Cordwood Building With Paper-Enhanced Mortar

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Cover Courtesy New Society Publishers
"Cordwood Building" collects the wisdom of more than 25 of the world's best practitioners, detailing the long history of the method, and demonstrating how to build a cordwood home using the latest and most up-to-date techniques, with a special focus on building code issues. 

Cordwood masonry is an ancient building technique whereby walls are constructed from “log ends” laid transversely in the wall. It is easy, economical, esthetically striking, energy-efficient and environmentally-sound. Cordwood Building: The State of the Art (New Society Publishers, 2003),edited by Rob Roy, is a key resource on cordwood building, bringing together the wisdom of 25 of the world’s best practitioners. The following excerpt is a chapter on paper-enhanced mortar, or PEM, also called “papercrete.” 

You can buy this book in the MOTHER EARTH NEWS store: Cordwood Building: The State of the Art.

Yes, the writing is in the wall. It is winter 2002. I have just finished my first year’s work building a cordwood home in southeast Minnesota, using newspaper along with sand and Type N masonry cement. I call this mixture “paper-enhanced mortar” or “PEM” — a more accurate term, I think, than the term “papercrete,” used rather casually with any cement and paper mixture.

Although the concept of using paper by-products in a cordwood mortar mix is still in its infancy, it is my opinion (rather than fact) that my current mix is “buildworthy.” My experiment is being conducted in a cordwood wall that has a post and beam frame. My house is a two-story, 16-sided type made in combination with the Double Wall Technique. With load-bearing cordwood walls, I would be reluctant to use this mortar due to its lower strength.

I started my first wall, following in the footsteps of Jim Juczak — well, sort of. I’ve never seen Jim’s shoes, and unfortunately, I am not close to a paper mill to get any free paper sludge. (Can you tell I’m jealous?) So instead, I was able to work out a deal with the county’s recycling center for 75-pound (34-kilogram) bales of shredded newspaper. The first bales also contained office paper waste, which was hard to slurry. But after some “case of beer negotiations,” the guys at the recycling center were happy to supply me with “pure” newspaper.

My first mix was a combination of two parts slurried newspaper to one part masonry cement. No sand. I’d tried this mix on a test wall with success, so it seemed okay to use on the house. The mix was very wet and hard to point; it had a slight cottage cheese texture to it. After I completed the first 8-foot square (2.4-meter square) panel of our home, I left it to dry for a couple of weeks … then a couple more weeks … and then a couple more weeks.

While drying, the mortar color changed from dark gray to light gray, then to light green, and finally to a pleasing white. After six weeks, the PEM was pretty much dry. This wall was on the north side of the house, so the mortar probably would have dried faster elsewhere. Nonetheless, it was a slow process.

About one-third of the mix was type N masonry cement; the rest was slurried paper, made by soaking the bales of shredded paper for 24 hours in a 55-gallon (210-liter) drum.

The mortar had no cracks in it, but I noticed a widening gap between the frame and the cordwood masonry. There were no gaps around individual log-ends, but the entire cordwood wall appeared to be shrinking.

Because of the shrinkage, I decided to try a more traditional cordwood mix, using sand with the paper and type N masonry cement. All subsequent walls have now been built using the following formula (by volume):

• 2 parts sand – 2 parts slurried paper – 1 part type N masonry cement

I love this mix. It has a very nice puttylike feel to it, dries in a couple of weeks, and looks great. Like Jim Juczak, I have been brush-finishing the mortar with a small foam painting brush, and when it dries you cannot detect any curing lines between batches. When I built my test wall, I didn’t lay cordwood inside of a frame like I did on the house. Had I done so, I would have discovered the shrinkage problem ahead of time and tried other mixes before building the real thing. The gap in the first panel, the one with no sand in the mortar, is now about a half-inch, and I can see daylight between the 6-by-6-inch post and beam frame and the wall. The 8-inch (20-centimeter) cordwood wall is still solid, as it is tied to the frame with frequent timber screws projecting two inches (five centimeters) into the solid mortar joint. Still, I’ll probably rebuild the non-sand panel this spring (2002). I’d feel a lot better with a sand and paper mortar there.

Pros and Cons of Paper-Enhanced Mortar

Pros of PEM

• What a great way to use recycled newspaper! There’s so much waste in the world — why not use it in an Earth friendly way?

• Paper doesn’t have to cost you a penny. If you can’t get it for free at a recycling center, just ask your friends, family, and neighbors to save it for you.

• The latest mix retards the set enough to eliminate most, if not all, mortar cracks but does not take forever to dry. All panels since the first have turned out fine.

• I don’t really know the R-value of the PEM, but with its 40 percent paper content, I would assume that it’s higher than more standard mortars.

• The PEM is visually appealing. You’d never know there is paper in the wall, and the mortar is a uniform, light gray color.

Cons of PEM

• Having to slurry the paper adds another step to the process. (If you are as lucky as Jim Juczak and have a friendly paper mill nearby, this extra step is eliminated.)

