Using a Modified Stick Frame to Easily Work with Straw Bales
A straw bale house is about so much more than stacking a few bales, and slapping on some plaster. If you desire to have a home that is comfortable, energy efficient, code compliant, and modern in its amenities, it requires careful planning and foresight.
Recently, we completed our own addition using straw bales and a clay/ lime plaster system. We used a modified stick frame for ease of building and cost constraints. This post details how we did the framing. Remember that the less experience you have framing, the more details you’ll need from your engineer or designer. This is not an article on how to frame. It is an article to explain how to modify a stick framing system to accept straw bales. If you’ve never done any framing, we strongly recommend getting the help of a professional.
Framing a Straw Bale House
First, when I say framing, I’m specifically referring to the details involving the straw bale parts of the house. I would define this as starting where the wall connects to the foundation, and includes windows, doors, up to the top of the wall, where it intersects with the roof assembly. The roof plan is a separate component of the overall building, though it is still part of the framing process. All I will say about that now is it is worth sorting out a highly insulated roof, to match the insulation value of the walls. In our project, we had a vaulted ceiling, and in our area, code only requires R28. We bulked that up to R50.
Sill Plates to Wall Framing
We did a rubble trench foundation, comprised of drain rock filled from the frost line to grade. At grade, we poured a concrete bond beam (the details of the foundation will come in a future post). When the concrete was wet, we inserted anchor bolts to serve as the attachment point between the foundation and the walls.
Once the concrete had set, we proceeded to attach the sill plates (first using a thin piece of foam gasket to help seal against air infiltration), and bolt them down.
Before we got going with bales, we insulated the sill plate using leftover Roxul (the rigid Comfort Board product) from the insulating of our foundation.
Wall Truss System
To frame the walls, we used a series of Larson trusses created by connecting two, in this case, 2×4’s together with two gussets, just pieces of 2×4, per truss. Using the trusses allows for the pretty standard framing techniques and materials, with some modifications. The trusses give structure to the walls, and they almost eliminate thermal bridging. This system has the added benefit of being an affordable method as well.
We built all the trusses on the ground, hammering the nailing plates ourselves. The trusses are the width of a bale, so that the framing would end up flush with the bales, and the plaster could seamlessly flow over the whole wall. In this case, to save on space, we oriented the bales on ‘edge’, that is, the narrowest dimension (14”). The other orientation of a bale is referred to as on ‘flat’ (18”). There are pros and cons to each.
The Larson truss system works well only if you’ve paid attention to spacing.
Each Larson truss was placed every 36” (or 37.5” on centre).
This is an average length of compressed bale. It is important that the bales fit tightly in order to make sure they are snug to the frame, and aren’t wanting to fall out. Around windows, you just have to make due with resizing bales. This is totally fine, it is just that when building the walls, it is so much faster to not have to resize every single bale. We put in a great amount of effort to make as many 36” bays as possible, and those efforts were well worth it when it was time to bale.
We also carefully planned the final height of the west wall to be exactly four courses high. We added up the grade beam, the sill plate, and figured in the slope of the roof to arrive at the exact wall height we wanted and it meant a very snug, solid, and efficient wall. If your last bale fits too loosely, you’ll have to stuff to make up the difference, and stuffing is never as good as a bale. Why would you go to all the trouble of these amazingly insulated walls, only to have a 4” cold gap along the top of the wall?
A good rule of thumb is to count how many courses you’ll be going up, and then subtract 2” in the framing. This allows you to compress in the final bale. 2” because, if you’re working with good quality building bales, that is about how much the wall would naturally settle.
The design of our building required that we have three structural posts in the exterior walls.
To deal with the change in dimension from the Larson truss to the post, we built insulated box columns. Basically, the structural element was specified by our engineer, we used 4 2×6’s ganged together to create the ‘post’. To create a box, we simply added a top and bottom plate, and built it out to the width of the wall. We used Roxul batt insulation to fill it.
Top Plate and Fire Stop
Once all the Larson trusses and box columns were in place, we attached a double top plate to connect all the framing together. At this point in the build, we started working on the roof assembly. Once that was all in place, and the metal was on, we were nice and dry underneath, and we could continue with the walls.
Where the roof trusses connected to the wall, there is a gap where the roof insulation could actually just fall on top of the straw bales in the wall. Because the top of the walls are angled to match the roof, there would be a bit of stuffing of straw at the top of the wall, and we wanted something solid to work against.
For this reason, and a few others, we used a fire rated 5/8th gypsum board cut tight to fit flush between the top plate, and came back with fire caulking to seal it. Fire is another topic (and there is lots of great information on how resistant and safe straw bale walls are when it comes to fire, resources can be found at fire engineering), but the point here is that we created a break between the wall and the roof.
For windows we followed standard practices around king and jack studs, as well as header sizing. What is unique though, because of the thickness of the wall, each window was basically framed twice. Once on the outside plain (where the window itself was mounted) and once the inside plain.
The west wall of the project was designed to take much of the load from the roof, so there wasn’t too much space to play, but we added a layer of plywood between the king and jack studs on the outside plain, but not on the inside. This allowed for the window to have a bit of a flared opening. On the south and north walls we were actually able to offset the framing the width of a 2×4 by omitting the jack stud on the interior plain of the window. This allows for light to travel deeper into the space, and has a soft appearance, without things getting too round.
Interior of window rough openings. Window on the left has a double stud on the exterior plain and single stud on interior plain. This is how we created the flared opening. The window on the right is on the main bearing wall, so it has a double stud on the interior as well.
We chose to use wood for the window sills and lintels, but we wrapped the plaster into the sides of the window wells for a nice sharp, clean look. We could have used wood on the sides of the windows as well, but our budget was very tight, and we thought the plaster/ wood combination would look great. It is also possible to plaster the sills and lintels on windows, but in this case, there was no space to hang a bale above any of the windows to plaster onto, and having a wooden sill is nice for setting plants and books on. We cut each board to match the flare of the window wells, and then left a bit extra to meet the plaster.
Here are some photos of how it came together. Next up, we’ll share a post about the baling process, and the detailing we focused on to ensure high performance, healthy and comfortable results.
So, when you are designing and planning, and working out your framing system, it is so important to think like a bale, and think about how you’re going to get those bales to do what you want.