found this while doing some research. Might prove useful - I'm still trying to digest the info :-)

http://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary

Think we have covered most of this on the Irish Study. It is why I took the stats approach to the problem (and it worked on my roof (N=1)). This article touched on it by using the average (which average, I know which one I would use) for Dec, Jan and Feb as your worst case temperature. I seem to remember then I looked at the Climate Zone (called Climate Regime over here) methodology it was a pretty good descriptor of what is really happening.
Does boil down to better insulation and airtightness though and the choice of materials, basic stuff but hard to implement.

Tom
I wish I understood what you mean when you talk of "Euler-derived" and "Euler-diagram", please explain, just to set the scene?

Posted By: fostertomJust googled it and it means something different from what I thought - so cancel that, and I'll find the correct term. Go back and edit all prev references to euler, for the archive?!

Good man, we will make a scientist of you yet.

ST do you have a link or can you provide more details on the approach you used? A lot of this is over my head at the moment but I'm working to gain an understanding of it. I've mentioned on another thread that I might have to use IWI on some walls so understanding how all this works is critical.

Bit I struggle with:
I'Ll try to explain

Just because theres a point in the walls which is below DP, means vapour will condense, but doesn't mean water will build up. Because if theres a route to a point with even lower water vapour pressure (like outside) then the water will re evaporate and diffuse outside.

I dont understand how DP calcs take account of this, i suspect they just say (if below DP then fail) which seems bit simplified.

Example: lath and plaster stone walls. The vapour resistance (stone) is outboard of the thermal resistance (air gap) so water should condense. But there are enough paths for the water to evaporate and diffuse to outside through mortar and air leaks.

I' m not expert / qualified in this, explanations would be welcome!

Speaking of liquid water how do you then take into account moisture on the exterior from say rain. Is it assumed that this would just dry to the outside?

Great link Will. I particularly like the pic where someone has insulated the nails

Yes North America, but the criticism of the prediction method is just as relevant here isn't it? It's basically saying that a build up of condensation which occurs at the dew point (which may be within insulation for example) will subsequently re- evaporate and continue to migrate until it comes up against a more significant barrier.

Of course this depends where the VB (or VCL) is. It could be the vinyl surface of a painted wall which has been internally insulated, or a rich waterproofed cement layer of external rendering

Looks like it. Are the numbers the 'count' of days that dew can form or the temperature that dew forms?

Yes get some iButtons, they take the guessing out of it and can be used for all sorts of experiments.

So you have 42 days where the conditions where met, or about 11% of the time, pretty low and considering that it is not 42 consecutive days there is probably not much of a problem from ambient humidity turnign to water.

Tom, "Euler" ?.. for the sake of the angles maybe?
Is there something about your background (that I don't know about) there?
Sounds very 3D :-/
You're not one of those simulation people are you? ;-)

Googling "Glazer diagram" suggests "Glaser diagram". First sensible hit for that is to a Google Book which says:

Chopped from a conversation I've been involved in here http://www.linkedin.com/groupItem?view=&gid=3612272&type=member&item=171377696&qid=fad9e01b-d3b0-448f-b9ff-f6046a03b65d&trk=group_most_popular-0-b-ttl&goback=%2Egmp_3612272

The Glazer method is that used for most simple Condensation Risk Analyses.

Referring to the Glaser method s, BS EN 13788 states:

'This standard gives calculation methods for:

a) The internal surface temperature of a building component or building element below which mould growth is likely, given the internal temperature and relative humidity–the method can also be used to assess the risk of other surface condensation problems.

b) The assessment of the risk of interstitial condensation due to water vapour diffusion. The method used assumes built-in water has dried out and does not take account of a number of important physical phenomena including:

• the dependence of thermal conductivity on moisture content;
• the release and absorption of latent heat;
• the variation of material properties with moisture content;
• capillary suction and liquid moisture transfer within materials;
• air movement through cracks or within air spaces;
• the hygroscopic moisture capacity of materials.

Consequently the method is applicable only to structures where these effects are negligible.'

In terms of simulating such conditions Hygrothermal simulation (such as WUFI) is a more accurate method for evaluating the combined risk of water and vapour transfer within such wall structures.

‘European standard EN 15026 provides minimum criteria for simulation software used to predict one-dimensional transient heat and moisture transfer in multi-layer building components exposed to transient climate conditions on both sides. The standard lists model equations and pertinent material properties which shall be used for computing heat and moisture transport phenomena. These model equations allow for the following storage and transport phenomena:

• heat storage by the dry materials and the absorbed water,
• heat transport by moisture-dependent thermal conduction,
• latent heat transfer by vapour diffusion,
• moisture storage by vapour sorption and capillary forces,
• moisture transport by vapour diffusion,
• moisture transport by liquid transport (surface diffusion and capillary flow) and the following boundary conditions:
• indoor and outdoor temperature,
• indoor and outdoor humidity,
• solar and long-wave radiation,
• precipitation (normal and driving rain),
• wind speed and direction.

So a much more accurate method of evaluation.

Hope this helps