A building science mystery in Hawaii
I read with great interest Martin Holliday’s recent blog post “Summertime Condensation Near the Peak of a Cathedral Ceiling” (09/28/18) and the comments it elicited. I have a very similar problem with a small house on Maui. The IECC unhelpfully puts the entire state of Hawaii in climate zone 1-A (Hot Moist). But the area where the house is located gets just above 15″ annual rainfall. Using the Koppen climate classification the area is more properly characterized as “As” (Tropical Savanna Dry).
The house consists of three linked pavilions, one larger than the other two. The pavilions have pyramidal, standing seam metal roofs with a relatively steep 8 in 12 pitch. The peak of the roof is 17′-9″ at the smaller pavilions and 19′-2″ at the larger pavilion. The roof sandwich is supported by 5 1/8″ x 12″ glulam hip rafters and 3 1/8″x 9″ glulam jack rafters at 3′-3″ o.c. These are connected with large, custom made steel brackets. The roof deck is 2x t&g Alaskan yellow cedar. All this is visible from below and forms the ceiling treatment. Above the t&g is 3″ of rigid insulation (Thermasheath-3) with an r value of 20.3 and foil on both sides of the sheet. Although the manufacturer recommends installing this product as a continuous sheet it was installed with 2x4s on side to form a 3 1/2″ cavity. The insulation is tightly butted up to the 2x4s. One half inch plywood is attached on top of the 2x4s creating an unvented 1/2″ air gap above the rigid insulation (a code requirement?). Above the plywood is a layer of roofing felt and a standing seam metal roof. The roof color is medium brown.
In all three pavilions there is water dripping from the ceiling. The roof has been checked and re-checked for leaks. It is tight and dry above the t&g deck. The dripping tends to start at about 10:00 in the morning and pretty much dries up as the sun goes down. The dripping occurs under every condition we could observe, sunny days, cloudy days, dry days, humid days, air con on, air con off, windows open, windows closed, ceiling fan on, ceiling fan off. We have run a dehumidifier in the larger pavilion bringing the indoor humidity down to 40%. The ceiling still drips. We have inspected the underside of the ceiling and find no mold, rust, dry rot or water damage of any kind on any of the ceiling components. We have taped down rosen paper on the floor and have discovered that the dripping is most intense at the center of the ceiling and at the connections between the longest jack rafters and the hip rafters where there are not only metal connectors but also inside corners where hot, humid air could collect. Condensation definitely forms on the metal brackets, we have photographs of it, but it also forms on the t&g at the apex of the ceiling, beading up at board edges. The gaps between the boards at the apex of the ceiling are noticeably tighter than lower down indicating a higher moisture content in the wood.
We have instrumented the ceiling and taken spot readings of surface temperatures. Typically what we find is an air temperature of 82 degrees (un-airconditioned) at the lower part of the space and an air temperature of 90 degrees at the apex. The highest spot temperature of the ceiling surface is 90 degrees. The spot temperature of the brackets is 82 degrees. The relative humidity below, where the dehumidifier is running is 40%. At the apex it is 60%. I have only used approximations to derive the dew points for these conditions. The dew point below is 60 degrees +/- which seems about right. But the dew point at the apex should be 75 degrees +/- which is too low for condensation given the readings we are getting. Perhaps there is some problem with our measuring devices. In any case it is clear that surfaces at the apex are below the dew point of the surrounding air. it is also pretty clear that there is considerable air stratification both as to temperature and water vapor content.
What Should We Do?
There are other structures on the same property that have the same roof shape and material, but they are constructed differently. They have flat ceilings with vented attics and standard truss construction. The standing seam roof sits on roofing felt on 1/2″ plywood. Insulation is in the attic above the ceiling. We plan to add some more at the underside of the roof. There is no sign whatsoever of water intrusion or of condensation in the attic, even though the venting is only in the blocking between trusses. There is no venting at the hip or at the peak.
Our most promising solution at the moment is to introduce an attic between the roof and the living space at all three pavilions. The attic would be configured to slope the ceiling at a pitch lower than the pitch of the roof, perhaps 4 in 12. The ceiling would consist of one layer of taped gypboard and a finish layer of cedar t&g. We would drill holes at the existing blocking between jack rafters and back these with copper screen. And we would provide an exhaust vent at the apex, which could be either a passive or a powered vent. And, of course, we will add additional insulation at the new attic to bring the total r value up to r 30 or 40. r 20.3 is totally inadequate.
Will this work?
And are there other thing we should do? Remove the rigid insulation? Reinstall it with no air gap (raises r to 24). Reinstall it with no 2x4s as the manufacturer recommends? Apparently a plywood nailing surface is not even required when installing standing seam. Should we thermally isolate the metal brackets by spraying them with closed foam insulation (like a thermal sleeve for a beer can)?
Or do we need to rip off the entire roof structure and start over?
Any advice would be most welcome.
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