Straw Bale House WITHOUT COB (or plaster)??

I have been interested in straw bale building in Ohio for a long time, my most desired pieces of it is the insulative value of the structure, and also it being fireproof! However I also live in southern Ohio, we have some really wet humid times of the year. I am curious if there are any ways to use straw bale on the external walls of an existing wooden lathe and plaster farmhouse, without having to use plaster or cob to cover the outside of the structure, while still being able to fight off the moisture and make it insulated. What ideas or experience do you have? I already have a bit of a problem with mice so i am wondering is here is a more solid way to keep the mice and bugs out of this area, what about metal siding? Wooden external siding with house siding over that? Please provide any thoughts or ideas.

Why am I opposed to cob? I just know that the moisture around here does bad with cob, and i have never had a good experience with the trowel. Things crack and it will take way too long.

You can use lime plaster, which I think is more frequently used with strwbales anyway. Good overhangs will also help.

Joshua, too many questions!
As Christo mentioned, lime plaster is used most times.
Straw is not always fireproof, check that out.
Mice are usually not a problem either, if straw and not hay is used.
Cladding is possible, but not common.

We live in a pretty humid part of Texas and don't have any issues with clay & lime plaster on our light clay straw walls. You could attach larsen trusses to your existing wall, infill with straw bales and cover it over with wood siding. I would avoid using metal siding or any other material with low vapor permeability so that any moisture that gets into the wall assembly can escape and not cause the straw bales to rot.

The downside of this approach is rodents and insects will probably eventually find a way to get around the wood siding and make a home in the straw bales. I second Cristobal and John with the lime plaster and large overhang suggestions. It dries slower than concrete once it sets it is pretty impervious to fauna and the elements. It's also vapor open unlike concrete.

I believe without cob or plaster that fire proof might go away.

Seems to me that a metal, wood or brick with regular 2 x 4 framing would work.

Put up the exterior walls then stack the straw bales inside before doing the interior framing.

Hi there Joshua!  Sounds like what you're proposing is something called a "straw bale wrap."  It's do-able, but not often done for the following reasons.  

First, the bales themselves are quite heavy compared with more conventional insulations--upwards of 7 lbs. per cubic foot. Each 2-string bale will weigh around 65 lbs., and each 3-string bale could be closer to 80 lbs. You'll need to stack the bales to whatever wall height you have...which is a lot of weight!  While a Larsen truss system might work, those trusses will need to be really well attached to the farmhouse framing, and I'll wager that the existing framing isn't up to the task of cantilevering 100s of pounds of straw bale additional framing, plaster, and new siding weight per linear foot.

Most bale-wraps have a completely new concrete foundation poured around the existing building's foundation (and also attached to it if in a seismic region) to support the additional bale weight (plus the weight of any framing, plaster, and siding)..and that alone gives lots of people pause.

Then, if you get beyond that hurdle there's the roof overhang to consider. If the exterior walls of a building are suddenly 15" or 18" or 23" thicker, the existing roof may not provide adequate coverage for the new wall surface. As Christo and John mentioned, a good roof overhang is important, and you need to be guided by your typical rainfall in this regard. If you have very little wind-driven rain you might get away with 12" or so of roof overhang, but I prefer at least 24" and usually built homes with 30" - 36" of overhang, especially for taller walls. So your retrofit may require extending the rafters, and adding new roof decking and roofing (and a new gutter). On sloped roofs this will lower the roof line enough to drop it below the top of some windows, which impacts sightlines from the inside, and also reduces how much sun light enters through those windows. This has proven to be another challenge that dissuades people from pursuing a bale-wrap.

The final major impediment I have run into is the question about what to do with the windows. If the exterior walls are now much thicker, should the windows remain in the plane of the original exterior wall (which makes for very deeply inset windows as viewed from the exterior?  If so, the exterior window reveals need to be detailed against moisture intrusion--think really deep window sills and really good flashing!  Or could the windows be moved to the exterior of the new wall surface? Yes, but then you'll also need the framing from sill-plate to top plate to support the windows so they don't rack....unless you want to go with "floating" window bucks....

Here in the arid west I have seen quite a few straw bale walls that have metal siding, but the metal isn't up against the bale surface. When the bales are stacked we make sure there's framing in the exterior wall surface to support whatever siding will be installed, then we plaster the bales--a thick, leveling scratch coat--to give them some protection against insect intrusion, fire, etc.  After attaching 3/4" or 1" furring strips through the plaster to the now-embedded framing to create a "rainscreen gap" we attached either cement board or metal siding. The top and bottom of the air gap has insect screen. This rain-gap air space addresses the concern for moisture moving through the wall from inside and condensing in the bale wall, or moisture from humidity or wind-driven rain penetrating beyond the siding.

