www.inhabitat.com

I just received an email in which a man asked about using bales below grade, as in an earth-bermed house. He wants to use bales due to their high insulative value, but is concerned about the effects of moisture on the bales. I too would be concerned about the moisture. We are trying to figure out a way to make it work as it would be a great marriage for sure!

I suggested that if he could find a way to use a material that would provide a break between the bales and the backfill, it may be possible. In other words, he would need to place the backfill against a structural element (what is yet to be determined, but perhaps concrete and a plastic waterproof membrane?) that would provide the strength and protection from ground moisture. He could then stack bales (the back side pre dipped in plaster for fire protection) slightly away from that wall so to leave a ventilation channel between the bales and the backfill assembly. That channel may need mechanical help in providing adequate air movement to protect the bales from moisture.

Seems a bit over the top to me, but I would like to see what other options you may have for such a job. I would really like to see this as a possibility in the future. Thanks for any input you are willing to share.

Here’s another reason that your foundation deserves the highest focus and attention to detail: your entire house sits on top of it! If you skimp here and something fails, it’s not an easy fix. Is it worth the extra few dollars to add a bit more rebar to the slab? Yes. Ensuring that the foundation is built to the very best standards possible is very much worth it. Here are some simple things to look out for when building your foundation and/or slab.

1. Make sure that the rebar is the right size and is laid out in the right spacing for your soil/geological conditions. A typical residential house in the United States uses #4 rebar in the foundation walls and #3 in the slab on an 18”x18” grid pattern. The layout for the foundation rebar depends on the size of foundation wall and the local codes.
2. Do not allow any rebar to “daylight” or even come close. No rebar should end or be placed within 4” of the edge of the concrete. Rebar closer than that can draw moisture from the outside and rust. That rust will creep down the rebar and, over time, render the entire rebar system useless.
3. Pay extra attention to the layout of your foundation and slab. Make sure that the corners are square and level. The closer to perfect you are, the better, but in no case should you be more than 1/4” out of square or level for a roughly 2000 SF house. Use a laser level if you have one or the best standby of all time: a water level. It’s the cheapest level you’ll ever buy. It’s basically some clear plastic tubing, water, and a little red food coloring to help you see the level lines better. You can even use this set by yourself.
4. Wait until the water has evaporated off of the top of the slab before you start finish troweling. If you press that water back into the surface of the concrete, it will weaken it. Allow it to cast off the water it doesn’t “want” and then get on it for the finish work.
5. If you plan to acid stain your concrete, don’t over finish the surface. If you polish the surface too much, you will seal it beyond what the acid stain can react with. You can definitely get the surface smooth, don’t get me wrong, the key is not to power trowel the heck out of the slab.
6. Use a stepped foundation when applicable to minimize concrete use. If you have a sloped site, step the foundation up or down the hillside to work with the topography. Be sure to measure the steps and keep them in line with the bale courses so you can step the bales too down the road if that works with your design. This won’t apply in all cases, but if it does, it’s great to get it right when stepping the foundation to keep your bale work easy.
7. Spend some extra time around your foundation bolts when finishing the slab. Many people don’t put a lot of attention here because they figure “it will be buried in the wall so who cares if it looks good.” This is one way that mistakes compound. When you add your 4×4 toe ups to a series of foundation bolts sticking out of poorly finished concrete, you will quickly find that the 4×4’s won’t sit flat. The thick 4×4’s won’t bend like a 2×4 to fit flat either, so you will be left with a toe up that’s up in the air in some spots and flat in others. This not only allows for air gaps through which bugs can also travel, but also messes up the framing before you even start it. Finish those areas well and you will be happy you did.
8. Use Wedge Bolts or other “after cure” anchors for the interior toe ups. By adding the interior anchor bolts after the concrete has been finished you can get a better finish on the concrete (not only for the bolts as described in #7, but also for your floor which will come very close to the anchor bolt locations). Using the drill in bolts also makes the layout and installation of the interior toe up a lot easier and more accurate.
9. Be sure to vibrate your form boards to eliminate “honeycombing” of the concrete. This not only improves the strength of the wall, but increases the beauty. This can be as simple as pounding a hammer against the form boards while the concrete is still wet. Do this BEFORE you finish the surface as the vibration can make the surface of the concrete drop a bit.
10. Use adequate bracing for your pours. There is nothing worse than having a form board blow out during a pour. It means more concrete will be used and your nice straight line will be shot. Use lots of diagonal braces to support the forms during the pour.

