![]() I've put 1mm PVC for the vapour barrier that seems the closest approximation for the polythene I've got in reality I've stuck with 50% RH inside, does that seem reasonable? This is not a bathroom or kitchen. I've assumed 25C inside (again I think it's unusual that I would have the flat any warmer than this, and it would usually be more like 20-22 I reckon) This is for a building in central-ish London so I'm working off a worst case scenario of -10C outside (it's unusual for it to be as cold as that, certainly any colder would be very rare) with 100% RH Here are my assumptions (if anyone thinks anu of these are in error please let me know!): ![]() Thanks - this is pretty much what I'm after. The alternatives to this are good ventilation in all the effected areas and choice of materials that are less effected by the effects of damp. If it takes 5 minutes to form, it will take 10 minutes to vanish once the temperatures have gone back to the pre-shower range. So for any given day, say that shower for 15 minutes and condensation appears on your window. To do that you need to do the reverse and double the amount of time because of the difference in the energy needed to condensate and evaporate water. The above will only give you the conditions that condensation will form, not how long it will last once formed. Here you can estimate from the Modal Temperature Differences and the Modal RH values. The hard part is the Temperatures and RH's between the extremes. You can also estimate the amount of time that it is warm inside, cold outside and raining, and say that is a Dew condition (a single glazed window is a good indicator of that). ![]() There is nothing wrong with estimating the amount of times that the condensation will form, every time you cook or bath/shower you can say that it is happening. So to calculate the Dew Points for your dwelling you need to know the RH and the External and Internal Temperatures and work out the probability that for any given temperature difference Dew will form. This leads to condensation forming (hence a Dew Point is closer to the temperature when the RH is higher). What usually happens in a wall or roof space is that the temperature drops faster than the RH. The risk of condensation forming is therefore 'how often does this happen'. What causes condensation is really quite simple, when the RH is at 100% then the Dew Point is equal to the Temperature. ![]()
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