celotex insulation of floors and ceilings

[Stephen]

Thank you. Now I understand. So it would not be a problem if I insulated under the joists on the basis that the insulation would stop the joists getting cold and condensation forming?

[John Rumm]

Basically you are trying to avoid interstitial condensation - i.e. where warm damp air can meet something cold in the fabric of the building and condensation can occur. This can lead to different approaches depending on circumstances.

For example, a traditional flat roof would have insulation on the under side of the roof joists (or stuffed between them) - meaning the timbers of the roof were in a cold space (a so called cold deck roof). This meant that if any warm moist air escaped from the building into this space it would condense and lead to wood rot unless the space was also ventilated. More modern roofs tend to be of the warm deck design where the insulation is on top of the timbers just under the final roof covering. That way the timber is inside the thermal envelope of the building and condensation should not occur even if moist air were to reach them.

[Dave Liquorice]

Donno but being wood shouldnt be a problem as one would tape the joints of a stud wall. That wood of course be modern fairly smooth timber, old rough sawn joists might be more of an issue.

But one could tape along each joist with the tape edges on the center line of the joist and then tape that gap/join.

[The Natural Philosopher]

That is what you do.