To Seal or Not to Seal Cordwood

Rob Roy is Director of the Earthwood Building School, which has specialized in cordwood masonry instruction since 1980. Rob and his wife, Jaki, have built four innovative cordwood homes for themselves since 1975, including the Earthwood home where they have lived for over three decades. Earthwood is a 2400 SF two-story round, load-bearing cordwood home, earth-bermed and earth-roofed. Details of construction are in Rob's Earth-Sheltered Houses (New Society, 2006) and Cordwood Building: The State of the Art (New Society, 2003) two of fourteen books he is written in the alternative building field. Rob and Jaki have taught cordwood masonry in Earthwood Building School all over North America, as well as in Chile, New Zealand, Australia and Hawaii. They have helped scores of owner-builders with their cordwood projects, including homes, saunas and outbuildings. Earthwood has produced a 3.25-hour DVD on cordwood construction, which, with his books, can be accessed through the Earthwood website, or on the Cordwood page here. Rob is considered to be one of the leaders in the field of cordwood construction and earth sheltering. He does individual consultations at a flat rate of $75/hour, but answers questions here without charge. See also Earthwood Facebook.

Questions and Answers

Q: Dear Rob, I spoke with you briefly a couple weeks ago about my wife & I bought 65 acres in central Missouri, and are thinking of building a cordwood home with hardwoods. My question is why can't we seal the outside, or for that matter the inside too with linseed oil. We're thinking of either building a post & beam or perhaps a custom steel building and filling in with the cordwood. I am searching for a portable band saw mill. What are your thoughts about this situation?

A: As you probably know, hardwoods are not my first choice with cordwood masonry, but when it is all you have, you can certainly build with it, as many have done successfully, if you pay attention to a few cautions. The problem with hardwoods is that they are more prone to both shrinkage and expansion than most softwoods. You are doing the right thing by building under the cover of a roofed post and beam (or metal) frame. This affords some protection to the masonry work and it allows you to work in the shade and out of the rain. I am not experienced with metal frames. Obviously, a consideration is that you do not want to have a direct conduction of heat from the inside of the building to the exterior, what underground housing guru Malcolm Wells calls an "energy nosebleed." There needs to be a thermal break somewhere in that framing system to prevent condensation on the inside and wicking of heat (or cold) in both directions.

Do not over dry the hardwood, a mistake that we made once, 23 years ago. Very dry hardwoods can take on moisture from a variety of sources, including the mortar and a driving rainstorm. The wood can swell and break up the wall. Also, guard the bottom courses in particular from excess water. For this reason, I do not advise building with hardwoods on a slab. Water can collect there and cause swelling in the first course of cordwood masonry. How long to dry it? With unsplit wood - rounds - three months is plenty at log-end length. With split wood, half that time is enough, then build. The wood will shrink anyway, even if you dry it a year, but this is only a cosmetic problem which can be fixed a year or two down the line by a variety of methods described in Cordwood Building: The State of the Art. Wood expansion is a structural problem.

I do not treat wood for several reasons. One, you want to avoid a constant maintenance situation. That is one of cordwood's attractions: it has virtually no maintenance. The sun's rays and other weathering conditions will break down the linseed oil or any other coating, stain, or preservative that you apply to the exterior. Let it weather. It will look good, a bit like stone masonry. On the interior, without "benefit" of any kind of coating, the wall will look the same twenty years later it did the day it was laid up. Also, any coating, including varnish, urethane, or linseed oil, will darken the wall. Cordwood is already a light-absorbing surface; you want to do everything you can to make the wall as light and bright as possible (including, by the way, using a light colored mortar, as mortar is typically 40% of the wall by area.) Finally, watch out with linseed oil. Some people are very sensitive to it. This is a point made by healthy house proponents.
A portable bandsaw mill is a good way to cut your own trees into posts, beams and rafters. I do not see it as particularly useful for cutting cordwood. A chainsaw or cut-off saw would be better for that.

Q: I just recently purchased a cordwood camp in Northwestern Ontario and found that the construction took place with hydro poles as the logs. It was built approximately 12 years ago. As you can imagine, there is a lingering creosote smell from the logs on the inside. I would imagine that a good vapor barrier and covering the inside of the walls with a 2X4 construction wall would eliminate the odor, but we would like to keep the look and atmosphere as is. Could you recommend a sealer or some sort of transparent coating that would eliminate the smell, but keep the beautiful look to the walls?

