insulation/moisture barrier question

Ok, I hope this is the best place to pose this question (mods, if its not, please move)

I'm building a walipini built on a foundation that kinda cuts the structure in half (half in soil, half with block walls with soil up against them), it will be more than 75% underground with the roof sticking up. My main question lies in my understanding of dry soil is insulative and wet soil is a heat sink, so I want to keep the wet soil off the outside of the block walls. Someone suggested that I put styrofoam insulation up against the block walls to keep them dry, but I had an idea of putting billboard vinyl underground at a slope away from the structure to run water away from it 5 foot or more (kinda like an underground roof on slope). It will have a sloped roof on it with an overhang of unknown size.

Anybody want to chime in with experience on this? I'm wanting to also know which would be best to put IN the blocks ; sawdust, perlite, styrofoam peanuts.....

Thanks in advance!

Hi Matt, sounds like a fun project. I see you are not far away in Canton, actually drove through there yesterday, fall colors are kicking! There are many interesting threads here that touch on what youre asking about. Search for PAHS and the Wofatis are attempting to use billboard vinyl in the same manner youre proposing.

Since I know your climate well, I have to question your assumptions on achieving dry soil and curious what kind of R values youre expecting to get from it. Its a lot easier for some climates west of the Mississip to start off with dry soil but here? I dont think its an easy thing to do. So even if you did manage to dry out a massive amount of earth and keep it that way till you place it around your foundation, I dont think you can ignore whats going on below and beside the dirt. The dry dirt will take on the moisture levels of whats next to and below it if there are not barriers in place to prevent the transfer. In other words, youre going to need a lot more than an umbrella above.

Yes you want to keep youre block wall dry. You also want to keep it warm hence the recommendations for insulation. Without insulation, your wall will be closer to the ground temps. Even if that ground is bone dry, I believe it will be much closer to the average ground temp than our preferred comfort range of 65-80F. This is a good thing in the summer but not the winter. Even in the summer, cold wall surfaces will not play nicely with our mostly humid air.

Filling your block with any insulation will probably be a waste of resources and if you fill it with anything that could rot, youre asking for serious trouble. I think vermiculite is the best choice but again, its probably a waste because CMU block has too much thermal bridging to make it worthwhile. Probably the best thing to fill the block with if youre backfilling that heavily is rebar and concrete. You might want to involve an engineer depending on the details. Block walls can work well but are notorious for failure.

For the best energy performance, you want insulation to the exterior of the block. Foam is tough to beat for this but mineral wool and foamglass are other options. Insulation to the exterior keeps the thermal mass coupled to the interior air and keeps the walls above the dewpoint.

Most US climates have termites which leads many building experts to recommend keeping basement insulation to the interior. You lose the thermal mass but its easier to install and protect from termites and the elements. Thermal mass is really not that important for most energy efficient homes and is not nearly as important as air-sealing, insulation and fenestration. Those are the big 3 in terms of good energy performance.

Even if you insulate the block walls (you really should IMO) you still need to follow other best practices with such a buried wall. I recommend a more traditional sprayed damproofing and permeable drainage board or backlfill. The vinyl could work but you need to be meticulous with the connection and edge details particularly the bottom, it should drain into a well designed exterior foundation drain sloped to daylight, slotted PVC wrapped in gravel and silt fabric being one of the best common approaches.

I dont mean to dismiss using vinyl or other vapor barriers as underground umbrellas as I do it often, more as underground shed roofs but would create more of an umbrella with a bermed roof. They are another tool in a belt and suspenders approach but shouldnt be relied upon as the only means of protection.

Check out this thread if you havent already. https://permies.com/t/39802/toxin-ectomy/mold-remediation-house It would be nice of you to avoid the problems of leaking basements and the risks of giving future inhabitants such headaches down the road. Underground water and moisture are powerful, persistent forces that will probably need more than some hastily placed foam sheathing and billboard vinyl. Based on your questions and research it looks like you understand the value of good details.

Brian, I'm not sure whether my question was not well worded or didnt include all the valid details, or we have a misunderstanding of what I'm trying to do. Here is a link to the construction that I've got going on. webpage

It is a greenhouse, so your question of validity of whether its worth it to worry about having dry walls or not may hold water. There will be high humidity on the inside, so I reckon that I may just accept that I will have wet walls. I wouldnt mind mossy walls one bit, in fact I think that would be rather cool. I'm going to berm up the soil against them on the outside to direct water away and provide thermal mass to harness the warmth of the soil below 4 foot deep or so. For moisture (you may notice the stairwell in the pics) I'm going to drain the stairwell through the footer straight into a drain running down the center of the greenhouse (U - shaped concrete drain, with wood top for path). This will provide a place for me to build a walkway on so I wont be tromping down the soil in there for no reason. This will drain out the low end of the greenhouse under the foundation on that end.

I've tossed around ideas for thermal mass in water barrels as well, but the question of taking up so much floorspace comes up. Many things I'm going to have to just run with and see how well they work out if I can change them afterwards like that.

I'm going to put a rocket mass heater in there too, but i'll have to get crafty with ways to keep it dry.

Thanks for you answer, you did give me more to think about. This block laying is so slow, but I'm done with one half of the walls and on to the next side which I would hope may only take me a month or so. . If you are ever out this way and want to check it out, holler at me. We can talk thermal mass, cows, chickens, or whatever.

Thanks

Well dang, now I feel even dumber than usual. Great work, thread and documentation over there. I agree that with a greenhouse, keeping the walls dry or warm is not nearly as important as if it were to be lived in. Any extra added humidity concerns are overshadowed by the plants and dirt their growing in.

The main concerns become keeping hydrostatic pressure from toppling or bulging the block but it looks like youve got that handled. The long north wall is not that deep but I would include a footing drain there if possible. The deep wall is short and supported by the stair wing walls. Youve got the appropriate planned drainage going just be sure the areas in those corners have drainage access to the that drain in the middle under the stairs. You also seem to be towards the top of a hill which helps alleviate concerns of migrating ground water during periods of heavy rain.

Obviously, greenhouses arent my deal but if I were to imagine a perfect setup, I would think about replicating what you have going. The thermal mass considerations do get trickier in these situations. I tend to agree that floorspace would be more valuable than extra TM with your setup. Are you planning on painting the interior block black? You might want to fill those blocks with sand or a concrete mixture with high sand proportions to increase the TM and ground coupled TM. I hope to see this thing in action in person. Great work!

We are AT the top of the ridge, probably 250 to 300 feet above the holler. Our well is 300 some odd foot deep, so if we have groundwater come up this high, we'll have a whole new set of problems! It will have dirt bermed up and away from it from the stairs to the bottom side entrance. In fact, the top stair will probably stick out 2 inches above the soil level and then slope away and down from that. I'd LIKE to form the soil so the valley of drainage slopes away from it and is no closer than 5 to 10 feet from the walls. I'm also trying to be realistic with the build cost - the first blocks I got were recycled at $1 per, but I wasnt able to continue finding them clean. I can get them for not much more than that new, so I quit worrying about finding them and then chipping off the old mortar.

I've got one piece of rebar drilled into the footer and running up through the blocks and poured at least one on every step on the foundation, and every 3rd step in the stairwell. You reckon that should be sufficient? I see what you are saying about the thermal bridging with the cement blocks, but its great now in the sun - you can stick your hand in the blocks a few hours after the sun goes down and it feels really warm. As for the north wall being painted black, I was going to run with it and see what it felt like after I got the roof on. I had thought about putting a row of barrels there painted black, and I still might put a few, but we'll see.

Thanks for the interest - the block work is going faster now as I've gotten back into the swing of it.