I have a 1960’s mid-century modern in the Pacific Northwest, post-and-beam construction with a low-slope (2:12) roof and vaulted ceilings. The rafters are 2×6 running 16” o.c. and sitting directly on the beams, with sheathing on top of those and asphalt shingles. There is batt insulation filling most of the 6 inch cavity. So although there are four soffit vents on each side and a ridge vent at the peak, and some 1 inch holes drilled in the rafters every few feet for cross-ventilation, I suspect that there is not much airflow.

The house has a huge, 8’ by 4’ basalt chimney near the center of the house which goes out near, but not at, the peak of the roof. Every spring, we have days here in March/April where it will be 40’s and rainy at night and sunny and warm (up to 75) in the day. On those days, heavy condensation will often form inside on the ceiling around the edges of the chimney, particularly on the “under” side or downslope side. This year (our third in the house) it’s gotten much worse and brown liquid has started dripping down the side of the chimney, on both the “downslope” and “upslope” sides.

Ripping open the ceiling drywall next to the chimney, we of course found the batt insulation soaking wet, as well as the sheathing above it. Mold is growing between the insulation and drywall.  

There are three possible sources for the moisture in the chimney that I can think of.

1.       There could be a roof leak caused by faulty flashing around the chimney (not sure how much actual water intrusion, if any, but we will be mitigating either way)

2.       The moisture could be coming from inside the house, as the drywall seams in the ceiling were not taped and mudded, but covered with trim (no caulk). This is also being mitigated, as the new ceiling will have rigid foam insulation with spray insulation around it to air seal the ceiling.

3.       And here is my question: is it POSSIBLE that the moisture could be coming from the soffit vents pulling cool, damp air in from the outside on these spring days when the sun hits the roof and heats up the small air cavity? And that when that hot wet air rises toward the ridge and hits a cold chunk of basalt rock chimney 8 foot wide across it causes a ton of moisture to condense? This would explain the problem being worse on the “downslope” side of the chimney.

The reason I ask is because it impacts our proposed solution. Right now the plan is to replace the entire roof (it’s time anyway and asphalt shingle roofs at 2:12 slope in the PacNW are nothing but trouble). Our contractor is proposing to rip out all current sheathing and place a grid of 2×4’s lying flat on top of the rafters and running perpendicular to them. Then sheathing on top of that and either a steel or membrane roof. That will give us an extra 1.5 inches of space for a total of 7 inches (including 5.5 for the 2×6 rafters. After that we’ll rip out the ceiling and put 4 inches of polyiso foam board with air sealing around it, leaving 3 inches remaining above the insulation for airflow. With this plan, if moisture is only entering via #1 and #2 above, we’re all taken care of. But if #3 is ALSO happening, then I’m out of luck and worse, I won’t even know that moisture is still accumulating for quite some time.

I’m thinking that what I should do is have my roofer install an EXTRA layer of rigid foam (maybe only ½ inch or 1 inch) ABOVE the new sheathing so that there is insulation on both sides of the roof cavity. This would not only provide an extra thermal barrier but also dampen the hot-cold variations in the roof cavity. That way, on sunny days the difference in temperature between the warm air in the cavity and the cold rock of the chimney won’t be as great, and condensation shouldn’t be as bad.

Thoughts? Is adding additional polyiso just a waste of money? Am I going overboard? Or is there something else I haven’t thought of? I’ve had several contractors who don’t seem to want to touch this with a ten foot pole.