Basement floor & wall insulation with perimeter drain

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Smunderdog

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Getting ready to put foam insulation down on the floor and up on the walls of my basement. I've read through all the other great forum posts but didn't see this addressed. 1957 ranch with a full basement. Previous owners had a perimeter floor drain installed that takes care of the ground water when things get soggy outside - basement stays nice and dry

So what is the best way to address the floor/wall interface with the foam board insulation? Take the 1" foam board right up to the drain and then drop the 2" foam board down the wall right to it completely covering the drain? Or should I stop the 2" insulation short and leave a small gap so ater can get into the drain should we ever have a flood type event in the basement?

I plan on using 7/16" T&G OSB on top of the 1" insulation as the subfloor. Once that is down I'll build the 2x4 perimeter walls on the OSB, put fiberglass insulation between the studs and drywall.




 

Dana

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Make your floor insulation higher than the lip of the drain, and run the wall foam down to the top of the drain. If that's not possible to go that thick on the floor due to headroom issues, you can still proceed, but the risk to the OSB is a bit higher. If you're staying at 1" for the floor, leave a ~1/2" gap between the drain lip & floor foam, but still bring the wall foam all the way down.

Then install 6-mil polyethylene over the top of the floor foam, extending at least 6" up the wall foam. Seal the poly sheet to the wall foam with housewrap tape tape, reinforced by painting over it with duct mastic. For the seams between sheets of poly at the floor, overlap by a foot, with the edge taped & reinforced with duct mastic.

Install the subfloor over the poly, running it all the way to the wall foam (or at least within an inch of the drain lip, leaving a bit of gap), and butt the 2x4 framing right up against the wall foam.

For a basement with a history of flooding do NOT use polyisocyanurate since it can wick moisture up at the cut edges. Both XPS and EPS can withstand damp much better. EPS is much "greener" product than XPS, due to the much lower impact blowing agent (pentane, instead of HFC134a. ) For any location in Indiana, 2" of EPS + R13 in a studwall beats IRC 2012 code min for basement wall insulation with good margin.

The most likely flood scenario is seepage at the foundation-wall/footing or at the slab edge, in which case the poly sheeting will protect the OSB.

If an innundation leaving an inch or three over the floor level is likely/possible, you shouldn't be using moisture-susceptible materials below the anticipated high-tide mark. That includes framing, wallboard, kick boards, and certainly including any flooring materials. It's fine to use cementicious tile backers over the foam and lay down ceramic floors, or skip the floor foam and put asphalt or ceramic tile directly on the slab. Stacking foam under the studwall plates to above the high-tide mark works to protect the framing, but use only cement-board or PVC trim-board for the kick board at the floor.
 

Smunderdog

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Make your floor insulation higher than the lip of the drain, and run the wall foam down to the top of the drain. If that's not possible to go that thick on the floor due to headroom issues, you can still proceed, but the risk to the OSB is a bit higher. If you're staying at 1" for the floor, leave a ~1/2" gap between the drain lip & floor foam, but still bring the wall foam all the way down.

Then install 6-mil polyethylene over the top of the floor foam, extending at least 6" up the wall foam. Seal the poly sheet to the wall foam with housewrap tape tape, reinforced by painting over it with duct mastic. For the seams between sheets of poly at the floor, overlap by a foot, with the edge taped & reinforced with duct mastic.

Install the subfloor over the poly, running it all the way to the wall foam (or at least within an inch of the drain lip, leaving a bit of gap), and butt the 2x4 framing right up against the wall foam.

For a basement with a history of flooding do NOT use polyisocyanurate since it can wick moisture up at the cut edges. Both XPS and EPS can withstand damp much better. EPS is much "greener" product than XPS, due to the much lower impact blowing agent (pentane, instead of HFC134a. ) For any location in Indiana, 2" of EPS + R13 in a studwall beats IRC 2012 code min for basement wall insulation with good margin.

The most likely flood scenario is seepage at the foundation-wall/footing or at the slab edge, in which case the poly sheeting will protect the OSB.

If an innundation leaving an inch or three over the floor level is likely/possible, you shouldn't be using moisture-susceptible materials below the anticipated high-tide mark. That includes framing, wallboard, kick boards, and certainly including any flooring materials. It's fine to use cementicious tile backers over the foam and lay down ceramic floors, or skip the floor foam and put asphalt or ceramic tile directly on the slab. Stacking foam under the studwall plates to above the high-tide mark works to protect the framing, but use only cement-board or PVC trim-board for the kick board at the floor.

Dana - thanks so much for your insights both here and in all the other threads on this forum - much appreciated!

The top of the drain lip is as high as 2" in some spots - a bit lower in others. I only have 7' from the concrete floor to the bottom of the joists up above, so I need to keep my floor foam to 1".

So in this case are you saying I should run the wall foam all the way down to the floor foam which completely covers the drain? And then put the poly on the floor foam running it up the wall foam which would completely seal the drain off?

Or run the wall foam down to the top lip of the drain thus leaving about an 1" gap between the floor and wall foam...but then covering the floor foam with poly running it up the wall foam which seals off that gap?

The only water this basement has seen since the perimeter drain was installed was when we lost power and didn't have any backup pumps. We now have a water powered backup pump so it would be very unlikely we'd see any type of heavy water. Either way I plan on using XPS or EPS. No light seepage of any kind in the 8 years we've been here. The perimeter drain collects excess ground water that would normally seep in through cracks really well and it is pumped well away from the house.
 

