Green Building Forum - U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine Tue, 19 Dec 2023 05:54:46 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172269#Comment_172269 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172269#Comment_172269 Mon, 01 Apr 2013 17:58:23 +0100 Ed Davies
Designers tend to settle for the simple metric of the U-value of the window, the heat transfer coefficient of the whole system including conduction, convection and radiation. It represents the heat flow in watts per hour through each square metre of the window for a 10°C temperature difference between the indoor and outdoor air temperature.

What?

I really expect and hope Prof. Roaf did not write that. I bet she's more than a tad fed up with whoever mangled her article in this way.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172306#Comment_172306 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172306#Comment_172306 Mon, 01 Apr 2013 22:41:13 +0100 Timber
There was an advert on the front of the latest TTJ (timber trade journal) for Caberdeck flooring with a massive clanger in it! I think - as you suggest, these things get mangled by an editor who doesn't know the technical aspects and thinks that what they have re-written has the same meaning.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172308#Comment_172308 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172308#Comment_172308 Mon, 01 Apr 2013 22:46:24 +0100 Nick Parsons U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172396#Comment_172396 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172396#Comment_172396 Tue, 02 Apr 2013 18:02:49 +0100 fostertom Posted By: Ed Daviesheat flow in watts per hour through each square metre of the window for a 10°C temperature differencethat the tickle - but what about
Posted By: Ed Daviesthe U-value of the window, the heat transfer coefficient of the whole system including conduction, convection and radiation
AFAIK, that's another kind of clanger, that may well have come from Sue Roaf, or almost anyone.

Rolling radiation loss into Uw or Ug figures is seriously guesstimatey. Radiation loss is greatly de-coupled from conductive/convective loss.
The latter is proportional to difference between internal and external air temp,
whereas the former is proportional to the difference between the fourth powers of the abs temps of internal bodies (not air) and external scenery, sky etc (not intervening air).
Very different, and barely correlated.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172399#Comment_172399 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172399#Comment_172399 Tue, 02 Apr 2013 18:49:21 +0100 Ed Davies U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172404#Comment_172404 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172404#Comment_172404 Tue, 02 Apr 2013 19:29:49 +0100 Ed Davies
So from the U-value point of view I'm not so sure that windows are special - for any part of the building some account needs to be taken for radiation to/from temperatures other than those of the air on each side. Hmmm…

¹ or reflected - one of the points of low-e glass coatings.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172405#Comment_172405 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172405#Comment_172405 Tue, 02 Apr 2013 19:45:24 +0100 fostertom
For opaque fabric, radiation loss (if any) is from internal room/objects to the internal wall/roof surface, then from external wall surface to scenery/sky etc. The temp gradient from inside room/object to scenery/sky is divided into three, in which the middle segment (thro the wall) greatly dominates, leaving v little gradient to drive radiation in the first and last segments.

Whereas for transparent fabric, the full temp gradient from internal room/object to scenery/sky etc is in full force, driving the radiation loss. It's true that the glass, and its coatings, do obstruct that radiation loss, by being relatively transparent to hot (high frequency) incoming solar radiation, and relatively opaque (a lot or a little) to tepid (low frequency) outgoing radiation from room/objects.
However that doesn't make it same as opaque fabric - the full temp gradient applies, albeit the resultant radiant heat flow encounters resistance at the glass line.

What's more, generally, those internal room/objects (incl the room's heat-source surfaces) that can 'see' the outdoors, thro the glass, are warmer than room air temp,
while those internal room/objects (incl the room's heat-loser surfaces such as external walls) that can't 'see' the outdoors, thro the glass, are cooler than room air temp.
So the room/objects that emit the radiation-loss are generally, by definition, warmer than would be expected by simply assuming that radiation loss is proportional to room air temp (same as conductive/convective loss is). Thus radiation loss is generally under estimated.

Similarly, when the weather is critical, the scenery/sky is generally colder than external air temp.
So the scenery/sky that 'pulls' the radiation-loss is generally, by definition, colder than would be expected by simply assuming that radiation loss is proportional to external air temp (same as conductive/convective loss is). Thus radiation loss is again generally under estimated.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172412#Comment_172412 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172412#Comment_172412 Tue, 02 Apr 2013 21:11:23 +0100 tony http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=1258&page=1#Item_2

I did try to talk about this a good while age, may be we weren't ready then, but are we now?]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172415#Comment_172415 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172415#Comment_172415 Tue, 02 Apr 2013 21:33:42 +0100 fostertom Presumably, in hot box type tests, the internal surfaces of the box on the hot side will equilibriate at about equal to hot-side air temp, likewise on the cold side.
Then they measure the actual heat transfer from hot side to cold side, at given maintained delta-t, without distinguishing between conductive, convective or radiant modes of transfer.
That must give an over-optimistic heat transfer coefficient U, because as I've suggested above, in real life
the delta-t between object temps, as affecting radiation will be different from
the delta-t between air temps, as affecting conduction/convection.

