Various insulation questions

[Time Waster]

I've been thinking about insulation recently and have a few general questions. What types of insulation situations would require a barrier layer, moisture / vapour barrier?

Which side of the insulation would the barrier go?

Example, two potential insulation requirements. One is underneath the house in the ceiling of an undercroft / under house garage. The other is internal insulation in a shed to create an outside room (so called shoffice). I'm guessing they would meet a barrier membrane. Where would that go?

In the undercroft ceiling case would the membrane go under the insulation or on top of it between the insulation and the ceiling / floor of the room above?

In the shed would you put the barrier layer to the outside of the internal insulation?

Anything else to consider? I'm guessing air gap too is important.

 

[lazybloke]

You don't want warm moist air from inside the building into the insulation and condensing where it reaches colder temperatures.
Best way to prevent this is an impermeable vapour barrier on the inside (waam dies) of the insulation. Joins and any holes to be taped.

Sheds don't have the temperature swing from inside to outside, and no source of moisture; so far less risk of condensation; probably no need for vapour barrier.

Caveat - I'm only a "diy builder"

 

[si_c]

As noted above, you generally only need a vapour barrier when there is a risk of moisture passing from inside to outside and condensing on the insulation. This is particularly a concern if the insulation is a soft type likely to absorb it. Generally for something like a home office if you are installing insulation in a wall you can apply a barrier membrane over the top to act as a vapour barrier, for PIR rigid insulation you'd cover any gaps with tape. The vapour barrier should always be on the warm side of the insulation.

Again caveating that I'm not a builder or insulation expert.