Basement Subfloor and Wall Insulation

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Mike Boley

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Hello,

I have spent close to 20 hours reading through every thread on here that I could possibly find that relates to my situation. At the conclusion of that I have decided on what I think is the best solution to my situation.

The zip code is 62341. I have a poured concrete foundation that is a walk out basement. The east side of the house is the walk out portion and is above grade. The north and south sides are on a slope where at the west side it is about all below grade and at the east side it is all above grade. The west side of the house is totally below grade.

I had discovered some minor, typical cracks in the foundation that had produced some water over the years due to insufficient gutters/downspouts and poor grading. Those issues have since been remedied. The cracks in the foundation were sealed with a 2 part epoxy and the dirt around the foundation has been graded for proper runoff. Seamless gutters and oversized downspouts with extensions were added.

The previous setup included poly on the walls between a standard 2x4 framed wall with faced R11 insulation. The floors were 2x4's laid flat 12" on center, with 1.5" of regular Styrofoam in between them with poly on top of that and then 3/4" T&G plywood. I am removing everything and starting with a clean slate.

After looking at the various products available and extensively reading up on their pros and cons, I have decided that I think my best option is to utilize XPS for the walls and floor. I do have some concern with the chance of small amounts of ground water surfacing through the floor in cases of severe rain. So, I thought I could lay down 6 mil poly on the floor, 1" XPS then 3/4" T&G. And secure that with Tapcon screws to the floor. I will be installing laminate floor as the finished product.

For the walls, I thought about 1" XPS there as well and then butt up a standard 2x4 framed wall that will be filled with R13 unfaced fiberglass. I already have the fiberglass, so if possible I would like to be able to use it. I have read where this system has been recommended for other threads, however it appears that your climate can dictate what will work the best so as to not build up condensation on the cold side of your framed wall. I would like to build all of my walls on top of the subfloor so as not to have any wood on the concrete to help reduce the chance of any mold/rot. I would atleast do this on the walls that are entirely or partially below grade.

I guess first, will these materials work for my situation? Second, do I need to do anything at the floor to seal where the floor and wall XPS meet? And what would I do to seal the top of the XPS where it meets the floor joists at the rim joist? Is there a recommendation as to how many screws to put in per piece of subfloor? Is there a specific tape that seems to work better than others to seal the seams? And should I leave any gaps around the perimeter foundation walls? I guess some of these questions are probably a little premature as I'm not even sure that this process will be applicable to me.

So any advice I can get will be greatly appreciated. I'm looking forward to the responses.
 

Smooky

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How long ago did you do the grading work and fix the gutters? If that was done recently I would want to wait and see how dry your basement will stay through a wet period, such as through the winter, if you get enough rain. The down spouts and ground surface swells should move rain water away from the house and will help with surface water from a rain. It will not help with ground water. If you do not have a good foundation drain I would be worried about ground water entering on the west side or on the sides that are partly below grade.
 

Mike Boley

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The basement has never taken on any large amount of water. Even with the issues that are now corrected. In a major rain event, it may have leaked half of a cup of water total. Mainly through 2 cracks that have been sealed. In addition to the other remedies taken. So far, since the work has been done, it seems to have done the trick.
 

Cacher_Chick

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When I did mine, I used 2" XPS secured to the wall from top to bottom with tapcons through 1x2s spaced evenly across the foamboard.
Because the foundation wall is not perfectly straight or plumb, the interior stud walls are built 2-3" to the inside of the foamboard with a treated bottom plate set on 3.5" wide strip of CBU to keep it off the floor. I elected not to use any insulation in the studwall because i know that at some point the basement will get wet.
I also used the 2" XPS cut to fit in each joist bay, adhered to the joist band and sill plate, with the sill plate foam sealed to the wall foam using spray foam. The sill plate and joist band work is time consuming to get everything tight, but it is the biggest leak point in most homes.
I did not do the floor as i could not afford the loss of ceiling height.
 

Dana

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Tape the seams of the XPS with housewrap tape, and paint over the edges of the tape with fiber-reinforced duct mastic for a more permanent air seal.

With the leak/flood history, it's worth sealing the slab with an epoxy-based floor sealer as a primary vapor retarder & waterproofer, but you'll still need the polysheet.

At the slab-wall intersection, lap the floor vapor retarder up the wall between the wall-foam and concrete. Then use housewrap tape to seal that joint, and seal the tape edges with mastic as with the wall foam.

In central & southern IL (US climate zone 4, warmer side of zone 5) you could get away with less than R5 wall for the wall foam. Using 1" EPS (R4.2, for 1.5lb density goods) is less environmentally damaging than XPS and would have but a modest effect on the overall thermal performance. Going with R15 rock wool or R15 fiberglass in the studwall would more than make up for that hit.

With the laminate floor and the flooding history it's better to "float" the subfloor rather than punching holes in the poly with TapCons. Any liquid water that seeped through would eventually end up in the subfloor. Using t & g goods, and staggering the seams of the foam with that of the plywood would limit the amount of "potato chipping" edge curl & waves, but only use TapCons as a last resort for keeping it flat.

The studwall plate should rest on the plywood, not directly on foam, and 24" o.c. and single top-plates are just fine, since it's not a structural wall. The wider spacing and single plates enhances thermal performance by reducting the thermal bridging of the framing through the fiber insulation layer.
 

