The perfect basement floor, or a future disaster?

[Zombie Burger]

I’m in the process of replacing a 450 sqft floor in a split-level basement living space. I have a dilemma that I’m sure lots of people have faced in regards to basements, but I’m still unsure of a solution.

The basement has a history of water seepage around the walls (not flooding, fortunately. Just enough to stain the bottom of the paneling). This has been largely remedied by the addition of exterior drain tile around the foundation. However, should the drain tile ever fail, I don’t want a floor that will be susceptible to water damage. The room is currently heated with off-peak electric baseboard heat. This limits the placement of furniture, so I plan on converting those circuits to an in-floor heating film. That necessitates a floating floor instead of tile. I’d also like to insulate the floor to improve efficiency; that means some kind of subfloor or underlayment will be needed. The last requirement is that the subfloor must be at least a 1/2 inch thick. This is to conceal some condensate drain lines that currently cross the room to a floor drain on the other side of the house.

After going over this 100 times in my head, I’m considering adhering 1/2 inch XPS to the floor for insulation, and then gluing 1/2 inch rock board on top of that as a subfloor. Above that will be a thin underlayment pad, then the heating film, a moisture barrier, and finally a vinyl plank floor. I think this will do everything that I need. Has anyone ever done something like this? Are there any better options that I’m not thinking of? This “magic sandwich” of materials works perfectly in my mind, but there’s always the possibility of it not being what I think it will be.

[Spork]

I'm not in construction... just a normal DIY guy.  Don't take this as "that's wrong!"... just a question.  I've never done this...

Would you really glue the rigid insulation down and adhere the cement board to it?  I'd think you'd make little risers out of treated wood -- the same height as the insulation, screw those down to the concrete, put the insulation between them, and screw the cement boards to them.

[Zombie Burger]

I've heard of the furring-strip design, but I was under the impression that it was not necessary when using XPS due to the improved compressive properties compared to EPS. The strips also compromise the thermal break and moisture barrier. My reason for gluing was to reduce flexing should any spots in the floor not be perfectly flat. Sort of an improvement over floating plywood subfloors such as this:

https://m.youtube.com/watch?v=4jSuWbdJy5A

[dilinger]

You probably want to look at these two documents:

http://www.buildingscience.com/documents/bareports/ba-0309-renovating-your-basment
http://www.buildingscience.com/documents/bareports/ba-0202-basement-insulation-systems/

I wouldn't glue down the XPS, and the thickness of XPS would be based on your climate.

[Spork]

I've heard of the furring-strip design, but I was under the impression that it was not necessary when using XPS due to the improved compressive properties compared to EPS. The strips also compromise the thermal break and moisture barrier. My reason for gluing was to reduce flexing should any spots in the floor not be perfectly flat. Sort of an improvement over floating plywood subfloors such as this:

https://m.youtube.com/watch?v=4jSuWbdJy5A

You're probably right... I've seen them put rigid foam under poured concrete.  It must be pretty sturdy.  It just seemed odd to have it be glued down.

[big_owl]

I'd use something like Dri-core.  It's really easy to install and would avoid most of the water issues.  We just put down about 1500 sq-ft of the stuff as we started finishing our basement and it has worked out very well.

[Zombie Burger]

You probably want to look at these two documents:

http://www.buildingscience.com/documents/bareports/ba-0309-renovating-your-basment
http://www.buildingscience.com/documents/bareports/ba-0202-basement-insulation-systems/

I wouldn't glue down the XPS, and the thickness of XPS would be based on your climate.

Thanks! There's some great info on that site.

1 inch of XPS would be great for insulation, but I'm trying to conserve as much headroom as possible. Part of the ceiling is only 7 feet above the slab, not including the light fixtures. I found another document "read-this-before-you-design-build-renovate" that I think explains the 1-inch minimum. XPS is semi-permeable, so thin sheets of it will permit moisture through and damage the plywood subfloor.

I might be able to solve that with a layer of polyethylene under the 1/2 inch XPS. A floating plywood subfloor might then work for me. It might even be cheaper than fiber cement.

