Why does the morning dew on under slab EPS remain all day?
This is just a curiosity question, to see if any of the building science gurus here have an answer. We are building a “near Passive” house on Vancouver Island, British Columbia, Canada, and are in the under slab prep phase. It is a slab-on-grade scenario, and yesterday we put down half of the house’s first 6″ layer of EPS (Type II), and about ¼ of the house’s second 6″ layer. This morning, there was a heavy dew outside, and of course every outdoor surface was soaking wet. However, by noon, it was quite warm outside, and very sunny, and everything outside was now totally dry. Except, the EPS laid out was still absolutely soaked, and remained somewhat wet right up until the end of the day. (Note- the EPS had been stacked outside uncovered for a few days beforehand and did see a bit of rain.) Here are the reasons why I was mystified enough by this “everlasting condensation” situation to post the question!
1. Even though the EPS is a highly reflective surface, why wouldn’t the water droplets sitting on top absorb enough solar energy to eventually evaporate? Water doesn’t just absorb heat via conduction from the surface below it (the top layers in deep lakes and oceans absorb solar radiation and warm up). Why didn’t it?
2. Since the EPS is 12″ thick, I doubt the ground temperature below was affecting the surface temperature of the foam much. At the very least it should have been air temperature (about 18 degrees C), but was also in full sun, so should have even been slightly warmer, which should have at least helped the water evaporate. Why didn’t it?
3. Why didn’t the breeze dry off the foam?
4. If it was simply the moisture from the previous days’ rain coming out of the foam, why wouldn’t the moisture drive be downwards, towards the cool soil, rather than upwards towards the warm(er) sunny side of the foam?
5. This is not an isolated event. Rigid foam always seems to do this, unless it is bone dry in the middle of summer, with no morning dew.
6. Some XPS we had in place as a slab edge thermal break was doing the exact same thing, and it was only 2″ thick applied to the inside face of the foundation walls.
7. Does the same thing occur with mineral wool left outside in the dew? (Didn’t have any around to do an experiment!)
Weird science! Any ideas?!
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