• I’m not sure how high up on a wall you could go in one day without the masonry compressing on itself a bit. If two people are laying, it might be wise to work on separate walls.

• PEM is not time tested yet. Only decades will tell how well the mix will hold up. I am encouraged, however, by a visit to a cordwood shed built by Paul Reavis of Brodhead, Wisconsin, in 1988. Paul’s recipe started with the standard mason’s mix of three parts sand to one part masonry cement. He bulked out the mix with unslurried paper, so that the final product was about 60 percent paper. Because the paper was not slurried, it did not become as integral a part of the mix. Nevertheless, the integrity of Paul’s mortar is still good after 14 years.

Another PEM Formula by Tom Huber

Overall, my PEM experience has been similar to that of Alan’s and Jim Juczak’s, but my PEM recipe is a little different. I used various containers to measure out ingredients. Here it is by volume percentage. The whole numbers in parentheses are a close approximation of the mix in terms of proportion.

• 67 percent (17) shredded and soaked white office paper
• 21 percent (5) Type S “Mortar cement”
• 8 percent (2) mortar sand
• 4 percent (1) hydrated lime

The paper comes free from the college where I work. Type S Mortar cement is a masonry cement — not a “mortar mix” — which is masonry cement mixed with sand. Since I already had the lime to treat the sawdust insulation, I added a small amount to whiten the mix and give it a little more plasticity.

I initially “soften” the white shredded office paper by soaking it for a few days in a 55-gallon (210-liter) drum. I wring out the excess water from the paper and then mix it together with the dry ingredients in a wheelbarrow. I’ve used this mix M-I-M style — a 16-inch (40-centimeter) wall with cedar sawdust insulation — and as a solid mortar matrix laid transversely right through the wall.

We used PEM on the back wall of our “lodge” building. The only evidence of mortar shrinkage is a slight gap along a couple of posts, which I have caulked. I believe this gap was caused by the wicking action of the dry, rough-sawn pine posts, which draws the moisture out of the PEM quite quickly. I’ve also noticed a few thin tears or rips in the mortar after it completely dried in the M-I-M constructed wall. Again, the smaller volume of mortar in the M-I-M part probably dried faster. In regular cordwood mortar, such perforations might have manifested themselves as continuous cracks from log-end to log-end, but the reinforcing quality of the paper seems to stop continuous cracking.

I surmise that PEM concoctions of various kinds have good non-shrink characteristics, due to their moisture absorbent, slow-drying qualities. PEM has good “squishability” — more like bread dough than regular mortar — which makes it easy and pleasing to work with. It does take more time to mix, though.

Paper-Enhanced Mortar: Observations from Rob Roy

My only experience with PEM came from putting down a few globs of the stuff at Jim Juczak’s house during construction of his second panel. Jaki and I discovered that the stuff was quite pointable, and I think Jaki shamed Jim somewhat into pointing the subsequent panels by showing how attractive it could be. In the first panel, Jim just smoothened the PEM in a rough sort of way with his rubber gloves.

For this book, I have tried to keep to the “state of the art” subtitle by giving the latest PEM results from the new pioneers: Jim, Alan, and Tom. We are also encouraged by the success of walls built by Paul Reavis in 1988. None of us claims to be an expert on this technique, which must be considered to be still in its infancy.

Studying the writings submitted and personal interviews with the three primary PEM researchers have led me to the following observations:

• The primary difference between the three “new pioneer” mixes is the sand content, which varied from none (Jim) to little (Tom) to a fair amount (Alan). Sand, obviously, makes the mortar harder and stronger but also denser. Sand increases thermal mass. The non-sand or low-sand mixes dry more slowly and would appear to have better insulation value.

• The question of insulation value is still out with the jury. Obviously, PEM has a higher R-value than does regular cordwood mortar. How much higher is unknown, and the answer is one we all hope to have in time for the next cordwood conference. A related question is whether or not to go with a “solid” mortar joint through the wall or to employ the ordinary M-I-M style used with cordwood masonry. The performance of Jim’s house through its first full north country winter in 2003 will go a long way toward answering that question.

• Does PEM save time? Depends. With Jim’s ready-mixed paper pulp and 45-second mixing time with a high-speed paddle drill, yes, mixing PEM is faster than mixing regular cordwood mortar. But both Alan and Tom report longer mixing times — Alan because of the extra time preparing the paper slurry, Tom because he hand mixes it like cob. Time spent on any mortar is largely a function of availability of materials.

• Paper-enhanced mortar makes use of a waste material and shows promise as an insulative mortar that can be used in cordwood panels within a post and beam frame.

It is too early to say whether or not PEM is suitable where the cordwood masonry is load-bearing. A test panel is advised with any new cordwood project, but is particularly advised in the case of PEM. And the test — or tests: try different mixes — should be conducted a full month before you want to start the actual project, due to the slower curing time.

Reprinted with permission from Cordwood Building: The State of the Art edited by Rob Roy and published by New Society Publishers, 2003. Buy this book from our store: Cordwood Building: The State of the Art by Rob Roy.

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