Mike Boley

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I appreciate all of the advice! I'm still trying to sort out my options. I apologize but I have another plan that I would like to throw out there. Would it be OK to use 1.5" EPS on the walls and the floors? Here is what I would propose. On the walls, the R6 EPS with a studded wall on that with unfaced R13 fiberglass in the walls. Seal the seams of the walls with house wrap tape and paint on mastic at the seams. I just wasn't sure how the EPS would do if any water were to find it's way inside.

I have taken out the subfloor and the walls in the areas I will be replacing and only found 2 areas where there were cracks in the foundation that caused minor water infiltration and one spot in a corner where the wall and floor met. The caulking at that joint was missing in that spot where some water had come in. I didn't notice any water throughout the slab anywhere.

On the floor I was going to use a poly (4 or 6??) and run it up to the wall and tape it to the wall and seal the seam with the mastic paint. I would then use the same 1.5" EPS on the floor and put the 3/4" OSB T&G on top of that. The EPS isn't T&G. So, should I use the Tapcon's to make sure I don't end up with any buckling? Also, do I tape and seal the seams of the floor EPS just like the wall? Do I need to spray foam any of the edges or anything?

I'm trying to find a balance of affordability and functionality. I had a spray foam contractor provide me with a quote to spray the walls with 2" of closed cell and it was $2,100. Right now, I can get the 1.5" EPS on sale for $6.99 a sheet and that creates and absolutely huge price difference. It appears as far as the R numbers are concerned, I can reach the levels that I need to with my plan. I just don't know how the materials I am suggesting will be in the event any water makes its way in. I don't foresee any major water events, but as with any basement, there's always the chance for some water to find a way in.

One last thing, where can I find the plastic channel that you put along the seam of the wall and floor? That may be able to help me with any water that would come through that area. I just can't seem to find where it is sold commercially anywhere outside of basement repair companies. Thanks again for all of your help!
 

Dana

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Closed cell foam typically runs about 17-20 cents per square foot per R (installed), whereas EPS runs about 10 cents/R-foot. With sale-priced EPS & DIY labor the out of pocket costs are WAY lower.

Any grade of EPS is fine for the walls, but for the floors you normally want 1.5lb density "Type -II" or denser, for the compressive strength. At $7 for a 1.5" x 4' x 8' sheet it's most likely going to be 1lb density "Type-I" goods. The labeled R of Type-II for 1.5" stuff would be R6.3, whereas Type-I would be R5.9-R6. With the rigidity of a full 3/4" subfloor Tapconned to the slab, it might cut it, but be sure to stagger the seams of the subfloor with those of the foam or you'll get compression at the foam edges.

Use at least 6 mil poly for the vapor retarders.
 

Reach4

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Use at least 6 mil poly for the vapor retarders.
So for basements, the vapor retarder/barrier would go on the concrete side of the insulation rather than the inner side as is done on the rest of the house in colder areas. I guess the vapor barrier would go to toward inside for the top plate. Right?
 

Dana

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For basement WALLS with foam insulation between the concrete and the studs/fiber there is no point to inserting a vapor retarder, unless you skimp on foam-R relative to the whole-R, in which case you want a "smart" vapor retarder on the interior side (not polyethylene sheeting. The R-ratio requirement is climate-dependent, but if you use the IRC 2012 chapter 7 prescriptive by climate zone for above grade walls (or higher) for walls without interior vapor retarders you'll be fine. Northern IL is zone 5, southern IL zone 4.

Even though the prescription for zone 5 is R7.5 min you'll still be OK at R6 foam for foundation walls with at least half of the wall below grade, since any wintertime condensation events in the above grade section are on foam or fiberglass/rock-wool, which are not damaged by moisture, and the below grade half never condenses- the moisture accumulation burden inside the assembly will not reach mold levels. The code prescription presumes up to R15 cavity insulation for 2x4 construction, which means as long as the foam R is greater than 1/2 the fiber R, your mold risk is very low. The prescription also presumes mold-susceptible sheathing between the fiber & foam, which you DON'T have in the basement stackup, which gives you even more margin. In zone 5 I would install R15 rock wool in the studwall even if the foam was only R6. YMMV. If you're really worried about that, you can go with R11s instead of R13s or R15s, which would give you more wintertime dew-point margin on the above grade section due to the higher foam-R/total-R ratio.

The foam itself is a vapor retarder, the value of which varies with foam type & density & thickness, and whether it has a vapor retardent facer (foil, vinyl, polyolefin are all common facers). Unfaced Type-I EPS would have a vapor retardency of about 3 @ 1.5" which is still more vapor tight than latex painted wallboard (typically 3-5 perms). Any ground moisture that diffuses through the foam into the stud cavity will still dry toward the inteior rather than accumulate in the studwall. A more vapor tight interior finish such as foil/vinyl wallpaper would be a BAD idea, since there is no way for the cavity to dry into the damp soil below grade.

For the basement vapor retarder it's somewhat better to put the poly sheeting between the foam and subfloor, since there is less space for water to collect and puddle on top of the poly in that configuration. The foam is just fine in the moist environment of the slab/subsoil. If there are any voids/sinks cracks where liquid water can collect above the vapor retarder, there is a risk of damaging the finish floor in the event of moisture spillage or minor leakage at the wall/slab boundary. With the poly sheeting pressed between the foam & subfloor, the amount of reservoir volume created is miniscule.
 
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