1″ GPS Insulation between Hardiboard lap siding and air and weather barrier.
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I’m preparing for an exterior renovation of a wood framed house in Brooklyn. It has 2×4 studs with mineral wool batt insulation inside. I want to add 1 inch of rigid insulation outside. My preference would be mineral wool because it’s vapor permeable but the contractors don’t use it. GPS seems to be a decent alternative, with a higher perm rating than XPS or Polyiso, in addition to what appear to be some other benefits. However, I ran across a post on this site that recommended putting it behind the air and weather barrier because it is relatively absorbent. I would rather not go this route since it is less typical. The literature on Neopor seems to show it directly under siding and over the air barrier. Could this higher water absorption be a problem?
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As a rule of thumb, I've heard very smart building scientists say that with most cladding systems and good flashing details, roughly 1% of the rain that hits the facade gets through to the back side. If there is at least a 3/8" gap between the back side of the cladding and the face of the insulation, as there usually is if you're using furring over the face of the exterior insulation, then 1% of that water reaches the insulation. (Not relevant here, but only 1% of that water reaches the WRB that is typically installed on the face of the sheathing. And another 1% gets through the WRB.)
In your case, that's very roughly about 0.1% of the water that hits the face of the cladding reaching the foam--that's not a lot. Although GPS will absorb liquid water, it will also drain out, so I don't see a need for a WRB over the face of the GPS. It certainly wouldn't hurt to add one, as long it has a high permeance rating. Just be sure your flashing details are good.