Exterior Rigid Insulation Layer Cake
Gerk1
| Posted in General Questions on
I’m planning a new build with Zip system as the main control layer. I want to install exterior insulation over the Zip. I’m toying with the idea of layering different types of rigid insulation like a layer cake. One idea would be a progressively more vapor open types as you move outward (mineral wool over Polyiso ). The mineral wool would protect the poly from bugs and allow water to drain. Since Polyiso degrades its r value as it gets cold, the Roxul would keep it warmer. Any thoughts?
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Replies
I wouldn't bother. Polyiso doesn't really drop in R value as much as some think, and it's not worth the added expense of the rockwool here. I don't see any value in the varying levels of vapor permeability, either, since it's the sheathing that needs the drying ability, not the insulation, so the increasing amount permeability as you move outwards doesn't really help you.
In my experience, critters get into the foam from the edge, not the face, so you should concentrate your critter-proofing efforts around the perimeter using screen or perforated flashing. Extra protection for the face of the insulation doesn't hurt, but I don't think it really helps all that much in this regard either.
My own preference would be to put two layers of polyiso up and call it done. Two layers lets you stagger the seams, which is a plus. I usually like to use a thinner inner layer (against the sheathing), and a thicker outer layer, since this helps to keep the exterior surface flat -- the thicker material is less likely to bow out, and it helps to keep the thinner inner layer pressed tightly against the sheathing.
Bill
Your cake is bland. At least mine has a crunchy frosting. Give me a few flavors. How about some blue stuff then some pink stuff. It's also a matter of cost. There would also be a capillary break behind the first layer at the Zip. Also which type of insulation serves as the best location for the condensation point. I agree about protecting the edge. Maybe there is rock wool around the edges. A frosting of rock wool. Use the cheap stuff as the main cake and use the more expensive stuff sparingly and strategically. EPS doesn't like getting wet but it's cheap. That's the main part of the cake. I'm just spit balling here.