Unvented attic assembly and rafters ghosting through roofing material
I recently completed a 2,500 square foot two story home in climate zone 3C, about ten miles inland on the coast of California. The climate is very moderate, with few days ever exceeding 85 degrees, or falling below 45 degrees. It does not rain often here, approximately 12″ a year.
The house was sheathed in zip panels, and all seams were taped to manufacturer’s specifications. It’s a crawl space foundation, with fiberglass batts between the floor joists, and a 12mil vapor barrier on the floor of the crawl space. The roof was also paneled with zip product, seams taped, and backed up with GAF tiger paw synthetic roofing felt.
Insulation in the walls is dense packed cellulose (2×6 wall construction), and the roof is unvented, with R-30 open cell icynene spray foam sprayed under the 4:12 pitch roof.
Air conditioning and a forced air furnace are present; however, they rarely run due to the moderate climate and seemingly functional insulation. During the hottest days of the year, the air conditioner runs for approximately 2 hours.
An interesting phenomenon began directly after occupancy. The upstairs bathroom ceiling would condensate quickly, a Panasonic 150CFM fan was used with an external humidistat. No amount of exhausting seemed to curb the condensation on the ceiling. The fan is located directly outside of the shower area, the bathroom is approximately 10×15.
The fan was checked during HERS testing and passed. I figured that it must be undersized, so I upgraded it to a Panasonic 380CFM fan. There is sufficient makeup air via several windows, and the door is not closed. I have not noticed any rusting, or other signs of moisture damage in the bathroom. There is no visible fog of moisture during a shower; however, the ceiling seems to condensate rapidly. Furthermore, the walls begin to drip after an extended period of time. I have never installed a fan larger than 150cfm.
I installed two hygrometers, one at the bathroom ceiling and one in the attic against the spray foam. The RH in the attic seems to remain constant at about 55%, which seems high to me. The bathroom ceiling hovers at the same RH until a shower is taken, where it can rise up to 80%. The interior temperature of the house is around 70 degrees, and the attic is somewhere around 72-74 degrees. The RH in the attic does not seem to climb more than a percent or two during a shower.
The ceiling condensation is one issue. The newest issue I have noticed, is that the rafters are visible from the outside when looking at the roof. They are ghosting through the triple laminated shingles, there is no physical bump, just visual evidence that some type of moisture or thermal bridging is occurring during the summer. I’m not sure if this is of concern or not, or if it indicates roof rot or not.
It would seem to me that in California, an RH of 55% is a little high. I have debated installing a supply duct into the attic area; however, since the HVAC is rarely running, I don’t see how this could help. I could install a fan into the attic space to exchange air, or I could install a dehumidifier which also seems extreme for this climate zone.
I welcome any and all advice:
How do I solve the ceiling condensation issue? I have the largest fan with makeup air available, and it doesn’t seem to be helping. Is the RH prior to a shower too high to begin with (55%)?
Are the ghosting rafters a sign of roof rot and moisture movement due to the spray foam? I thought I would see a higher RH in the attic (55% all day) if that was the case? See attached photo.
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