Basement floor assembly question
Hello GBA,
There will be a bathroom in the basement and the design I’ve settled on is shown in the “Bathroom Floor” picture. I’m trying to minimize any threshold height differences, and have two assemblies I’m trying to decide between for the rest of the basement. Both Option A and B get me within 1/32″ of the bathroom floor, which should be fine. The main difference between the two options is A uses 1/2″ rigid foam and two layers of 19/32″ OSB (similar to the “Stay-Dry, No-Mold Finished Basement” article). Option B increases the foam thickness to 1″ but uses a single layer of 23/32 OSB.
Does anyone have thoughts on a preferred approach? Cost-wise they are very similar. Option B has thicker foam which should help with thermal efficiency a small amount and is less layers, so less labor. I believe the main purpose of the double layer of sub-floor in option A is to prevent cupping, which could be a concern with only a single layer of ~3/4″ OSB?
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So, doing some more research it looks like the single layer of OSB is endorsed in some areas by FHB/GBA. The Steve Baczek article “Build a Risk-Free Finished Basement” shows a single layer of subfloor in the low risk scenario. Additionally, user yacpro13 reported good success with option B approach in https://www.greenbuildingadvisor.com/question/basement-subfloor-assembly-details#comment-139296
Looks like my main issue is going to be securing the OSB to the concrete. Tapcons are not universally loved, so I'll have to decide it its worth the cost of going with something better, like GRK or CobraTap.
Any thoughts or experience from the community on this approach?
Thanks,
--Jamie
Cupping and potato-chipping is indeed an issue with a single layer of subfloor, even 3/4" goods.
A dual-layer of half-inch inch CDX or OSB glued & screwed to each other with seams overlapped by a foot or more can be floated, and doesn't need to be screwed to the slab.
Thanks Dana. I've never seen the floated floor approach mentioned. Do you have any articles or good posts that talk about it?
I've floated a single layer of 3/4 OSB above grade. No issues provided the surface underneath is flat.
I don't think I would float anything in a basement though. Water leaks will always happen, you want the assembly to handle at least a small flood.
I would go for a single layer and fasten it down.
Thanks Akos. Dana sent me down a rabbit hole with the floating floor suggestion :) so I've been doing some reading. Sounds like for LVP/T, carpet, and laminate the floating floor might be a good approach. I do admit it kind of kills me to be poking holes in my foam by screwing the OSB down. I have a poly layer underneath the slab, so I'm not worried about puncturing the vapor barrier, but I would think each one of those screws are little thermal bridges.
However, I'm looking to use an engineered hardwood as my finished floor, and it looks like the floating approach could be an issue for that finish. I would guess it depends on if you glue down the engineered hardwood or not. There are products you can float, but then you would have a floating finished floor over a floating subfloor...
--Jamie
There is no problems with engineered click in the basement over a proper setup (VB, concrete, foam, subfloor). Just watch out for length and width run limits as some are limited to 20' rooms. This is sometime hard to find but very important when floating a large surface.
I personally like LVT, it is one of the best finishes in a basement.