Slab on Grade / Substructure Construction

I’m looking at question 22 of practice exam 2 for PDD. The question gives a scenario of a slab-on-grade poured over plastic sheeting (vapor retarder) and gravel, as well as waterproofed basement walls.

The purpose of the gravel is to keep moisture away from the underside of the slab (to prevent capillary rise of groundwater). The plastic sheeting additionally acts to keep moisture away from the slab. The layer of gravel and plastic sheeting will also help with radon control. The waterproofing on the basement walls serves to prevent the passage of water through the wall assembly. All of these points are made quite clearly in the reference noted (Fundamentals of Building Construction, 6th Edition, pp 66-73, 567).

However, I cannot find any mention of a gravel layer, plastic sheeting, or waterproofing helping with heat flow. Whether under slab or in a basement wall assembly, thermal insulation would be used to prevent heat flow.

Can you clarify how the assemblies noted in the question would help control heat flow, and possibly point me to the chapter or pages in textbook where I could read more about it?

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@coachchrishopstock can you please take a look into this question and see if you can help @Smith.Marks out?


Hi @Smith.Marks thanks for writing in!
This one is definitely overly tricky and we’re going to replace it. The assembly described (gravel, vapor barrier, and waterproofing on a concrete foundation) is not generally deemed to prevent heat flow - I agree with you that insulation would need to be added in order to achieve heat flow reduction.
This ‘check two’ question is technically correct because, of the 6 options given, heat flow and radon mitigation are the only two that are mitigated in any sense by this assembly. Since each component has a very low, nominal R-value, technically the assembly as a whole does reduce heat flow through the wall. Again - that’s an overly tricky interpretation so we will replace it with a better question.

Thanks again for bringing this to my attention and good luck on your next exam!

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