Internal Shear- T&G Cedar Walls

All of the internal walls of our straw bale house are shear walls- meaning that they are constructed to be structural panels that can resist lateral forces acting on them. Lateral forces are those that are parallel to the plane of the wall, and are typically wind and seismic loads. In simple terms, lateral forces could push over parallel structural panels of a building were it not for perpendicular shear walls keeping them upright. In our case, the external structure of the 2×4 framing with bales stacked as infill for the walls, does not meet the structural requirements that protect against racking, particularly as we live in a high seismic event zone. Our structual engineer calculated that we needed 36feet by 8feet of shear wall, which amounted to all of the interior walls in our house. After building our walls with plywood on one side and a LOT of nails, as well as metal brackets from the foundation to the end of each wall and strapping over blocking in the ceiling rafters of the main room, we were good to go.

Mostly, these internal walls occur around the bedrooms, the utility room and staircase. The lower room of the house is basically one big open area, with shear walls separating the mudroom and utility room. We decided to buy some tongue and groove cedar and cover all the plywood with it, so we found a whole lift of 5inch wide cedar that was rather rough, meaning lots of knots (and knot holes) cracks, scrapes, scuff marks, discoloration, but in a general state of usability and affordable!

For the upstairs bedroom/hallway zone we pulled the best of the pile and used a clear natural stain product called Osmo as a finish. Luckily, with the solid plywood wall it really didn’t matter about the length of the wood. We finished the passage that connects to the upper house, which then took on a bit of a sauna feel, seeing as the ceiling is also cedar.

One of my original ideas for covering these walls was to use old pallet wood, and so to create a rustic, upcycled look by painting or colour staining the pallet material. This seemed like a good cheap option but ultimately we decided that it would be way too much work for the amount of wall we needed to cover. However I was still interested in adding a coloured stain to the cedar, which was all rather rough looking anyways. I experimented with creating multiple colour tones using blue, white, and grey water based stains from Saman, which was really easy to use and gave a nice palette. I prestained a whole bunch of boards, finished them afterwards with the same Osmo natural oil stain, and then assembled them on the walls from the mudroom, into the kitchen and then up the staircase on either side. Because the ceiling was made of uncoloured cedar, I decided to arrange the colour tones from dark at the bottom to light at the top, effectively fading into the clear boards before they hit the ceiling.

It was a fun experiment, and I think the blue, grey, and white palette creates a fun feature amoung the neutrality of the white/grey tone of the natural plaster which will cover the straw bale walls. I hadn’t really pre-thought the interior decor of this house, at least not as a priority, but as we move along into the finishing phases it is really fun to choose materials not only for their price and availability, but also for their attributes of atmosphere and tone.

We had a lot of this cedar leftover, so we also used it to finish some of the framed bathroom walls which create the toilet corner. Also, the we used it for the walls inside the utility room, which didn’t need to be beautiful, and despite the roughest boards being used in this room, looks surprisingly decent for a utility room.

Framing~ The Bones

roofDespite a very frigid December (for the west coast), we managed to continue building just in time to get the roof sealed up and water tight before the wet west coast winter starts in.  Here is a brief account of the structure of the frame, and the way in which we are going to insert the straw bales.

first wallOur decision to use a stick frame method as opposed to a timber frame (as we did for our first house) was mostly a compromise of time and money.  Stick framing is super fast, and we hired an experienced framer friend (thanks Danny!) to work with Colin and his design with the curved roof lines.  Dan helped Colin put his drawings into a model of the house in Sketch-Up, a computer program for architecture, so they could get accurate measurements for all the framing, especially where the lengths of the 2×4’s change subtly with the curves.  The curved roof was the main reason to use stick framing- it was eaiser and faster to frame the studs under the curved beam which is essentially two 2×8’s (cut out of 2×10’s to get the curve) overlapping all along as one continuous header the length of the roof- this also alleviates the need for headers over doors and windows.  Framing started on December 2, and was done in a month, despite a week break over Christmas.  We still have some interior shear walls to frame, as we were focused on the walls that were necessary for the roof.  Also, window and door framing will be cut in sometime in the spring.  Many people have asked us about how 2×4’s can be the only thing holding up a living roof, but the engineer says they are strong enough with the 3×8 beam on top.

The exterior walls are framed to 18 inch centres, as we will be standing the bales on end between the 2×4’s.  A lengthwise notch will be cut down each side of the bales to fit them snug around the framing, and thus hide the wood frame down the centre of the wall of bales.  The new international building code for straw bales has published findings that the bales placed 14 inches wide in the wall is the same insulation value as their 18 inch wide option, due mostly to the orientation of the straw.

All of our interior walls are shear walls (plywood on one side, and attached to the foundation directly), as required by the engineer, since our exterior walls are straw bales and not considered to have any shear strength.  The shear strength is the load that an object is able to withstand in a direction parallel to the face of the material, as opposed to perpendicular to the surface.  In walls, it is usually plywood or cross bracing that provides the shear strength, preventing any side to side movement.  So our internal walls are (or will be) sheeted with plywood and continue down to the foundation.  At the foundation, the walls are secured with hold downs to resist any upwards movement in an earthquake.

The roof is standard construction with 2×4 strapping over 2×12 joists, though in the more curvy parts of the roof we had to use double layers of 1×4.  On top of the strapping is standard 1/2″ plywood decking and a 4″ curb all around the edge to keep the dirt in.  We decided to go with a double layer torch on roof membrane this time, which should easily last a very long time, longer than us… The roof was torched on in early January and we are now secure and dry for the rest of the winter.  Colin is back to work in his shop for the next few months to get caught up with his ThujaWoodArt projects, but come the spring we hope to do the infloor heating and plumbing under the concrete slab on grade subfloor, and prepare for installing the straw bales in the summer.  We will be offering workshops for installing the straw bales and plastering in the summer through the local Heartwood Folk School, check their website for more info as we get closer!

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