R-Value

I've always been interested in cord wood, but have heard it just doesn't do well in cold temperatures. (-20 F.) Does it lose more heat than a log cabin because of the material between the wood? I guess I should really read Rob's book.

From an engineering perspective the wood is a good insulator, but not the cement mortar usually used. Given that, I see no reason why cordwood construction would not be good enough for cold weather given a good design. As long as the space is not too large, the logs are not warped to the point that the composition of mortar is too great and also that the walls themselves are made relatively thick then you should have desirable results. Because cord wood is not very selective I see no problem making this happen.

Iam hoping Rob or others can enlighten us more on how cordwood can be built to block air movement. To me, the checks and cracks in the logs allowing air leaks is a bigger problem than the lack of R value.

Do I remember seeing a double layer cordwood wall with an air barrier/insulation in the middle? This is the only way I can imagine cordwood performing well from a thermal and energy efficiency standpoint.

Brian Knight wrote:Do I remember seeing a double layer cordwood wall with an air barrier/insulation in the middle? This is the only way I can imagine cordwood performing well from a thermal and energy efficiency standpoint.

That's how I would do it. That would make a nice thick wall to keep the heat in.

R-value of cordwood masonry and air infiltration: two very important questions.
First, R-value. There is a popular misconception that the mortar of a cordwood wall (which can be 40-50% of the cross-sectional area of the wall), goes all the way through, from interior to exterior. This would be a thermal disaster! The mortar would conduct the heat right out of the house. Thankfully, this is not the way we build cordwood masonry walls. There are two separate mortar joints - an inner joint and an outer joint - separated by an insulated space. In a properly built cordwood wall, the R-value of the mortared portion of the wall is actually greater than the wooden portion. As an example, our Earthwood house in Northern NY (9000 degree day climate, like Montreal) has 16" northern white cedar cordwood walls. On end grain, the cedar is good for R1 per inch, so R16. However, the mortared portion of the wall, with six inches of sawdust between the inner and outer mortar joints, is worth R22. The average value of the wall meets NY's R19 energy code requirement. But, of course, the thermal performance also benefits from the exceptional thermal mass.
As for air infiltration, it is important to season the wood to minimize log-end shrinkage. This is a big topic, and species specific. so impossible to address here in a short answer. We spend about 45 minutes on this topic at our workshops. If the wood does shrink, you can caulk the gaps with a good siliconized clear caulking. Best to wait a year or so, at least through one heating season, before doing this. You don't want to do it twice. Incidentally, we have done very little caulking on our various buildings at Earthwood, because we have usually seasoned our white cedar for a year before using it.

Thanks Rob! Thats very helpful and iam sure the classes and book will help fill in many of the details.

I think that cordwood construction is going to have a tough time meeting current energy codes (IECC 2012) and especially future energy codes.
The main reason will be the airtightness requirements as required by passing a minimum blower door test. Airtightness is more important than R value!

Building climate zones 4 and above (NY falls under 5 & 6) require a blower door minimum of 3 ACH50. I suspect this will be challenging for cordwood. Any past blower door test results from cordwood construction that people have would be greatly appreciated and will hopefully disprove my concerns. Most high performance home builders are shooting for less than 1.5 ACH50.

Air leaks account for around 1/3 of our home energy and environmental costs. Its an easy thing to measure and can have much bigger impacts than R value. R value can be harder to measure especially with something as variable as cordwood. Any rules of thumb for those of us wanting to be exact with our R value measurements concerning the ratio of mortar to wood (in those layers)? What about the wall thickness ratios of Mortar:sawdust(or other):Mortar?

Table r402.1.1 of IECC 2012 shows what the R value requirements are for wood frame walls and mass walls. For wood frame walls, zone 5 requires R20 or R13 + R5 insulative sheathing. For zone 6; R20+5(insulative sheathing) or R13+10.

Mass walls are trickier to calculate especially with Cordwood. If a properly constructed 16" cordwood wall truly achieves R19 (seems high to me) then it might meet code with the R13/17 requirement. Zone 6 requires R15/20 for mass walls. The second #(after /) must be the R minimum "when more than half the insulation is on the interior of the mass wall." I would think that the second number would apply in cordwood's case.

Thanks, Rob, for answering. We are in Zone 2 at a pretty high elevation in the mountains above Anchorage. I know that R19 isn't going to cut it, however, if it has a lot of airflow, I wouldn't have to spend all the money I am right now on the electric HRV unit to move air in and out of our very tight house now. I like the idea of putting an air space in the middle---cool idea. Or maybe straw/clay in the middle. That breathes.

