Asking again on how to raise my living room floor

Question

I was working on the house last year and asked about this project previously. I never got around to tackling the living room as other rooms had priority. But now I am returning to this.

Previous post here:

How to raise my living room floor by 3 inches?

Started a new thread as it's been so long. Reading comments there again, I see that people had asked for more info, which I'm providing. Let me know if any additional info would help.

House built in 1975. Room is about 16' square built on 2x10 joists spaced at 16". House and room are structurally very solid but the existing subfloor is "wavy" - specifically due to some joists being up to 3/4" lower than others. None of the joists are at the same height. By my determination, it was just built that way. Floor was originally carpeted (on top of 5/8" particleboard that I removed) and you couldn't tell it wasn't flat. The plywood subfloor is 5/8" and is nailed (was existing) and screwed (I added) down to joists. I can access the floor from below (prev post was incorrect - basement is finished but not below living room). The joists have wood cross-bracing.

The hallway had been tiled in marble by a previous owner. This raised the floor by nearly two inches. Obviously it was already a big step as there is now a 9.5" step down from hallway level to living room subfloor level. I am putting down LVP atop a new 5/8" AC plywood underlayment - both of which I already have as I'm doing same in 3 bedrooms. In bedrooms, I only had to raise the floor by 1.5", which I did with strips of 3/4" plywood below new 5/8" underlayment.

There are four approaches that I've considered or had suggested to me. Looking for feedback - including pros and cons.

1. 2x4s on edges, orthogonal to joists, every 8", scribed and cut make floor flat

2. 2x4s on edges, parallel to joists (one on top and one halfway), scribed and cut make floor flat

3. 2x4s on sides, orthogonal to joists, and shim to flatten

4. cut with circular saw 1.6" wide channels, orthogonal to joists, for 2x4s on edges to fit into, trimming joists at intersections as needed to make level

Regardless how how I shim up the new underlayment, I plan to have supports under the new underlayment at no more than 8" spacing.

I already have LOTS of shims cut of various thicknesses of plywood from 1/4" to 3/4" for use in approach #3. I already have the AC plywood and LVP.

Someone in previous post had suggested "sister 2x8s beside the existing joists". I don't think that would make sense. Too much work - including removing cross-braces from joists.

Answer 1

I'll go outside your box a little bit.

First, level up the existing surface using sheet goods. You can get plywood, OSB, MDF, hardboard, cement-board, etc. in various thicknesses. Do what seems best to get a cheap solution that lifts "most" of your surface to level.

Depending on how uneven things are, you might want to infill with self-leveling cement. That is, fill the big areas with cheap bulk flat materials, then get the edges, crevices, etc. with leveling cement. At the end of this you should have a flat floor.

Now "raise" the floor by using rigid insulating foam. This is just foam, no coating, that is typically some pastel color like pink or green. Might be white, but watch out for weak foams in white! You want something that proudly announces a load-bearing capacity in pounds per square inch/foot.

Those foams are available in various thicknesses. Usually with an accompanying R-value/U-value printed on the surface in big letters. You don't care so much about the insulation (but you'll need to remember it if you ever decide to do underfloor heating or the like). You're more interested in the foam spacing up your surface.

Feel free to combine thicknesses in individual layers. You may or may not want to apply a surface layer of wood sheathing. If you're just using vinyl planking, then you may not care. But if you want to use carpeting, you'll want a wood surface to nail/tack the carpet down to.

Also, be VERY mindful of "transition strips." Anyplace you change material or change elevation, there's likely to be some kind of finish product that needs to be nailed, screwed, glued, snapped, clicked, velcroed, or something down to the underlayer, and you'll want to make sure you have that ability. Foam doesn't hold nails or screws very well.

Finally, I'd suggest you make sure, regardless of whether or not you have a wooden top, that the outer perimeter have a border about a foot wide, maybe less. This would be just a tricky way to cut the foam (cut a 1-foot strip, then install however-many 4x8 sheets, then cut the other side to leave a 1-foot strip, then install the 1-foot strip). The idea is that since you have already finished the walls, if you ever need to dig down three inches for a repair job, you want to have a small-ish area you can dig down in. 1 foot might be too wide, even. Maybe half that is okay. Just enough to reach down and do whatever repair work needs doing on the walls.