The catch is that how your roof is built is perhaps one of the most important details in the entire project. Why? There are several reasons. Here are a few.

1. Insulation Value. Everyone talks about how amazing straw bale houses are for creating a super insulated home. That’s true, but only as true as the roof is insulated. I think it was grammar school where I learned that heat rises.  That law of physics is still true today and if your roof is not properly insulated, that law will ruin your super insulated home. There’s simply no point in building a house with R-40+ walls and an R-30 (code for vaulted ceilings) roof. Over insulate whenever you can. I find that two layers of R-21 insulation fit perfectly into a cavity created by 2×12 framing or 11 7/8” engineered lumber. Those are the two most common rafter sizes in vaults, so there’s no reason you should not have at least an R-42 roof. If you are using trusses, the insulation can be even higher.
2. Strength. As I mentioned above, the most common rafter size these days is 2×12 framing or 11 7/8” engineered lumber (BCIs or TJIs). Don’t skimp on design or implementation here. I remember reading an article several years ago about a man who was killed when his “straw bale house collapsed” (according to the newspaper). When I read further, I saw that the straw bales had, in fact, not collapsed nor had they caused any damage to the house. Instead, a large snow load had caused his under-sized rafters to collapse under the weight of the wet snow. A tragedy for sure, and one that could have been avoided if the rafters were sized properly.
3. Protection. The roof obviously does a great job of protecting you during weather; however, the protection I’m talking about is more about proper passive heating and cooling design. Be sure to take the time to properly design the roof overhang to provide the proper shading and solar gain for your latitude. Every site is different as is every design (I hope), so it’s really important to take in account all of the factors you have on hand. For example, site slope, latitude, potential shading from neighboring structures or landscapes, standard weather patterns for the area, and so on. The more data you can amass, the better.
4. Slope. The slope of the roof can impact many aspects of the home. The steeper the roof slope the quicker an overhang will encroach on window views as it is extended, the faster water and snow will slide off of the structure, and the less practical roof repairs will become. If the roof slope is too low, water and snow will build up and won’t drain away quickly. This can cause leaks and damage may occur to the house. A medium sloped roof may be the perfect choice not only for the above details, but also for the inclusion of solar panels to power the house and heat the water. As always, the exact slope will depend on the design and the natural impacts of the site.
5. Overall Design. The more complicated your roof design, the more expensive it will be to build. It’s hard enough to build a good roof with even the simplest design. If you add lots of valleys and hips (intersections of other roof lines, etc…) things get complicated quickly. After all, the roof is built up in the air. That’s not easy to do. Then once framed, the complications continue when t comes time to flash and waterproof it. Finally, the installation of the finish material itself is also complicated. What you use for the finish material can also have a large impact on the functionality of the roof.  Standard asphalt shingles don’t make a good surface for water collection, metal roofing (although great for water collection) can reflect the sun and heat into neighbor’s properties, natural materials like slate have pros and cons as do the recycled “fake” counterparts. Check out all the details before you make a decision. Also, don’t look only at sticker price. Consider the lifecycle cost of the material. If a metal roof will last you 50-150 years, will provide water collection, and will reflect heat away from the home, then perhaps the added sticker costs are worth it.

This is just a sampling of the things I encourage you to consider when thinking about and/or designing your roof. There may be other site-specific things I have not listed and those are always important to pay attention to.

Below are some of the affects that climate has on a bale home. Some of these may seem obvious while others may not. I’m sure I’ve missed some, so feel free to add your own in he comments section.

