I have been reading Canadian Wood-Frame House Construction to get an idea how houses are built and function, and am really confused about one aspect.

External floors are built just like external walls, just on their side. Internal boards->Vapour Barrier->Studs/joists with insulation. Which makes a lot of logical sense in walls and ceilings. But since water drains down, I have no idea how this is supposed to work for the flooring. I am using an example of a wood framed external floor, but the same principal extends to concrete slabs that have a vapor barrier built in. Assuming I spilt some water, or a pipe broke, a few gallons of water got on the floor. How does this standard design allow this excess water to be shed by the house?

The entire house is encased in a plastic vapor barrier, that is sealed and caulked at all the seams, it is one giant plastic bag. This house now has several gallons of water under the floor boards. What happens next? Or is a vapour barrier not a liquid water barrier?

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3 Answers 3


How does this standard design allow this excess water to be shed by the house?

You are correct in seeing the flaw in a plastic vapor barrier. It doesn't allow the house to breathe. I personally don't like the idea.

The only way to shed the water is by evaporation through ventilation. Some of the water could drain to the ground since vapor barriers are not necessarily water tight but the tighter you make the house envelope the harder it gets to allow it to breathe.

  • 1
    The idea of vapor barriers is to 'near fully' control the separation of the home environment an the external environment. When done right, vents exist in moisture areas to get direct vapor sources OUT, and venting in the roof allows the conglomerate of those remaining vapors out. The part that is important and often missed, is that a source has to allow air in, to keep the pressure equalized. (the solution has to be a full system, the whole home, not just analyzing the pros and cons of a barrier excluding the rest of the home design).
    – noybman
    Aug 19, 2017 at 16:33
  • @noybman point taken. I would still rather allow the house to breathe naturally instead of adding mechanical ventilation to compensate for plastic. Plastic promotes mold. The only place I would install it is under the concrete in a basement floor.
    – ArchonOSX
    Aug 19, 2017 at 16:36
  • I used to think of it like you do too, until I learned that overall, the introduction of the mechanical ventilation is the "weak point" and does its job when installed correctly. When lacking, then other parts of a home that are NOT really designed to be for it to breathe, become sources, and then you have no control over the losses, or the various tuning you would have to do to compensate
    – noybman
    Aug 19, 2017 at 16:39

None of this is a good idea. Better to seal the entire crawl space water tight with very low vapor permeance. If possible insulate the crawl space as well (not the floor joists). This is usually cheaper than insulation for the joists. Then go ahead and allow a small amount of building AC into the crawl. This resolves all the issues and the sealing would be done according to the climate zone for heat/humidity. You also end the issue of stack affect.

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    Hello, and welcome to Home Improvement. The OP never said his floor was over an easily-sealable crawl space. And, you should probably take our tour so you'll know how best to contribute here. Dec 26, 2019 at 23:21

Water is not supposed to find it's way inside the structure. The "wrap" is there to keep a barrier between the outside elements you want kept out, and the inside kept in. (This includes bugs etc). That wrap is perforated with nails from siding for example, but the siding and flashing, and windows and doors are installed to drive water DOWN, not in. Thus, leaks in a home from outside occur when the barrier material wears down, or is installed incorrectly. Moisture can find it's way in - gravity.

With this said, the same principle applies to floors and sub floors, we cannot eliminate the water table, and all concrete is porous. So the same principles are employed to keep the water that wants to get in out. Via barriers, drain tiles, pumps, etc.

If gallons of water are spilled on the floor, *(it doesn't matter what the source is), the source needs to be eliminated and the water cleaned up as fast as possible to avoid damages. Some water may get into crevices where it doesn't belong. It will evaporate, it will absorb into materials that absorb water etc. The humidity in the home should be low, so it will not stay a liquid for very long, but since it got somewhere it didn't belong, it 'COULD' mold/rot/damage that area. Thus, why you clean it up immediately.

Gravity will act upon the spilled water. so will anything that can absorb it.

And by the way, a vapor barrier is rarely perfect... water can get in and out, but honestly, it shouldn't find its way there in the first place.

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