2x8 over concrete slab new construction
Last Post 26 Sep 2019 10:47 AM by newbostonconst. 12 Replies.
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nate848User is Offline
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27 Jan 2018 03:52 PM
Hello,

Build a new 1500 sqft home in PA slab on grade home. I do not want cold or hard floors so this is what I was thinking. 10mil vapor barrier and 2" foam under the slab. Then over the slab use 2x8 or mabay 2x6s to raise it up (acting like sleepers) and get some insulation above the slab, fiberglass or blowin and then put the plywood over it so I would not have a hard or cold floor. Also was thinking mabay frame the house and them go back and put the 2x8 or 2x6 in so I can get good insulation in the corners of the house. It will be a very well insulated tight house. My architect is worried about getting mold if I put wood over the slab. I will also be using a heat pump to heat the home.

What are your thoughts or what do you recommenced? I do not want radiant heat since I will still have a hard floor and its not cheap to have an electric boiler or something to heat the water.

Thanks

jonrUser is Offline
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27 Jan 2018 04:55 PM

Perhaps you should just build with a sealed crawlspace. This makes wiring, plumbing and ducting easier.
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27 Jan 2018 05:13 PM
I don’t have any first-hand personal experience with the floor assembly you are considering. However, I will pass along a couple comments and clarifications that you might find helpful and perhaps want to consider.

One normally only uses PT lumber when wood comes in direct contact with concrete. I think the bigger issue is creating an organic floor cavity that is unvented and could become moist and moldy. Sealing a cavity to absolutely ensure it will not see moisture can be very challenging. Maybe it is more appropriate to place the vapor barrier on the top surface of the concrete slab if you do this.

I personally prefer permanent and solid concrete slab floors over wood floor creaky and flexible structures. You can polish or stamp the concrete. You can easily finish a raw concrete slab with carpet, engineered wood flooring, and tile. One doesn’t normally stand for long periods of time in a home and carpet/rugs can be used in those areas where you desire more softness.

HR radiant heating can be as efficient or more efficient than any other heating approach. There are very efficient and expensive heat pump systems that can be used for HR heating if the ROI makes any sense. Perhaps more importantly, a small 1500 sf home that is well sealed and insulated is going to have low heat load and will be relatively cheap to heat no matter how you do it.
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nate848User is Offline
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27 Jan 2018 06:41 PM
what if you put 2" rigid foam xps over the slab and then 2x6s. The rigid foam plus the 10 mil vapor barrier under the slab should stop any water or vapor from coming up through? I would still need to drill though the foam to attach it to the slab if its over the slab but could tape over all screw holes

I need ac so heat pump is alot less since I need duct work.

thanks

DilettanteUser is Offline
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27 Jan 2018 07:11 PM
1: You don't want to put fiberglass or cellulose directly on concrete due to lack of moisture control on the slab.

2: Put down a moisture barrier panel system like DRICore or Amdry. Put down rigid foam board above that. Then your flooring of choice.
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28 Jan 2018 07:03 PM
Putting insulation above the slab isolates the conditioned space from the beneficial thermal mass of the slab. Adding insulation above the already-insulated slab is likely to increase heating/cooling energy use. Getting the slab-edge insulation fully up to snuff (or higher R than the code-min R10 , 2' deep for most of PA) would be better, and likely cheaper.

XPS is the least environmentally friendly foam insulation in common use, due to the HFC blowing agents, which have extremely high global warming potential. As the HFCs diffuse out the thermal performance falls from the labeled R5/inch, to about R4.2/inch at full depletion, the same as EPS of similar density. EPS is blown with hydrocarbons (usually pentane at about 7X CO2 rather than >1000x CO2), most of which is out of the foam and recaptured while still at the factory, often burned for process heat, and has no effect on the short or long term performance of the foam.

Using 2" of Type-II (1.5lbs per cubic foot) or Type-VIII (1.25lbs ) EPS under the slab would be R8.4, now, and R8.4 in 50-100 years. With virgin-stock foam 2" of EPS would still be financially rational in your climate, but if you went with reclaimed roofing EPS (usually Type-VIII, sometimes Type-II) even doubling to 4" (R16.8) that might still make financial sense, the foam would still be half to 2/3 the cost of 2" of virgin stock EPS, and LESS than half the cost of 2" virgin stock XPS. Used foam is available from several vendors in PA. Any grade of EPS or used XPS is fine to use under the slab, but not the footing. Do NOT use polyisocyanurate foam under the slab or for slab-edge insulation, since it can wick ground moisture, and would perform at only about half the rated R when waterlogged. In super-saturated soil EPS can lose as much as 15-20% of it's performance when wet, but unlike polyiso it gives up moisture readily when the tide goes out. (Polyiso is fine for above-grade insulating sheathing, or basement/cralwlspace interiors, as long as the edge doesn't rest on soil or slab.)

