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Why does the morning dew on under slab EPS remain all day?

Burke Stoller | Posted in General Questions on

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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  1. GBA Editor
    Martin Holladay | | #1

    Good question. GBA readers: any ideas?

  2. Charlie Sullivan | | #2

    I'm surprised too, but I think there are a bunch of things working together here:

    1) There's essentially no solar energy absorbed by the water or the EPS. You can visually see that water doesn't absorb significant visible light energy (until it's many feet deep); it also doesn't absorb near IR. You can also see that the EPS reflects the visible; I'm not sure about the near IR.

    2) That leaves convection and radiation heat exchange with nearby objects as ways that the surface and water drops might warm up. The convection heat flow we need is downward to a horizontal surface. That's the condition in which convective heat transfer is weakest, so although that happens, it's slow and doesn't supply heat to the water very fast. If we compare that to a leaf, for example, the leaf is smaller, so air can flow around it not just across it, it's not perfectly horizontal, and it has air flow on both sides and the heat from the bottom can easily conduct through to warm water on the top.

    3) Radiation heat exchange with nearby objects might be minimal if it has a clear view of the sky--you could be losing heat by radiation to the sky rather than gaining it from heat exchange with nearby objects.

    So the only way it can warm significantly is by convection of heat downward to a large flat surface, and that's not very effective. Whatever breeze was present certainly helped and was probably the determining factor in the rate of drying.

    And there's one more effect that I think contributes significantly:

    4) After some of the dew dries on, for example, a car, there remain some drops, each separated by some bare surface area. Because the surface (a piece of sheet metal in this case) conducts heat pretty well, the surface around the drop acts as a heat collector, and the heat conducted in the surrounding area gets conducted to the drop to help it evaporate. The EPS doesn't do that well.

    I imagine mineral wool wood absorb sunlight more and would not behave as much this way.

  3. David Meiland | | #3

    Do an experiment--weigh a chunk of dry foam, soak it in water for a while, then weigh it again. If it can hold much water, then your answer may lie in the "recharge" effect of water that has been absorbed (either rain or maybe even in manufacturing). My suspicion is that the moisture drive would be upwards toward the air, rather than downwards.

  4. Burke Stoller | | #4

    Interesting thoughts- thanks for the responses! We had a really neat effect on site today. Yesterday we had placed peel and stick down on top of the foundation wall, and then wrapped it over the top of the XPS insulation acting as the slab edge thermal break (so that we could seal the under slab poly to the peel and stick and have it act as the capillary break between concrete and framing). When we showed up, there was a perfectly distinct dew line on top of the walls: no dew on top of the peel and stick over the concrete, and VERY heavy dew on the 2" of peel and stick running over top of the XPS! That made perfect sense, as the concrete had stored enough thermal mass, and was conducting enough heat up from the ground below that the peel and stick on the concrete never quite got down to the dew point, whereas the XPS has zero thermal mass and was the same as air temperature, so had definitely reached the dew point. Still, it looked pretty neat. Some of the unusual things one finds when building super-insulated houses!

  5. Expert Member
    Malcolm Taylor | | #5

    Where in the Island are you? Perhaps you can post some pics as things progress.

  6. Burke Stoller | | #6

    We are building for a client in Qualicum Bay, which is just south of courtenay. I will try to get some pics up at some point. Is there a spot on the site that would be a good place to post?

  7. Alan B | | #7

    Whats the humidity in this scenario?

  8. GBA Editor
    Martin Holladay | | #8

    You can post photos here, on the Q&A pages. If you ever want to submit a guest blog, you can contact me by email: martin [at] greeenbuildingadvisor [dot] com.

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