Cavity fill: rockwool or EPS?

[MikeStout]

Which one is better for a new built: rockwool or EPS? Rockwool doesn't look sturdy enough. Will it fall down with time? EPS, on the other hand, needs to be fixed firmly to the inner skin. Will it stay like that? Yes, the question may seem silly, sorry.

 

[^woody^]

Which ever has the best u-value

 

[MikeStout]

The U value of both is basically the same, the difference is within the margin of error.

 

[tony1851]

I think the full-fill batts are easier to fix, needing less skill than the rigid boards. If a rigid board becomes loose, it can fall against the outer skin and channel penetrating rainwater inwards.

 

[MikeStout]

rockwool = full fill? what about good old gap for ventilation?

 

[tony1851]

The full-fill batts are coated with a water-repellent finish which prevents penetraing rain from getting to the inside skin.
Ventilation is therefore not possible. In fact, ventilating the cavity partially defeats the object of the insulation. If for example you use rigid boards against the inner skin, you want the residual cavity to have no air movement; if there is a draft in the cavity, the air movement will reduce the overal U-value of the wall.

 

[^woody^]

EPS is crap and the least efficient for any given cavity width. Does that help?

 

[MikeStout]

Does that mean that a weep hole is not needed?

 

[^woody^]

Weep holes have nothing to do with insulation. Weep holes are required above lintels and cavity trays

 

[Notch7]

Will it fall down with time?

rockwall batts are reasonable stiff, anyway they wont fall down because of wall ties.

I didnt know EPS was even used for cavity insulation.

I believe PIR insulation can now be used for full fill -but I dont know how it works.

 

[tony1851]

@ OP, you wouldn't use EPS - PIR boards are the correct choice (EPS is only about half as thermally efficient as PIR).

 

[garyo]

I've always felt I must be missing with this. PIR takes up the same amount of space to deliver a similar U value (by the time you've allowed for ventilation) , more expensive, harder to fit, and likely that the attention to detail required will mean that the U values will be much poorer than it says on the tin. So why does it exist as a concept?

 

[pilsbury]

Rockwall batts are very widely used, effective, cheap, easy to install and meet the required u values for building control.

 

[tony1851]

I've always felt I must be missing with this. PIR takes up the same amount of space to deliver a similar U value (by the time you've allowed for ventilation) , more expensive, harder to fit, and likely that the attention to detail required will mean that the U values will be much poorer than it says on the tin. So why does it exist as a concept?

No, PIR would take up less thickness than polystyrene for a given U-value (I think you would only need half the thickness IIRC) and you don't want ventilation of the cavity.

If the cavity is vented, you effectively lose the insulation value of the still air-gap, and also the small contribution of the outer skin, plus a little from the surface resistance of the outer skin.

 

[garyo]

and you don't want ventilation of the cavity.

Ah so that's the source of my confusion then. Whenever I've seen it discussed, and I'm sure I've seen pictures, there's 50mm of PIR pinned against one side of a cavity, then a 50mm clear air gap, to give the equivalent of 100mm rock wool.