Practice exam question - slab on grade capillary break
How can you have a capillary break under a slab? The answer they give suggests the slab is hovering in the air. Also could have sworn the Amber Book said you can't have air gaps under a slab... or am I missing something?
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Official comment
Based on the comments here and other feedback we've received, we've flagged this item for review and revision by our volunteer item writers to be updated and released on a future version of the practice exam.
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In essence it's a vapor barrier, I think the wording is akward but capillary movement equates to water withch translates to a vapor barrier which goes between the under slab insulation and the slab itself in cold climates. Might be a case of trying not to use a word or phrase that doesn't lead you to an answer directly but at the same time making you think about the concepts of what they are asking. Also, there is no air break here, it's just graphichs so you can clearly see what they are trying to show. If you try and draw a vapor barrier in a detail and you don't seperate it from the adjacent solid lines, it will get lost when printed, then reprinted, then copied, ect.
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I just want to say; option A is pretty much the only way i've ever physically seen slabs produced. imagine this as a constructability problem as well. usually the plastic barrier is the absolute last thing there; and they fill it with the slab ontop.
That being said; I think you're missing/messing up the term used. "capillary movement should be reduced" - this is done by the gravel. they're basically saying it just needs gravel to separate it from soil. Soil is notoriously wet... it sucks up water and if it's touching anything the water is going to move though it quickly.
And. i absolutely agree with you. a capillary brake is a poor term, they should be using the term weather barrier; water barrier; or waterproofing, or even the term "membrane" is much more "correct" than the term they used. This exam has some pretty serious errors when it comes to terminology and regional use. The writers try to make it the most generic phrasing possible. but it just comes across as unprofessional to me.
I guess the graphic give away would be generally speaking; a broken line represents waterproofing or membrane layers in most graphic conventions. that's what made it pop out to my eyes immediately
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Sorry for being late to comment here: but A is pretty much how all slab on grades everywhere are constructed (minus the insulation in warm climates) so it's safe to just memorize this layering and call it a day. This will hold for all conditioned buildings.
(As an aside, there are only two exceptions I can think of, one common and the other very uncommon: (1) Unconditioned garages. These might not be insulated, and they might not have a vapor barrier (poly) or capillary break (crushed stone) under the slab -- garage uses are often considered not worth the trouble. (2) Ice skating rinks. There are a bunch of different ways of designing cold buildings, but if the ice is right over the concrete slab, the poly vapor barrier will need to be *under* the insulation, not over it. But apart from these exceptions, in all other contexts the vapor barrier under a slab will ALWAYS be in direct contact with the underside of the slab. Like I said, it's pretty safe to just memorize the assembly shown in "A".)
Anyway! Where this gets confusing is the terminology. Often people use the terms "vapor barrier" and "capillary break" interchangeably, but they're not quite the same thing. A vapor barrier prevents the transmission of water in the vapor form via molecular diffusion. A capillary break prevents liquid water from wicking through the pores of a material (think of dipping the edge of a paper towel into a cup of water).
In this example (and in real life) we use polyethylene sheeting under slabs. We often (usually) also include a bed of crushed stone under the polyethylene. The polyethylene is both a capillary break AND a vapor barrier. The crushed stone is a capillary break (because the spaces ("pores") between the stones are too big for the wicking effect to occur). Why, then, do we have two capillary breaks under slabs? Couldn't we just specify the poly without the crushed stone and call it a day? Yes! But the crushed stone is not terribly expensive and gives us more forgiveness in the assembly. Plus in addition to being a capillary break, it also acts like a big pad that redistributes water under a slab making it less likely we'll have water problems.
Another part of the question that's a bit confusing is the stated desire to include a "thermal break" under the slab. The insulation *is* a thermal break in that all insulation provides a thermal break, but I've literally never heard it referred to that way. We just call it insulation. The term "thermal break" is usually applied to systems where it's hard to make the insulation continuous like the frames of glazing systems or metal cladding attachments. In these cases we still call the insulation insulation and we call the small spacers or gaskets or "thingies" that provide small interruptions in otherwise continuous attachments "thermal breaks". So a thermally broken storefront glazing system will have little rubber or plastic spacers installed in between the frames separating the inside part of the frame from the outside part of the frame to reduce heat transfer. (Btw: the thermal breaks can be excellent... or not super terrific. Just because a system has them doesn't necessarily make it "good" or "efficient". In other words, sometimes a thermal break can make a glazing system or cladding attachment system a lot better, and sometimes it makes the system only a very little bit better.)
Christine Williamson
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Oh! One more thing: it only looks like there’s an air gap under the slab in A. It’s just the graphic representation. (If there’s ever an intentional air gap in an assembly it will be called out). In reality, the insulation will sit right on the crushed stones, the poly vapor barrier (dashed line) will sit right over the insulation, and the slab will be in direct contact with the poly. In fact, the slab usually sticks to the poly. The question never specifically says the dashed line is poly - it calls it a capillary break (see my comments above) - but with very rare exception, it’s polyethylene. To get these types of slab questions right, you’ll want to know the layering of 99 percent of slab on grade assemblies (which is what’s represented in A, and the warm climate variation is exactly the same layering but without the insulation) and you’ll want to know that the poly is both a vapor barrier and capillary break and the crushed stone is a capillary break, too.
