How has this roof assembly not rotted?
Looking at an early 1900’s home where the homeowner wants to add some insulation in the attic. Cape code construction with knee wall, slanted ceiling then flat ceiling with attic above. For the slanted ceiling, the stack up from outside to inside is asphalt shingles, synthetic underlayment, OSB decking, 1x planks with up to 1″ gaps in between, 4″ dense packed mineral wool, lathe, plaster. Cardboard used as “membrane” for mineral wool at the bottom of the slanted ceilings at the knee wall. No ventilation at the eaves, continuous ridge vent. This is BZ 5, Wisconsin.
Guessing mineral wool has been present for a number of decades. Poking around, the stack up has seemed to perform remarkably well. No signs of water or condensation.
Everything that I’ve read on GBA about unvented cathedral ceilings makes me believe this roof assembly should have failed by now. Why has it not? Trying to understand better before adding insulation/ air sealing in the attic.
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Replies
Johan,
When we talk about risky assemblies we usually mean they have a much higher chance than safe ones of failure, not they they will necessarily fail. Roofs often get saved by being in direct sun, air-leakage sufficient to keep them dry, or very low interior humidity. The dilemma for renovators is knowing which can be removed and still have to roof function.
Thanks Malcolm. That is the question now as the homeowner wants to add insulation in the attic. Thoughts on what would be the best strategy?
It's unrealistic to redo the slanted ceilings at this time due to homeowner's budget/ disruption.
I’ve been in your shoes more times than I care to remember.
Are you talking about an open attic when you mention best strategy for adding insulation in the attic?
Yes. There is currently 3-4" loose mineral wool in the attic above the flat ceiling. Homeowner is hoping to increase.
Johan,
You may find this discussion useful: https://www.greenbuildingadvisor.com/question/2x6-rafter-insulation
Thanks. Unfortunately that article doesn't apply in this case. The house has a knee wall, slanted ceilings, then a flat ceiling with attic overhead. Homeowner does not want to remove the lathe and plaster of the ceilings. So I see the options as 1) Do nothing. 2) Add cellulose in the attic without baffles and hope. 3) Add baffles in the attic to create an airgap and then cellulose, however there won't be continuous airflow from the eaves as the slanted ceiling portion is stuffed with mineral wool.
JohanH,
Right now the house appears - against probabilities - to have established an equilibrium where it doesn't have moisture issues. Any intervention you make that doesn't guarantee a well performing roof assembly (options 2 & 3) leaves you open to responsibility for any future failures. Given the restrictions placed in the work, I wouldn't touch it.
Am I missing something or wouldn't this assembly just dry to the inside? Combine that with potential lower interior humidity and a not-too-cold geography, and it should be fine.
Chris,
In heat dominated climates the vapour-drive in roofs and walls is to the outside. Unfortunately It doesn’t simply reverse itself and dry inwards when there is no good path to drying to the exterior.