It A Matter of Lime

Water and straw do not go well together...at least not when that water is precipitation and the straw is your wall. Therefore, it is extremely important to design a straw bale house with features that keep rain and snow away from the straw walls.

Our house, with its eight-foot porch, demonstrates one way to do this. However, on two sections of the house, the north kitchen bumpout and the south sunroom bumpout, there is no porch. There is only an eighteen-inch eave overhang. This is perfect for passive solar design but not so good for shedding weather.

In order to protect these two vulnerable walls, we will use lime plaster rather than the earthen plaster used under the porch. Earthen plaster will erode when precipitation hits it. In contrast, lime plaster chemically sets up in such a way that the plaster becomes hard enough to shed water without eroding.

We actually have two such scenarios to deal with. On the south sunroom bumpout, where we have three, nine-foot by five-foot windows, we decided against using any straw at all. You might ask why. Of course, straw bales have excellent insulative value, but, as mentioned above, you must protect the bales from moisture. There is only room for one bale of straw beneath these huge windows. To us, it seemed that one bale would provide minimal insulation and, as an extra bonus, would present all of the challenges of moisture exposure. So, no bales there.

Instead, we framed this section with 2X6s and our timber frame. We covered the exterior and interior with oriented strand board (OSB) then insulated the air space between the OSB walls and the ceiling with icynene foam (see June 23, 2011 - More Insulation post). We stapled expanded metal lath to the OSB to provide a tooth for the plaster. Now we have great insulation, no moisture issues and an opportunity to use lime plaster on the inside and the outside of the south sunroom bumpout.

Scott made our exterior plaster with Type S lime purchased in bags from our local building supplier, Loa Builders. He added water, sand, polypropylene fibers, pumice (purchased  from Hess Pumice in Malad, Idaho), kaolin clay and soda ash.

There are two main challenges to using lime. One, it is extremely caustic so we needed to wear  rubber gloves, dust masks and eye protection. Two, lime plaster does not like wind or direct sun, so we needed to wait for a calm day. When two such days were in the forecast we set to work. The 18-inch eave overhang was enough to shadow the wall for nearly the entire day, so the wall was shaded as we worked. Once we finished the first lime plaster coat, we protected the lime from sun and wind by leaning boards against the lower part of the wall. The upper part was shaded by the eaves. For the next two days, Scott misted the lime periodically to prevent it from drying out too quickly. The result was perfect...no cracks and the first of two lime coats finished!

Scott applying plaster under the eaves

A section of wall with the first lime coat

Pete helping

The entire wall with the first lime coat finished

Adobe Wall

As I write this post, I am reminded that our initial building plan was to complete this house in just three years. I am also reminded of the construction adage: building will always take more money and more time than you think. This is indeed true for yet another pair of home builders - us.

Scott and I have been back in Torrey for one week now. However Scott completed a big project here while I was wrapping up loose ends in Logan. He built the adobe wall behind our masonry stove. Our thinking is that our masonry stove will heat our big main room and, at the same time, heat the adobe wall. The warm wall will then heat the air in Scott's office on the other side.

So, in early May, Scott contemplated this stack of 151 adobe bricks, which we made last summer with the help of my brother John's family: Wesley, Casey, Kelli, Jessica, Barbara and John.



 

First Scott framed an 8-foot X 8-foot wall.

He then set up a story pole on the edges of the studs. The purpose of a story pole (also called preacher board or jury stick) is to keep the wall level and plumb as well as to keep the courses at the correct height. He stretched a string aligned with the marks on the story pole and stacked the wall.

One concern we hoped to address was the stability of such a narrow, tall wall - 5 1/2 inches wide by 8 feet tall. To tie the entire wall together, Scott placed a masonry ladder (a welded-metal lattice) within the mortar every other course. The ends of the ladder were attached to the frame on each side of the wall.

The mortar recipe was essentially the same mix used in the plaster on the exterior of the house - high-clay-content dirt from our property, a little added masonry sand, finely chopped straw and water. 

One month later, the mortar has dried and the wall seems perfect. The only thing we have left to do is plaster both sides. That should happen this summer.

A Mouse in the House

When discussing straw bale construction with folks who are new to the concept, we hear some pretty predictable questions.

One of the first things people want to know about is fire danger. While loose straw will burn, baled straw less easily supports flame. That’s because the straw in bales is densely packed. This inhibits oxygen flow needed for fuel combustion. And baled straw covered on all sides with thick plaster is nearly fireproof.

What about bugs? Straw is not like hay, so there is not much nutritional value in it. And once the bales are plastered, there is no avenue for bugs to enter the walls.

Mice? Well, there is grain left in the straw when it is harvested and baled. This does attract mice, especially when they discover what a warm, comfy place it is. That is why it is important to get a coat of plaster on the walls as soon as possible. We were able to get plaster on the exterior of the house last year, but the interior had to wait until this summer. When we visited Torrey last winter, we worried that we would find a lot of mouse damage but we were lucky. This was really the only spot we found evidence of uninvited guests.