In warming weather, the radiant delta-t between object temps may actually be smaller than
the conductive/convective delta-t between air temps,
but when it matters, in weather that's getting colder, and/or when the sun goes in,
the radiant delta-t may be v much greater than the conductive/convective one, on which hot box testing is based.

So when it matters, radiation loss thro windows may be much higher than anyone has suspected.
That has great bearing on the typical assumption in passive solar design, that heat stored into heavy masses inside south windows, actually stays there - maybe it's re-radiated away much quicker than expected.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172482#Comment_172482 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172482#Comment_172482 Wed, 03 Apr 2013 17:37:29 +0100 Ed Davies
However, in the real world windows tend to much poorer insulators than the surrounding walls. This means that with the window the resistance provided by the interface between the air and the glass on each side is a larger proportion of the total insulation. It's this resistance that radiation effects bypass so windows will be disproportionately affected.

On the outside the interface resistance is usually taken to be small, because at least some wind is assumed, so the relative effect will be smaller.

What I wonder is how much difference this actually makes. I've no idea how much energy is really transferred by radiation at typical temperatures.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172486#Comment_172486 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172486#Comment_172486 Wed, 03 Apr 2013 17:58:26 +0100 fostertom Posted By: Ed DaviesOn the outside the interface resistance is usually taken to be small, because at least some wind is assumedThat's another, comparable approximation that we accept trustingly. How long ago were these assumptions and approximations established, and do they still hold good in the new regime of ultra-low U values? Are all our so-precise calcs (esp in e.g. PHPP) in fact way out?
Posted By: Ed DaviesI've no idea how much energy is really transferred by radiation at typical temperatures
We ought to know, shouldn't we.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172487#Comment_172487 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172487#Comment_172487 Wed, 03 Apr 2013 18:11:04 +0100 Mike George Posted By: fostertomAre all our so-precise calcs (esp in e.g. PHPP) in fact way out?

No idea about PHPP but many studies I and other have conducted using Tas demonstrate that dynamic results bear little relation to steady state u-value calcs...]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172532#Comment_172532 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172532#Comment_172532 Wed, 03 Apr 2013 22:24:43 +0100 fostertom
How do dynamic results compare with U value based calcs using degree-days i.e. averaged over a 'typical' month (assuming the 'typical' month that's described in the degree-days figures is derived from the same date-range as the 'typical' month that's described in the weather data fed into a Tas calc).]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172533#Comment_172533 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172533#Comment_172533 Wed, 03 Apr 2013 22:34:15 +0100 fostertom Posted By: Ed DaviesI think glass is pretty opaque to long-wave IRNot so sure - AFAIK it's just a %age opaque. We scorned the old hard-coat on Pilks K-glass (remember that?) because it was so weak in blocking IR compared to the European soft-coats, which themselves came in different grades of effectiveness.

Do we have info on the %age of IR that's blocked by different coatings? Remember the coatings have no relevance to the conductive/convective transmission thro multi-pane units - they're purely about radiant transmission.

Isn't it true that even a top grade coating only blocks - I dunno - 65% of the IR? and more typical soft coats perhaps 50%?
If so, then it is true that
Posted By: fostertomthat doesn't make it (transparent fabric) same as opaque fabric - the full temp gradient applies, albeit the resultant radiant heat flow encounters resistance at the glass line
]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172535#Comment_172535 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172535#Comment_172535 Wed, 03 Apr 2013 22:39:24 +0100 SteamyTea So you are probably only talking about 10 to 20°C for a few days rather than a couple of hundred °C.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172538#Comment_172538 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172538#Comment_172538 Wed, 03 Apr 2013 22:46:05 +0100 fostertom U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172539#Comment_172539 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172539#Comment_172539 Wed, 03 Apr 2013 22:46:22 +0100 tony U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172544#Comment_172544 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172544#Comment_172544 Wed, 03 Apr 2013 23:09:42 +0100 SteamyTea 40 K it is 0.13 W/m^2

I think, so pretty minor]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172551#Comment_172551 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172551#Comment_172551 Thu, 04 Apr 2013 08:24:35 +0100 fostertom U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172552#Comment_172552 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172552#Comment_172552 Thu, 04 Apr 2013 08:29:43 +0100 tony U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172557#Comment_172557 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172557#Comment_172557 Thu, 04 Apr 2013 09:01:07 +0100 SteamyTea Posted By: tony200K for a window with a view of the horizon.I am not so sure it is that high when looking at the horizon, there is a lot of warm air and particles before you reach cold space.
I think that if it was a serious effect then we would be getting windows icing up in air conditioned buildings in the West Indies.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172564#Comment_172564 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172564#Comment_172564 Thu, 04 Apr 2013 09:40:33 +0100 Ed Davies Posted By: SteamyTeaI am not so sure it is that high when looking at the horizon, there is a lot of warm air and particles before you reach cold space.Indeed. From pointing an IR thermometer upwards on a clear night I think the effective radiant temperature of the sky then is around -40 °C or so. Difficult to be sure as those thermometers aren't brilliant at measuring temperatures much lower than themselves but seems about right.

