Subfloor in basement which is correct

[kjcred1]

What a wealth of information! I had an identical question to the op and the internets brought me here. After reading all of the posts I wondered; if headroom is valuable in a basement that sweats, could one use 1/4" durock, instead of OSB/plywood. The sandwich would be poly, 1/4" fan-fold XPS, 1/4" durock, mortar, tile. Finished floor height would be a bit above 3/4". Thoughts?

 

[Dana]

R1 isn't quite enough for summertime dew point control for something like a throw-rug on that tile in a KS climate, at KS deep subsoil temperatures.

Deep subsoil temps are in the 50sF, and summertime outdoor dew points average above 60F from June-September. Anything you put on the floor that has an inherent R value greater than 1-ish will potentially have very high humidity nearly saturate high relative humidity air on it's bottom side. Mold risk really soars when the temps are above 60F and the relative humidity is over 70F. Going with R3-R4 ( 3/4" or 1" EPS ) would be better than any fan-fold from a mold risk or "musty basement smell" point of view.

Rather than 100s of TapCon penetrations of the polyethylene sheet in damper basement, use foam board construction adhesive (not standard construction adhesive, which would compromise the foam) to glue the Durock to the foam. Be sure to weigh down any waves or springy sections due to slab un-evenness.

It's useful to figure out the source of the moisture in the basement and reduce it before finishing it. If the walls need an interior side perimeter drain to keep walls from wicking moisture up the walls from the footing you'll be cutting into the slab to make that happen. Simply finishing everything with waterproof materials can be risky, and having to do it over again is expensive. In basements that "sweat", many will need a dimple mat against the foundation behind insulated finished walls that drains into a perimeter drain to keep finished walls from becoming mold farms.

 

[Sponsor]

 

[kjcred1]

Let me check to make sure I understand what you are saying and have already said.

Dimple mats on the floor are pointless when the space could be occupied by poly and rigid foam for a better end result. The foam provides a barrier, separating the colder soil and the warmer basement atmosphere. Any moisture from the poly up will be able to dry inward. From the poly down, the concrete won't care if it stays wet.

Why then put a dimple mat on the wall? I suppose if the wall needed water proofing maybe, that way water coming in would, as you say, go down between the floor and the wall into a perimeter drain. But I can't help but to think that would be less than desirable for two reasons. 1) if you have water coming in through the walls, the dimpled mat is more addressing the symptom, not the problem. 2)Wouldn't the same principles for the floor, apart from the poly barrier, hold true for the wall? Put the rigid foam directly against the wall without the poly. That way the wall can dry to the inside. Wouldn't the dimple mat act a vapor barrier much in the same way the poly would?

 

[Dana]

Dimple mats aren't a great solution, but way better than nothing. It's definitely attacking the symptom rather than the source of the problem, ergo "...figure out the source of the moisture in the basement and reduce it before finishing...". Yes, dimple mat is a vapor barrier, but only the above grade portion of the wall can ever dry toward the exterior. Below grade moisture moves only from the ground toward the interior, and if the concrete is moist enough to visibly sweat, it really needs to isolated with a vapor barrier.

Rigid foam offers only modest drying capacity toward the interior. It's fine for walls that don't have obvious moisture issues, but for those that do, it's not enough.

When sweating is from saturated soil & concrete that can't be mitigated on the exterior, a dimple mat offers a drain space and doesn't increase capillary draw, whereas poly sheeting would block drying toward the interior while increasing the capillary draw (moisture wicks upward slightly faster at the concrete/plastic boundary than in the middle of the concrete wall), raising the moisture content of the concrete where it meets the moisture susceptible foundation sill. If there is a powerful capillary break (like EPDM, butyl flashing, or 10 mil polyethylene, etc) between the top of the foundation and the foundation sill that won't matter, but if the wood rests on the concrete or is only separated by a foam or laminated fiberglass sill gasket it matters a lot.

FWIW: I've never personally been involved with a project that had to go the dimple mat route, but am aware of some that had no better alternative. eg One home built on a flood plane area with the slab a foot above the normal water table. To address it on the exterior would have required digging down to the footing along the entire perimeter, back filling with washed stone protected by landscape fabric. Dimple mat and an interior perimeter drain was a much cheaper solution.)

 

[kjcred1]

So given the scenario where the dimple mat is the best viable solution; the mat would need to be taken up the wall at least to height of the exterior soil grade. Rigid foam would be placed against it and above it on the wall. In that manner the wall could dry both to the interior and exterior? Or would it be preferable to carry the dimple mat all the way to the sill with the rigid foam backing against it?

