Hello everyone, I am new here and there is so much stuff to learn and absorb, great info.

I built my underground home a little over two years ago and this is my second winter here. I live in middle Tennessee. Winter has really been a cold one here so far. Now that I have been here for two very cold winters, I am trying to figure out how to keep my south facing open wall from sweating. first some info. I built my home by using the stacked block method. filled with re-bar and concrete, on a monolithic foundation. I sprayed the walls inside and out with SBC concrete, trawled down to smooth, then coated two heavy layers of liquid rubber, then a layer of foam sheeting for wall protection, french drains, before backfilling the structure I have 4 windows across the south facing front with a large double door. the three other walls are underground. When it gets really cold, like below freezing, and I am cozy and warm inside, the South wall sweats like crazy. I also use a de-humidifier in the house, but it cannot keep up with the sweating even though it is designed to handle more than my 1320 sq ft home. None of the walls underground sweat, only the South exposed wall. Hind site suggests that maybe I should have built the Southern facing wall out of wood so that it could be insulated well and not sweat. Being block and concrete, it transfers the outside cold and the heat hits it and whamo, moisture. Now I am working on fixing this and could sure use some ideas. Summers are no problem at all. I have given thought to lining the outside wall with Styrofoam and overlaying it with wood, but I am not sure about this yet. Anyhow, I thought I would ask some of you if you have experience with this and if anyone had suggestions to remedy my problem. I hope I have given enough information, if not, ask away and I will answer to the best of my ability. Thanks everyone,
Jim

I'd expect the solution to a wall with major condensation would be major insulation, with the question being how to provide such.

I'd also expect a good seal to the existing wall would be advisable; a permeable layer of insulation on the inside would still have air move through it, hit the cold bricks, and then you'd have condensation between the concrete/insulation. Recipe for mold.

Perhaps a better solution would be insulation on the outside of the south wall. Strawbale? You'd need to address things like roof and footing, but you wouldn't lose any space inside, and it would allow that front wall mass to be inside the thermal envelope.

Thanks Dillon for the reply. My thoughts as well. I thought about 2" foam, with wood siding on that. It would keep the look we were after.

James, I would not jump to costly conclusions yet that may not solve the issue. It does not sound like cold condensation on the inner wall to me. Lets make sure we understand your design first.

You used dry stacked AAC blocks or the heavier concrete?
What SBC product what perm rating? Both sides.
What mastic or rubber coating and perm rating? Both sides?
Rigid Foam on both sides/ EPS?

What is the humidity inside and outside when the walls sweat? It would best to get a relative humidity reading inside and out to determine vapor drive direction, dew point temps, and where the greatest content of moisture is coming from. Is it higher low on the wall, by windows and doors, etc....A meter like this would be very revealing: https://www.youtube.com/watch?v=qat9Nqs8KDw

Or at least a moisture and temperature meter and hydrometer.

It sounds like moisture is trapped inside the wall, trying to dry inwards where the humidity is lower and temperature higher but low permeable barrier(s) are condensing it not evaporating or drying fast enough, but that needs to be verified. You can not fix something until you fully understand it. The other walls are drying to the exterior I guess, the meter would reveal this too.

There is nothing natural about petroleum based mastic's and foam, I'd stop using them, it appears you have barriers on both sides of the wall. Perhaps mold. An IAQ reading would be good too.

Hi James,

Being block and concrete, it transfers the outside cold and the heat hits it and whamo, moisture.

Perhaps I'm putting too much weight on this bit; I took it to mean the wall is quite noticeably cooler than the interior? Can this be quantified at all?


Terry, sounds like you have a more in-depth understanding of this subject; what do you think about insulating a smallish portion of the wall to observe results? Or is this misleading due to the rest of the wall remaining as is?

