One of the Frequently Asked Cordwood Questions (FACQ) is: “What type of wood can I use?” At one time, the answer was, “Anything you got.” But over time folks realized that there were good, better and best options for wood selection. To narrow down your choices let’s place the woods on a continuum. Hardwoods are dense, and have a tendency to swell and crack mortar joints; thus making softwoods the better choice There are quite a few choices, but the best one is a naturally rot-resistant, disease-free, decay-resistant, insect-resistant softwood with a low shrinkage rate and a high R-value.
With all that taken into consideration, cedar stands out as an ideal wood for cordwood. Are others that work? Absolutely. The pines work very well, but they have a bit more shrinkage and are prone to insect infestation and the wood must be peeled immediately and dunked in a borax solution.
Types of Wood for Cordwood
- Low-density softwoods like Cedar and Pine work best. Their cellular structure has more airspace and provides better insulation and less shrinkage.
- High-density hardwoods like oak and maple are heavier and more compact and swell when subjected to water/humidity.
- Cedar is rot-resistant, smells good, is lightweight and has a pleasing color.
- The following is a list of woods that can be successfully used within a cordwood wall (pay attention to the volumetric shrinkage rate, which will tell you how much your wood will shrink after cutting).
Volumetric shrinkage (total shrinkage) of cedar, pine, tamarack & aspen
- Northern White Cedar volumetric shrinkage 7.2%: Cedar is the ideal wood for cordwood because it possesses so many of the qualities needed to make a successful cordwood building. However, other woods can be used with confidence. Often the final result is in the proper preparation.
- Red Pine volumetric shrinkage 11.3%
- Virginia Pine volumetric shrinkage 11.9%
4. Lodge Pole Pine volumetric shrinkage 11.1%
5. Aspen volumetric shrinkage 11.5% rated as non-durable With aspen (poplar) the face of the log will often turn black from UV and rain. This is a good candidate for staining the exterior log face.
If you leave the wood round, it is going to develop a primary check. You can fix this (by stuffing) before or after the wall building.
6. Tamarack volumetric 13.6% moderately durable Tamarack is a very dense softwood and if not split and dried can have the same effect as a hardwood (swelling, cracking mortar joints, pushing walls out of plumb). Tamarack is best used for posts, beams, and boards.
7. Red Cedar volumetric shrinkage 7.8% Aromatic, decay/insect resistant, durable, allergen
- One of the best sources of information on shrinkage, durability, rot resistance, etc. of various woods is at www.wood-database.com It is the best site I have found. The best wood(s) for cordwood should be the best-rated you can find for
- 1. Shrinkage
- 2. Durability
- 3. Rot Resistance.
- Go to the website http://www.wood-database.com and use the search bar to type in the wood. In order to be considered Good for Cordwood, the Volumetric Shrinkage Rate should be near 7-8% (Cedar is just a little over 7.0%), and the wood should have some rot & insect resistance, and be listed as durable. You can use woods that are not highly rated but you must 1. Split 2. Treat 3. Place them “up” off grade 4. Use a damp-proof membrane 5. Build large overhangs and gutter your roof.
Recent woods of interest to readers:
Shrinkage: Volumetric: 12.4%
Rot Resistance: Rated as non-durable, and also susceptible to insect attack.
EASTERN RED CEDAR
Common names: Aromatic Red Cedar, Eastern Redcedar
Volumetric shrinkage: 7.8%
Rot Resistance: Regarded as excellent in resistance to both decay and insect attack, Aromatic Red Cedar is frequently used for fence posts used in direct ground contact with no pre-treating of the wood.
WESTERN RED CEDAR
Common names: Western Redcedar, Western Red Cedar
Shrinkage Volumetric: 6.8%
Western Red cedar has been rated as durable to very durable in regard to decay resistance, though it has a mixed resistance to insect attack.
As you can see, the cedars excel in every category.
If you are going to use wood other than cedar, you will want to determine whether you have to treat the wood with borax (a non-toxic, naturally occurring product) to stop or prevent insect damage. A solution is made of hot water and borax (1 gallon of hot water to 4 cups of borax, mix thoroughly and dunk the log ends for 30 seconds each. Let it dry.
What if you only have access to hardwoods?
Hardwoods are not the ideal wood to use with cordwood. They have a tendency to swell (because they are so dense) and move walls out of plumb and crack mortar joints. If they are the ONLY thing you have and you understand the inherent problem, you can use the following method to help ameliorate the problem.
• Build up “off of grade” and if you are able, put your hardwood on a pressure-treated bottom plate (like two 2 x 6″s). This will help stop any moisture from migrating from the foundation into the mortar and causing the wall to swell.
1. Split at least 70% of your hardwood. Leave some round because several pieces of round wood look nice in a cordwood wall, especially at eye level.
2. Use larger mortar joints than normal. 2″ mortar joints or larger. This helps with the pressure that hardwoods create when they swell. They will even swell during high humidity.
3. Use large overhangs (3′) if possible and make sure you have no splash back on your cordwood or any way for water to migrate up the wall (use plastic or rolled roofing) between any block, cement or mortar at the foundation.
4. Use a post and beam framework.
5. Use a forgiving mortar mix (there are several choices). My mix of 1 Portland, 1.5 Type S Hydrated Lime, 2 soaked sawdust, and 3 sand, is forgiving. But it needs to be covered tightly with plastic after each work session so that it can dry slowly and cure properly.