If you were willing to go through all of this I think it's possible to do a bale-wrap, even in your area.  I'd strip the existing siding off the walls to expose whatever insulation and framing you have there, do any needed electrical, plumbing, air sealing around electrical and plumbing perforations, and rodent-intrusion work, then pour a new foundation, extend the roof, add whatever framing needed to support the new roof extension and windows (if moving them to the exterior wall plane), stack the bales right up against the now-exposed original framing, and proceed with whichever siding system you like.

If your farm house was built pre-plywood (circa 1950s) it probably has let-in diagonal bracing to provide shear. You might want to check that this is still OK--now would be a good time to upgrade if necessary!

I'll also note that the original bale wraps were done on CMU (concrete masonry units) walled homes in the American Southwest, where the combination of interior thermal mass (8" of concrete) and exterior plastered straw bale insulation made for really comfortable and stable interior temperatures.  

If your farmhouse has lath-and-plaster interior walls it has quite similar thermal storage capacity to the 1"+ of plaster used on the interior of a typical straw bale wall assembly. The original plaster may have been gypsum or lime (which are vapor permeable), but it has also probably been painted and repainted many times over the decades?  The many layers of paint will function as an air barrier. You may want to remove it (be sure to check for lead paint and do it safely--paint applied prior to the early 1970s in America probably had lead in it...) so that any moisture in the form of water vapor moving towards the cool dry interior (assuming you run air conditioning) during hot, humid summer days doesn't get trapped against the inside surface of the interior original lath-and-plaster walls).

Complicated, right?  You're building an entirely new wall assembly--including a new foundation and extended roof--around your house and using the original exterior walls as an insulated chase for electrical and plumbing.

I have no doubt that you'll achieve all of the thermal efficiency you had hoped for, but if that's your primary goal there are other ways to achieve it. For example, Larsen truss walls that are only 4" or 6" thick and filled with a lighter weight insulation, and whatever siding you want. Or you could use wood-fiber exterior insulation panels (a lot more environmentally friendly, though not as "efficient-per-inch" as rigid foam) attached to existing sheathing, then add new siding. You still need to deal with detailing the windows to the new wall plane, but that's a lot less involved that adding a new foundation and extending the roof.

That's why so many people who consider a straw bale wrap end up not doing one--there's a lot to it!

Jim

Many Hands Builders

Hi Josh,
I don't have extensive building experience, but I built a straw bale addition on my home in south eastern PA that has been up for about 5 years. We probably have similar climates. I used clay based plaster that I coated with Beeckosil Fine from beeckmineralpaints.com. The paint has held up really well, and can take a direct spray from a garden hose. I've only had to repair sections where the plaster cracked or sustained a physical blow. With either, the underlying plaster does wash away with rain.

I also have really long roof rafters to lessen the chance of rain damage, but it seems the paint is the special sauce. I believe it was developed for restoring older buildings in Europe.

We do occasionally get mice inside from a space near a sliding door that I haven't fully finished yet. They don't ever seem to dig through the wall though.

"Wet humidity" is zero problem for either clay or lime plaster. Rain, on the other hand, is. As others have noted, overhangs are critical. My SB in the BC Interior is clay, and vulnerable to rain but protected from vertical rain with 4' overhangs and from horizontal rains with landscaping. (It's also passive solar so the overhangs are built for the latitude.)

The suggestion of Larson trusses is good, but you might get better insulative continuity from something like lime-based hempcrete or papercrete which can be made very lightweight with perlite (so no need for weight-bearing foundation).  Breathability would be a consideration but if the inside is painted I'm not sure I'd worry too much-- permeability requirements depend on whether  you have more heating hours or cooling hours.  If more heating hours, paint's impermeability is good. If more cooling hours, it's not.

You have a lot of questions but not enough info posted to answer them adequately. The one answer I can say unequivocally is don't use SB without plastering it-- it is definitely neither fire resistant nor rat resistant without plaster.

Weve been in a strawbale since year 2000 in California foothills.  Good drainage, good boots, good hat with ample overhang, and clay/lime plaster should work.   In kitchen/bath areas that can be moist, we also used lime plaster to avoid mold/mildew and it seems to have worked.   If there are areas that get direct or driven rain then those take extra consideration with roof, external porch wall, garden walls, etc to help protect bale walls.   Cob (as compared to earthen plaster) can hold up quite well and is easy to maintain if needed.   We ended up lime plastering a Cob solar/battery building that is quite exposed ro driven rain and it's doing well.  Our perimeter foundation is 12inches above ground with 1.5inch cutouts on both inside and outside to allow plaster to go below bales and provide support/drsinage for plaster.   Use gutters to minimize water sprayback to walls and keep downspouts as far from bale walls as possible.

Our straw bale house is very tight. It rained on the inside of the windows until we installed our HRV. We went with a smallish unit just to pull moisture from the shower area now the glass is dry in the winter. Never had a mice issue and we live in a place where mice are abundant. They don't chew through bales. Bats made a nest between 2 bales on the exterior right under the eave, we let them stay. We have not finished our exterior plaster - upper  half the house is still the lime plaster scratch coat. Has not been a problem. We had our garden hose leak, it sprayed up and on the exterior wall of the house for 24 hours. It dried very fast and no issues found yet.  

