We recently placed 3.5" of RMax foil-faced polyiso foam board (air and vapor impermeable) neatly cut and placed between the rafters of our cathedralized ceiling home in the Santa Cruz Mountains of California (hot dry summer, very wet and rainy winters, never much below freezing). Home is roughly 80 years old, built originally as summer cabin (very leaky, etc.). The roof is unvented, medium color asphalt shingles in good condition, with no roof leaks after three years living in the home. Roof deck is tongue-and-groove 1x8 pine, painted from below as a finished ceiling several years ago.

3-days after installing and sealing the perimeter of each sheet with can foam, I felt water drip on me while working at home mid-day (~2pm). I started paying attention, and we got many, many more drips, right below the ridge blocking. This occurs only in mid-afternoon from roughly 1pm to 3pm on hot sunny days, and is most evident on the South-sloped roof surface. This is a handful of drops, not a deluge of water, but still clearly unexpected and not good. 

We opened up two south-facing panels yesterday at 2pm. We found a completely dry and hot roof deck, and completely soaked ridge blocking and structural metal hardware. The ridge was beaded up all over with condensation, and the drops I observed were coming from the Simpson hardware at the ridge. I count myself lucky to have seen this issue prior to sheetrocking the whole home. I've added some photos below to show context, install quality, and observed condensation (note, this shows beaded water on the hardware, but it also covered the entire ridge block, just hard to see when painted white). 

I have a clear hypothesis of what's happening in my mind, but I want to know what others experience has been with this type of assembly and insulation strategy. Is it common in your region to cut and place foam board between rafters in an unvented roof assembly? It meets the letter of the model codes and local CA codes.

Have you ever experienced or heard of this issue, specifically condensation on hot summer days in unvented roofs insulated with polyiso or other foam boards? Or moisture stained sheetrock or rotted ridges in such roofs. (Note: I'm very familiar with moisture issues in these roofs in general, but am curious about this group's experience with this specific strategy).    

I also have a sense of how to fix it, but what would this experienced crew suggest? If removed, what would you recommend replacing this insulation with, prior to sheetrocking? 




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    • The "blocking" is more like an undersized ridge beam running the length of the room. I'm sure it's thermally bridging and represents a hot surface, but cool enough to support condensation at likely very high vapor pressure. I think the air up there probably has a dew point >100F, and we're at that kind of temp. The hardware might be cooler, or it might just absorb less water than the painted wood, so it beads up more and drips (thankfully). A/C is not on/operating at these times, but the indoor temp is <75 F. Outside air is very dry here in CA in the summer, so I don't think its leaking from outside. My hypothesis is that water vapor is emitted from the dry wood roof deck, because it is so hot, and this water vapor is driven to the ridge by buoyancy, where it condenses even on very hot surfaces.     

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