Top Five Mortar Mixes for Cordwood Construction by Richard Flatau
It is very important to understand that cordwood as a building technique is continuing to evolve. Changes and improvements have been significant over the last 50 years. Here are a few points to consider:
Cob mortar with cordwood (aka Cobwood).
- 95% of the cordwood built nowadays is non-load bearing. Therefore the mortar matrix is not holding up the roof (then it is called load-bearing). The post framework holds up the roof and code officials love it. The mortar can be weaker (in this case, weak is a good thing, because you can add more organic matter like, sawdust, hemp hurds, clay, vermiculite and the mortar beads can be thinner (3 inches on the inside and the outside). This leads to better R-value and a better set and cure of the mortar.
- Do not rely on OLD books (use at least a 2020 copyright date), articles and videos. Some of them are just plain wrong or misleading. Investigate using recently published materials and videos.
- Experiment with your mix on a test wall or a small shed. This will help you fine-tune up your mortar.
- There are at least 5 basic mortar mixes with dozens of variations. Choose the one that meets your personal preferences.
- You do NOT need to use Portland cement in your mix. There are several different mixes that have no Portland and some that have less than 10% Portland for the total volume.;
Top Five Mortar Mixes for Cordwood Construction
1. Traditional Mortar Mix
3 parts Mason’s sand, 2 parts soaked/drained coarse softwood Sawdust, 1.5 parts Type S Hydrated Lime, 1 Part Portland Cement or 9 parts mason’s sand 6 parts soaked/drained sawdust 4.5 parts Type S Hydrated Lime 3 parts Portland Cement
2. PEM Paper Enhanced Mortar
6 parts drained, slurried paper 6 parts fine sand 3 parts Type N masonry cement CEM Cellulose Enhanced Mortar by Tom Huber 5 gallons moistened· cellulose 2.5 gallons mason’s sand 2 gallons masonry cement 1.5 gallons hydrated lime
3. Lime Putty Mortar
Careful quality control has produced good results, whereas there have been failures where variables are not kept within best practices. Making the Lime Putty Mortar 4.5 gallons water to 50# Bag of Type S Hydrated Lime The LPM must be made at least 5 days before making the mix. Mortar Mix 1 part Lime Putty Mortar 2.5 parts washed masons sand.
Building a 95-foot-long cordwood classroom in Guatemala.
4. Cob Mortar
The term cob is used to describe a monolithic building system based on a mixture of clay, sand and straw. The construction uses no forms, bricks or wooden framework; it is built from the ground up. To be sure, there are as many different cob mixtures as there are cob structures. Do your homework and find a mix that works for you. Cob Mix (one of many) • Soil Containing • 30% Clay • 70% Sand • Straw (not hay) • Water
5. Hemp mortar
Hemp is being used as a full fledged “total infill” (no insulation cavity). What the current research is partially focused on is making a hemp mortar that will set and cure well. This process is still ongoing. The hemp hurds/shiv mixed with lime can be used TODAY to fill the insulation cavity. This is a very promising mix and batches are being tested throughout the world. We used hemp hurd insulation and mortar at the Entrance Center at the University of Manitoba.
Insulation The most common insulation used for cordwood is sawdust mixed with lime. Some folks have been experimenting with hemp hurds (the waste product from harvesting industrial hemp). Others have used perlite (heat expanded glass), vermiculite, cellulose and spray foam. These mixes and formulas are presented in more detail with pictures at https://cordwoodconstruction.org
Should you wish to learn how to build a cordwood cottage, cabin, or home, please visit www.cordwoodconstruction.org While your are there, click on the pictures, read the brief articles, check out the latest workshops and newsletter, and if you are interested, click on the book cover to see all the cordwood literature available in print and ebook format.
If you have questions that aren’t answered on the website you can email me at richardflatau@gmail.com
Readers have requested a brief bio, so here goes:
“Richard & Becky Flatau built their mortgage-free cordwood home in 1979 in Merrill, Wisconsin. Since then, they have written books, conducted workshops, facilitated the 2005, 2011 and 2015 Cordwood Conferences and provided consultation for cordwood builders. Cordwood Construction: Best Practices DVD, Cordwood Construction Best Practices (print) and Cordwood Conference Papers 2015 are the newest publications available from their online cordwood bookstore.” www.cordwoodconstruction.org
