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Community and Q&A

Basement insulation (what are we doing wrong?)

misunderestimated2 | Posted in GBA Pro Help on

Hi all,

Seattle, 1910 craftsman home with unfinished basement, I believe we’re zone 4A.
So, we have been working on finishing our basement and decided to go with the rigid foam approach preferred here and by Joe L.  We managed to source GPS Platinum 1″ and 3″ to get a total of R-15 (and this stuff is really nice).
We installed foam in August and noticed dramatic differences in the interior environment.
Then the rain came. 
I noticed a small water leak in one of the basement walls so I removed some of the foam- turns out it’s not just a small area. The entire surface along all walls have moisture between the insulation and the concrete.  I remove the foam and the walls dry out right away. Put insulation on the walls and they wet up right away. UGH!
I’ve been experimenting now and we have coated the walls with Xypex one area Drylok another (thinking it’s partially wall leakage) and still get the same results (foam off = dry / foam on = wet). 
After doing more research of our existing conditions, it turns out the foundation wall has no footing, the concrete floor slab is pored up to the wall. Could this be rising damp? 
Granted this wall assembly is not complete and will get an additional furring wall with mineral wool insulation + GWB. Will that be enough to stop the moisture?
Is this moisture condensation or vapor diffusion, and what should the strategy be?
We are now working on exterior drainage, and will be tying all downspouts to rigid pipe> catch basin> off site (hopefully). We can’t do an interior drain system (because no footing). I could maybe do what they call an interior baseboard gutter, and tie that to a sump but the products look really finicky.
Any other suggestions? Perhaps some sort of dimple mat and then the insulation? Just create an airspace?
I was also researching “rising damp” and in Europe they inject material through masonry walls to effectively eliminate dampness.  There are injection epoxy kits available in the US for crack fill and waterproofing concrete walls (drill a matrix of holes, add packers, and inject polyurethane foam). this process seems very similar to the Euro approach and might have the advantage of some residual R-value on the positive side.  This would be a lot of additional work, but easier than adding a footing.  

Pictures tell a story, see attached
Overall assembly is to be: Concrete wall w/ -side waterproof coating> R-15 GPS foam> 1.5″ wall furring w/ mineral wool> 1/2″ GWB + PVA> latex paint
This is a concrete stem wall about 42″ high with 2×6 framed cripple wall above.
The cripple has wood siding> tar paper> 3/4″ plywood > x6 wall with R-23 mineral wool> 1/2″ plywood + tons of nails for the earthquake.

Please advise. 

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  1. DCContrarian | | #1

    I can think of three possible explanations, it's important to determine which one because the solutions are different.

    Most likely rain water is trickling through the wall. This is supported by your observation that you only see water in the rainy part of the year. It could be that it was always there, but without insulation it dried before you noticed it. The solution is to either divert the water on the outside, or divert it and give it a place to go on the inside.

    Possibility two is that your insulation is not air tight and warm humid air from the inside is reaching the wall. The insulation is keeping the wall colder than it was, which causes the moisture to condense out as liquid water. You're only seeing this during the rainy season because the interior air is more humid. The solution to this is making sure your insulation layer is air tight and vapor tight.

    The third possibility is dampness in the soil is wicking through the masonry, creating high humidity along the wall, which condenses. The solution to this is a vapor barrier on the interior.

  2. DCContrarian | | #2

    A quick test would be to put the insulation back with plastic sheeting between it and the wall and see which side the moisture accumulates on. That will tell you which side it's coming from.

  3. misunderestimated2 | | #3

    I just added a sheet of plastic to my mock-up assembly and will let you know soon.

    Another part of the house does have a footing and isn't having these problems. I'm thinking the base of wall is soaking in the moisture since it can't be treated and is diffusing through the waterproof coatings.
    So we wanted these walls to dry inward to make this design work, and understanding that subgrade walls will always be wet. Your suggestion it sounds like that is not possible.
    Is Drylok a vapor retarder? We are getting wet on the Drylok walls.

