dead air.
Get your head out of the box.
"If you could make your attic absolutely airtight, it would sit there as a dead air space bubble, with all the R-value of that dead air."No it would not. As long as there is air and not a pure vacumn, then that air will be moving as it gets warmed near warm surfaces and cooled near cooler surfaces. Warm air rises and cooler air falls so a convetion heat loss loop is established, transferring heat to the ccooler surface of the bladder
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if sealed well, then there is no convection heat loss, true.But there is still radiant heat loss. a Window would be equivalent and the best you are going to get is maybe R4 or so there.
Being as this is at the ceiling level where there is more heat to begin with, seems the actual heat loss ccould be more, but then the air wash effect might be less in some locations
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R-value of prabally 1. A ceiling here has a R-value of 25-30
And every inch of still air inside the tube has an R value of about 0.5-1, or about R10 the foot.
There's probably some conductive loss, since the tube spans inside to outside. But, at that level of inspection, a ceiling assembly with R30 insualtion in it, really has an R value of R17-20, after ciphering in all the penetrations, voids, conductive framing members, and such.
Like as not, it balances out as being no worse, for heat loss, than an idle bathroom exhaust fan. With the definite gain of daylighting evey day the sun is out.Occupational hazard of my occupation not being around (sorry Bubba)
The problem is, how are you going to prevent convection currents inside that tube? In the winter, the warm air will rise, the cold air will fall, and no more R value.
Convection inside the tube itself would be negligible.And nighttime is the only time you'd be concerned about that negligible amount at all, anyway. During the daytime, any sunlight at all, will actually add heat to the air that is inside the tube.
Get your head out of the box.
prevent convection currents inside that tube? In the winter, the warm air will rise, the cold air will fall, and no more R value.
Well, that will happen probably any time the sun comes out. The 'trick' of it is that the air is sealed in the tube, unlike the air swishing about the attic (presuming the attic is ventilated).
As a guess, given how many W there are in surface-level sunlight, the air in the tube will heat to some sort of maximum, and generally stay that way, with some Brownian motion from conductive temperature differentials (and some convective ones, too).
Every one of these I've been around was hot to warm, not cold. Now, I'not been around any north of 29ºN latitude, either (my attic, today at lunch, was down to a brisk 89º with an outside ambient of 82º). The bathroom exhaust fan probably has more 'cold' air in it, at rest, than the solar tube.Occupational hazard of my occupation not being around (sorry Bubba)
Typically, any air gap greater than about 1" will support convection currents and the R value goes down.
In the winter, the bottom of the tube is warm and the top is cold - perfect recipe for convection currents.
I think it is 5/16" whih is why dual pane windows are no thicker
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If convection currents can be defeated (negated?) with air-sealing than why don't we have IG windows that are 1-1/2" thick?The thickest double pane window I've seen had a space of 7/8", with most being somewhere between 1/2"-3/4". I always thought this was because any more thickness had a negative effect.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Anyone have access to a thermal imaging camera? I would like to see a picture of a sun tunnel when it is 30 degrees out.
I hate listening to arguments where both sides are smarter than me - no room for me put my 2 cents in!View Image “Good work costs much more than poor imitation or factory product” – Charles GreeneCaliforniaRemodelingContractor.com
"I hate listening to arguments where both sides are smarter than me "That has not stopped 3/4's of the people posting in the forum..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Right, the operative phrase is convection loop
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Now what would make you think that the air in the tube is sitting still?
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you think that the air in the tube is sitting still?
Well, because it's a relatively closed system (with a bottom lens in place, that is).
Once that air heats about 5 degrees, it will have a phenominal amount of lift. Ok, there's almost no "K" value from the smooth sides of the tube, but most folks wrap the tube in insulation. So, there will be some falling air along the sides as it gives up its energy to the cooler side walls. But, against that, the very low K means that air is likely to build up in strata parallel to the tube. That will increase the effective R value a bit.
There's likely, at least my thermodynamics egghead so opines, some practicall level where the air will just find an inertia point, as it has neither entracne or exit.
He alos says there's a simple answer as to whether a closed tube is a heat sink or not. Just stick a thermo couple on the celing, and another on the lens of the tube. As long as the lens is warmer than the ceiling, the energy is going from higher to lower. Now, we cannot argue (agreeing with MEs can be so tedious) whether the top end might have some effect on a roof. Might be a hot spot, from the sun heating the air under the dome. Or not (stinkin' engineers). Occupational hazard of my occupation not being around (sorry Bubba)
I have a question that may or may not be worth discussing but it just came to my mind when I read that some wrap the tunnel with insulation.If you have a 16" hole in your ceiling (covered with a lens) and a 16" hole in your roof (covered with another lens) what good will it possibly do to wrap the tunnel with insulation? Let's say X BTU's can escape through the hole in the ceiling, wouldn't the same amount escape through the roof?Another way to look at it would be like poking holes in the side of a drinking straw. If you put that straw in a glass of water so the original hole and the poked holes are submerged, I would think you would be able to blow the same amount of air through compared with a straw that had no holes poked in the side.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
In this case, the heat loss would be a function of he surface area exposed to colder than the room temp.So by increasing the cold surface area ( tube sides un-insulated) you increase heat loss
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So could there be a point where adding length to an uninsulated tunnel would not increase heat loss? Or would the heat loss keep increasing as the tunnel length increases?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
could there be a point where adding length to an uninsulated tunnel would not increase heat loss?
