Trying to design a house with 3-D printed concrete Wall with strawbale exterior insulation.

Hello awesome people!! I recently joined the forum to research and ask you experts about insulating a concrete wall with strawbale. Please be kind and hear me out if my plans will work.

Because most 3-d printed concrete walls are insulated by injecting closed-cell spray foam in between the 3-d printed cavities of a wall, when the building reaches the end of its lifecycle, there is no possible way of recycling the concrete for other uses, not to dismiss the use of petrol derived spray foam in the first place. So, I at least wanted to make the insulation part of this project sustainable.

I understand that in standard strawbale wall cases where wooden framing or structural bale walls are used, it is best to use vaper permeable finishings on either side of the wall.

However, as I am stacking the strawbales on the outside of a 3-d printed concrete wall (which has a mortar consistency/grain size for printing) (I also heard from the 3-d printer people that the concrete they use is vapor permeable, not sure to what degree), and because we live in a somewhat colder climate, northern New York state, I am worried about vapor intrusion and it condensing inside the strawbale during winter times when we don't have much sunshine to dry it off.

Are my worries totally unfounded? Is it best not to use a vapor barrier in cold climates for whatever construction type? What if the concrete cracks and introduces a massive amount of moist air inside the bale wall during our almost 6-month-long winter? Wouldn't it be a smart idea to prevent a failure that is destined to happen?

As of now, my wall design consists of, from interior to exterior,

""concrete wall - vapor retarder/barrier - 2x2 furring strip for air gap - 18" strawbale - clay plaster or vapor permeable house wrap - rain screen finish""

I tried to deal with the condensation that might happen on the surface of the vapor barrier during the humid summertime by creating an air gap between the barrier and the bales.

Will this system work? Or is it a better idea to replace the vapor barrier with a vapor-permeable membrane? like the one that will be used on the exterior of the bales? and get rid of the airgap because now even though the condensation happens, it can dry out to the interior. (although I am not too confident in concrete being able to dry that moisture out...?)

I will attach a drawing of the wall section here also, please give me some advice on how I should treat the bottom of the wall where it meets the foundation.


I really appreciate the help!!!

Concrete can be recycled nowadays
Permeable vapour barriers prevent moisture build up and I would use them.
Have you thought of using earth plaster on both sides and eliminate the 3 D print?
There is a great recent topic on the matter of plasters and moisture.
https://blog.allplan.com/en/straw-bale-house

Wow, thank you for the reply! I will swap out the vapor barrier with a permeable one. I would love to try building with structural strawbale, but this project has always been about 3-d printed house, not a strawbale house, the strawbale part is my minimal effort to make this project slightly more sustainable and to use locally sourced materials
.

If there will be moisture that can condense in or on the straw bales continuously (six months of the year, and then humid summers) the straw will decompose and collapse without structural supports.
Organic material+water+oxygen=decomposition (aside from some very rare and special circumstances which you probably don't want in your house)
The airgap you are describing is a good idea in itself but the environmental circumstances you are describing would call for a complete water isolation of the straw if you want to keep it from decomposition.

Maybe a continuous dry air circulation in the airgaps could mitigate this effect, but now we are getting into complexities that may defeat the purpose.
Under certain (rare) circumstances wet straw can even self combust.
I am not sure what would be the right solution here to preserve the straw.

In my opinion straw bales are best when there is perfect water barrier on the outside, and on the top, and on the bottom,  and breathable surface treatment on the inside.

I'd be interested in reading about the reasons why you wanted to do a 3D printed structure to begin with. I know very little about those.

Hi Erik, Thank you for the reply and advice.

Yes, complete water isolation is why there is a waterproof but vapor-permeable house wrap on the outside of the bale wall, and then a rain screen to further protect the wall. Also, 2.5ft overhangs on all sides.

My concern is that, if I go this route, I will be creating a rather airtight wall assembly. Then I assume I can't promote dry-air circulation within the wall? is this correct? or is the convection created by heat from sunlight or of the summer promoting air circulation within the wall, even though the wall is airtight? (The wall will still be vapor permeable on both sides, even though it is not the highest permeability)

The reason behind the 3-d printed wall is that my client is extremely interested in it and it is a new experience for us to build with 3-d printing technology.

Hi Jay Yoo!

Thanks for posing this interesting question about building with straw, and for being aware that spray foam (as constituted) is among the least environmentally friendly materials available for building—high embodied energy, difficult to recycle anything it sticks to.  

There is a somewhat similar assembly called a “straw bale wrap” (straw bales wrapped around an existing CMU wall) that I believe was first tried in New Mexico and Arizona where un-insulated CMU walls were common in residential construction.  You might contact David Eisenberg with the development Center for Appropriate Technology in Tucson, AZ (www.dcat.net) for some thoughts on this.

