There seems to be some differing opinions on whether to use construction plastic in/under the foundation or not. Some manufacturers indicate, for example, that construction plastic should be placed between the layers of foam insulation when casting a slab. Others say it should be directly against the ground if making a crawl space.
The purpose of the construction plastic is, of course, to prevent ground moisture from rising into the foundation.
What I'm wondering is what happens to the moisture that gets "trapped" under the plastic. Will it stay there, or will it move?
If you lay plastic directly against the ground under the foundation, can you completely skip the layer of capillary-breaking macadam that is normally used?
The purpose of the construction plastic is, of course, to prevent ground moisture from rising into the foundation.
What I'm wondering is what happens to the moisture that gets "trapped" under the plastic. Will it stay there, or will it move?
If you lay plastic directly against the ground under the foundation, can you completely skip the layer of capillary-breaking macadam that is normally used?
When it comes to crawl spaces, I have always (=since I became aware of the problem) believed that the recommendation was macadam/gravel + plastic. Without being knowledgeable in the area, I would believe that moisture under the plastic partly stays there and partly seeps up at the edge of the plastic, i.e., allows some of the moisture to pass through.
Nope, I don't think so, at least not in my opinion.tlundberg said:
The reason should be to reduce exposure to any ground radon.
We've built in a radon-safe area, I skipped the plastic anyway, but added a radon pipe.
edit:
Yes, when it comes to a crawl space foundation, I believe the plastic is/can be used to stop both ground moisture and radon.
Well, eventually a balance should establish itself, which isn't very different from what it would be without plastic.
When I ponder about moisture, I usually consider the ground always at 100% RH, but only water in vapor phase (which proper drainage should ensure). And then we have a different temperature/humidity climate above the slab. Eventually, all parts of the construction should reach a certain temperature and thus, eventually, a certain equilibrium moisture level.
But of course, without plastic, there might be continuous diffusion leading to a slightly different equilibrium state, but concrete is quite diffusion-resistant, so I don't think it should differ too much.
Or, well, perhaps RH above the plastic can be somewhat lower.
Okay, I admit, I haven't thought about this much...
Additionally, there's the reverse moisture migration that can happen when the floor heating is turned off.
Yes, it's not entirely clear what happens, still...
No, I don't think so.
I think it's always important to allow free water under the house to drain away and out.
Yes, that's what I find a bit strange. If, like Mikael_L, you assume the ground has 100% RH, then there's 100% humidity directly under the plastic. If more moisture rises, either condensation will form under the plastic, or the moisture (in vapor form) has to go somewhere else. The only way it can move is along the plastic until the plastic ends, and then continue up through the ground. If the plastic ends on the inside of the edge beam of a slab, or on the inside of the walls in a crawl space, it should mean there is very high moisture load right there.bonnier said:
If the moisture has nowhere else to go, shouldn't all the moisture come up into the foundation/slab despite the plastic? The difference is that it comes up more concentrated along the outer edges rather than evenly spread across the entire surface.
The trick is that the plastic creates its own climate zone between the ground and the plastic, and underneath it, a local equilibrium is created that reduces evaporation etc. from the ground.
A little does escape at the edges but not as much as if the plastic weren't there.
A little does escape at the edges but not as much as if the plastic weren't there.
I think maybe a light bulb went off for me now
I have always imagined that moisture in vapor phase constantly rises through the ground. With a "plastic cover" over the ground, it would eventually become "overflowing" and the moisture would "spill over" past the edge of the plastic. Like pouring water in a bowl, but upside down. When the bowl becomes full, just as much water will spill onto the table as it did without the bowl. The difference is that now it runs along the edges of the bowl instead of straight down from the pouring jug.
But if you think that the reason moisture in vapor phase keeps filling up is that it constantly disappears upwards through the ground. Then it has to fill up from below to maintain 100% RH in the ground. If you then put on plastic, the transport of moisture upwards through the ground stops, and it also stops filling up with moisture from below. It becomes "full," but never "overflowing." Next to the plastic, it continues as usual, at the same rate, with moisture in vapor phase being transported up from the ground.
Does anyone understand what I mean? Am I right?
I have always imagined that moisture in vapor phase constantly rises through the ground. With a "plastic cover" over the ground, it would eventually become "overflowing" and the moisture would "spill over" past the edge of the plastic. Like pouring water in a bowl, but upside down. When the bowl becomes full, just as much water will spill onto the table as it did without the bowl. The difference is that now it runs along the edges of the bowl instead of straight down from the pouring jug.