1. Rain can saturate walls if they are not properly protected. You may have heard the saying “Big Hat and Big Boots” referring to a good roof overhang and a large raised foundation. These are two very good ideas if you live in an area with high rain totals. Also consider adding a waterproof membrane above the bales, just below the bale stop or box beam. The challenge is avoiding punctures in this membrane during construction. Make sure it laps over the edge of the bales a couple inches. Too little overlap and water can still make it down into the bales. Too much overlap and your plaster will not adhere to the bales at the top of the wall.
2. Humidity, unlike rain, cannot be designed out of the equation with a hat and boots because it permeates everywhere and it can cause moisture to build up in the walls if not handled properly.  The best approach to high humidity is two-fold. First, build with quality materials that can help in removing excess moisture from your air/wall interface. Hygroscopic plaster such as lime or earth is a great idea as it will naturally help keep a constant moisture level in the wall. If there is excess moisture in the air, it will absorb it and hold onto it (until it reaches saturation of course). Once the air dries out below the level of what the plaster is holding, the plaster will release the excess moisture back into the air. The second approach is a mechanical one. Install an Energy Recovery Ventilator (ERV) to help keep the air in your home fresh and to remove excess moisture. These are simple to install and work very well. They are also very energy efficient. Don’t rely solely on them as anything mechanical can fail (power loss, broken parts, etc) so be sure to use them as part of the two-fold approach.
3. Cold weather can have all kinds of impacts. One that is often overlooked is the condensation of moisture in the walls. Because cold climates require us as humans to heat our indoor air space to stay warm, we create uneven climates from one side of a wall to the other that are quite drastic. If the warm and moisture laden air from the inside of the house pushes into the center of a bale wall where everything is cold, the moisture can condense on the straw. The best way to win this battle is to not heat your home and live in the cold. Not very likely option I suppose, so another plan of attack is to make sure you seal any penetrations into the wall and seams between the wall and other surfaces. The most common areas are around electrical and plumbing installations and at the floor to wall and wall to ceiling transitions. Be sure to pay special attention to these areas and you’ll be fine. Use foam gaskets at the electrical and plumbing installations and vapor tape and/or caulking at the wall to ceiling/floor transitions.
4. Hot climates pose their own set of issues. Of course, hot climates that are associated with high humidity must be approached in accordance with #2 above; however, dry climates have a different set of concerns. One that is often not considered is the life of the plaster. Natural plasters are not built with chemicals designed to help them resist cracking and thus must be installed carefully. Be sure to plaster the house when it is protected and ONLY then. Hang tarps from the eaves so that you can plaster in the shade and out of the wind. These two aspects of a hot and dry climate can ruin a plaster job. Keep in mind that your plaster is not only the “look” of your house, but also its protection. If it is compromised, so too is the substrate, in this case, your bales.

Like I said at the beginning of the post, this list is in no way complete. There are many other climates that should and will be considered and many other affects of each of those climates. This is a list to do one thing: get you thinking. I hope it does just that. As always, I welcome your comments and input.

What’s also cool is that this holding barrel is hydraulically lifted, so there’s no strain to your back. You simply hit a switch and the hold thing lifts up and dumps the material into the mixing barrel. Add your water and you’re off. While the plaster is mixing, you can get the next batch pre measured and into the holding barrel. This is especially useful when using a material like Natural Hydraulic Lime because that material has to actively mix for 20 minutes. While it’s mixing, you prepare the next load. It’s a great use of time. Otherwise, a 20 minute mix takes 30 minutes start to finish. This way, it’s 20 minutes on the nose.

Another aspect of working with plaster is moving it from the machine to where you need it. Most machines require a wheel barrow, a string back, a well inflated tire, and some relatively good balance. This is usually not a problem (save the well inflated wheel barrow tire), but it is work. This machine would simply pump the mix from the mixing drum directly to where you need it. In fact, it can pump the material up to 10 stories up! Most of us won’t need that, but it does help to send the mud up a steep driveway or hill where a wheel barrow might end up costing you hundreds in chiropractic repairs.

So now we have mixed our plaster, placed it within inches of where it will be used, and the only thing we have had to lift is a bag and half of lime at 35kg each. Not bad. So how about applying it, any savings there? Of course. The machine uses the same compressed air that drives the mix up the hill to run a plaster spraying gun. This speeds application and does a great job of both leveling the wall on the first shot and penetrating the straw.

The down side to this machine? It costs about $8,000. If you can find one to rent or borrow, go for it. Otherwise, just think about how cool it is and marvel at what we have been able to build as humans!