 

[Dana]

I imagine it would be easier to just run the dimple mat to the top the foundation than to guesstimate grade level on the other side. The additional drying capacity toward the interior through foam is miniscule compared to the drying capacity toward the exposed above grade exterior. There is usually a small ledge of foundation between the foundation sill and the interior. Topping that with foam that extends out over your wall foam makes it easier to air seal the whole thing. Can-foam any big gaps, use housewrap tape to seal the seams of the wall-foam.

Run through this bit o' bloggery before committing to a dimple mat wall solution.

In a KS location you'll need at least an inch of EPS for wall-foam for wintertime dew point control (1.5" is better), with unfaced or kraft faced batts in the studwall to hit IRC code min. (In a basement with a wet wall or flood history unfaced batts is definitely better.) The floor foam should extend under the bottom plate of the studwall, of course, and the floor poly sheeting should lap up the conditioned space side of the dimple mat. (If lapped between the dimple mat & foundation a leaking wall would drain to the top side of the floor poly rather than into the perimeter drain.) The dimple mat should extend under the slab edge repair, and over the perimeter drain. This isn't the best detail drawing for what you're building but the principle should be clear:

 

[Reach4]

I was thinking that the water under the dimpled stuff would end up at a floor drain.

 

[kjcred1]

Thank you dana for answering my questions, and everyone else in the community who contributes their knowledge. I have read maybe 4 dozen more articles and blogs from terrylove, gba, bsc, finehomebuilding, and others since my last post. Here's where I'm at. Single story home constructed in the early 60's on a poured concrete basement without rebar reinforcement. Hydrostatic pressure had caused buckling. I excavated the entire perimeter, jacked the house, pushed the walls strait, and installed a new exterior footing drain to daylight. The exterior basement wall was pressure washed & waterproofed with a paint-on sealer, then wrapped with a peel and stick membrane (hydroseal3000). I also put in an interior footing drain to sump well. The existing cracks will be epoxy injected to resist movement from lateral pressure, but the main strength of the now compromised walls will be 4" I-beams spaced every 6'. The I-beams rest on the footer. Pouring the repair section of the concrete floor secures them at the bottom and a 2x8, running the length of the house lag bolted to every floor joist, secures each at the top (I-beam reinforcement methodology gleamed from Daniel J. OConnor of Hydro Armor Inc.).

My question: After seeing figure 3 in this article, and reading about thermal bridging, and reading comments by GBA Advisor Martin Holladay, "your steel studs are much worse thermally than wood studs; they act like radiating fins, conducting heat through any insulation you install between them"; I began to question what effect the I-beams would have. Would it pull heat/cold from the slab, footer, concrete wall, or wall cavity? For better or worse? How would you recommend insulating with these I-beams?

I'll be doing the 1/2" XPS, 1/4" durock, mortar, tile sandwich on the floor as previously discussed, and I'm planning on rigid foam insulation on the walls. Would you place the I-beams directly against the concrete wall and air seal around them? Or would you place rigid foam between the wall and the I-beam? (see attached photo for examples of both)

Is it more important to separate the I-beam from the concrete, or separate the I-beam from the winter warmside of the interior?

The other component of the same attached photo is a sketch of what I plan to have based on my research. The drawing includes two components that are on top of the footer which I have circled. I understand them to be rigid foam, but what is their purpose? How do they function? Thanks!

 

[kjcred1]

Thoughts?

 

[Dana]

(Took a vacation week away from internet... ergo the delay)

Martin is right about the thermal conductivity of steel- it's a problem. Don't sweat the thermal bridging of the I-beams from the slab, but DO pay attention to how it's treated at the walls. The total cross sectional area of the steel in contact with the slab is quite small compared to what it would be at the walls. In Boone County MO the deep subsoil temps are in the mid-50s, not perma-frost, so the heat loss and condensation potential will be small at the slab, but potentially large a the walls, particularly the above-grade portion.