We set some traps to capture the little critters, caught a dozen and didn't have any in the house after that. Now our cats are with us. One of them has slain at least 20 mice in the yard around the house.
 He's not called Peter Panther for nothing!

Sometimes, with a little chuckle, people make reference to “the big bad wolf.” Keep in mind that wolves are on the endangered species list, so there are not many of them around anymore. And we’ve yet to hear of huffing and puffing as a cause for the collapse of a straw bale house.

Built-ins

While we have been researching cabinets and shelving, we’ve often come across the term “built-ins.” They probably aren’t referring to built-in table saws though.

Last summer, before we put even the first bale of straw in place, we set our very heavy, cast-iron table saw inside the house with the help of our tractor. This saved us time when we needed to cut the many, many, many pieces of wood required to make window bucks, door trim, lath and more. All summer long that table saw sat in Scott’s office doing the work it was created to do. As the days passed by, bales were stacked, plaster was applied, and doors and window installed—the house rose up around it.

Now we are ready to plaster the inside of the house, but we had to get the table saw out. The size of the saw was slightly smaller than the width of the door (we actually did measure to make sure this was true before finishing the walls). But once the walls were up and the doors on, the saw seemed huge. One part of the saw did need to be removed and then a 32-inch-wide, 400 pound mass had to be ever so carefully eased through a 36 inch door without bashing the wood trim. Scott moved it himself, working very slowly and carefully using basic tools—inclined planes and levers—and got it out the door and down to the porch where the tractor could pick it up and move it to the garage. The door trim still looks good.

Table Saw in situ

Table Saw "out situ"

Cutting Edge

As I have mentioned earlier, one of the interesting aspects of strawbale construction is learning how to solve problems. I'm guessing that folks with years of building experience could have avoided many of the problems we've had the pleasure to unravel, but now we have one more under our belts.

Two years ago when we installed the sill plates for the bales of straw to sit on, we didn't have a clear picture of how those plates would be integrated into the interior walls. Now that we have moved on to plaster preparation, we discovered that we should have set the sill plates farther back from the doorways.

Here you can see the 4 X 4 treated-wood sill plate beneath the bale wall. The door is visible on the right. Two inches of plaster will eventually be applied to the wall as well as to the sill plate. We also need to put trim around the door. It is clear that there won't be enough room for the plaster and the door trim. In addition, Scott and I want bull-nosed corners on all of the walls. How will that work on a square sill plate?

Earlier we had purchased a handy Dremel Multi-tool. It is great for sanding tight corners. Its small saw feature can also be used instead of a chisel and mallet. Scott went to work and cut away two inches of sill plate and we now have room for plaster and trim. The cement under the sill plate is rough and unstained, but now we have another opportunity for a design detail. More on that when we've figured it out.

It Could Have Been a Big Mistake

One morning as I walked from the house to garage to get some tools, I noticed wet plaster just below a window sill. This is not something you want to see on any house, but it could spell impending catastrophe on a strawbale one. I hollered for Scott to reassure me that there was nothing to worry about, but the look on his face told me there was plenty to worry about.

We walked around the entire house, inspecting each of the 16 windows. Three of them had wet plaster under the sills. This was puzzling since there hadn't been any measurable precipitation. Plus, how could moisture like that accumulate when we have that strategic 8-foot porch around the entire house?

Fearing the worst, we checked the still-exposed interior bales of straw below each window. That's when we realized we had made an error that really could have spelled trouble with a capital T. After installing the windows last summer, we should have filled the gaps between the window frames and the window bucks with insulation. Instead, in preparation for future plaster work, we covered the windows with plastic and forgot about them. Now that we were in the house and heating it with our gorgeous masonry heater, moisture in the air had condensed on the frames then dripped down to the bottom of the window bucks and out to the exterior plaster. Again, a big problem for a straw house and something of a duh! moment when we realized what we had almost done.

Luckily Loa Builders was open when we arrived to purchase cans of that handy expandable foam. Scott filled the gaps. This solved the problem. No more condensation on the window frames, no more dripping, no more wet plaster. Disaster averted. Whew!

Christmas Hearth

Scott and I spent the Christmas holiday in, yep, Torrey. We had a short list of things to do...all of them related to preparation for a spring plaster festival. (Extra hands, energy and heart are always always welcome to join us.)

The most exciting event of the entire week was arriving to discover Wade had completed the facade for our Tempcast stove.

This is what greeted us when we first arrived.

Wade came over to make sure we were happy with his work. Who wouldn't be. It is absolutely beautiful. You can find more of Wade's work here.

It kept us toasty night after night, just as advertised. It is entirely possible that this stove could warm the entire house even without the radiant heat in the floor. However, I'm sure we'll be glad to have both systems in place.