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Our unfinished basement has a concrete floor and poured concrete walls. It has stayed completely dry all year for a decade since the house was built. I will be framing the exterior walls 1" from the concrete with 2" of closed-cell spray foam as insulation and moisture barrier. I plan to insulate the floor with 1" polystyrene foam-board glued to the cement, then 3/4" pressure treated sleepers screwed to the foam, then 3/4" subfloor screwed to the sleepers, then hardwood or laminate flooring on top of that.

Must the interior walls be framed first and nailed directly to the concrete floor, with the foam and subfloor built around them? Or can the foam, sleepers, and subfloor be put down first with the interior walls sitting on top of them? The latter would obviously be far easier as the foam, subfloor, and sleepers would not need to be cut around a very large number of walls in a small space. The difference could be a five day job becoming a five week job (I don't have a lot of free time to dedicate to this project each week).

I understand that nailing through the subfloor would not provide the same stability as nailing directly to concrete, but these are non load-bearing walls that will likely support little-to-no weight. Are there any other disadvantages, or things I need to look out for or address?

  • Even with foam insulation it would be best to anchor the walls with a ram set for a newer home if it is older the cement will break up and then it would be best to use Tapcon screws to anchor the wall. Don't want it to move if bumped. – Ed Beal Mar 11 '16 at 23:20
  • @EdBeal Thanks Ed. I have a powder nail gun (Ramset Cobra 0.27 cal) and would like to use it to attach the walls. However, it only accepts up to a 3" pin, and I'd need over 4" to get through a 2x4 footer and the 2.5" of subfloor/insulation. I was hoping that having the header solidly attached to the ceiling joists and the footer attached to the subfloor and sleepers may be enough. – Nicholas Mar 11 '16 at 23:48
  • I missed that with the frame on top of the sub floor just nailing will hold it in place just fine – Ed Beal Mar 12 '16 at 10:24
  • It would take a massive amount of force to move a bottom plate nailed to even just 3/4" OSB. One nail every 16" would be great plenty. On another note, I don't know why you'd glue down the foam or screw the sleepers to it. Gravity will keep everything stable. If you're worried about noise due to movement, address any concrete irregularities first. – isherwood Jul 7 '16 at 16:55
  • @isherwood - is completely correct. I didn't touch on this because it isn't really the question but gluing or screwing is an extra step only to cause you future pain. What do you suppose you would do if you had a leak to take the subfloor apart? All of this should be left floating, many people tape the sections together. Also again a nail every 12-16" is plenty. If we are floating under the framing though we do use screws so that the framing can be moved if there is an issue (also two screws per 8 feet on the header). – DMoore Jul 7 '16 at 18:56
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I think you can do both, but framing walls on top of subfloor is potentially less work.

If you use a product like DriCore for the subfloor, they even explicitly recommend you frame on top of their subfloor product. They do, however, recommend you screw the framing through the subfloor into the concrete with something like 3" Tapcons spaced every 4 ft through the bottom sill of the framing.

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I am not sure about your location. If you are in the far north it is a great policy to insulate your basement floors - rigid foam sheets and OSB is certainly a great way to do it and probably the easiest.

However if you are in a mixed climate, I am not sure what you are looking for out of the insulated floor. The energy savings will not be there as cool months will be offset by warmer months. Also with a well insulated finished basement, you shouldn't even feel a "cool" floor in the winter unless you are tiling. I live in a moderate climate. When it is 0 degrees Fahrenheit, my basement carpet is not cold.

So first think about why you are doing this. By having this "waterproof" barrier on your floor you are not fixing water issues, you are just making it longer before you notice them. If my basement sprung a leak I would want to know right away, not after a few weeks when I start noticing mold grow everywhere. Also note that you will have more work to do if you get a leak because now you have to move all of the subfloor from the area being worked on - and dry the subfloor. You may have to remove a lot of the subfloor to dry it out or get fans to it.

As far as the install I have done both methods - laying out the sublfloor (or dricore which I have used many times - bigger pain than the foam+plywood) first or framing then subfloor. It really doesn't matter which you do. Either way your drywall is well above where water should get to and that is the main organic material you are worried about. (you obviously don't install subfloor after drywalling)

Now my preference is framing everything first, even if I have to make more cuts to my sheeting. Framing on top of dricore or foam+plywood is a pain in the ass to put it mildly. The dricore teeters - note much of your framing will be along the outside walls and of course that means you will be framing on the edges of your dricore or sheets. The sheets teeter, but not as bad as the dricore, however they smoosh. Your cuts have to be super accurate, as if the wall is too tall even by a 1/16" it will push down board, which pushes up other end. Then your next section of wall is too short by 1/32" and it is sort of floating, while its plywood may pull up when nailed/screwed and the plywood next to it is smooshed so now there is a slight ridge (so we start shimming out the damn framing like cabinet). Don't even get me started on getting the wall at a perfect 90 degrees and not realizing you have pushed the plywood in a certain direction by a 1/4", you push that back and damn it your sections don't line up right. Don't get me wrong it isn't rocket science and you can frame on top, just takes longer and prepare for at least some mild frustration.

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In many modern homes in cooler climates theres a vapour barrier ( 6 mil up to 30 mil) under the concrete which prevents ground moisture from wicking through the concrete floor. This is a preferred start for any kind of basement development because it eliminates dampness and potential mold caused from water seeping into your bottom plate or subfloor.

Any subfloor is subject to expansion and contraction which means it needs a bit of space around the perimeter to expand and/ or contract. With non- load- bearing walls floated on the subfloor there is always the possibility of the floor cupping slightly during periods of expansion.... mainly because the bottom wall plates interfere with the expansion of the floor itself. Over the span of 10 feet, the expansion could be as much as 3/8ths of an inch. I would classify the potential movement as a nuisance rather than a major issue.

One subfloor I prefer is made by Amvic/ Amdry which uses 2 foot by 4 foot OSB panels fused to one inch thick styrofoam with a waterproof layer over the foam. It snaps together with special plastic strips. Its about 30% more money than Dri-core ; its easier and faster to install and wont capture water if you have a minor flood.

I'm fairly sure the main drawback of float-framing on top of a basement subfloor is the expansion/contraction issue. With that in mind, I recommend floating the perimeter walls on the concrete using a pressure treated plate on a tar paper strip. Float the framed wall about 1.5 inches off that plate usine 6" galvanized ardox spikes ( pre-drill). After that install your sub- floor leaving a gap of 1/2 to 3/4 inch to the perimeter plate all the way around. After that, lay out your interior walls and float them on the sub- floor. In my opinion, you should always float basement walls as any floor lift will cause structural problems such as jacking your house. This problem is especially likely where homes are built on clay soil.

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