Concrete vs. Concrete-Free Slab Cost Comparison?
Hi, I’m working with an architect and builder on a one-level ~1800 sq’ PGH design for a site in Southern NH, climate zone 5A. We chose PGH because the budget isn’t huge, and we need to make intelligent tradeoffs to balance the energy and climate impact of the building with the cost. We can’t afford to check every ‘green’ box but we’d like to get the most green bang for our buck.
I know reducing concrete is a major way to reduce the carbon footprint, and concrete-free slabs are attractive for that reason. But if this method costs, say, 3x that of a concrete slab, then the slab will win.
My question is, in 2023, what are the rough relative costs of a slab on grade (with the polished slab as the finished floor) vs. a concrete-free slab, with a wood finished floor?
I realize there are tons of variables here, including labor and materials costs, as well as (for the concrete-free slab) the choice of finished flooring, the exact design of the house, etc., but I’m hoping to learn if the answer is one of the following:
a) Polished concrete slab is usually/always cheaper
b) Concrete-free slab is usually/always cheaper
c) Could go either way, major factors swaying one way or the other are X, Y, Z.
I suspect the answer is choice a, based on a comment to this article: https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home
Where Michael Maines states
“I don’t have specific cost information but when I present this idea to people, I typically say that if you want the least-expensive system, go with an exposed concrete floor–just use pozzolans to replace some of the Portland cement. But if you require a wood or tiled floor, consider skipping the concrete, as it’s just not necessary”
That was back in 2019 however, and I’m wondering if that’s still true. Thanks.
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Replies
I too was intrigued by the concrete free slab and dug into it deeper. In Northeast Ohio CZ 5A, considering current (January 2023) concrete and lumber prices I found that they were pretty much neck and neck. The foam for the concrete free slab really adds up quickly. Although lumber has come down quite a bit, it adds up pretty quickly too when you are doing two layers, which I would definitely recommend. Then, factor In a high quality vapor barrier and tape/sealant and there isn't much savings and in my case was a little more than concrete, wire mesh, vapor barrier and a foam glass base. My cost estimate was based JUST on material cost as the labor for either option wasn't going to effect me.
If you are able to do the slab free floor yourself, then yes, it would definitely be cheaper than hiring someone to poor the slab. One thing I personally didn't care for about the slab free floor was getting the base perfectly level. I felt this would be extremely tedious and I wasn't sure how I felt about it. Another consideration was how potential future buyers may feel about it. Likely, they wouldn't know, but it does pose a potential issue as well. I can't speak to the cost comparison of a finished concrete floor compared to other flooring options, ie Vinyl,tile, wood, carpet ECT., But I would suspect that a polished concrete floor would be less as it doesn't require additional material, and the labor would be less than installing more traditional flooring. Lastly, I would use an online calculator to determine how much embedded carbon you would really be saving. I read a couple of articles that suggested the savings was minimal compared to a traditional slab. I personally did not make the calculations for my project but it's definitely something to consider. As always, talk to you AHJ about any non traditional building practice. In my experience, the AHJ is very leery of non traditional systems and typically will require an engineer to sign off. In the end, they don't want to be liable for something that isn't explicitly called out in the code. Good lucky with your project!
Thanks Bwiemels, that's good data. In my case labor cost does matter, since I won't be doing the work myself.
I have a hunch that finished flooring is a major component that will push the concrete-free approach to be more expensive, since it would run into the low 5 figures for a house this size.
Interesting about the possible "not much" carbon benefit.
You have a good point about the AHJ, this is a small town and they are... picky. So I'd need to plan for the expense of an engineer to go with a non-traditional approach.
I certainly would love to see more data from others / other points of view, but thank you for responding, you've provided very valuable information.
There are too many variables to say for sure. Polishing (aka grinding) adds cost over simply sealing the troweled concrete; I recently heard $6.50/sf as an estimate for polishing. A concrete-free slab with two layers of Advantech will cost more than one with one layer of Advantech over 2x4 sleepers. Wood flooring varies in cost.
Generally speaking, I'd say the least expensive is sealed, unpolished concrete; next is polished concrete; most expensive is a wood floor, whether built over concrete or a concrete-free slab. But there may be some overlap depending on the details.
The price for the entire job one way or another may also be different depending on where you are in the country (concrete slab vs. slabless). One thing that slabless is allowing me to do more easily in this 2-layer slabless design that I'm working on right now is to more easily match finish flooring thicknesses, for zero threshold doorways and transitions from wood to tile or other materials. Adjusting the thickness of the tile backer board on the floor is easy in this case as Schluter (and others?) provide boards in a variety of thicknesses. It seems to me that a staggered two-layer system will be more resilient under tile especially on the panel edges. (In my design: foamed glass, sand, Stego or similar, 1" XPS. On top of that Advantech and then either another layer of that or tile backer.) The foam could probably be left out and I'm still wondering about putting landscaping fabric between foamed glass and sand to stabilize the sand. Thoughts?
Any use of XPS is questionable from a “green”
standpoint. The GPW of XPS is horrific.
My understanding is any XPS sold in USA/Canada now has a tiny GWP.
I hope you'll forgive me, I was speaking in generalization. What I should have specified was that I'd use is the NGX foam, which has a reduced negative impact on the environment.
I am in the process of planning an ADU with these exact questions.
The only price difference is 2x subfloor and finished flooring vs concrete as finished floor.
Everything under "slab" is the same.
I have considered these options in the past. For me the decision point ended up being longevity; a properly done slab should never need care and outlast the house in just about every circumstances. One issue with a wood floor at grade is water.
Even if you are not in a flood risk area, there is always a risk of large amount of water ending up pooling at the lowest level of the house from a broken pipe, defective washing machine or water heater, fireman intervention, etc. With a concrete slab, it's just a matter of pushing the water out. With an inaccessible wood frame, dozens or hundreds of gallons of water on the floor could be a bigger issue.
Thanks everyone for the input, all good thoughts and some things I hadn't thought of.