Concrete-free slab – Mesh under Advantech subfloor?
Some background: I’m building a small addition in Maine, Zone 6A. The floor will be concrete-free, with layers as follows (from bottom to top):
*compacted stone
*geotextile fabric
*Glavel (~12″)
*geotextile
*screed layer
*stego 15mil VB
***mesh??
*2 layers of Advantech
*finish flooring
I’ve read/watched most of the various articles/videos for concrete-free slab construction. There was one video in particular that used a plastic mesh product under the Advantech. I like this approach, however, I’m not sure which product would be suitable, and have not been able to re-find the video. I’m hoping that someone here might be able to recommend a product or steer me to the video.
I’m interested in adding the mesh for 2 reasons: If any moisture accumulates in the assembly it would not sit directly against the Advantech, and would have improved drying potential. Also, I’m considering adding a small access panel (in pantry) to allow pumping in the event there was ever flooding (plumbing leak above floor).
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Replies
Steve, do you recall what the mesh was? There are only a few people sharing details of their concrete-free slabs: Josh Salinger, Randy Williams, Steve Demetrick and Steve Baczek, Elizabeth DiSalvo, and me are the ones I'm aware of. If there's more, that would be great but I don't know of anyone. I know that Nat May has used them as well but he hasn't written about them. Do you recall who the video was by or about?
Thanks Michael. Unfortunately, I really don't recall what it was or who it was by, it might have been 6+ months ago that I watched it. It's even possible that it was a video for floating a subfloor over a slab, but I'm somewhat certain (within a few IPAs ;-) that it was for a concrete-free assembly. I'll do some more searching.
Steve, I was thinking that "mesh" was a 2-D material, but based on other responses it sounds like it's a 3-D product? I understand the desire to keep the Advantech out of potential water, but there shouldn't be water on the poly. I can also imagine it as an attempt to level out uneven surfaces, but like Josh and Malcolm wrote, it's probably too squishy to be a good idea.
Michael- I believe it was a 3-D product, but I'm not certain. It would need to be to allow any transfer horizontally. If I ever find the elusive video I'll post a link.
Do you happen to remember what color it was? Different manufacturers brand their products with distinctive colors so that might be a useful clue. And was it a random filament style like this:
https://www.homedepot.com/p/39-in-X-61-5-ft-Roll-Roof-Ventilation-Mat-CEDAIR/311222526
or a more structured mesh like this:
https://plasticomponents.com/product-category/ultra-lath/
Hi Charlie- I'm not certain of the color. Thanks for the links- some of the products from the plasticomponents site look interesting.
Hi Steve,
We did a bunch of video work for Fine Homebuilding on a concrete-free slab about a year ago, but I don't think it has been edited/published yet. I could be wrong, though. In this particular home we used a weed barrier mesh between the 3/4- no fines gravel and the 1/4- screed layer. I doubt this is what you saw, though.
FWIW, the mesh you are considering wouldn't be necessary as the Stego is a Class 1 vapor barrier and the Glavel will be your capillary break, taking care of both liquid and gas phase water. The Advantech will dry out to the interior of the home via diffusion if it is placed directly on the Stego. I don't think it would hurt anything, but I'm skeptical it would add any practical benefit. I imagine the thinking is that a small air cavity will allow for drying or give enough of a gap that liquid phase water won't touch the Advantech... Either way it would dry via diffusion as this wouldn't be enough of a gap (and it is horizontal) to promote convective drying. If anything, the capillarity of the Advantech will spread out the water within the OSB and allow for drying to the interior. Again, I don't think it really hurts anything but your pocketbook to add it.
Josh Salinger
Hi Josh- Thank you for your feedback. Your videos/articles regarding concrete-free slabs have been an invaluable resource!
steve41,
As a general rule of thumb, it's not a good idea to create air-spaces in assemblies unless they have a good drying path. They can accumulate moisture where if they didn't exist none would occur.
My worry with the mesh is that the structure of concrete-less slabs relies on the subfloor layers bearing directly on perfectly level, compacted substrate. Introducing a layer of somewhat compressible plastic may interfere with that.
You're not thinking like a DriCore subfloor, are you?
DC- no this was a rolled mesh product. Perhaps similar to some of the thin wall rain screen products but with enough structural integrity to not crush within a floor assembly.
Thanks for your replies- Josh, Michael, Malcolm. Your points are well taken. I actually had the same concern regarding the compressibility of the mesh, either within the mesh itself or against the Stego. It sounds like my best bet is to stick with the proven system, i.e. without any "mesh".
Are there any reasonable flood mitigation options for this type of assembly? I'm concerned with a plumbing failure above the floor. It seems I'm working hard to make a nicely sealed swimming pool....but I never want to fill it.
Steve,
Thanks for the kind words!
Haha, yes-- we joke about it being a swimming pool, too. Except during the construction which is why we complete it after the building is dried in.
We have discussed putting in a drain and primer and connecting it to the poly for the rare flood occurrence, but haven't executed it yet. I don't see why it wouldn't work, but one wouldn't be able to create a slope to the drain. Floods are rare and easily dealt with. In my opinion the more dangerous situation is a slow leak that goes unnoticed over a long period of time. That being said, cutting out and replacing a section of plywood is way easier than cutting out and replacing concrete.
Steve Demetrick has a great story about how a CFSOG in a basement he built flooded and he was able to dry it out and the assembly was in perfectly fine shape. I believe this project has been going strong for 20 years, even with the flood (don't quote me on the timeline, its my best recollection).
Josh Salinger
The way swimming pools are drained is with a drain at the lowest point.
If you have perimeter drains you can leave a small gap so anything that gets under the floor ends up in the drain. Advantech is pretty water resistant. And basements are normally finished so that if they flood all the finished materials are ruined.