Underslab Myths
Unfortunately, this industry is filled with misinformation. Most of it from benevolent ignorance and hopefully very little from malevolent intentions. Scroll to see them all or or click a link below to jump right to a specific section.
Always specify a minimum thickness
Vapor drive is constant in every building
Vapor drive is a function of indoor/underslab RH and temps
Insulation does not affect vapor drive
Installation accessories are all the same
You Need a Vapor "Barrier"
There is a bit to unpack here. Let's define which version we are talking about. There are three:
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Standards-based
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Industry coined version
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Dictionary version
Standards like ASHRAE, ASTM, ANSI, etc. define the lowest level of permeance / highest level of protection to be 0.1 U.S. Perms.
The industry (manufacturer's selling low-perm products, define a barrier as 0.01 U.S. Perms.
The dictionary would say a barrier let's NOTHING through.
So, like Santa Claus, the dictionary version doesn't exist.
The manufacturer's version has an agenda to sell over-engineered products.
The standards-based version of 0.1 Perms is a pretty good number, but can be adjusted depending on the rest of your slab design and building scenario.
So what do you need? Typically 0.1 Perms is just fine. Specifically for your project? Let Hygrothermix help you design the right-sized solution.
Always specify a minimum thickness
Minimum thickness specs made sense when every material was the same. If for example your spec called for basic LDPE, it would then make sense to call out a minimum thickness. That thickness would then translate to specific material properties.
However, technology has changed quite a bit since the 1980's. We now have new resins constantly being developed. Instead of minimum thicknesses, we should instead specify specific properties and allow the manufacturers to innovate and create thinner, value-engineered products that meet those requirements. This is exactly why ASTM E1745 was created. Specify Class A of ASTM E1745 and let us manufacturers create the thinnest and most cost effective product that satisfies that standard and the rigors of underslab installation along with in-service environments.
Installation accessories are all the same
Be warned- we arguably have an agenda here as we design/manufature and sell our own accessories. We do this because it lowers your cost and becuase we understand the importance of quality adhesives and sealants to your underslab installation.
The manufacturers in this industry don't make their own accessories and they beat up the actual manufacturers to keep their costs as low as possible. We would know as we have made (in the past) accessories for many of the industry leaders. The result of this "drive out cost" mentality is a constant cheapening of the products.
Since we create our own, we can both keep quality high and provide a competitive price. Additionally, we have chemistry-matched our adhesives to our TECKFilm membrane creating the strongest bonds in the industry. So no, our accessories are not like our competition. Our adhesives outperfrom the industry and seal/bond in all temperatures and nasty consttruction environments.
Installation is not important
Installation is crucial. Any location where a vapor retarder installation is not complete or correct, can result in localized 100% humidity regions within your slab. These "hotspots" can cause localized issues within and above your slab.
Insulation does not affect vapor drive
Insulation might be the most important factor in terms of the vapor drive an underslab membrane will be exposed to. This should not be overlooked as you design your hygrothermal protection. Hygrothermix can help choose the correct vapor retarder / insulation combo for your projects.
A 99% complete installation is 99% effective
The validity of this potential myth is dependent upon the rest of your slab system design and deserves a longer conversation. For the sake of complete and responsible installations, please don't use this faulty argument to not repair damaged areas or gaps in your next installation. Hit us up if you want to understand the nuances of this subject.
Vapor drive is constant in every building
Manufacturer's like to quote 70F and 40-50% RH inside and 55F and 100% under the slab to come up with the vapor drive through the membrane. This is false and not just becuase some buildings operate at different temperatures. The fact is that the use of insulation, the selection and size of the stone capillary break, and the degree of slab drying allowed has a lot to say about the values of temperature and humidity below, within, and above the slab.
ASTM E1643 is just an installation standard
Just look at the title and you'll see how much more than an installation standard ASTM E1643 is:
Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs
This is important as it sets the stage for the starting point of proper specifying of underslab membranes. Take a look at our "standards/how to specify" section to learn more.