Superinsulated roof drying to the inside
P. 84, 85 of the current issue. A superinsulated roof with rigid insulation over the roof sheathing and permeable batt insulation betweent the roof joists. So the idea is that it drys to the inside, with no vapor barrier. But depending on humidity conditions could not some of the condensed vapor drip through the batt insul. and right on the top side of the ceiling? The diagram on p. 85 seems to show exactly that with a blue arrow and drip bubbles right up to the edge of the batt insulation.
Replies
The foam IS the vapor barrier on the exterior of the roof deck. The condensation point is within the foam layers, so the deck under surface doesn't get cold enough to condense. And vapor that passes into the batts can pass back out again and isn't trapped.
More or less
But I would not do something like this without a complete heat exchange and moisture controled HVAC system in a house
My coffee hasn't kicked in yet this morning...
Is there a way of testing humidity levels in the house over a period of time? Or would you rely on some method of observing vapor penetration in an existing envelope?
I ask this question because in this case, the insulation of the roof deck is a renovation to an existing structure. It could be 10 years to 100 years old. All sorts of air and vapor sealing techniques have been used during this time frame. Whether they work or not is also in question, of course.
So, without knowing what the rest of the structure is like, you would AUTOMATICLY say that an HVAC refurbish/replacement would be in order with a major insulation project like this?
I'm not saying you're wrong, I just want to understand the connections from your point of view a little better!
OK now
since i don't take the magazine any more insert disclaimer for blindness here.
But I read this "depending on humidity conditions could not some of the condensed vapor drip through " to tell me that the OP probably does not understaand the concepts fully yet.
With the roof surface fully insulated and sealed with a good closed cell foam there will be NO condensation. What causes that is when the warm damp air hits a cool dry surface, lowering the ability of the air to carry moisture.
Your Q points out to me that a whole humidity and air control system may not be needed in the house of th emagazine