  4. misunderestimated2 | | #4

    As far as air sealing, we had created a seal with expanding foam top / bottom and lapped the joints, all butt joints also foam sealed. No vapor barrier as I thought that is a no-no.
    "No interior vapor barriers should be installed in order to permit inward drying"

    This pic is near top of concrete wall, where the foam has been cut away.

    1. DCContrarian | | #5

      The question of drying in basement assemblies is hotly debated.

      The article you linked to is by Joe Lstiburek, who has since recanted and now feels basement assemblies should not transmit vapor at all.

      In no case should your basement wall assembly allow water vapor from the outside to go through to the inside. You'll never be able to dry out the soil under your house, for practical purposes it's an infinite source of moisture, moisture that you want to keep out of your house. So there should be a vapor barrier on one side of the foundation wall, in new construction it goes on the outside typically but in retrofits there's no choice but to put it on the inside. The question is whether there should be a vapor barrier on the interior side of the wall.

      The two schools of thought: on the one hand, if you have a vapor barrier on both sides of the insulation, any moisture that gets in is trapped, you have to have a pathway for drying and that can only happen to the inside. On the other hand, no drying is possible anyway, moisture moves from wetter to dryer and from warmer to colder, and the air inside the basement is always warmer and moister than the insulation.

      I'm of the second camp. But this means that your insulation -- your entire wall assembly, really -- has to be impervious to moisture, because if any gets in there it's not getting out. But I'd argue that it would never get out anyway because there's nothing to drive it out.

      If you're in the first camp, you still need a vapor barrier to protect the cold foundation wall from condensation. It has to be roughly within the interior 2/3 of the insulation to stay above the condensation point, and everything to the exterior of it has to be impervious.

      1. Jon R | | #8

        > Joe Lstiburek, ... now feels basement assemblies should not transmit vapor at all.

        An incorrect portrayal of old hearsay. Very recent material from Joe L is clear - "The recommended permeance of the interior rigid insulation layer is approximately 1 perm".

  5. Roger Berry | | #6


    DC is right to test the source with plastic taped tight to the cement wall. This will show pretty quickly if the wall is the moisture source or not. Your image of the moisture shows a pattern that echoes the foam bead structure of the GPS. One could be led to think the wet pattern is due to room humidity permeating the foam and condensing on the wall. I suspect that the reverse is true, slightly drier parts of the wall are due outward release of the water vapor.

    I used Xypex additive in my entire foundation footing and walls to avoid rising damp from known water sources , courtesy of the rock I am built on. It has worked very well and I do not fear putting my hardwood lumber or panel stock against the walls. Just the same, when discussing the additive with the local agent, he advised that while the crystals formed in the presence of water would seal cracks up to about 1/32" to prevent bulk water from entering he could not say that all moisture would be prevented from transpiring through the wall if I had standing water against. (think swimming pool-like)

    I do have slightly elevated humidity in my basement shop all year, but this could be a function of the perpetually cooler temperatures as much as any rising moisture from below the slab level. As I understand it, the key feature of Xypex and similar coatings is preventing intrusion of water and waterborne pollutants. Parking garages and highway maintenance being a major market. I would confer with the Xypex people to see if coating the wall will have the intended effect.

    I have used both UGL and Thoroseal to retard water leakage and moisture in cement block. I would suggest you look up any sheets you can find on them. As a practical matter, both keep out bulk water. UGL use to have store displays composed of a plastic tub, a fountain pump and a cinder block half painted with UGL. The pump filled the cinder block's cells with water. The side of the cinder block coated with UGL would fill up with water to the top and the un-coated half's cell would weep water through the block so fast it never got over half full.

    Ultimately for my own wet basement, I went with the Thoroseal powder which mixed into a very, VERY heavy paste that filled the pores of my cinder block much faster. My wrist did not enjoy the experience, but I did manage to dry off the walls. Might be overkill for you, but it was effective. I still would check on transpiration properties.

