The last Barn. (maybe)

[EddieWalker]

In my opinion, the term vapor barrier is misleading. People seem to think that water in the ground is turning into vapor somewhere under their building, and then that vapor works it's way up through the ground, and then through 4 inches or more of concrete. Then it turns into water and puddles up on top of the concrete.

Vapor barrier, or plastic, is important under a slab when pouring the concrete is the soil draws out the moisture in the mix while pouring the concrete. If your soil does not do this, there is no need for the plastic.

I see the same with house wrap. People seem to think it's to keep the rain out, but it's really to keep the wind out.

Purple primer for PVC is not really a primer like used in paint, it's the first part of a two part chemical treatment to melt the PVC together. The purple primer softens the PVC and then reacts to the clear cement to melt and weld the two pieces together.

Sealer on grout doesn't actually seal grout. It hardens it. There is a chemical reaction to the sealer that causes the grout to become harder then the tile, and locks the floor together. This is also why layout of tile is so important, once the grout is hardened, it can weaken long tiles if the grout line of neighboring tiles is centered on each other. This applies to cement based grouts. The new polymer type grouts do not use "sealer" to reach maximum hardness. Some sealers might have some stain blocking, but it's mostly useless and just a sales gimmick.

Names are used to sell products, and don't actually do what the name says they do.

 

[deezler]

For the low price of some basic plastic sheeting, I don't know why you wouldn't put it under your slabs, though.

Concrete is not perfectly impermeable. It can and does wick moisture. Why do people apply waterproofing on the outside of a poured basement wall? It's exactly the same with your slab. That being said, if you have perfect drainage away from your building/pad, your soil under the building shouldn't be a swamp that is trying to send moisture inside. So it might work out fine for you, sure, but for a couple hundred bucks of plastic.... peace of mind.

 

[ovrszd]

For the low price of some basic plastic sheeting, I don't know why you wouldn't put it under your slabs, though.

Concrete is not perfectly impermeable. It can and does wick moisture. Why do people apply waterproofing on the outside of a poured basement wall? It's exactly the same with your slab. That being said, if you have perfect drainage away from your building/pad, your soil under the building shouldn't be a swamp that is trying to send moisture inside. So it might work out fine for you, sure, but for a couple hundred bucks of plastic.... peace of mind.

If I have enough water under my slab to saturate it and come thru my concrete a plastic film is the least of my problems.

Of all the steps involved in pouring a slab, a plastic film is at the bottom of my list.

 

[EddieWalker]

For the low price of some basic plastic sheeting, I don't know why you wouldn't put it under your slabs, though.

Concrete is not perfectly impermeable. It can and does wick moisture. Why do people apply waterproofing on the outside of a poured basement wall? It's exactly the same with your slab. That being said, if you have perfect drainage away from your building/pad, your soil under the building shouldn't be a swamp that is trying to send moisture inside. So it might work out fine for you, sure, but for a couple hundred bucks of plastic.... peace of mind.

Anything below grade is going to have pressure that is forcing water into the building. This is not the same as a concrete slab that is above grade.

Wicking happens when the bottom of the concrete is in standing water. Not vapor coming up through the dirt. Common thickness of a concrete slab is 3 1/2 inches. If you put a piece of concrete that is 3 1/2 inches thick, in a one inch puddle of water, the top of the concrete will never get wet. You might see some moisture up the sides above the water line a little bit, but nowhere near the top.

I agree that plastic is cheap. It doesn't hurt anything if somebody wants to use it. I just don't think it accomplishes anything, and I have plenty of other things to spend money on.

 

[ponytug]

We have worked on buildings that had insufficient vapor barriers on the concrete floor slab. With exposed concrete or vinyl tile, it was not much of an issue, but epoxy lifted right off due to the moisture below. The buildings were all above grade, but the water table locally was high.

To @EddieWalker's point about mislabeled materials, polyethylene is not impervious to water vapor, it just slows it down. As an example, water moves through a LDPE film at the rate of 1-2gm/100 sq.in./24 hours/mil of thickness.
Water Vapor Transmission Rate

YMMV...

 

[EddieWalker]

I'm from Hayward California and I know a little bit about building in the East Bay. How long after the slab was poured did you have moisture issues?

Where was the building with the high water table? I worked on some houses in San Leandro that where built on landfill next to the bay. They where moving around so much that every water supply line and sewer drain line had to have a flexible section to deal with all of the movement from the high water table and unstable soil. None of those homes had any moisture issues on their slabs at that time, but I'm not aware of plastic being used under the slab. It wasn't common practice to do that back in the 90's

 

[ovrszd]

Anything below grade is going to have pressure that is forcing water into the building. This is not the same as a concrete slab that is above grade.

Wicking happens when the bottom of the concrete is in standing water. Not vapor coming up through the dirt. Common thickness of a concrete slab is 3 1/2 inches. If you put a piece of concrete that is 3 1/2 inches thick, in a one inch puddle of water, the top of the concrete will never get wet. You might see some moisture up the sides above the water line a little bit, but nowhere near the top.

I agree that plastic is cheap. It doesn't hurt anything if somebody wants to use it. I just don't think it accomplishes anything, and I have plenty of other things to spend money on.

Years ago I helped my BIL pour a slab inside a pole building. Outside temp around 50F. Had plastic under the slab. Started pouring it at 7 a.m. In an hour we had it poured. We could finally finish trowel it at 3 a.m. the next morning. What an ordeal.....

 

[cat fever]

It's been a week since the wood was delivered, it's all blocked up and in the barn. We are tired!
We can now split at our leisure.
This was the last log.

 

[EddieWalker]

That has to be the cleanest, most orderly stack of rounds that I've ever seen!!!

 

[cat fever]

That has to be the cleanest, most orderly stack of rounds that I've ever seen!!!

I can't take credit for it, my bride did all of that. She's a trooper when it comes to hard work.