This stuff can be confusing...Most homes vapour drive is inward this time of year since the relative humidity is higher outside than we want inside around 50. TN looks like it is high outside. The biggest driver for vapor are differences in RH (high to low), second temp and pressure differences across the wall a breathable wall equalizes, barriers make large and small holes draw more moisture through them. At the same time hot air is going the opposite direction trying to get outside. A well designed breathable wall will dry the vapor before it reaches the inside, and will not loose significant heat through the wall. Also the inside material will have an ability to store and release vapor, AAC block would be a good example due to it's open pore structure and it is up to 80% air, but it is very Capillary open to wicking from ground water. A dense impermeable SBC coat would prevent these functions, so would a low perm mastic barrier, that slow down vapor transport or can cause it to condense on a hot surface depending on that surfaces dew temp a meter could read....Foam for sure is a barrier except at it's seams.

I'm not clear if the foam is to the interior and what finish is to the interior air like a low perm paint that is sweating? It may be of interest to take a section of interior coatings off and see if the wall is sweating at the block surface, or get a depth reading with a scanner. If not that would prove the inner barrier(vapor and air) is the issue not allowing drying by the dehumidifier, which in most cases it is. If it is sweating I'd start looking down too at capillary wicking up into the walls or a capillary break under a footing, stem wall, and slab if there is one, or around windows and doors, penetrations, from wind driven rain, wicking can reach miles in concrete. It may a combination of things the south wall is experiencing the others are not.

Foams and mastic and/or dense SBC coating's do not allow drying to the interior and often folks need whole house dehumidification or air recovery units like HRV/ERVs.....The problem with them is they may have no effect if the wall has a barrier(s) that won't allow the device to pull moisture out of the wall on the exhaust cycle. An open breathable interior wall is critical, more than the exterior, for mass effects. More exterior insulation or foam won't solve interior vapor locked wall issues.

Had the blocks been covered with a breathable lime or clay plaster, wood, the issue would more than likely not exist. Working with not against the forces of nature

heavy blocks, covered with SBC inside and out, rubber coating on outside only, thin foam on outside only. Those are the walls under earth, the south wall is heavy block as well, SBC coated(1/8") and painted with water based paint. All walls are concrete filled cores. .Sorry, I don't know the perm ratings on the sbc or rubber. the sbc was a quickcrete product and I don't remember the rubber coating brand. Sweat only occurs on south open to air walls. As a side note, I inspect for mold and have never found any yet, anywhere.

Hi James,
It sounds like your wall's dewpoint is reached when temps outside drop below freezing. Any time a surface reaches the dewpoint(100% RH) in a room, it will start to condense water and if the surface can not diffuse or absorb the water it will sweat. The RH of the room will determine the dewpoint.
So, to prevent the sweating that you mentioned, you must either; raise that walls temperature or lower the RH in the room( in order to actually accomplish this you must cool the room, not just dehumidify) or utilize a material that can deal with all that water.

I would suggest that you build a hygrothermal mass wall just inside the original wall with a small, sealed air gap between them.

Bill, thanks for the reply and info. Can you point me to the ,"how to build", info on the hygrothermal wall? I have been researching, but if you have a good go to, I would appreciate that. Thanks again.
Jim

Bill, I don't think I asked that question properly, should have asked for example of how to build that wall in this existing structure. I figure you probably already understood that, but for the record.....

James, here more than likely the problem...Quickcrete perm rating is zero! http://www.quikrete.com/PDFs/DATA_SHEET-Quikwall%20SBC%201230%201231.pdf

Water permeability - Falling head permeameter
Uncoated Too High to Test
1/8" (3.2 mm) QUIKWALL® SBC 0.00 ml/cm2/hr

Meaning you may as well have a plastic barrier inside and outside of your wall which is a no-no. The dehumidifier can not get past it nor can heat to dry it. I'll let Bill explain how to put a breathable interior up although it would be best to take the SBC of the inner wall at least. BTW you can not always see mold but once it forms the wall section have to be taken down.

Hi James,
There are many many ways to build an insulating, hygrothermal mass wall. It really depends on your skill set, environment and available materials. I like adobe, but that is a traditional method where I live, there may be better options for you.
We "experts" don't have all the answers, because the best advice is usually site/builder specific. Look to traditional means, methods and materials that are appropriate to your tastes and skills as well as available materials.
There are many amazing materials that have been overlooked by most because of corporate guided disinformation aimed at selling hi dollar products, when locally sourced freely available materials are often better.
You will also find great pleasure in working with these ancestral techniques.