I don’t know for certain the answers to your questions—there’s a pretty good chance that the wall assembly you have drawn hasn’t been built and experienced, or modeled and evaluated, so the answers I offer are speculative, based on how I understand the way heat, and moisture in water vapor form moves through materials.

I see a few challenges with the wall assembly as designed, which are basically taking two different kinds of walls placed side-by-side but functioning as a single assembly. That alone should be a red flag for both complexity and high cost.  I would reconsider combining the two for whatever reason.  

If I were on the design team, as a builder I see challenges with (1) the vent space functioning as you anticipate, (2) the overall assembly offering the thermal performance you expect, and (3) the structural challenges of connecting the two assemblies (plastered/clad bales and 3D printed wall.  

Several other people replying to this thread questioned whether this makes sense—I agree with them.

Moisture. I think you’re right to wonder whether moisture could be a problem in this assembly. Portland cement concrete at 4” thick has a permeance that ranges with the mix, but it’s usually well below 1 US Perm.  I’m guessing that a 3D wall made with Portland cement that’s 6” or 8” thick will be vapor impermeable—the only moisture escaping the concrete will be whatever is in the mix when the wall is printed, and that could take quite some time.  And it could be quite a lot of moisture—I’m not familiar with 3D printed walls but I understand that the mix needs to be quite wet with very fine aggregate particle size for it to pass through the 3D printer nozzles—and that’s a recipe for excessive cracking.  I’m not sure what kind of reinforcement against cracking is possible with 3D printed walls—poured Portland cement walls have rebar or rewire or some other “web” of material, and sometimes synthetic fibers that offer greater tensile strength to resist the cracking. You’re right to think cracks might channel moist interior air into the straw bale walls. The vapor barrier applied to the exterior of the 3D wall—whether painted on or peel-and-stick—is a precaution that guards against water vapor migrating from interior activities (showering, cooking, breathing) into the adjoining straw bale wall.

The vent space between the straw and 3D wall might seem like a good idea, but straw bale walls with another kind of ventilation cavity have been researched.  Kyle Holzheuter’s “Hygrothermal Environment of Straw Bale Walls in Japan and Building Practices to Control Interstitial Moisture.” Nihon University, 2010 takes a look at part of your question.  Some straw bale structures in Japan employ a vent space, open to the exterior, directly under a plastered straw bale wall. This design element was intended to address a concern for moisture build-up. His research concluded that this feature had the undesirable effect of providing a pathway for moisture to enter the wall cavity; he recommended that builders avoid this practice. The vent space you have drawn doesn’t appear to be open to the exterior except to perhaps drain at the bottom, so this may not be quite the same thing, but my point is that you probably don’t want a vent space there, at least not as drawn.  If you go through with this, consider installing a fibrous rainscreen mat (commonly used behind veneer stone and stucco in humid climates) between the bales and the 3D wall to replace or supplement the furring strips; it gives the bales something to lean against across their entire surface, perhaps reduces the convective looping that might occur in an unobstructed gap, and prevents condensation that forms on the vapor barrier (from exterior moisture migrating through the bales towards the interior being blocked by the vapor barrier and condensing), from soaking into the bales.  Extra materials and labor not usually associated with either wall system.

I don’t think you need to slope the exterior foundation wall.  The sill plates under the bales could be filled with an insulative drainable material (e.g., lava rock, perlite, rock wool) that can handle damp conditions without deterioration.  Just don’t seal the underside of the exterior sill.  Any water vapor passing through the bale wall and condensing on the vapor barrier on the 3D wall would theoretically drip down and work its way out of the assembly by flowing through the intermittent gaps between the concrete footing/foundation wall and pressure treated lumber sill plate.  

Structure.  Another concern is finding some way to connect the two walls. You’ll need a way to secure the bales and rainscreen/siding system up against the 3D wall (with fibrous drain mat sandwiched between).  You could connect them at the top with a box beam, but there needs to be periodic lateral connection as well to keep the bale wall up against the 3D wall.  If you secure the interior gap’s furring strips with Tap Cons (concrete screws) you might be able to slip baling string behind the strips and use that to sinch the bales tightly to the 3D wall.  You could also drill holes for tie strings or wires after the 3D wall is printed, or secure the bales with plywood plate washers and all-thread rod. All do-able...but more extra materials and labor not usually associated with either wall system.  If you penetrate the interior of the 3D wall with any lateral connections you’ll probably need to cover perforation with something more attractive, like plaster.  To make sure the exterior of the straw bale wall is an air, insect, and rodent barrier you’ll need to plaster it at least one, or possibly two coats, and make allowances for letting 2x blocking into the bales for the firring strips that will support the siding.  This is how we typically detail a straw bale exterior that will have a siding finish—the steps aren’t “extra” except that you already have another rainscreen between the two walls to address the condensation concern.  I can hear the cash register ringing!