Dana

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It doesn't matter if there's an air gap between the wall foam and the lip of the drain, but leave a hint of a gap between the floor foam & drain to take on any seepage that might occur. Even if that means there is a 1-2" stripe of missing foam at the bottom of the studwall, that's is neither a thermal or moisture problem, as long as you have the poly sheeting there rising at least to the high-tide mark of the last flood.

Grid power failures are always the very real possibility of power failures, as well as pump failures (even rarely used water driven pumps). Even if it's a once in 50 years event convergence of events that would lead to significant water over the floor, having floor materials that are water tolerant is the prudent way to go.

Following an ice storm at the beginning a December cold snap 5+ years ago I lost power for 10 days, as breaking trees had wiped out a lot of powerline infrastructure over the region. It was cold enough during that period that a water-driven pump could have iced up (in fact, the plumbing in the house very nearly did freeze, and only by jerry-rigging the ignition on the tankless hot water heater was I able to run daily tubs full of hot water as thermal mass to keep that from happening). I was glad to have stopped the moisture-susceptible polyiso wall foam 6" from the floor, and used vinyl trim board for the kick boards, since I had over 3" of standing water in one part of the basement for days. When power was restored the cleanup took only a few hours, but if I'd had to demolish the floors or part of the walls it would have been weeks or months.
 

Smunderdog

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A bit more detail before I start putting wall insulation up. In a prior comment I said I was going to use 2" XPS on the walls but I found 4x8 sheets of 1 1/2" polyiso faced with foil on both sides for $13 so I've decided to go with that. So as the diagram shows my stackup from the concrete block out is as follows:

Wall stackup:
- Concrete block foundation walls that were professionally waterproofed 10 years ago and have shown no signs of moisture.
- 1 1/2" foil faced polyiso that stops 6" short of reaching the floor. Seams between each sheet of polyiso taped with aluminum tape.
- Non weight bearing 3 1/2" wall with unfaced fiberglass insulation in the stud cavities. Bottom plate of wall rests on the OSB (incorrectly drawn in the diagram below where I show it resting on the floor foamboard)
- 1/2" drywall that stops a few inches short of the OSB
- base trim (resting on top of laminate floor (incorrectly shown in below diagram as resting on OSB)

Floor stackup:
- Concrete floor. An interior perimeter drain was installed 10 years ago that collects ground water during heavy rains which drains into sump pits and is pumped into the easement that is located over 50' away from the back of the house.
- XPS foamboard with seams taped with Tyvek tape
- 6mm poly that runs up between polyiso & stud framing (not shown in diagram below)
- 3/4" tongue & groove OSB
- wood laminate flooring

From what I've read in some of Dana's other posts on this forum and others - foil faced polyiso can be used as long as the top/bottom non-faced edges are sealed. So in this stackup the poly plastic seals up the bottom and the XPS used to seal up the rim joist seals up the top - right?

Anything else here that needs revision?
 
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Dana

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It's easier to install your base-trim if you extend the OSB subfloor to at least 2" under the bottom plate of the studwall, since gives you the full bottom-plate thickness to nail to. As drawn you would have to nail the trim to the studs only, which may be too wide a fastener spacing to keep the trim piece nice & tight, with no bowing or gaps.

At the top of the studwall it's worth installing R15 unfaced batts (high density rock wool or fiberglass, preferred for greater resistance to convective air flows) up against both the horizontal & vertical pieces of cut'n'cobbled XPS, which should be sealed at all of the joints- take particular care to seal the band joist XPS 360 degrees to the subfloor, sill plate, and joists. If the dimensions are right that a cut high density R23-30 batt would fill the whole space extending completely over the top plate of the studwall it's probably worth it. Since any condensation that occurred on the XPS has nowhere to go except the warmer interior where it re-evaporates quickly, there isn't much mold risk even if it leaks a bit of air, though an air-barrier between the joists doesn't hurt, as long as it's vapor permeance is at least 2 perms to promote rapid drying.Even some cut-up corrugated cardboard air barriers caulked & stapled in there would be enough.
 

Smunderdog

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Thanks Dana - I actually incorrectly drew the baseplate on the floor foamboard - it will rest on the OSB as you describe. I hadn't quite made it to asking about that gap between the ceiling joists, XPS/Great Stuff foam sealed band joist and the room yet since it will be a little while before I'm to that part. Part of my "problem" is that I intend on leaving the ceiling open (not drywalling the ceiling) since there is already limited headroom I want to leave that open to create the illusion of a higher ceiling.

So I'm not exactly sure how I plan to "finish" the spaces between the ceiling joists on top of the wall yet. Open to ideas there. Doesn't have to be a super polished look...

I test glued my first piece of the foilfaced polyiso yesterday and used a sheet of plywood leaned against it to hold it into place while the adhesive dried. So far so good!
 

Dana

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Be sure to tape the seams with 2" FSK tape before getting out the can-foam for sealing the rest of the foam togther.

If you cut in wallboard with notches to accommodate the joists as an air barrier in the joist bay areas and caulk/foam seal it, that would be an ideal air-barrier, and you can just paint it when you paint the rest of the wall.
 

Smunderdog

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That was the only idea I had as well for the wall/joist/ceiling interface - notch the drywall and then caulk the edges.

Is FSK tape any different than this foil tape at Lowes? If so, where does one source FSK tape locally (what type of store)?
 

Dana

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I wouldn't trust any tape that had only a 1-year warranty with no temperature ratings. A purpose-made FSK tape designed for sealing foil facers or a premium all-foil tape that has a specified operating temperature rating is more appropriate.
 
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