Slab on Grade Construction

Got another one for this team. A question in exam two asks about the role plastic sheeting plays when its placed under a slab (but on top of a layer of gravel). The purpose of this sheeting helps prevent water vapor from creeping into concrete. To quote fundamentals of building construction, chapter 14 casting a concrete slab on grade on pg 555, " a moisture barrier, usually consisting of a heavy plastic sheet, is laid over crushed stone, to prevent water vapor from rising through the slab from the ground beneath"

Based off this definition, why isn’t condensation included in the answer for the exam question?

@coachdarionziegler @coachryanutter @coachmarkbailey

@Shikha another good question! In my professional experience, the term moisture barrier generally refers to a foundation / slab condition. Anything that occurs below grade. Once above grade, the term becomes either vapor retarder or vapor barrier. The term vapor directly alludes to designing around condensation and the passage of moisture from the air.

In my practice, we specify two different types of products for both of these conditions. A “below slab moisture barrier” is intended to prevent rising groundwater from seeping up into the slab after a gully-washer of a storm. The vapor barrier and vapor retarder are either a liquid applied product, or a similar “house wrap” style product that are generally less mils than the underslab moisture barrier.


Makes sense! However, how does a below grade slab moisture barrier prevent heat flow?

1 Like

Hi @Shika !
To be clearer, a below grade moisture barrier does not prevent heat flow. If there is the need to prevent heat flow, underslab and perimeter rigid insulation board must be installed. For example, if you install perimeter insulation board (down to the frost line) around your entire building, you are insulated under that slab. The moisture barrier would then prevent ground swells of water or moisture from infiltrating the slab and causing degredation.

1 Like

Ok sounds good. I’d ask for you guys to take a look at that question on Exam form 2 for PDD.

1 Like

Hi @Shikha ,

Just to expand on Mark’s explanation, ‘condensation’ isn’t an answer on the exam because condensation occurs when water condenses in the air due to humidity and temperature.

The under slab moisture barrier doesn’t stop condensation (which occurs in the air), it prevents water from being being forced through the concrete through hydrostatic pressure.

Check out this resource I found!
Hydrostatic Pressure & Moisture Vapor(called%20hydrostatic%20head,the%20pores%20in%20the%20concrete.

Hope this helps!

1 Like

I can see why condensation is incorrect. What I’m not necessarily grasping is that the answer is claiming that the moisture barrier helps with heat flow, which is contradictory to what @coachmarkbailey is saying.

I’m so glad you guys respond! One of my favorite features of black spectacles.