The most common places for such infiltration is through the back of electrical plugs and other wall penetrations, at joints around partially exposed framing (picture interior timber frames exposed, yet also in contact with the plaster), and the floor to wall joint. For the back of the electrical plugs, either use a sealed electrical box typically used in exterior installations or shoot some minimally expanding foam into the box after the wiring has been completed but before the switches and plugs have been set. For the joint along the timber frame, be sure to reinforce the plaster with blood lath where it meets the wood and then consider using either a bead of caulking after the plaster has been applied and has cured, or a seal similar to the one used at the floor to wall joint.

As you can see in the picture above, the floor joint is covered with an adhesive flashing that folds from the floor to the toe up, sealing the gap that would otherwise be left to move moisture into the walls. This works very well, but there is one draw back…okay, maybe two. First, the floors and toe ups have to be very clean to allow the material to stick and seal the joint properly. That’s hard to accomplish on a job site. Secondly, if the floor is  finished slab, or some other finish material, you risk installing the adhesive material where it will be seen or leave a sticky residue on the floors.

A material I like to use in place of the adhesive flashing is a sill sealer. It’s a simply roll of foam that is installed underneath the toe ups at the time of their initial installation. Because it is a malleable foam, it fits any shape (within reason)  and seals it tightly. It’s also really easy to install, and the floors don’t have to be so clean you could eat off of them either.

Sealing these areas will increase your building envelope’s efficiency greatly and will extend the life of your bales. Two really good things to have in your house and for your peace of mind.

Picture: BNPS

If you live in a coastal area or mountain region, you probably have more experience with big wind gusts than someone living in a quiet little valley (except for those screamers that whip down the valley from time to time). The point is that wind is different wherever you go and building codes reflect those differences. Some areas in the United States, like Florida, Texas, and other Southern Coastal states, have to design their homes to withstand hurricane force winds while areas in Tornado Alley have to build their homes to handle twisters.

This morning I was looking through the internet in search of facts about straw bale construction and high winds and I was shocked to see that the hard data is far and few in between. This seems to be an area in which some studies have been done, with promising results, yet little follow up and publications exist. I hope that I am wrong with this assessment and that, in fact, there are studies and papers out there that I am missing. To that end, if you have any leads I should follow up on or if you know of specific resources in regards to high wind/straw bale studies, please let me know. The following is what I was able to discover and, like I said, it is very promising.

Several years ago wind tests were performed on a straw bale wall in a wind tunnel similar to that described on the Nordic Wind Tunnel website. The test placed the equivalent of a 75 mile per hour gale force wind on the wall. In order to pass the test, the subject wall was not allowed to move more than 3/8″ from its original position. In the initial testing, the wall passed and, in fact, it did not move at all.  The test crew increased the wind load up to a gale force wind of 100 mph. Even with the increase in speed and force, the straw bale wall performed really well, moving only 1/16″, far below the limits provided for in the test.

In Serious Straw Bale, a book by Paul Lacinski and Michel Bergeron published in 2000, reference is made to a study performed in conjunction with the Canadian Mortgage and Housing Corporation (CMHC) in which hurricane force wind loads were applied to a test straw bale wall panel. The findings suggest that straw bale walls are vastly superior to conventional walls in resisting wind loads. The results showed that the panel withstood wind loads of 153 pounds per square foot (psf). This means that the straw bale panel could be rated at more than seven times a standard hurricane design load. That’s impressive!

As amazing as this all sounds, I think we need more. I would like to see more research done on the strength of straw bale walls and their ability to withstand high winds. We are seeing bigger hurricanes, more often here in the United States, and no one would likely deny that the same is true for tornadoes.  We may be sitting on a building technique that can protect thousands of people from the forces of nature and yet we are not in a position to take that technique to those who need to hear about it because we don’t have the scientific testing to back up our claims. It doesn’t matter how amazing these structures appear to be. We need to know without a doubt that they are as great as we believe and the only way to convince the “powers that be” in the construction world is through independent testing.

I offer you a call to action. Are you in a place to take part in that testing? Are you a professor or someone else with access to a wind tunnel? Do you have grant writing skills and/or the desire to headline this effort? There are so many things I want to do to increase the popularity of straw bale construction around the world, but I realize I just don’t have the time or ability to do them all. That’s why I’m asking for your help. If we can get the right people together and the right resources lined up, we can make this happen. It’s possible that the results of our efforts will help save lives in the future.