On the walls, assuming it hasn't already been backfilled, how easy would it be to use 1-2" of rigid EPS on the exterior, with some Z-flashing slipped behind the siding & WRB? The above grade portion of the EPS can be protected from the elements & insects with a cementicious EIFS material such as Quikrete Foam Coating (below grade you can usually just leave it alone, but in termite infestation zones coat the whole thing.) That would be a more ideal thermal break than boxing in the I-beams. But if it comes down to Option 1 vs Option 2 in your drawing, Option 1 is the right way to go, using 1.5" foil-faced polyiso, taped at the seams with a high quality temperature rated foil tape (eg Nashua 324a, sold in box stores, but there are others.) The temperature to which it's rated is unimportant, but simply have a rated temp is an indication of a higher quality adhesive that will go the distance and not come undone in a few years, unlike some lesser no-name tapes. If not foil tape, housewrap tapes will work as well.

 

[kjcred1]

Thank you again for all your help!

 

[gopack]

My municipality is forcing me to use sleepers, must be a rigid connection to the concrete, no exceptions.

2003 house, very dry basement, zone 5 (milwaukee area), have never seen water in sump crock in 2 years living here, house is pretty much on hill top with good grading. Verified poly installed below basement concrete when the broke up for bathroom rough plumbing (assuming at least 6 mil).

I have approved plan to use 5/4 treated for sleepers, with 5/8 T&G OSB spaced 24"oc, with 1" XPS between sleepers. Finishing only about 1/2 my basement, about 1000 sq ft. 1" XPS glued to walls with 2x4 walls (1" also installed outside poured concrete wall). Absolutely will not allow any proper method mentioned here..

Should I add 6 mil on concrete as base layer below sleepers given there already is 6 mil below concrete, or would that create more of an issue sandwiching the concrete with poly? Should I also consider some sort of sealing paint before installing the sleepers? Appreciate any advice.

 

[Dana]

gopack: At the 1" XPS on the exterior is closed to being a class=II vapor retarder, impairing the ability of the foundation wall to dry toward the exterior. Concrete is completely tolerant of moisture, but the foundation sill on top is not. Hopefully you have an adequate capillary break material (EPDM sheeting, or metal flashing) blocking moisture exchange between the foundation sill & foundation.

Sheet plastic over the slab is fine, and provides a capillary break for the sleepers. Concrete works fine even when fully saturated-wet (like 99% of the bridge foundations in North America.)

The requirement for sleepers is just silly- there is no good reason for them to be there.

 

[Reach4]

The requirement for sleepers is just silly- there is no good reason for them to be there.

If you do put in sleepers, they could make it easy to run electrical conduit. That may not be something you need.

 

[Le_Pion]

Hi everyone, i been reading this post for quite a while and i've have gathered a lot of information thanks to it, but i was hoping someone could answer the following question.

I plan on using PT sleepers for my subfloor because the slab is uneven, i'll be putting the poly underneath the sleepers and the XPS between them, my question is concerning the thickness of the foam.

Why is everyone suggesting 1 inch foam when 1.5 inch could fit in there, is there any reason why i would need the 1/2 inch air space between the xps and my T/G ply?

thanks in advance!

 

[Dana]

The only "benefit" of the air space is a slightly softer floor, since the subfloor can flex a bit.

Clearly 1.5" of foam works, but use EPS, not XPS. Over the lifecycle of a house the performance of XPS falls to that of EPS of similar density, as it's climate damaging HFC blowing agents leak out.

 

[Le_Pion]

thank you for the quick answer, i'll make sure to do that.

 

[Le_Pion]

I had another question, i've reading about all the different methods, and of the disadvantages of the sleeper method is that it could created moisture buildup which would in turn rot the plywood.

what do you think about installing it this way :

6mil poly
PT sleepers
1 inch foam
0.5 air space for a total of r6 vs r7.5 for 1.5 foam

5/8 t/g plywood

i would then cut channels and leave some spaces in different areas of the sleepers to allow for some air movement under the floor, and i would put 2 dummy air traps in the front and back of the floor for this particular room, to allow for the extra moisture to leave the subfloor?

what do you think of that?

 

[Dana]

I think you're overthinking this.

With the plastic sheeting as a capillary break there is effectively no moisture transfer from the slab to the sleeper.

The foam is an additional capillary break (and thermal break) between the slab and the subfloor.

Adding an air space between the foam and the subfloor is THIRD (and completely unneeded) capillary break.

The bottom of the sleeper is cool enough to take on moisture from the room air in summer if you ventilate the basement with outdoor air, but the subfloor will never be that cold. If you keep the basement's relative humidity below 60% in summer there really isn't any risk to the sleepers, and even less risk to the subfloor.

 

[Le_Pion]

thanks for the quick answer!!