Mass.  It’s possible to over mass a structure.  Too much mass and it can take a very long time for a room to feel comfortable.  Distributed thermal mass in the form of 1” – 2” of plaster covering a good amount of insulation (like a straw bale wall) is usually more than enough in both modeling and real-life energy efficient structures I have worked on or been in, at least when it comes to moderating internal temperatures as outside temperatures fluctuate.  Over massing means that the structure is going to always feel cold unless you make an effort to heat it, and it takes a lot of energy to bring an 8” or more thick thermal mass to room temperature and keep it there.

There’s also the question of dew point in an extreme cold climate.  I have no idea where the dew point is on the wall assembly you’re contemplating, but if you were thinking of a stand-alone straw bale wall I’d be advising you to consider what’s called a straw-cell wall.  This is a straw bale wall with a 2 x 4 frame attached to the exterior filled dense pack cellulose.  It’s like adding a sweater to a building (described and discussed briefly in Straw Bale Building Details: An Illustrated Guide for Design and Construction, CASBA’s book published in 2019).  Chris Magwood with Endeavour Centre in Ontario and Jacob Deva-Racusin with New Frame Works in Vermont pioneered this straw bale wall assembly for extreme cold climates specifically to address the dew point issue.

If you want to proceed with this 3D printed/straw bale wall despite the considerable extra cost, not to mention the really high carbon footprint of using a presumably Portland cement-based material for the 3D wall, consider investing in modeling that offers an educated prediction of how this unusual wall assembly will perform before you build it.  A WUFI analysis simulates heat and moisture transfer through materials assembled in a wall—I don’t know what they cost, but certainly a lot less than trying to fix something that doesn’t perform as expected.

Good Luck!

Jim Reiland
Many Hands Builders

FROM What is WUFI?
"WUFI is a Windows-based program for the hygrothermal (i.e., heat and moisture) analysis of building envelope constructions.
WUFI is an easy-to-use, menu-driven program for use on a personal computer which can provide customized solutions to
moisture engineering and damage assessment problems for various building envelope systems. "
Thanlkyou for the post Jim.

And thank you for the assist John.  Sometimes I forget about the design and building jargon, especially acronyms!

OMG Jim, this is such a detailed answer to basically all the questions that I had in the last month. Thank you so so so much for taking the time to address my questions and even more. I have not thought about over-massing would be a problem. I normally do not build with such massive wall either!

I totally, wholeheartedly agree that this double wall system is redundant, over-massed, complicated, expensive, labor-intensive, quite incompatible, and still carbon-heavy. But the client is really excited about the idea of using three different materials, straw, wood, and concrete/earth, especially the permanent/durable nature of concrete/rock. He must have taken the story of Three Little Piggies a little too seriously.

We are currently looking for a way to print earth instead, but most projects that 3-d print earth seem experimental and funded for research rather than this private project.

I really appreciate the research and resources you recommended. These are fascinating! especially the Japanese example. Your suggestion of using fibrous rain-screen matting is very appealing. It can minimize the air gap that can introduce moisture but also separates condensation from strawbales.

Another much debated topic is the use of earthen render or lime plastering of the strawbales. The owner was apprehensive about plastering the bales' exterior because...
1. the Client will DIY a lot in this build, and does not want to prepare and apply plaster. He is more inclined to use mechanical/dry fastening systems.
2. Could be cheaper/easier/faster to apply a housewrap to achieve air-tightness and waterproofness.

So, the properties that are missing in tyvek, but present in plaster is that 1. fire retardant. 2. Pest Prevention
Is it absolutely necessary to include plastering because of these two reasons?

We are currently in the process of running environmental simulations to see what would be the best course of action.

Thank you again for your time, and your advice cleared a lot of things up for us.
Best,
Jay Yoo

Hi Jay Yoo,

I wonder if you can talk to your client about using these different elements--3-D printed wall, straw bales, etc., in the same building, just not in the same wall assembly?  For example, a Trombe wall on the south side of the building, bale insulated walls elsewhere?  What you're proposing is novel (in my experience); if people think it's expensive to build a house, then they have a pretty good idea of what it takes to re-build all or part of a house that has failed.  That concerns me most with novel wall assemblies.  On the one hand, experimentation pushes the envelope--whether they succeed or fail we can learn something so long as designers and builders talk about it.  On the other hand, when they fail, they can be very costly to repair or replace.  The question I'd be asking is "if this doesn't perform as expected, or outright fails, can you afford to do it over?"  