Pouring the stemwalls in a minimal 2"+ 2" EPS insulated concrete form would get you to R16.8, well above the code-min R10. Some ICF vendors start at 2.25" + 2.25", which would be even higher.

Installing a standard subfloor on 1x3 sleepers 16" o.c. would have the same amount of "give" as on a typical joisted floor, and wouldn't isolate the thermal mass of the slab from the conditioned space interior.

greentreeUser is Offline
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28 Jan 2018 08:45 PM
Ive built what you are asking about and have torn into some as well. They are actually pretty common in garage conversions.

You cant lay wood flat, especially any 1x because any variations in the slab will give when you step on it, and it will be a wavy hot mess. The slab will absolutely not be dead flat, it just wont.

Put any depth white wood 2x on edge like floor joists and if its not span rated, like you want to use 2x4 or 2x6 shim with rips of pressure treated plywood on the slab and shims to get the floor flat. The tricky part is what to do between your sleepers and slab. I have done poly sheeting over the slab, probably the most common, the ones Ive ripped into have had poly over and no poly and none have had any moisture issues, not saying that cant happen. Most of them are filled with fiberglass kraft faced batts stapled to top of joists or just batts basically laying on the sheet of poly, Im not advocating, Im just saying that is what is out there and not rotting.

In your case since its new and you should have a good draining base under your slab plus put 2-4" of subslab foam with a vapor barrier the slab should stay dry as long as the slab edges are insulated as well. I would have a way to monitor humidty just in case, since this a bit unconventional and a plan to dry it out if need be. If you build first then install, you will need to have a perimeter rim or blocking to seperate the floor system from the walls, blocking will be easier.

I also framed one over a garage with I joists in hangers, maybe 2' off the slab. The architect had a sheet of poly over existing slab, not sealed just laying there and a blown fiberglass fill which was about 3' deep. I was just framing and finish so I didnt sweat it but wonder how that one is holding up.
nate848User is Offline
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30 Jan 2018 11:36 PM
thanks for the great info and advice
newbostonconstUser is Offline
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31 Jan 2018 11:44 AM
I agree with sailawayrb build a solid radiant floor and use rugs where you stand a lot. Much simpler/safer then what you are trying to do. The old Kiss principal.

I do wish you luck with what you are doing, building is great/tough adventure. Also being that it may not be a normal thing for the building inspector to see you might have a hard time with them. That could cost you time and money.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
ModernHausUser is Offline
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14 Nov 2018 05:51 PM
I'd suggest the following (Ground, up):

1) 6" Stone
2) 4" EPS (Not XPS)
3) 10 mil Poly (Minimum), sides brought up 18" on the wall and secured (Creating a bathtub effect).
4) Slab

If you're paranoid about moisture and mold, I'd suggest building a lip around the perimeter of the foundation to span LSL's across, on which you can build your subfloor / flooring. This will allow airflow. That is, if you have a few extra bucks to spare. Sure, you can also use lumber (Treated when in contact with concrete) but as others have noted, creaking / uneven flooring is the worst (One of my pet peeves). No need to insulate since you have installed 4" of EPS under the slab. Also, be sure to 'connect' the EPS under the slab with EPS on your wall to create a sealed envelope.

One thing I suggest that you do not do, is, place Poly under the slab and then place another vapor barrier above the slab; moisture will have nowhere to go.
MildredCookUser is Offline
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14 Sep 2019 11:02 AM
For the work of asphalt paving and the different construction of the foundation, always try to give the contract to the company who has the experience of the work related to the construction and the repair of the pavements. I heard about the paving contractors Queens provides the best and effective services of the asphalt paving and its repair. One can contact them and check for the different services according to your need and requirements.
alulkoUser is Offline
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26 Sep 2019 03:18 AM
I thought in PA you need a 3 foot foundation. You can build on a slab legally?
newbostonconstUser is Offline
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26 Sep 2019 10:47 AM
Posted By alulko on 26 Sep 2019 03:18 AM
I thought in PA you need a 3 foot foundation. You can build on a slab legally?


With the new code there are insulated foundations that are now allowed. 12 inch deep or more foundations are allowed if designed correctly.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
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