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Heating calcs: wind chill; degree days
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Heating calcs: wind chill; degree days

funcrusherposted on 12-02-07
Can anyone kindly cast light on either of the following aspects of assessing domestic heating requirements?


The 'degree-day' methodology in effect includes a 3C temperature 'credit' IE, it is said that because in winter an unheated building is considered to be 3C warmer than external ambient, the effective temperature deficit to be supplied by heating is reduced by 3 degrees.

However, in calculating actual energy requirements, heating calculations have traditionally disregarded this 3 degree credit. This seems contradictory.


Wind chill on buildings seems under-recognised. It seems possible that it could be a major factor when houses have an irregular and extended shape - gables, dormers, extensions etc.
Paul in Montrealposted on 12-02-07
Where do you get the 3C credit from?

Heating degree days are defined by an external temperature of 18C (and so are cooling degree days for that matter). You might not need any heating for the first few degree-days due to the internal gains of the occupants, but the number of heating degree days per year is a good measure of the severity of the climate. We have an average of 4800 HDDs in Montreal. Also beware the US uses fahrenheit Degree Days for their climate figures - same temp, but in degF so the numbers are 1.8x higher.

As for windchill, it does have an effect, but it is a different effect than the psychometric effect on humans (due to evaporation of sweat). No doubt a building on a windy day will have a greater heat load than on a calm day - air leakage exacerbates it too. In Canada, windchill is sometimes expressed in watts per square metre - the highest I've heard is around 2000w/m^2 ... at that rate, you get frostbit within a couple of minutes at most. The psychometric "feeling" of such a figure is around -45C.

Paul in Montreal (where the highest windchill we've seen this winter was "only" -35C ... for an air temperature of -24C ... in the daytime!)
Tonyposted on 13-02-07
Q2 When wall get wet an excessive amount of energy is drained from a house in the process of evaporating away that wet. Is this what you mean by wind chill for a building?
Nigelposted on 13-02-07
The SAP 2005 calculation takes into account whether an elevation is exposed or sheltered. Also an awkwardly shaped building will have a higher ratio of external envelope to floor area and will therefore be less energy efficient.
funcrusherposted on 13-02-07
The standard calculation for degreedays in the UK stipulates a 3C credit on the basis that an unheated building is on average 3C above external. I can see that - up to a point. EG ground gain, solar gain in daytime, reduced radiation loss at night = frost protection etc etc. But surely the same logic should apply to requirements for heating energy needs? I can find no reference to this inconsistency.

Regarding windchill: Ignoring radiation, heat is ultimately lost by convection, not conduction ie, the external air convects the heat from the outer surface of the house. Convection is much affected by the boundary conditions ie surface layer of undisturbed warmer air. In some respects, that's how clothes work, and why woollen clothing is far less warm in a wind, and why hairy mammals suffer wind chill. Heat exchangers are more efficient when surfaces are rough and there is forced cooling via eg fans. All hot objects cool much more quickly in quite modest drafts.
I think there is even some research that shows creepers growing on external walls provide significant reduction of heat losses, supposedly from reduced wind chill.

So surely, insulation values (or at least there effectiveness) must be influenced by external wind chill. It is often said that certified standard insulation values never seem to be attained in practice, no matter how rigorous the experiment. Is it because the famous hot-box has no forced cooling?

Allowances for severity of exposure probably have more to do with reflecting air leakage than wind chill, as quite low wind speeds have dramatic effects on cooling rates.

Shouldn't we factor this in somehow?

Are external creepers the ultimate green measure?
Paul in Montrealposted on 13-02-07
Windchill cooling of a building is really conduction, not convection, but it amounts to the same thing - surface transport of heat via a fluid (perhaps with some latent heat removal too).

Problem with covering a building with a green surface ... like ivy, is that it ruins the the masonry. The mortar gets damaged, as well as the surface of the brick/stone and it's a veritable highway for insects to get into places you don't want, as well as moisture. We found all sorts of roots and shoots going into the roof of the 109 year old stone/brick house we renovated (which had had ivy in the past). Of course, our climate is more severe and freeze/thaw is much more of an issue than the UK.

By the way, I'm pretty sure hot2000 models wind effects as it is part of the air infiltration model - I think hot3000 goes even further. Anyone in the UK using this? It was jointly developed by Natural Resources Canada and the University of Strathclyde and is definitely a dynamic modeling program that uses actual daily climate data.


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