Posted By: tonyThey

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Posted By: tonySome people I am trying to help -

OK, so who wrote the report? (Was what I was meaning in my original question, not who received it!)

It depends on the roof build-up whether or not you can get interstitial condensation problems in the summer.

For example, you have tight-fitting roof finish such as metal or interlocking tiles with no ventilated void below then it could happen on a warm humid summer day when you get a rainstorm which suddenly cools the roof leading to condensation just below the roof finish. If the void isn't ventilated that can cause a problem.

Sloping ceilings are the work of the devil.

The only traces of condensation I found are due to badly fitting insulation, so that warm moist air can get to the breather membrane which obviously does not breathe enough in winter. In summer I cannot imagine how there could be any condensation there, it is very hot and very dry.

In the coming couple of weekends I am hoping to attack the sloping ceilings in room 1 out of 3: rip out lathe & plaster, re-fit existing insulation, add some more, fit airtight membrane, put plasterboard over.

Not at all, if done with technical understanding. Summer or any time. Totally economic, practical, spacially efficient and interesting, valuable way to build - I do it for preference, by far.

Posted By: tonyThey have a report indicating interstitial condensation will be a high risk in sloping ceilings in the summer

Anyone seen problems in the summer?

Without some details of the building/climate and the computation, we can only guess.

My guess is that the outside air is warmer in summer and so has higher partial pressure of water vapour (for any given RH%). So there is less vapour pressure gradient from inside to outside, so less diffusion, and ventilation is less effective, so it can't carry away the water vapour as fast as it is generated. This does actually happen in our house, but in Spring and Autumn, not in Summer when all the windows are opened. And condensation happens on the windows and solid walls (our weakest links thermally) not on the ceiling or roof, which are solar heated in daytime in Summer. Maybe their sloping ceilings are their weakest thermal link and their model doesn't include solar heat.

My other guess is that the outside air has been modelled as much warmer than the inside air, but with high RH, like summer in Devon. Then the vapour pressure drive is reversed and water vapour diffuses inward from the outside, until it meets a vapour barrier on the inside.