Recent high levels of insulation bury the bottom chord of a truss in the insulation where it stays warm and dry year round now. The upper chord and struts become very dry and shrink during the summer from higher attic temps (8–110+) and then pick up moisture from the cooler and more moist (when cooled at night) outside air that enters through vents. Thus these chord members expand causing the stresses in the diagrams.
When the moisture and air leakage from the house contribute to part of the increased moisture load in the attic wood, the first attack is to dry out the house so that there is not excess moisture in the air and then airseal (create an air barrier) at the ceiling level to stop house air containing moisture (and heat) from getting to the attic.
The lack of a vapour retarder/barrier has never been shown to contribute to the problem. An old style unsealed (read: very loose) ceiling retarder could still help contribute to the attic moisture if there was a very wet/damp house below due to a variety of problems; unvented dryer, bathrooms, kitchen; wet/damp basement/crawlspace, high numbers of plants,aquariums, etc; interior stored wet firewood. The unsealed nature of the retarder allows a significant flow of house air containing moisture up through the ceiling and loose fibrous insulations to the cool sheathing where it may condense…but it, at least, adds to the general RH of the attic and to the wood expanding due rise in moisture content.