I’ll give you an example of an experimental wall that failed.  Back when my wife and I built our straw bale house I followed what turned out to be not great advice, at least not for my climate and house design. Applying a lime-over-clay plaster was, in the 1990s and early 2000s, a much-talked-about environmentally friendly way to lower building costs and use more site-found materials. The suggestion to take this approach even appeared in a book about natural plasters written during that period.  Lime is more costly than site-found clay, less environmentally friendly, but more durable as an exterior finish. The thinking of the lime-over-clay plaster approach was to apply a scratch and brown clay-based plaster covered by a much thinner lime finish.  Doing a bit of due diligence, we plastered a 50 square foot test wall, which looked pretty good after a few weeks, so we did the entire house with this regime. That wall still looks pretty good, but it was also the most sheltered and least visible wall on our project.  Had we chosen a more exposed wall and waited several years for the temperature and moisture extremes prevalent in our climate to have an impact, we would have decided differently.  

This technique may work in some climates, with some building designs, and using more lime-compatible clays, but in our case, we had significant delaminations.  It turns out that many, if not most clay plasters have very different moisture and temperature expansion-contraction characteristics than most lime plasters.  Reports of success using this regime—lime-over-clay—have been spotty and difficult to replicate.  Success appears to be associated with dry, temperate climates, buildings with very large (protective) roof overhangs that shield the walls from sun and wind-driven rain, and highly local (and thus not widely available) clay soil types.  As walls are wetted by wind-driven rain that penetrates the thin lime plaster the clay expands a bit.  When dry conditions return the plaster dries and shrinks. Similarly, the sun beating on south and west facing walls (in the N. hemisphere) heats up the plaster layers which respond by expanding very slightly...but moving at different rates.  The bond between the two layers is mechanical—the lime is hanging onto just the lath of a scratched and roughened clay plaster surface. Over time the movement from moisture and temperature changes causes the layers to decouple.  In our case--and the numerous straw bale homes in my area that had the same problem--large sheets of lime plaster delaminated from the most exposed walls.  I suppose I could say we have had a 90% success rate with this system since after fifteen years only 10% of our wall surfaces have failed, but who knows what things will look like in ten or twenty more years?  Although I’m retired I’m still physically able to replaster the walls using a proven method (because I don’t want to do this when I’m too much older!), but if I had to hire this done it would easily set me back $25,000 in 2023 dollars (for rural S. Oregon—probably $50,000 in more urban areas) to change out the plaster system (to all clay, or all lime) on 1,300 sq. feet of wall surface!  

So, word of caution—beware untested wall assemblies unless there’s a back-up plan and the resources to do them over.                                                                                                                                                                                                                                                                                                

To your question:

"So, the properties that are missing in Tyvek, but present in plaster is that 1. fire retardant. 2. Pest Prevention
Is it absolutely necessary to include plastering because of these two reasons?"

Plastering the exterior of the straw bales functions as a fire retardant, prevents pest intrusion, is an extremely effective air barrier if the plaster doesn't crack (which usually means a scratch and a brown coat), is sometimes part of the structural design, and when it is visible, can be quite beautiful.

In your proposed wall assembly it sounds like you’re assuming the 3D printed wall will crack, but a painted-on or peel-and-stick membrane on the exterior of that wall should be an effective air barrier at least for air travelling between the interior and exterior.  I don’t know whether air will move through the bale portion of the wall—my understanding is that air moves because of air pressure differences between spaces separated by a barrier (wall); the narrow space between the interior of the straw bales and the 3D wall may not really be that different from the exterior, so perhaps there’s no air movement there.  The same is true for water vapor, which mostly travels on air currents through openings in walls.  If you also do a good job air-sealing the exterior wall surface I don’t think that much water vapor will make it into already-dry straw bales.  Two coats of plaster would be my first choice to accomplish this.

The challenge with using a building wrap like Tyvek is how to attach it to straw bale walls. Consider letting 2x framing into the bale walls at 16” or 24” centers.  Or use an exterior framing system whereby bales can be stacked between the studs. Staple and tape the building wrap to these studs.  Theoretically you now have an air barrier that’s also vapor permeable, and it would also likely resist pests, too.  Some might attach siding directly over and through the wrap and into the studs, but a better practice nowadays in conventional wall systems is to affix furring strips over the wrap-covered 2x framing (so there’s a rain screen gap) and attach siding to that.  Be sure to use some kind of porous barrier or screen at the top and bottom of the wall so air can circulate and liquid moisture can drain down and out, but insects and critters can’t get in.  

Decades ago some straw bale walls were built with a building wrap attached to bales using landscape staples. Perforating the wrap with a bunch of tiny holes somewhat defeated its function as an air barrier, but it’s still a common and even recommended practice on straw bale buildings for the lower course(s) of bales where deep snow might sit against the bales all winter long, or when designs call for un-guttered roofs which will splash water on the bales at the wall’s bottom.  I have seen (and repaired) lots of damaged plasters at the base of straw bale buildings because original owners or builders didn’t want to use gutters.

Jim
Many Hands Builders