I will share your responses with folks up here, so that maybe someone will try it out and do the research.

Dont confuse breath ability (good thing) with air infiltration (bad thing). Using an ERV/HRV with an ECM motor should help alleviate energy use concerns. They use about as much energy as a compact fluorescent light bulb.

For Brian: Richard Flatau's 30-year-old cordwood home in Merrill, Wisconsin, a cold climate, passed a blower test with flying colors. His cordwood walls are 12" thick. We can talk R-values and infiltration until the cows come home, but the essential questions really are these: Are the houses comfortable? And: How much fuel is required to keep them that way? I can report that our 2400-SF (actual usable; really 2800 SF gross) round two-story cordwood home is very comfortable indeed through our 9000-degree day climate on a fuel use of about 4 full cords per year. We buy most of our firewood and spend about $600 a year for fuel. For fair disclosure, it must be said that 40% of the cylindrical walls are earth-sheltered. (16" thick surface-bonded concrete block walls below grade.) The impact of thermal mass in combination with insulation value is still hotly debated, at least as far as fuel consumption is concerned. After 31 years inside of our 240-ton Earthwood house, I can tell you that I am a great believer in thermal mass, as long as there is good insulation between the mass and the ambient. Incidentally, I try to be conservative with my R19 estimate of our 16" white cedar walls with their insulated mortar matrix. I am only using R1/inch for the white cedar, although it is rated at R1.5 on side grain. I take a two-thirds value because of the greater heat transfer along the end-grain, and infiltration. My 2/3 factor may be a little high or a little low. Dr. Kris Dick, a P.E. in Manitoba, has tested a 24" single-wall cordwood wall, with the insulated cavity, and yielded results of R35 or R1.47 per inch. This seems high to me, as I'm sure it does to you, but he is the engineer, not me.
For Cindee in Alaska: You say, "I like the idea of putting an air space in the middle---cool idea. Or maybe straw/clay in the middle. That breathes." An air space, by itself is not quite useless, but almost. Engineers normally assign it a value of R2, like a double pane window. Not much help. When the space is filled with an insulation with an R-value of R3 per inch, a 12" insulated space would yield R36, plus R2 twice for the heat transfer through the two mortar joints, so: R36 + R2 + R2 = R40. As opposed to R2 + R2 + R2 = R6 for the double mortar joint with just an air space. For Alaska, I would recommend 24" cordwood walls. The mortar joints can be 5" wide, which would give you almost 14 inches for the insulation. As for the straw/clay in between, you would need to get some kind of estimate for its R-value per inch in order to do the calculations. Incidentally, we have had no deterioration in the sawdust insulation at Earthwood after 31 years. About ten years ago, and again last fall, I had occasion to go inside the cordwood walls (once for an expanded window, once for a vent for a gas clothes dryer). The sawdust insulation, treated with builder's lime, was as light and fluffy as the day it was installed. We treat the sawdust with lime at the ratio of 12 parts of sawdust to 1 part of lime, and always pass the sawdust through a half-inch mesh screen, to get rid of bark, grass and other junk.

Lots of good info Rob. We appreciate the hard work, time and especially sharing your knowledge and experience with us here.

For those of us interested in net-zero, high performance homes and correctly sizing HVAC equipment, knowing the air-infiltration and R-values are absolutely critical. Comfort is subjective. Every home and occupant will have different behavior, definitions of comfort and energy costs. Energy codes dont recognize anecdotal evidence of comfort.

Fuel use on the other hand is a direct output of energy modeling and design software. R-value and ACH 50 are major input numbers that can be plugged in to give us a very good idea of how much fuel a house will need to stay comfortable.

The WI cordwood blower door test example doesnt tell us anything without the actual result numbers or the energy code or efficiency standard that it passed. To be fair, air sealing is much more important at the floor and roof planes than the wall planes. I suspect the air leakage in most cordwood walls however, can be considerable. What are your feelings on finishing the interior with dry wall or plaster?

Your energy use examples are fairly good for a home in such a cold climate although Iam confused if the $600 fuel costs are in addition to the 4 cords of wood. Ive read about some homes in your climate that are designed to stay comfortable with only a 1/2 cord. Iam not a fan of any combustion heat sources when possible but cutting the needed amount down can lower the risks and concerns.