6. Plan on “going over” your walls with Permachink caulk or chink paint (permachink.com), or Vasari lime wash www.vasariplaster.com After a heating season or two, this will take care of any cracking, log loosening, differently mixed mortar mixes. Log Jam also makes a similar product.
7. Use a mixture of hardwoods if you have them. It will add interest to your wall. If you can find a cord of eastern red or northern white cedar, use it on the bottom row.
8. Some of the round logs will develop a primary check that will go all the way through to the outside. Stuff this check on the inside and the outside with white fiberglass (it matches the mortar) or Permachink.
9. If you want the building to last, use a Borax dunk to repel insects, fungus, and mildew.
10. Use keyways vertically to keep the wall from tipping out of plumb, should you have any problems with the entire wall swelling.
AS to Kiln Dried logs or lumber. Kiln-dried is good for your framework, paneling, window boxes, etc. What you need to know is that if the Equilibrium Moisture Content is 12% and your wood is kiln-dried to 7% it will eventually gravitate back to 12%. It’s just nature’s way :0) You can certainly use kiln-dried, just realize it will gain moisture over time. It will gain less moisture on the interior because you heat the inside.
Shrinkage information from the Wood Database http://www.wood-database.com/ These are white cedar round logs in a cordwood wall. Each has developed a primary check. The check runs all the way through from the outside to the inside. This lets air flow into the house. In this picture, the check is being stuffed with white fiberglass to match the mortar color. You can use brown paper bags, newspaper, caulk (it has to be a moveable caulk or it will CRACK) or any type of material that looks decent. The check needs to be filled on the outside and the inside.
It is important to check the moisture of your wood using a moisture meter. These are easily obtained online.
HOW TO DRY YOUR WOOD: The first rule of air-drying wood for cordwood is to get it in the SUN. In a single row, covered on top. If you can orient it north and south to take advantage of the sun as it goes from east to west (or as my science friends would say the earth is really doing that motion ). Then make sure it is covered on the tippy top. If you want to speed up the drying, split 70% of the wood. Split wood dries faster. If you can hook up a powerful fan to blow on the rows of wood–moving the fan daily can help dry the wood even more (this is a suggestion from the Wood Handbook put out by the USDA). A borax/borate dunk is also a good way to rid the wood of mildew, fungus, and bugs. Borax is a wood preservative, a fungicide, an insect repellant, and a fire retardant. Mix 4 cups of Borax (like 20 Mule Team Borax Laundry Detergent) with a gallon of hot/hot/hot water and dunk the log ends into the bucket/tank for a minute or two. Then stack them to dry. The bottom line is you are working to get the wood to its Equilibrium Moisture Content (that’s 12% in the upper Midwest, 7% in the desert). You can find the right % by asking at your local lumber yard or look at the EMC table in the book Cordwood Construction Best Practices on page 66. You should also get a moisture meter to check the sides and the end to make sure it is dry enough to mortar into your wall. If it is not dry enough it will shrink in the wall and create air infiltration gaps.
NOTE: It is important to note that the borax treatment (dunking the logs in a borax solution to prevent insect infestation) is an optional step in your cordwood wood prep. It can be used if you live in an area where wood-eating insects like to munch on wooden houses. The folks who have developed the process are horizontal log cabin builders/suppliers. If you are going to dunk your logs in a borax solution they should be DRIED FIRST (to at least 12% moisture content). The reason is that a wet, green log will not be able to suck the borax solution deep into the xylem and phloem tubes. Peel your wood, cut it into sections, dry it, and then dunk it. Not all wood needs to be treated. If you use a rot-resistant wood (cedar, cypress) you do not have to treat, it (unless you want to), since it would offer an extra layer of protection.
One way to keep the round “look” and not have the primary check or log loosening problem is to “explode” the round logs (split them) and then put them back together in the wall.
AVOID THIS RESULT. (Below) This is often the result when your wood is not dried to its lowest Equilibrium Moisture Content. The logs can be punched out of the wall.
In the photo above all the rounds have shrunk. Avoid this by splitting 70% of your wood and drying to 12% moisture content.
This is what we are trying to avoid. A log that was not dry enough, developed a large primary check and loosened severely in the wall. How to avoid this? Use softwood, dry it well, split most of it and use a slow setting, slow curing mortar as I suggest in my book Cordwood Construction Best Practices.
This is what mortar cracking looks like. What causes this? The mortar mix is too rich in Portland cement, it needs more sawdust to make it weaker (weaker being a good thing in terms of wood masonry). It was also mortared in the hot sun, which dried it out too fast. It was not covered after the workday to slow down the set and cure.
If you don’t use a “Best Practices” approach, your cordwood walls can be drafty and require significant maintenance to seal them properly.
Should you wish to learn how to build a cordwood cottage, cabin or home, please visit www.cordwoodconstruction.org While you 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 ONLINE BOOKSTORE 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 email@example.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 Workshop DVD (2018), Cordwood Construction Best Practices (print 2017) and Cordwood Conference Papers 2015 are the newest publications available from their Online Cordwood Bookstore. The books & DVD are also available as ebooks for a quick and easy shipping free download. www.cordwoodconstruction.org
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