Gravel in your toe ups. Is that enough?
It's the million dollar question you've all been wondering about: is it enough to have just gravel in your toe ups or do you need something more? Okay, it may not be a question worth a million dollars, but making a bad decision here can indeed cost you money. In some cases just a little extra cash, but in others, a large amount of moola! I'm personally a big fan of using gravel in my toe ups and I've had great success with it over the years. Are there other materials you could put in there? Of course there are. However, the question remains if they're worth the cost. Below I've set out the steps I use to fill my toe ups. I give some tips on how to get the most out of the toe ups for your money. I also discuss how to make sure they will provide years of structural integrity, bale protection and insulation value. We'll start by analyzing the photo to the right for some important details.
Size Matters
First of all, you can see that the toe ups themselves are 4x4s. This is very important. In "the old days" we used to use 2x4s, but that's no longer true. Testing has shown that 2x4s are more likely to fail under the stress of loads such as severe racking from seismic activity or wind. Being that the toe ups are the very base of your straw bale wall, it's vital to ensure that this base will survive even the toughest tests.
Another advantage of using 4x4s is that you have more nailing surface for any mesh you plan to apply to your wall. You may already know that I'm a HUGE fan of using 2" x 2" welded wire mesh on all of my buildings. That's a different topic that you can learn more about in my blog. The key, as it relates to this post, is that the 3.5" of wood provided by a 4x4 gives you lots of room to hit your target when installing mesh.
Finally, the extra 2" of height means you have extra time in case of a flood. For example, if you're away and a washing machine hose breaks, sending water all over your laundry room floor, you would have a lot of extra time to discover the problem before water found its way into the bales. This may seem far fetched, but I've seen it happen. Water damage can happen fast, so providing yourself with extra time is the same thing as providing yourself with peace of mind.
Anchor Bolt Spacing
The specific locations for your anchor bolts, shown in the picture above as the little round details on the toe ups, will be determined by the location of your house. This is because the spacing changes as your risk of seismic activity increases. For some areas, you can have a bolt every 6', while in others, you may be required to place a bolt every 3'. Either way, two things are always true:
You will need a minimum of two bolts per section of toe up. This is true on the main sections of toe up (either the interior or exterior main runs), but is not necessary on infill pieces such as those that form doorways. The requirement for two bolts stops the toe up section from spinning around a single bolt, thus increasing the overall strength of the toe up.
You need to have a bolt within 12" of the end of the toe up. This is true no matter the length of 4x4, assuming we are talking once again about the two main runs of toe up: interior and exterior.
Start Out Fresh
Although a small amount of organic material in the toe ups won't likely cause any problems down the road, it's a good idea to clean them out before moving to the next step of installation. Keep in mind that there is always a possibility that the toe up space will come in contact with water should there be a flood. As such, organic material in this space is at risk, whereas the straw that sits atop the toe ups would be safe. Take a few minutes to grab a shop vac and clean out the toe ups. Once again, you'll provide yourself some peace of mind.
Once things are clean, make sure that the roofing felt or plastic that is placed in the bottom of the toe up space is seated properly. You will want it tight to the toe ups, or even stapled up a bit to the inside edges of the toe ups. Sometimes, it's easier to install the felt before the interior toe ups are set. This way, you can cut the felt a little wide and simply hold it in place with the toe up. This is fine as long as the material is ultimately clean and well sealed. In the picture above, things are a bit too dirty for my liking so we got to work with the shop vac.
To Felt or Not to Felt?
Why bother with the roofing felt? Great question. In fact, I've even had this conversation with code officials who were not sure why the felt was required, just that the code says it should be there. So, let's solve the mystery. It's in place to stop any chance of capillary action from wicking moisture up from the concrete and placing it in the bales. Consider it a cheap form of insurance. The gravel will also act as a capillary break, but the felt sure doesn't hurt. You won't need the felt if you're building on a raised, wood framed floor over a foundation. You will need it if you are placing your toe ups directly onto the concrete.
Toe Ups at Doorways
One place where I often see mistakes in the toe ups is at doors. Very often, people place the connector piece between the inner and outer toe ups at a right angle. They run this piece out from the edge of the door frame. This absolutely locks you into a squared off doorway. This is a problem since most people want the gentle curves at doorways as a highlight of their walls. The best way to provide yourself more flexibility is to install an angle piece at your doors. I typically install it at 30 degrees. This allows me to finish the door with a gentle curve, a sharp angle, or even a right angle. Options are always best at this stage of the game, so give yourself as many as you can.
This end piece does not require anchor bolts and is simply toe-nailed to the main toe ups. You may also notice in the video below that a section of the toe ups has been carried fully through to the exterior in the corner. This is not necessary. In fact, it actually creates an area of thermal bridging and should ultimately be removed for best results. With an R value of R1 per inch, this 18" run of wood is better than many people's insulated walls, but it's still far below the R40 our bales are providing. Best practice is to stop the toe ups at the interior edge and then fill the entire space with gravel.
