I've convinced myself that it'll curve downwards (sag) so approximating it with a straight line is safe (conservative). My reasoning is partly driven by the apparent lack of thought given to the design of some standard details for things like SIP construction (and I know this has now been reconsidered by at least one SIPs supplier). My gut feeling is that it may not have been for conventional construction methods, but may well be for new methods of construction. The big question that I've never seen answered is whether or not it's worth modelling rapid changes like this. Having spent years modelling the dynamic conditions experienced when weapons hit the water at relatively high speed, I'd say that this is probably a pretty difficult thing to model accurately. I've not seen any interstitial condensation model that seems to allow for dynamic changes like this, as to do so requires more data than just the relevant material U values, the heat capacity of each layer within the wall also needs to be taken into account, as that will determine how long each element takes to heat up or cool down under any given set of changing conditions. I have a suspicion that this sort of dynamic effect may act to "pump" moisture into a condensing region deep in the wall, that may remain too cool to allow the moisture to evaporate out again before the next cold overnight period. Whether that then turns back into vapour and moves out of the wall structure depends on how quickly enough heat can travel through the wall so as to cause it to evaporate, as well as how vapour permeable the structure is outboard of where moisture may have condensed. The warm sun the following morning then heats up the wet outer wall and, because the core of the wall may well still be pretty cold, there would seem to be a risk that water vapour (from the near-saturated layer of air in the cavity) may condense out at some point deeper in the wall. The cold night then lowers the temperature of the wall. The wet day may result in there being moisture on, or in, the outer skin. The situation that springs to mind is a fairly typical one for this time of the year, where a wet day may be followed by a very cold, clear night, which is then followed by a bright, sunny morning. My concern has always been how a structure will behave under rapidly changing conditions, as I'm not sure that the tools often used for construction modelling can account for this properly.
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