[Zombie Burger]

I'd use something like Dri-core.  It's really easy to install and would avoid most of the water issues.  We just put down about 1500 sq-ft of the stuff as we started finishing our basement and it has worked out very well.

I actually considered this for a while, but I saw a lot of complaints online about having nowhere to vent the subfloor. My research seemed to suggest there was no point in keeping the slab itself dry; a moisture barrier in direct contact with it would create a very thin region of high vapor pressure above the slab, preventing additional moisture intrusion.

[big_owl]

Not sure I understand.  The primary purpose would be to keep your floor from being ruined due to water damage to a subfloor directly against the concrete in case of water ingress.  I suppose you might achieve the same thing by using XPS on the floor, but without any free space between the concrete and the subfloor, I'm not sure where the water would go if your basement floor were to experience a significant amount of water ingress.  And you're still facing a venting problem.  If you install the dricore correctly you are supposed to leave a space between the edge of the subfloor and the basement wall so that there is ventilation up the side of the wall.  I don't know how much water you really have to worry about - whether it's just 1/2 cup seeping in on a rare occasion or a gallon plus every year.  I suppose that would make a difference. 

[Goldielocks]

I've heard of the furring-strip design, but I was under the impression that it was not necessary when using XPS due to the improved compressive properties compared to EPS. The strips also compromise the thermal break and moisture barrier. My reason for gluing was to reduce flexing should any spots in the floor not be perfectly flat. Sort of an improvement over floating plywood subfloors such as this:

https://m.youtube.com/watch?v=4jSuWbdJy5A

You're probably right... I've seen them put rigid foam under poured concrete.  It must be pretty sturdy.  It just seemed odd to have it be glued down.

We put in a floating floor... Insulation on the slab, covered with plywood...  But we did not glue the insulation down.

[dilinger]

Hm, if you're skimping on the XPS, other alternatives include coating the slab with epoxy or using a plastic dimpled sheet.  I haven't tried either of those methods, but they're described here:

http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

Me, I'd go with the 1" XPS and enforce a height restriction on downstairs visitors! ;)

[Zombie Burger]

We put in a floating floor... Insulation on the slab, covered with plywood...  But we did not glue the insulation down.

Glad to hear that someone has actually done this! What sort plywood did you install, and how did you do it?

[Goldielocks]

I am trying to recall now.

I think we just cut 1.5" to 2" foam slabs, bought from manufacturer directly 10 miles from us.  Fairly dense foam.

Cut to fit to the wall studs.  Wall STDs of course on 2x4 floor plates Hilti- anchored to the floor directly, so foam just sits very close.  Max 1/4 inch gap in places. No glue.

I spot troweled and ground down humps in  the concrete floor first to level it somewhat.  It would have been $800 to have it properly leveled.

Plywood was three quarter inch. Cut to fit. Laid Directly on insulation.  No glue.  Some screws to floor plates, where it made sense but you could still lift most of the floor if needed in future.

So floating all over.

Laid membrane cushion base for laminate, then floating laminate click planks.

If there is any water from plumbing disaster in future, we could lift everything to check, if we take up the laminate and baseboards.

Floor was 100% dry, 40 years old when we started, so no water or damp issues there.  No insulation in slab when built.

Basement is now very warm, and the floor flatter.  Uses less energy to heat.

Working well 5 yrs now.

Bottom step of stairs is shorter though!

[Greg]

I wouldn't recommend a few of your ideas coming from my point of view as a design/build professional that does a lot or remodels and additions. 

If you have a space that has a history of water problems, an electric heat film system is a bad idea.  Look for other solutions.

Ditto anything applied to the concrete.  If there is a water issue in the future, it will wick up between panels of EPS, plywood, etc.  Dri-core is effective in these situations due to how it holds the subfloor material off of the slab.  You can use this with forced-air ventilation of the slab air space if you want to be extra careful.  In this case I would make anything you install easily removable (read: no glue) and if possible, modular so that one area can be removed while leaving unaffected areas in place.

Good luck!