My own strawbale house is located in upstate South Carolina, just a couple hours drive from the Blue Ridge, a temperate rain forest.  On paper, at least, I get 50" per year.  No problems to report, so I suspect the OP's concerns are overblown.  Yes, SB buildings were pioneered in arid and semi-arid climates, but they can perform perfectly well in humid climates as well, given the proper detailing.  I wouldn't try to build a load-bearing SB structure here, where the bale walls go up BEFORE the roof, but that isn't relevant to the OP's question.

I will second several observations already mentioned above.  For sure, lime plaster is the most common finish, as I have used on my home.  Which has stood up very well, BTW.  Exterior clay plasters on SB are a thing, but mostly in drier climates.  There is a reason I didn't attempt an exterior clay-based finish here, even though clay is a lot easier to work with than lime.

Also, while an SB wall is fireproof even without a plaster finish, I wouldn't recommend it.  Resistance to fire arises from the density of the bales.  But that doesn't mean the exposed surfaces won't burn if ignited.  The fire would resolve into a smolder and leave the bulk of the bales intact, because oxygen can't penetrate sufficiently to sustain a flame - there have been documented cases of this happening.  But that would be little comfort if the flames leaping up the surface of the wall set your roof alight before extinguishing themselves.

My SB walls remain unfinished on the interior, and I can report that there is no concern about mice or rats interfering with the exposed bales.  Insects might be a different story.  I've not had problems, but then the exteriors of my bale walls are finished with a very durable lime plaster that went on very well - almost zero cracks over the entire house.

There is no reason why you can't cover a SB wall with siding instead of plaster.  But it would take a lot of work; the framing would need to be considered very carefully.  I'd recommend wooden siding of one type or another.  Metal or vinal could be used, but I'd definitely recommend framing for an air gap between the siding and the bales, as described above, to account for condensation.  And if it were me, I'd still give the bales one good base coat to keep insects out and reduce their vulnerability to fire.  If you do that, you're a third of the way to just finishing the wall with plaster, so why not proceed?

But there remain the concerns voiced above about adding the thickness of a bale-wrap wall to an existing structure that wasn't built to accommodate those dimensions in its foundation or roof overhang.  I don't know the OP's situation, so I can't say just how much trouble that would pose.  For sure, you do NOT want to skimp on the roof overhang on any building, ESPECIALLY on a natural materials building like SB.

I suspect that installing a bale wrap is more trouble than its worth, given the complications.  Though as a natural materials builder it pains me to say it, if I wanted to retrofit an existing structure for extra insulation, that sounds like a good case for a few inches of rigid foam board.

This is a great deal of experience in one place.  I do have a question regarding areas subject to high wind in conjunction with heavy rain.  I worked in codes and building science for some time and remember in Florida, 2004, that multiple hurricane events overlapping the same areas proved too much even for concrete walls.  Modern homes constructed in the U.S.have an internal vulnerability.  High winds can develop very strong pressure gradients between the outside and the interior of the home.  What we saw in 2004 in Florida was the fact that concrete blocks subjected to multiple high wind events with heavy rain saturated the interior of the blocks.  That water began to seep through the walls after the storms.  My proposal would provide a nod to rainscreen techologies.  If a wall's exterior is coated or covered by a water resistant material such as plaster or cob, then wooden slats could be anchored to the bales and siding placed over the slats.  There should be weeps on each piece of siding's lower limit and infiltration areas on the top of each piece,  That provides an equalization chamber that allows wind pressure (which can force water at least 8 inches vertically) to reduce such that any water than enters flows down the back of the siding and is not driven into the main body of the wall.  The slats must be limited to smaller blocks by being broken up both vertically and horizontally.  This reduces wind pressure buildup between the outside and inside of the structure.

A note on secondary foundations if attempting to place straw bales on the outside of a wood wall.  One should construct the outer foundation to be at least an inch lower than the original foundation.  This helps any moisture that might seem from the inner wall or a void along the top plate to flow down and out of the home.  If level or above the interior foundation wall, the opposite can occur and that would be challenging.

For Matthew in Clemson, I understand mice may not make nests in straw bales, but what about insects such as fire ants?  Just curious.

Richard Henry wrote:For Matthew in Clemson, I understand mice may not make nests in straw bales, but what about insects such as fire ants?  Just curious.

It's a good question.  All I can say is that, after many years, I've noticed no problems.

The worst I can report is that I used to leave some clerestory windows open in my unfinished house with the bare-straw interior walls.  Birds would fly in and out and, I eventually realized, make nests in the cracks between bales.  I don't leave the windows open any more.

I don't personally know much about straw bale houses, but I have really enjoyed watching Home Grown Hand Gathered build theirs on YouTube. They have a video where they talk about some of the frequent concerns they hear about such houses: https://youtu.be/koHbfApBEHA?is=Pmw1uybs72kuktIS