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This question really got me confused. I was just about to create a new post about it and this popped up. So if we are comparing option A and B both of them contain the crushed stones which is an essential here but why would the configuration of option A be better than option B?
Thanks
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The question depends on knowing that what NCARB has identified as a "capillary break" (the dashed line) will actually be a polyethylene vapor barrier. This is not incorrect -- polyethylene vapor barriers do indeed act as capillary breaks -- but it is misleading in that it's not the term we use most frequently in professional practice for that material. And, in fact, the crushed stone is also a capillary break. What we're using the poly for is vapor control. So, again, not a good question. And perhaps a bit unfair, too.
As to why A is better than B: we don't want to include a polyethylene vapor barrier on the underside of the insulation because the insulation will not be able to dry down. (You'll notice that this also puts the vapor barrier on the cold side of the insulation). Nothing bad will happen to the building if you built B instead of A, but the insulation would lose maybe 25 percent of its thermal effectiveness from being wet. So A is more efficient and more comfortable than B.
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We can use the technique of exclusion here: Answer B is not a good choice because the gravels can damage the moisture barrier when placed next to it. Answer C is not as good as Answer A because sand tends to trap moisture, while gravels are ideal for drainage.
Gang Chen, Author, AIA, LEED AP BD+C (GreenExamEducation.com)
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Yes make sense. But the word “rain” in the question is very distracting because it make me think that I want to protect my rigid insulation from the saturated earth underneath. But now that I’m saying this to myself I would answer me back that the question is also asking about thermal break rather than waterproof.
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Hi Yvette - I didn’t see your image before.
Where you write “is this the slab?”, yes, that’s the slab. What’s under it in the image you attached is a bed of sand. Sometimes this is done in California. The reason they do it is to keep the slab more evenly wet (top and bottom) in the days after the slab is placed to control slab curl. The poly is located under the sand. This is NOT a good way of constructing slabs. The polyethylene is effectively useless when it’s separated from the slab like that. It will do nothing to prevent water transfer through the slab via either capillary transfer or vapor diffusion (because the sand can and does still get wet.) So while it is true that the bed of sand helps to control slab curl, there are other ways of controlling slab curl that still allow us to put the poly in the best location for water control during the life of the building (which is nearly always in direct contact with the slab).
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As of 2/24/2025 this question is still as poorly written (and explained by NCARB) as it was when this post first went up. The vapor barrier goes on the warm side of the insulation in a cold climate. Under a slab, the VB also acts as a capillary break preventing ground moisture from wicking up through the slab. Purely as a capillary break, it could be under the insulation, but that's not the place for a sheet of plastic in a cold climate. The fact that NCARB explains the answer by saying "to prevent rain water from being trapped between the insulation and the slab during construction" is baffling. We don't permanently install plastic membranes under concrete slabs for temporary conditions like rain during construction. As for the comments above about insulation being compromised from getting wet...it's rigid insulation, moisture is not a concern. And to the comment saying the insulation protects the vapor barrier from the gravel, well that's true, but that's not why the foam is under the plastic sheet.
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Chrislaggis: As of 06/04/2026 it is still a poorly written question. Even asking experienced architects here in my firm they were baffled by the response. If I come across this, I'll take your advice that depending on cold climate conditions, I'll put the break on the warm side of the insulation (A). In a hot condition, I'll do the opposite (B)
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Adam B it's baffling isn't it - buildingscience.com has some great details and whitepapers to go in-depth about this even beyond what Christine does above (she knows the BSC folks very well), but they're building scientists - NCARB, for whatever reason (I'm almost certain it's because the ARE is testing more or less to the standard of care, which is the average level of service you'd expect from an average licensed architect), only ever consults licensed architects for technical correctness of ARE items - not mechanical engineers for HVAC items or building scientists for these kinds of technical performance questions, etc. That said, knowing that fact is a help on the exams - when uncertain, as is the case here, the correct answer is the one that the average US licensed architect is more likely to think is correct - not necessarily the most technically correct one. Fortunately, this problem is rare enough that it shouldn't impact your ability to pass - even if you occasionally give an answer that is more correct than the "standard of care" option NCARB's volunteer architects thought was correct.
Note that the correct answer has the keynote numbers in numeric sequence, neither of the elements in B or C are. This is exactly the kind of bad question that the PhD psychometricians that make the ARE legally defensible as a professional gatekeeping exam have a hard time filtering out through statistical analysis - people who are generally likely to pass the ARE are also generally likely to have the deductive reasoning skills that make this question easy for them, while people who are generally likely to struggle with the ARE are generally likely to not answer this question correctly. Sometimes the answer really is that obvious - for completely non-technical reasons.
Best,
Ralph, the Amber Book Team
yep, makes sense:) I have to get out of my Floridian brain to see this in the question without you saying it:)Please sign in to leave a comment.
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