All steel building venting for home
We have an all steel building that we are turning into a home. It currently has a steel roof with fiberglass insulation with a vapor barrier on the warm side, there is no space between the insulation and tin other than the rib gaps of the exterior tin (the walls are the same setup as well). We are going to frame it with wood studs and add rock wool insulation in between the purlins (and girts for the walls) then put our ceiling/wall covering up which will create a cathedral ceiling. My question is how do we vent the ceiling with this setup? Can we leave an airgap between the current vapor barrier/insulation and the rock wool and install roof vents? We have gutters on the building already so i dont think we will be able to put soffits in. We currently show 70-80% humidity in the building…we are zone 5a if that makes a difference.
Any help and advice is greatly appreciated.
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Victoria,
The problem with this type of steel barn or warehouse is that there is no air barrier on the exterior side of the insulation.
If I were you, I would pull out all of the insulation and pile it in the middle of the building. Then you need to come up with a plan to install an exterior air barrier -- ideally with something sturdy like OSB or plywood, but perhaps with rigid foam.
Doing this isn't cheap, of course. You end up having to create a new building inside your building.
No air barrier on the exterior side of the insulation means that you aren't getting the full benefit of the insulation. But you are getting some benefit. So I would leave it in there, and just make sure that any new insulation you add is inside of an air barrier, so you can be sure you get what you pay for with it.
And you might not even need to vent the new cathedral ceiling. The challenge is that it's hard to tell what the effective R-value is for the insulation you have now, given the wind washing and thermal bridging that are undermining it. I might install vent like you suggest, but make them adjustable, and monitor the humidity in that space. Then only open the vents as needed if that humidity gets higher than you'd like.
This is similar to the not uncommon request for help turning a quonset hut into habitable space. Like that project, there are inherent challenges that stem from the choice of starting structure. You're basically building backwards, from the skin first and inwards from there, rather than building the structure first and finishing with the skin.
If I had to do this, I would get two inches of closed-cell spray foam applied to the interior of the metal structure, then I would build my interior walls, and finally fill the cavity between the spray foam and the interior walls with cellulose. You could use some of the existing insulation there in place of an equal volume of cellulose (or blown fiberglass, or more spray foam, or whatever). No vapor barrier needed with this approach.