But if you think that the reason moisture in vapor phase keeps filling up is that it constantly disappears upwards through the ground. Then it has to fill up from below to maintain 100% RH in the ground. If you then put on plastic, the transport of moisture upwards through the ground stops, and it also stops filling up with moisture from below. It becomes "full," but never "overflowing." Next to the plastic, it continues as usual, at the same rate, with moisture in vapor phase being transported up from the ground.
Does anyone understand what I mean? Am I right?
You get to think a bit about the driving force for moisture migration and water movements. The tricky thing about water is that transport can occur in different directions simultaneously. Water can, for example, be transported by capillary suction forces in one direction while water in vapor phase is driven in the other direction due to temperature differences.
We can probably disregard capillary forces. I assume that the entire foundation is separated from the ground with a completely capillary-breaking layer.
In your post, it almost sounds like temperature difference is the only reason that water vapor moves. But differences in RH also play a role, right?
In your post, it almost sounds like temperature difference is the only reason that water vapor moves. But differences in RH also play a role, right?
Sure, temperature obviously plays a big role. But it's possible to have air that has different RH at the same temperature, right? With the help of a dehumidifier, for example.
But if we return to temperature, and a previous thread where Mikael and I discussed moisture, http://www.byggahus.se/forum/byggnadsvard/219195-angelaget-2.html#post1811906, we might have the answer to why plastic isn't needed for moisture's sake.
RH in the ground: 100%
Temperature in the ground: 8°C
Vapor pressure: 1073 Pa
RH indoors: 60%
Temperature indoors: 20°C
Vapor pressure: 1402 Pa
The higher vapor pressure indoors means that the moisture in vapor phase rather wants to go from inside out, rather than the opposite.
But if we return to temperature, and a previous thread where Mikael and I discussed moisture, http://www.byggahus.se/forum/byggnadsvard/219195-angelaget-2.html#post1811906, we might have the answer to why plastic isn't needed for moisture's sake.
RH in the ground: 100%
Temperature in the ground: 8°C
Vapor pressure: 1073 Pa
RH indoors: 60%
Temperature indoors: 20°C
Vapor pressure: 1402 Pa
The higher vapor pressure indoors means that the moisture in vapor phase rather wants to go from inside out, rather than the opposite.
No, it doesn't play a big role - it is, in fact, crucial!
Temperature difference is the driving force for diffusive moisture movement. When calculating it, one can choose temperature or partial vapor pressure as the driving force in the equilibrium equations, but it is practically the same thing.
Temperature difference is the driving force for diffusive moisture movement. When calculating it, one can choose temperature or partial vapor pressure as the driving force in the equilibrium equations, but it is practically the same thing.
Self-builder
· Stockholm
· 8 227 posts
Without involving equations for moisture movement in the ground, the presence of the plastic is meant to act as a lid - that is, where the plastic lies, no moisture, or at least very limited, will get through.
Next to the plastic, moisture will behave as normal, or more/less - but then those equations must be involved, and I suspect that the increase/decrease will be relatively small anyway.
In other words, the building plastic is a lid; if you boil a pot of water and hang a lid above it (with a gap), the steam won't go straight up but will be forced to the sides - so what is directly above the lid won't be as exposed to steam...
/K
Next to the plastic, moisture will behave as normal, or more/less - but then those equations must be involved, and I suspect that the increase/decrease will be relatively small anyway.
In other words, the building plastic is a lid; if you boil a pot of water and hang a lid above it (with a gap), the steam won't go straight up but will be forced to the sides - so what is directly above the lid won't be as exposed to steam...
/K
I eventually chose to use plastic, as you can see in my project thread. http://www.byggahus.se/forum/socialt-projekt/220006-mitt-forsta-utbygge-tv-rum-och-sovrum.html
What I see is that there are water droplets under the plastic. But I can only see where I haven't added insulation yet. I don't know how it will be later when I've put insulation on top of the plastic and the temperature is the same on both sides of the plastic.
What I see is that there are water droplets under the plastic. But I can only see where I haven't added insulation yet. I don't know how it will be later when I've put insulation on top of the plastic and the temperature is the same on both sides of the plastic.
We have a basement with bedrock, the rock slopes with a difference of >30 cm. It's a bit damp on the ground, the builder suggests laying a Platon mat first and casting with EPS concrete to get an even surface. After that, you can lay foam insulation and then pour floor heating in self-leveling compound.
Theory: The small amount of water under the Platon mat should drain towards the walls and evaporate from there. The Platon mat should prevent moisture from rising.
Will the Platon mat hold?
Is it the right construction and not a risk construction?
What are your opinions or suggestions?
Theory: The small amount of water under the Platon mat should drain towards the walls and evaporate from there. The Platon mat should prevent moisture from rising.
Will the Platon mat hold?
Is it the right construction and not a risk construction?
What are your opinions or suggestions?