Fill the Space
Now this is where the question of materials comes up. Most people think that adding rigid foam will greatly increase the R value of the toe ups. As much as rigid foam has a high R value per inch, it also carries a high toxicity rating. This is something that most folks building a green home are trying to avoid. So is it really necessary? I say no. I have built with gravel for many years and have never had anyone tell me that it was a cold spot in their walls. We have done multiple thermal imagery scans over the years and they always show the same thing: no thermal bridging in the toe ups.
So how is that possible? Trapped air. It's all about the air. I use 3/4" rock when filling my toe ups and this helps trap air in the spaces between the rocks. That air acts as an insulator for the toe ups. The key here is the size of the gravel. If you use sand, or even pea gravel, you will likely not trap enough air to properly insulate the space. If your gravel is too big, it will be hard to install and may trap too much air, also reducing the insulation value. I have found that 3/4" rock is a perfect material to work with.
What's That Cost?
The next question takes us back to our opening statement: how is this possibly even close to being a million dollar question? Consider that the gravel, or whatever material you decide to use, is installed underneath the bales. If there is ever a problem with your material choice that requires removing it or replacing it, guess what else needs to come out? That's right, pretty much everything else that makes up the wall. The bales, mesh, and plaster could potentially all need to be removed. Suddenly, you can see where the dollars add up!
Another place that money comes into play is with the material choice itself. Rigid foam isn't cheap and installing it requires some labor. Mostly because you don't want to install it the easy way: flat. Instead, you need to rip it into 3.5" strips so that it can be installed vertically. This allows any water that finds its way into the toe ups to escape from below the foam. As a side note, I only add sill seal (an open cell roll of foam) under my INTERIOR toe up. This leaves the exterior toe up to drain water to the exterior of the home should a leak occur.
How about the labor for adding gravel you ask? Not bad at all. Check out the video above to see for yourself. Install the gravel flush with the top of the toe ups. You can even use a small piece of 2x4 as a screed board to make that installation perfectly flush. Install the gravel relatively tightly so it won't settle over time. However, don't pack it in too tightly as that will knock out the air we need for the insulation. A gentle tap here and there is all you need.
Add The Grip
If you notice in the above images and video, there is nothing for the bales to hang onto. No rebar impalers (thank goodness we got rid of those nightmares), and no nails to grip the bales. That's by design. We want to have a free and clear surface for the toe ups while we install the gravel so that we can use the top of the 4x4s as a screed base. Once the gravel is set, you can add your nails to create some grip for the bales. I use 20d, hot dipped galvanized nails for my anchors. I space those nails roughly every 6-8" staggered across each toe up (from side to side). Be sure to bring them in approximately 1" from the edges so you don't split the toe ups. This also ensures that the nail actually takes a bite of the bale and doesn't miss it on the outer edge. See the nails in the photo to the right to get a sense of the spacing.
If you have windows where the bottom of the sill is within one bale height or less from the top of the toe ups then be sure not to add nails to the toe ups under those windows. You can still add nails to the inner toe ups, but not on the outer frame. This will allow you to slip the bale under the window while still providing grip on the inner toe up nails. You only need to hammer the nails into the toe ups by roughly an inch. Just make sure they are solid enough that you can't wiggle them and pull them back out. The goal is to leave as much nail exposed as possible so that they can adequately grip the bales.
Set Your Bales on the Toe Ups
It's time to set the corner "stone", the very first bale in the house. This is a joyous event, so be sure to have people around to celebrate with you. In fact, once the celebration is over, you can get them to help with setting the rest of the bales!!! Start in a corner and work your way out in both directions. Make sure to overlap the corners as you go up in courses. With each bale you lay, use a tamper to drive the bales down onto the nails atop the toe ups. This anchors them well to the frame and will stop them from sliding around. You need only tamp the first course of bales. The remaining courses are not in contact with the nail bed and won't be effected by tamping. You will, of course, have to tamp the walls in and out in order to clean up the vertical plane of the wall. However, that comes later in the process and isn't relevant here.
Now Get To Work
I highly recommend that you stand back and admire your work each step of the way. After all, it feels great to measure progress. That said, you'll need to get to work now pretty quickly as you don't want to leave walls partially baled. The risk for water damage is high when the tops of the bales are exposed. Once the walls are fully raised, they are much less susceptible to water damage. Rain that hits the sides of the bales, for the most part, can dry off in time. Water that lands on the top of the bales and seeps into the bale center can be a problem. So yes, by all means enjoy the progress you've made. Just remember to make a bunch more progress before you walk away from the build for the day. As always, protect your walls from weather if there's a chance of rain. You'll be happy you did should it start raining. You can learn more about the bale stacking process in our video series and book if you're ready to dive in deeper.
Happy Baling everyone!
Andrew Morrison