Thanks for your kind comments, Brian. Yes, comfort is subjective. To be more specific, we keep our house to around 72 degrees throughout the year. (We’re in our 60s!) The house mass means the temperature is very slow to change, up or down. It is a giant capacitor.
Most of our students are owner-builders – or potential owner-builders - who generally want to build their cordwood or earth-sheltered (or combo) homes out in the country, all climates, from Georgia to Alaska. They may or may not have to go through the local permitting process, but, if they do, it is not usually a problem. They are generally not involved with serious energy code issues. Once again, the bottom line – for me - is fuel use. To clarify, the $600 is roughly what we spend on firewood each year, some years a little more, some a little less. We use only wood and passive solar for heat.
I know of no home in Clinton County (New York’s northernmost county) that gets by on a half a cord of wood per year, but I do know of a beautiful 1800-SF double-wall cordwood/earth-shelter built near us by our former students Bruce Kilgore and Nancy Dow, which is very “comfortably” (sorry!) heated with a about a cord. Incidentally, both Bruce and I burn our wood in massive masonry stoves, the most efficient means of burning wood, in my opinion.
As to finishing the walls with dry-wall or plaster, I have only seen two examples of this (plaster) where it was done well, one in Washington State and one in Ontario. I would certainly not do this until the log-ends have transpired their sap moisture. This could take a year or more. If infiltration is the issue, there are cheaper and easier ways to attend to that. Jaki and I have not found infiltration to be a problem at Earthwood, or at Mushwood, or summer camp. But we were very careful about the log-ends we used.

Just to clarify, what's the sq footage of your home and how many cords do you use?

CJ: The gross square footage, footprint including thick walls, is 2800. But we lose 400 SF in the 16-inch thick walls, so the actual usable area, that you can run around in, is really about 2400 square feet - or "round feet." (It is, after all, a round house.) We use a bit more than four full cords (about 12-plus face cords where the logs are cut to 16 inches.)
I notice that you are in Vermont. We are right across the lake in Northern NY. You would be welcome to come to our Open House on Saturday, May 4th, 10 am to 4 pm. For more info, go to www.cordwoodmasonry.com Click on Newsletter and scroll down for directions.

I'm having a hard time visualizing the loss from 2800 sq ft to 400 usable! Maybe you could post a pic! The 2 walls with an air gap someone mentioned above made lots of sense because air is a good isolator though I could see how that would use up lots of sq ft.

We built our house 17 years ago. It was designed by Adirondack Alternate Energy near Saratoga, I think. 2400 sq ft with an R value of 38 on 6 sides. We burn about 2 cords/yr more or less on the wood and the weather. Our heat capacitor, as you put it, is 10" of concrete below the house that has ducts in it. There's a fan that pulls warm air from the top of the house down to the concrete. We are off grid so I only use the fan on sunny days for a few hours.

Oops! Typo city. My recent reply said 400 SF actual usable. It is actually 2400 SF, now corrected above (I hope!) Sorry about that. Your home seems very energy-efficient, indeed!

OK that makes more sense!

We built out stack wood house with 18 inch logs and put saw dust between the mortar. It works great unless you too close to a window. The windows need some work.

pics?

Cj: I am unclear as to whether you are asking for pics of Earthwood or of Yone' Ward's house. At any rate, I don't know how to put pictures on these posts. But our Earthwood website, www.cordwoodmasonry.com , has several photo album pages that might be of interest to you, including pics of the various buildings at Earthwood Building School. See also our Earthwood Building School Facebook page, which my wife, Jaki, maintains with more current stuff.

Sorry, Yone Wards house.

There are a couple of ways to post pics though. One is to click on the Attachments tab (bottom left) when posting a reply. The other is to link to an image off site like Flickr.

Cj Verde wrote:pics?

I have these in my straw bale thread, but I'll share them here too.

Here is a close up of the mortar and saw dust on the 18 inch logs:

Here is a picture of my dad building one of the six walls:

Here is the finished house:

One of these exterior walls was turned into an interior wall and we finished it up with a surface grinder, some slab wood, a torch, and some varnish.

The only issue I have with the technology is the cracks that appear and the bugs that dig in after six years. I suspect this would be easy enough to fix though.

Like I said, the windows are an issue. I don't know if it's because they are aluminum, or because the installation was never finished and they just leak. Either way, I want to see if I can do something about them this summer.