poly under footing, but seams not taped
Last Post 23 Apr 2014 06:01 PM by central_lower_michigan. 8 Replies.
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central_lower_michiganUser is Offline
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22 Apr 2014 08:12 PM
Building an addition; footings were poured yesterday and exterior drain pipe under peastone was installed today.

Instead of putting a capillary break between the footing and wall, we wrapped 6 mil poly around the inside of the footing forms.  Cut it wide enough to meet up with the sub-slab poly on the inside and come up above grade on the outside.  Before filling in the peastone, I inserted vertical footing insulation (2.5" EPS type 2).  We will use the same insulation outside the block wall up to the cripple wall sole plate.  Will also place Platon dimpled HDPE from above grade down into the peastone, then aluminum flashing from siding down to below grade.  The excess poly sheet on the exterior will go outside the EPS and be taped to the inside of the Platon.  We have a significant ant population, so some physical protection is needed for the foam above and below-grade.

This is a shallow footing in a high groundwater area.  This spring, groundwater filled the footing forms within 2" of the top (max possible based on dam spillway downstream from us).  The ground is sandy and drains well.

This required four strips of the 6 mil poly.  The sheets are overlapped about 12 inches, but I forgot to tape the seams.  My concern is that when the groundwater rises above the bottom of the footing, it will seep in between the sheets, soaking the footing EPS and wicking into the concrete.  Then, 1-2 weeks later when the groundwater retreats, some will be remain trapped above the plastic, with no downward drying.  The drying paths will be to interior wall (1.5" continuous EPS + 1.5" mid-furring EPS) and the portion of exterior foam above the poly sheet.

The existing house footing goes down to the same depth.  It is a typical humid basement; we run a dehumidifier and there are some moderate mold issues but nothing catastrophic.

I would be grateful for any advice on how concerned to be about this, and suggestions on whether to:

a) leave it as is and tape poly sheeting as planned

b) leave it as is but with less poly / Platon on the exterior

c) dig out the peastone and drain pipe around the seams and tape inside and outside.  I have Tyvek, Gorilla and duct tape.

d) dig through the peastone down to the bottom of the footing and slit the poly for the entire length of the perimeter (so it still serves as a vapor barrier, but drains the high groundwater back out)

Thanks
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22 Apr 2014 08:17 PM
And a followup question. Should we now apply Thoroseal (or some other capillary break) above the footing to block wicking into the blocks / inside wall? Suggestions for something which could be found on short notice at local hardware or lumber yard?
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22 Apr 2014 10:16 PM
I'm thinking what I'll do is stick with plan A, but put the interior drain pipe lower than the bottom of the footing. The exterior pipe is centered even with the middle of the footing, but the connector pipe runs under the footing.
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23 Apr 2014 10:43 AM
With 12" overlaps there's no real point to taping the seams of the poly.  It will be fairly gas-tight as-is, and vapor impermeable. The odds of having absolutely no leak points for the high-water table flood condition is essentially zero anyway- don't sweat the seepage on lapped seams.

The entrained air in the soil is always at about 100% RH, so there's never any drying toward the exterior below grade. But there can be above grade, even through 2.5" of EPS as long as your protective finish for the EPS is vapor permeable (cementitious versions of EIFS work.)  Stopping the footing-poly a foot above the poly is fine, seal it to the foundation wall with the goop of your choice, but stop the dimple-board at grade- do not extend it above grade.  The above-grade portion of the foundation wall can (and should be allowed to) dry toward the exterior so be selective about how you treat that.  Any moisture that gets into the concrete during flood stage gets redistributed via capillary action, which will move the residual moisture up to the top and out, as well.

Aluminum flashing should never be in contact with soil- it will be toast in less than a decade if you do.

Infiltration air leakage in summer could easily exceed the effects of the high water table in a S.MI location, given the 60s & 70s type of outdoor dew points.  If you haven't already, air-seal the foundation sill & band joist of the existing house (and the new addition). Band joists & foundation sills comprise the largest untreated air leak in most existing homes (bigger than all door & window crackage leaks combined), even those with foamy sill gaskets. The heavy duty EPDM sill gaskets can do a pretty good job of air sealing between the concrete & sill (and recommended, since it's also an excellent capillary break), but they don't do anything for the band joist leakage.
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23 Apr 2014 11:31 AM
Thanks for the reply. To clarify, the planned aluminum is a coated product: http://www.quality-edge.com/trim-coil

Our builder and local building supply have a lot of confidence in it based on 25+ years track record around here. The current plan is to place the top edge of the Trim Coil just inside bottom of the siding (between foam and siding). It would cover about 8" of above-grade foam and then 4 or 5" into the ground.

That arrangement would provide no exterior drying potential, correct? Would you recommend instead a short drip flashing and then a cementitious finish like Quickrete Foam Coating? If so, would 32" below grade wall + 8" above grade be enough drying potential?

I had considered Tuff II acrylic trowel-on because it sounds more durable.  Would that not be vapor-permeable enough for outward drying?
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23 Apr 2014 11:59 AM
they have a 25 track record of using aluminum flashing below grade? In the dirt? What kind of a guarantee are they giving you against the aluminum disintegrating? (better be longer than 10 years) Never heard of such a thing, although I know people do use metal, but it will not last.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
central_lower_michiganUser is Offline
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23 Apr 2014 12:08 PM
The manager of our local building supply built his house that way 28 years ago. He told me the only issue he's seen is color fading due to UV.
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23 Apr 2014 03:38 PM
Aluminum is an extreme vapor barrier, that would inhibit the foundation from drying, leading to saturated concrete or forcing all drying to be toward the interior, which is less than ideal.  Aluminum flashing down to below the siding is fine, but below that it hurts more than it helps- there is no point to it.

Z-flashing properly lapped with the housewrap/felt to direct any water that blows by the siding outward, to  drip just beyond the exterior of the finished foundation foam just below the siding is far preferable to directing it behind the foundation foam.

Only 8" of above-grade exposure on the foundation isn't huge (12-16" is better), but it's WAY better nothing.

Quickrete Foam Coating is as good as any. It rejects liquid water and doesn't wick much, but is at least semi-permeable (unspecified in the data sheets- no ASTM E96 rating) to water vapor, which is what counts. 

Acrylic finishes vary in vapor permeance- the sell sheet for Tuff-II talks about it being "water resistant"  but"...not to be used in place of a traditional waterproofing", which makes me think it may be fairly permeable. Call them.
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23 Apr 2014 06:01 PM
Customer service reply:

"Tuff II is not a vapor barrier it will allow vapor to pass through. What you are describing is liquid water wicking up. I would not recommend Tuff II in this situation since you would experience blistering of the Tuff II from the moisture."

So I guess I'm back to a cementitous coating.
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