Basement floor: What thickness OSB?
Questions regarding a basement flooring system. General description on build is below the question(s).
Want to install a sub-floor consisting of 1” rigid foam board (EPS) and 2 layers of OSB (opposite directions, two layers screwed together, etc), but have the whole system ‘floating’; that is, I want to avoid attaching the OSB to concrete slab with Tapcons.
I have read a decent amount of posts here and elsewhere that have nuanced descriptions of this concept. Ity is clear that if you are going with 1 layer of OSB an(*and Tapcons), then you use ¾”. But when using 2 layers, the OSB thickness is not always mentioned.
a) can I get by using 2 layers of 5/8” OSB, or do I need to use two layers of ¾”.
b) t/g OSB? Or leave a gap between the edges for expansion (I read that in one post)
For the two layers and my sq-ft, I am looking at ca. 90 sheets (total). So there’s some cost savings to go with the 5/8” if (IF) it is sufficient.
https://www.greenbuildingadvisor.com/article/planning-and-prepping-for-basement-flooring (option2)
In addition, I also plan on building some interior walls (non-load-beariung), and the bottom plates would sit on top of (and be screwed to the OSB layer, but not all the way into the concrete). I read one place (that now I cannot find to cite) that you should attach the bottom plates directly to the concrete. But that was one article whereas all others I have seen suggest I can just screw into the two layers of OSB. Som I would love some opinion on this too.
General: New construction. Zone 6b. Walk-out basement. 3 walls ‘fully’ below grade. Concrete slab with R10 (2” foam board under total slab,) and a PE radon barrier. About 1500 sq-ft.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
m_elo,
I don't have a definitive answer, but in blogs detailing their projects Randy Williams, Christi Weber, and Josh Salinger all used 3/4" plywood.
T&G subfloors are used to span framing where you don’t have support below. Over a solid substrate, especially when there will be two layers, I don’t think it’s necessary.
Non-load bearing walls are fine just attached to the subfloor
Thank you Malcolm.
Seems like a lot of trouble for a slab that will already be insulated. Why not just go with dimple mat and LVL?
NYNick. Its a thought. We have not decided on a flooring yet. By LVL, I assume you mean luxury vinyl (flooring). If we decide to go that route, the dimple mat might be enough. The biggest thing now is the home is still quite under-construction and with all the supplies in the basement now, it is difficult to assess how flat the floor is (and hence how much bridging across inconsistencies I might need to make if we go with less flooring that would telegraph the slab inconsistencies more). The double-layer of OSB would seemingly help with some inconsistencies.
My very old basement slab (80-100 years?) is uninsulated and a little irregular. Only the new foundation floors have 2" of foam, but all the foundation walls were spray foamed.
I was a little concerned that the irregularity of the old slab would affect the installation of the luxury vinyl. I even considered self leveling concrete. But in the end I consulted with the flooring expert, who convinced me it would be okay.
I used DMX 1 Step as the dimple mat with a high quality luxury vinyl plank that looks like wide plank pine. The floor came out great.
I have a Minisplit down there to heat/cool the space. The minimum temperature you can set this unit is 60 degrees. That's just dumb, but it is what it is. As an experiment this winter I turned the minisplit off, left some sensors in the basement and went to Florida. The basement hasn't gone below 56 degrees since I've been away.
Unless there's another reason to install more sub-flooring (moisture, wood flooring, wiring etc), I'd just go with LVL. They make some really nice stuff these days.
Nick,
You are right. We kind of went off on a tangent thinking about this as a concrete-free slab, when it's really a question about what is an appropriate substrate for finished flooring over an insulated slab.
As far as I know, Andy Engel was the first to publish details for
floatinginstalling two layers of sheathing over foam, in this article: https://www.finehomebuilding.com/2005/03/01/the-no-mold-finished-basement. (FHB has compressed the article down to nothing so I'm attaching the original version.) He used two layers of 1/2" plywood.The second published example is here: https://www.jlconline.com/how-to/foundations/a-basement-floor-without-concrete_o. Builder Steve Demetrick went with two layers of 3/4" Advantech.
Since then, most concrete-free slab designers and builders have used two overlapping layers of 3/4" Advantech.
The third publication was my design, using a single layer of 3/4" Advantech over 2x4 sleepers: https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home. Advantages include reduced carbon emissions and lower material cost. I know of a few others built using this system, but I'm trying the double-layer out on a current project.
I appreciate all the input!
Andy Engel's article was one I had, but my reading suggested he attached the first layer of 1/2" to the slab with Tapcons. I'll re-read it gain.
It seems that the key to the floating system is 'mass'. Using 3/4" Advantech in even one layer of a floating system would be ... well, an advantage. But double the price. Might come down to how my Enron stock is doing the day I call the lumber yard.
You are correct, he did fasten the first layer with screws. We had various conversations about it, public and private, and he thought it would also work as a floating floor. But 3/4" material would be safer. 5/8" would probably be fine. Or 1/2" over 3/4".
The advantage of concrete is that it's heavy enough to compress minor fluctuations in the foam layer. The important thing with any wood-slab type floor is to ensure the substrate is as level as possible. The sheets may eventually conform to dips and humps but not readily. So I would say that the key is having a level substrate.
I did hear from one builder who used "my" system but used pressure-treated 2x4s "to be safe." Some of the 2x4s twisted as they dried, resulting in some uneven areas. They eventually settled out but I recommend sticking with kiln-dried material to limit wood movement. If there is any risk of moisture at that level, something is wrong with the design.
As a complete aside, since you mentioned Enron: a former girlfriend worked for an energy-based company; her job was to travel to various places and set up offices that were fully ready for people to start working in. She didn't understand their business model, just that her job was to set up these offices. I even visited her at work once, 3/4 up a San Francisco skyscraper. Of course the company was Enron. (She left for public sector work before they crashed and burned.)
Is this new construction?
The way I see it basement floors seem to find a way to get wet and putting stuff like wood and was wood AKA OSB on the floor is a very big bet that the floor will never find a way to get wet as all that stuff simple must go to the land fill if it does get wet.
You already have insulation under the slab so the floor should not feel cold. Consider staining the concrete or some LVT that can handle any water that may come.
Walta