Mon, 25 May 2026 19:20:43 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9083#Comment_9083 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9083#Comment_9083 Thu, 08 Nov 2007 12:34:06 +0000 tony
In other words are U values dependant on wind speed?

Heat is lost from the surface of a wall by; conduction straight into the air (this is small?), radiation ( this will be independent of wind speed ), and convection.

Convection losses can be increased by forcing air over surfaces, forced convection. How much of this goes of for walls?

Do hot box tests take account of any of this?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9085#Comment_9085 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9085#Comment_9085 Thu, 08 Nov 2007 12:40:16 +0000 Paul in Montreal
Here in Canada in winter, wind chill is measure in watts per square metre - extreme windchills are around 2-3kw per m2 - but these figures are usually used to determine how many seconds of exposure skin can stand before frostbite sets in.

For houses, though, wind speed is more likely to affect air leakage - the models in hot2000 ask for the "exposure" of the building to help determine the effect. I'm sure the air leakage impact is more than an order of magnitude greater than the "wind chill" effect.

I'm sure hot box testing doesn't account for any of this - unless it's done in a wind tunnel.

Paul in Montreal.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9087#Comment_9087 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9087#Comment_9087 Thu, 08 Nov 2007 12:53:06 +0000 jon
Do you think there may be an error in standard assumptions?

It does strike me as odd that a fast flowing turbulent gas does not carry away significantly more energy than still conditions but I have no data that shows otherwise. On the other hand I suppose that for gases as opposed to fluids, local Brownian motion within the gas may be much more significant than the macro scale turbulence and air movement.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9093#Comment_9093 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9093#Comment_9093 Thu, 08 Nov 2007 13:57:15 +0000 fostertom https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9110#Comment_9110 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9110#Comment_9110 Thu, 08 Nov 2007 17:20:22 +0000 steveleigh
A simple example is a hot cup of coffee which becomes drinkable far quicker when you blow on the surface.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9111#Comment_9111 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9111#Comment_9111 Thu, 08 Nov 2007 17:27:13 +0000 Paul in Montreal
the coffee analogy is flawed because the cooling of the coffee is accelerated by the evaporation of the coffee itself. Try cooling a cup of coffee by just blowing on the side of the cup (it will speed it up of course - but to a smaller extent).

This is not the same as wind blowing over a wall (unless the wall is dripping wet). The reality is somewhere between the two extremes.

The only real test is to build an actual house and monitor it over a period of time - the problem is that there are then many interlinked variables which makes it extremely difficult to separate them out.

Paul in Montreal.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9112#Comment_9112 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9112#Comment_9112 Thu, 08 Nov 2007 17:37:23 +0000 tony
So are we saying that U values are not a reflection of the real world? Indeed we seem to have already said that wind speed has an effect on heat loss.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9113#Comment_9113 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9113#Comment_9113 Thu, 08 Nov 2007 17:54:36 +0000 jon
Whether or not the surface condition emits at a slightly higher or lower rate is insignificant to most walls where the rate of energy throughput is determined by the most resistant elements (the insulation generally)

We're just discussing the margins here]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9114#Comment_9114 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9114#Comment_9114 Thu, 08 Nov 2007 18:02:21 +0000 Mark Siddall
Mark]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9116#Comment_9116 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9116#Comment_9116 Thu, 08 Nov 2007 19:49:33 +0000 jon
So, does this mean that for a sheltered (or exposed) condition, we may be allowing too low a U value for the external interface?

If so, what is the margin likely to be? From a boundary check, if we assume a norm of 0.35 for a low emissivity cavity then each interface skin at 0.175 form the upper boundary I would guess

Rso would be 0.03 normally or 0.11 for sheltered (at least that's what I've got in my software)
So the lower boundary for sheltered is 0.11 and upper 0.175

Doesn't seem a big difference so the impact of macro conditions must be overruled by mico effects of brownian within the gas?

Or have I missed something obvious?

jon]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9117#Comment_9117 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9117#Comment_9117 Thu, 08 Nov 2007 19:58:38 +0000 fostertom Posted By: jondoes this mean that for a sheltered (or exposed) condition, we may be allowing too low a U valueFor all conditions, we're allowing much too high a U-value, by a factor of 2.5, by assuming steady-state like the test conditions, whereas real-life dynamically fluctuating conditions result in a much lower effective U-value. This in addition to any variations caused by air speed and/or wetness.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9119#Comment_9119 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9119#Comment_9119 Thu, 08 Nov 2007 20:52:11 +0000 jon https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9129#Comment_9129 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=843&Focus=9129#Comment_9129 Fri, 09 Nov 2007 00:04:08 +0000 fostertom