what is the purpose of taping the joints of rigid insulation on an exterior wall?

Answer 1

Insulation resists heat flow due to conduction. It is very effective at doing so but it is of little or no use if heat can move through other means. If air can bypass your insulation, the insulation might as well not be there.

In some cases the insulation may act as a drainage plane. If there are spaces between the rigid, water can flow in by capillary action.

As far as the answers about vapor barriers, that all depends. Where do you live? In a cold climate the answers given are true. You would not want a vapor barrier at the exterior of your wall assembly. You want that wall to be able to dry to the exterior. To accomplish this use a building wrap that acts as a drainage plane for liquids but is vapor permeable.

However, if you live in a hot humid climate, the opposite is true. You would want to keep the outdoor, moisture laiden, warm air out of the wall, but also allow any moisture inside the wall to escape to the inside. In this case you want the vapor diffusion retarder to be on the exterior of the assemply.Also when talking about moisture and wall assemplies it should be noted that only 2% of the moisture that gets into a wall happens by diffusion. 98% gets in by air transport. This is why a continuous air barrier in conjunction with continuous insulation is so important.

Answer 2

Absolutely of no use whatsoever. That tape will deteroriate in a year or so and release from the foil. Its better to spend a few dollars on round head fastners and along those joints double the amount used to hold the insulation onto the substrate.

On moisture barrier, you do want moisture to pass and leave the house and not accumulate in the fiberglass insulation
between the joists. Still another reason not to seal up between the outside rigid insulation, just to let that moisture out.

Now, a total house wrap over the rigid insulation makes sense, because that has millions of small holes poked in it with a laser beam and that allows some air and moisture passage as well as a reflective barrier to reflect back heat and cold.

Answer 3

if it has a vapor barrier (foil faced) then tape will help keep the envelope sealed, preventing moisture from entering.

Answer 4

It keeps air and moisture from penetrating the joint.