    For a bit of history, while you may not have a (now) standard footing, you likely have a trench footing. Literally just a trench cut with a shove and filled with concrete. My wet basement home was built in 1930ish and had this feature. A very elderly home inspector filled me in on the method. Unfortunately, your floor slab is probably in direct contact with native soil rather than washed stone, so there is no capillary break under the slab. Maybe, if the floor was done in the 50's after removing a oil converted coal fired gravity furnace, you will be lucky and find they floor does have a bed of stone. In any case, perform the same plastic square test on your slab. This will help inform your choices going forward as you try to make the basement more useful.

    Depending on your particular site, look for ways to direct water away from the house as aggressively as you can. Also, look for abandoned drain tiles at the downspout locations. I discovered a system of fired clay tiles that originally connected the downspouts to feed a cistern behind the house, a fairly common feature even in urban areas. In my case, the village idiots decreed in the 60's, all such cisterns are a danger to be filled in. The partially removed gutter tiles now dumped water directly next to my walls. Much digging and cussing ensued. And that is the end of old home archeology.

  6. DCContrarian | | #7

    After much research -- UGL does not make this information easy to find -- I have found that just about all DryLok products are not vapor barriers. I think there is one product that is. Most of them have rather high perm ratings. So they keep out liquid water but do little to keep out dampness from the capillary action of concrete. (I had never heard the term "rising damp" before but I assume that's what you mean, as liquid water flows down but dampness flows from wet to dry in every direction due to capillary action).

  7. misunderestimated2 | | #9

    I just checked the can is Drylok original. Google says semi-Impermeable coating - keeps water and vapor out (<1 perm rating). It is easy enough to apply but I still need a vapor barrier coating. I guess I can add more paint, maybe something mold specific.
    The GPS is actually good for water absorption, but it has high vapor permeability. Maybe I should have gotten the foil faced material as it is a vapor barrier, although that would trap the moisture inside the assembly. I can always paint the foam. Maybe both sides.
    I'm also starting to really consider the PU injection mentioned earlier.
    Rising damp is talked about here:
    Our concrete is obviously not like the brick masonry mentioned, however it had lots of rock pockets, and the aggregate is literally river rock & pebble. We packed the rock pockets with Xypex concentrate and coated with their Megamix.
    It is definitely not on anything more than dirt, no footing, a common practice at the time apparently.

    So it is a dry day here today, I'll need to report back once I get this plastic to wet. I imagine will be as suspected, moisture on the concrete side

    Attached is why I think we are getting the rising dampness- as opposed to a foundation with footing

  8. Hammer 🔨 | | #10

    How long have you lived at this home and observed the basement? If you just moved in or you haven’t spent a year observing the dampness and moisture in basement this might have always been an issue. That would at least rule out some of the scenarios, and determine if it is an water entry issue that always existed or if it has anything to do with the new wall assembly.

  9. misunderestimated2 | | #11

    Verified today that the plastic sheeting wets on the concrete side.
    I also managed to install a new trench and catch basin for one of our downspouts that we suspected was having issues. 3 more to go...
    We've had the home for around 10 yrs.
    With the process of renovating the basement we have changed much of the envelope, like the cripple wall insulation and adding building paper.
    We definitly have higher interior humidity this year. Lots of schweaty windows. I suspect this will change once I can get the basement walls dry & insulated.

    So I guess now I just need to stategize on how to stop the walls from wetting once insulation is applied. I've read the Xypex coating will pass water vapor, and the Drylok is semi-impermeable.
    I think I should find an epoxy paint instead of the Drylok

    1. Tom Wheeler | | #12

      Don't do anything else until all the water is directed as far from the house as possible when it comes off the roof. 15 feet or more is best.

    2. DCContrarian | | #13

      The danger when applying waterproofing on the interior is you trap the water with no place to go. It builds up, the pressure grows and it just comes out somewhere else. In the worst case it can create enough pressure to damage the wall structurally. So you have to make sure it has somewhere else to go.

      Most waterproofing products are not vapor barriers. I've found it hard to get information, although DryLok does sometimes tout breathability as a benefit (it isn't).

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