Now we are back to needing thermocouples again.
Remember heat = high energy, and entropy always "wants" high energy to decrease.
If the sun heats a closed light tunnel to 20º more than the room below, it almost does not matter if the attic above the same room is 20, or 30º cooler--the bottom end of the tube will still be a "radiator" of heat. It's only after all of the ehat from the tube has dissipated, and it becomes cooler than the air and the ceiling adjencent that it can start to become a conduit for heat loss.
Mind you, the "heat loss" will be to the entire tube assembly, so some of the energy will be put into the equialibrium within the tube first. That's where the insulation wrap will help. The longer you keep that tube of air at however much over "room" temperature it gets to, the less likely it is to become a heatsink.
Now, take the bottom lens off, and make this an open tube exposed to room air, and it's a whole 'nuther ballgame. In that case, conditioned air is going to rise up to the top of the tube at every oppertunity. When that air cools, it's going to dump out of the tube, and then to the floor of the room. Now, the amount of air that draft "is" will be a small thing, but underwelcome down the back of anyone's neck.Occupational hazard of my occupation not being around (sorry Bubba)
we were a dealer for SunTunnel before Velux bought them.
all of our install were flex tube.. the solid tube only came out after Velux
we did a lot of the 14" and a lot of the old 20".. then they decided to max out with the 22"
the rim of the dome has vents.. so it is not airtight.. didn't want condensation fogging the dome
the bottom of the tunnel is either single gasket or double gasket... both very airtight
in our climate we always recommended the double gasket ( double glazed )....
the shortest tunnel we ever installed was the first one ....
about 9" (cathedral ceiling )....
the longest one was a 22" diameter with 24' of tube..... that was a pain in the neck and it voided all the warranties... but they had a first floor Dining Room on the front of the house... and they wanted the dome on the back roof above the 2d floor
most of our tubes were less that 9'
there is some convection currents within the tube and there is some heat loss
here's the calc i figger... the R-value is 2..... use a Delta-T of half what you normally would... ( 0 deg out .... 70 deg inside would be 70.. use 35 deg ) i sy half because the temp of the column of air in the tube is not going to be the same as the outside air... and it is not going to be the same as the house air temp... so half
the area of the glazing is Pi x 7" x7" or Pi x 11" x 11"
take that heat loss and balance it against the nice natural light
ALSO... keep in mind.. the light is normally brought to areas that cannot get the benefit of a window.... AND...
the heat loss will be about half of the same heat loss for a window of the same size ( if you agree with my calculations )
so... there you are... and here i am
i love SunTunnels for those areas you can't get windows to
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike,Did the tunnel with 24' of tube have practically the same light intensity as the 9" tube?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
it was such a dark room that the Suntunnel was pretty dramatic..
but i have no way of comparing it to anything else
BTW.. i wouldn't do it again... way too long , too much work.. had to come back twice to rehang the tube.. it kept pulling apart from it's own weight
we did another one about 14'... that was tough too, but i'd do that one again
i'm only talking about the 22" diameter ones.. i wouldn't push the length on the 14" onesMike Smith Rhode Island : Design / Build / Repair / Restore
I am thinking in terms of night heaat loss and winter skies where there is littel if any solar gain in the tunnel
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I think if the sides are well insualted and sealed the heat loss there is negligible compared to the loss at the top lense. But if the tube is uninsulated,, the longer it is, the reater the heat loss will be. It is also a function of the temp differential between the interior conditioned air and the exterior and or attic unconditioned air.Mac has a good point about solar gain in daytime, but he is in Texas where the buffalo roam, and the skies are not loudy all day.
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I guess I was thinking of the tube just like the straw I mentioned, where a mouthful of air blown in one end equals a mouthful of air coming out the other end.As things typically are in buildings, the system is more complicated than the sum of the parts. If we took the top lens off at the roof, insulating the sides would have no appreciable benefit. Since the top lens is somewhat of an air barrier (even if it does have small vents) the air in the tunnel is somewhere between the temp. of the conditioned space and the outside.I wish I knew more about physics...
Jon Blakemore RappahannockINC.com Fredericksburg, VA
"I wish I knew more about physics..."You have a lot in commmon with einstein
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"A lot" may be a stretch.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Maybe he meant you have messy hair...and like sailboats? ;o)