Layering EPS or any cheaper insualation over Rigid Rockwool for exterior
SebMaz
| Posted in General Questions on
Hello,
Is there any benefits from layering different type of insulation (something cheaper than Rockwool ex. EPS/XPS) over the Rigid rock wool on the exterior? or it will cause more harm than good ? will it block the permeability of the Rockwool?
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Replies
Rockwool is the most vapor open rigid insulating material you can get. ANY kind of foam is going to significantly reduce the vapor permeance of the overall assembly. I see no advantage here, only downside.
If this is a new build, and you want more R value, use either a thicker layer of rockwool, or a different insulating material that is cheaper to get up to the R value you want.
If this is an existing build, you should layer an additional layer of Rockwool Comfortboard over the existing. I would not add a layer of a different insulating material here.
Bill
What I'm trying to achieve is to not trap the moisture inside the house, the Siding that I will install is Epoxy + Quartz backed with EPS foam (2.2inch), I'm afraid if I install XPS / EPS under it won't dry to outside anymore. so will the rigid rockwool help if used under the EPS Siding? or its just waste of money and won't make difference
Usually wall assemblies insulated with rigid insulation use some type of foam (EPS, polyiso, etc.), and are designed to dry to the interior. Those assemblies work fine in heating dominated climates which I think might be where you are? It doesn't hurt to allow the wall dry in both directions, but it's not necassary, either.
In your specific case, I think the rigid rockwool (probably Comfortboard?) is a waste of money, because you've already got that relatively thick 2.2" layer of EPS going up. If you want more R value here, I would just put up some EPS instead, which would effectively give you a thicker overall layer of EPS in the assembly. If you put a 3/4" layer of EPS up, you;d be just shy of 3" total of EPS, which would be a little over R12 here. 1" of EPS would give you almost R13.5. I don't think you're gaining anything with rockwool here, and I think it actually introduces additional risks to the assembly as a whole.
Bill
It depends on the situation but in a heating-dominated climate there is risk of causing condensation at the interior face of the foam. It wouldn't hurt the mineral wool and it would eventually dry, but in the meantime it would ruin the mineral wool's insulating ability. It would be safer to put the foam against the sheathing and the mineral wool outside of that. Or just use mineral wool alone. Or better, rigid wood fiber.
my climate is hot in summer and cold in winter, with medium-dry humidity. (highland continental climate), my problem is I will install Epoxy+Quartz + EPS backed sidings, but it is just 2.2 inches so I don't want to risk and would make it at least 4.5inch, but not sure if the Rigid wool will make any difference to help not trap moisture inside. for wood fiber we don't have it here.