Romans making ice cream in the desert, etc.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172566#Comment_172566 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172566#Comment_172566 Thu, 04 Apr 2013 09:52:15 +0100 Seret Posted By: fostertomNot so sure - AFAIK it's just a %age opaque.

That's correct, about 2% of long wave IR will pass through glass.

Posted By: SteamyTeaAt a 20 K temp difference the losses are 0.0083 W/m^2
40 K it is 0.13 W/m^2

I think, so pretty minor


That's for a black body, in reality the emissivity will mean it's even lower. For example a cotton sofa at ΔT of 20K will emit about 0.007Wm-2. Low-e windows can have an emissivity of something like 0.05 IIRC, so you'd be looking at 0.0005Wm-2. There's a reason we don't really bother going out of our way to separately model radiative heat loss in the standard models for buildings. It's just not a big deal at the temperatures involved.

As for radiation to the sky (space being what, 3K or somthing?), as ST points out the atmosphere isn't particularly transparent, especially at low angles. If it wasn't so we wouldn't have seasons, and in fact life on Earth wouldn't be possible. The insulating effect of the atmosphere stops the Earth radiating away all the heat it gets from the sun and raises the average temperature from what it's supposed to be (about -15º) to a nice comfy 15º. Which kind of makes the idea of a "habitable zone" for exoplanets a bit moot, as technically not even Earth is in the sun's habitable zone. But I digress...]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172567#Comment_172567 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172567#Comment_172567 Thu, 04 Apr 2013 09:53:43 +0100 SteamyTea
Now did the the Romans use just temperature or did they use evaporation cooling, or even a chemical process. Often wondered what they did for us.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172568#Comment_172568 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172568#Comment_172568 Thu, 04 Apr 2013 09:58:32 +0100 SteamyTea Posted By: SeretThat's for a black bodyI used 0.92 as that is what is claimed for glass. But that is close to 1.
Would a cotton sofa at 20K shatter when sat on :wink:

Was a bit about 'habitable zones' in New Scientist and they have extended the zones now because water vapour plays a crucial role in climate, also explains, to some extent, Earth early climate.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172569#Comment_172569 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172569#Comment_172569 Thu, 04 Apr 2013 10:00:01 +0100 fostertom Posted By: Seretabout 2% of long wave IR will pass through glassSounds a bit too general - if that's true of ordinary glass, why bother with coatings?
Posted By: SeretThere's a reason we don't really bother going out of our way to separately model radiative heat loss in the standard models for buildings. It's just not a big deal at the temperatures involved
If so, then again, why do the coatings on glass (vs none, or low-grade ones) make such a big difference to overall heat transmission?]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172570#Comment_172570 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172570#Comment_172570 Thu, 04 Apr 2013 10:09:02 +0100 fostertom Posted By: SteamyTeaWould a cotton sofa at 20K shatter when sat on :wink:He actually said
Posted By: Seretcotton sofa at ΔT of 20K will emit about 0.007Wm-2

I'm suggesting that at times when these considerations really matter - i.e. when outside air temp is falling, or it's a cold clear-sky night, or just when the sun goes in, the delta-t as affecting radiation will be a lot more than 20K delta-t. It will be between the sofa at 20C, and
Posted By: Ed Daviesthe effective radiant temperature of the sky then is around -40 °C or so
- so 60K delta-t, or heading that way.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172572#Comment_172572 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172572#Comment_172572 Thu, 04 Apr 2013 10:23:25 +0100 SteamyTea
The reason for coating glass is that it can allow, to a certain extent to filter just one wavelength. If you remember I did some tests a few years back that shows that the coated glass works for trapping in air at a higher temperature.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172574#Comment_172574 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172574#Comment_172574 Thu, 04 Apr 2013 10:29:51 +0100 Seret Posted By: fostertom
Posted By: Seretabout 2% of long wave IR will pass through glass
Sounds a bit too general - if that's true of ordinary glass, why bother with coatings?


You're confusing emissivity with transmittance. Low-e coatings are to stop the window itself radiating, not to stop IR radiated from other sources. Ordinary float glass is perfectly good at blocking long wave IR, which is why it works for greenhouses, but it's emissivity is above 0.9, so it's an excellent radiator. Hence low-e coatings, they cut radiative heat loss from the windows themselves enormously.

I've just realised I did my maths wrong anyway, it's Thot4-Tcold4 not ΔT4.

So the sofa at 20°C, if it were taken outdoors into freezing temps would radiate at about 75Wm-2, but through the window of your living room it would only manage 1.5Wm-2. Reflection and the low-e coating on the window would mean that the window would itself radiate bugger all of the other 73.5W.

If the sofa were radiating to a -40°C sky through the window it would manage about 3.8Wm-2.]]> U-value description in Sue Roaf “Insider” article in Spring 2013 Green Building Magazine http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172577#Comment_172577 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10435&Focus=172577#Comment_172577 Thu, 04 Apr 2013 10:45:06 +0100 SteamyTea Posted By: SeretThot4-Tcold4not ΔT4.I just used an online calculator, so not sure how it worked it out. Both are small losses really, and then people pull curtains over windows and turn up the heating.]]>