How to fix my unvented cathedral ceiling roof assembly?
We tried to build a house with a fairly tight envelope based on reading I could find. Well, apparently I messed up, so really want some advise before I mess things up more.
House is in central Missouri( zone 4, but real close to 5). Small, 750 sf but with an open loft making it about 940sf. Sealed Concrete slab as the finished floor, and new construction ( under roof in may but moved in The next January). Red metal roof and one side of that faces north..
Walls are 2x4 and we used open cell foam with 1/4" foam inside the studs as an air ( not vapor) barrier.
The builder used 2x8 for the ceiling joists but we firred that down on the inside with 1 1/2" foam board and then 1 1/2" wood stud. We used 2" closed cell foam under the roof deck. Then r 30 rock wool under that. And, no drywall or air barrier and a tounge and groove pine ceiling attached to those studs we added. Yeah. I know( now)- bunch of mistakes in there.
So- we move in during an unusually cold snap (1 degree on one night). And notice a puddle on the north side. Turns out to be condensation in many places of the ceiling. We put a thermometer in a place we could get at between the rock wool and ccsf, and temp was around 37 degrees. So , 2" ccsf is not enough in general and certainly not enough to keep the foam face warm enough with the rock wool we have. At one time when this first started the rh showed around 60%, but we have been using a dehumidifier ( in the winter) to try to keep it under 35. ( we do have kitchen and bath vented outside). We now have a wood stove, which helps with a "dry heat", but also have a mini split, which is what we were originally using when this started.
It's a new metal roof so that isn't coming off- so need to fix this from inside. The house is now finished, so while I know this will get messy, the less the better. And of course now we are really house poor. We will find a way, but, the more reasonable the better.
I want to keep it at about r38 which is code here. Could I take down the ceiling and add 6" ocsf, assuming that since it is an air barrier at about 4" that it will stop the majority of the vapor? Or do I need to add another inch or two of ccsf ( which I was hoping to avoid- mostly because I don't want to take down and reinstall the rock wool, which is a pain). I can't afford full depth ccsf. Do I need to add either an air barrier or drywall before putting the ceiling back up? I would prefer something other than drywall ( just because of what that would do to trim details, etc), but not sure of our choices. Any and all options that would work and not ruin my house later would be appreciated.
Posted Feb 16, 2017 5:03 PM ET
Edited Feb 17, 2017 6:37 AM ET
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