A: This is a tough one! I'm not sure that I am qualified to answer. There are vapor barrier paints and varnishes on the market. I suggest that you go to a good paint supplier in your area and ask for a vapor barrier clear coating. It does occur to me that, because of the porous nature of end-grain log-ends, that you might need a couple of coats. I am sorry that I am unable to answer your question any better. Perhaps one of the other readers will be able to come up with something. I would be very interested in hearing of your results, however. Please let me know how it comes out, so that others can benefit from your experience.

Q: We are thinking of building an outdoor sauna and sinking part of the sides and all of the back into the ground. The ground is sloping and wooded. One company, Sisu, who make barrel type saunas said no way the wood would rot. They added the wood needs to breathe. But what if a stone wall, or concrete, was constructed and then the wood structure placed inside it, with a gap in between. The location is NE US.

A: Part of our round cordwood sauna at Earthwood is earth-sheltered, a small part, maybe 5 or 6 square feet. We used dry log-ends, then parged the exterior or the wall with two coats of plaster to return the surface to a smooth cylinder. Then we waterproofed with the Grace Construction Products Bituthene waterproofing membrane. Then, most importantly, we drained the area well with 4" perforated drain tubing encased in crushed stone. We have had no problem in the nearly 25 years that we have been using this sauna. See my book The Sauna at www.cordwoodmasonry.com. Your suggestion, in your message, sounds as if it would work, too.

Q: We are starting to harvest wood for our home, having used woodchip/clay for two previous buildings, and with good results, we are stretching towards cordwood, and looking at the perceived beauty of wedding cordwood and a cob/clay type mortar...a few questions. We really enjoy the earth plaster look and feel of our woodchip/clay home, and although there is one question that addresses stuccoing the interior, we are interested in possibly earth-plastering inside and out; would you foresee problems with that? Also curious about that internal material how would a straw-rich straw clay mix work? or cedar shavings?

A: I have no experience with earth-plasters and do not feel qualified to advise you on this. What I can tell you is that the exposed end grain of a cordwood wall acts like a capillary wick, sucking moisture out of any ambient with which it comes in contact: mortar, for example, and even air. We pour a pint of water on the hot stones in our cordwood sauna to make steam. There is a great hit of steam for a minute or two, but the cordwood soon sucks the moisture out of the air. (Which is why steam rooms are lined with shiny non-absorbant tiles.) Fortunately, the only place that the end grain comes into contact with mortar in a cordwood wall is when the stackwall corner pieces, called quoins, are set up against mortar. I always advice builders to seal the ends of these quoins with something like waterseal or Silcone Magic, greatly reducing the rapid transfer of moisture into the wood. This rapid transfer can cause the mortar to dry too quickly and then shrink and crack.     

Although I have not used earth plaster, it strikes me that if it is applied directly to an unsealed cordwood wall, it is very likely to dry very quickly, and, therefore, shrink, crack and (maybe) fall off the wall. All I can suggest is to test a fairly harmless area, maybe 3 feet square, and see what happens. Test another area where a sealer has been applied first. Let us know your results in these pages.      Remember that cordwood masonry's long life as a wall system has a lot to do with its breathability. Maybe your earth-plaster is breathable. I don't know. But I would caution against plastering it until after the sap moisture has transpired out out of the wall. This may not be until you've spent a winter of actually heating the building. And I am leery about doing both sides of the wall, even then. The wall needs to breathe, one way or the other (and preferably both.) 

Q: I have a technical question about Cordwood homes... my question comes to the aesthetic appeal.. can a person after building the house cover the interior and exterior of the Cordwood walls with stucco.. I am not sure if there would be a problem with the expansion and contraction of the logs.. I would love to know because I am extremely interested in the construction of such a home.

A: Several people have successfully covered the interior of cordwood walls with stucco. Plastering the exterior has been less successful. I have no personal experience with placing plaster over cordwood, so I am disinclined to give advice. You might want to put a posting on the excellent cordwood forum at www.daycreek.com and see if anyone out there, who has successfully stuccoed cordwood, can give you some advice. On a related note: To brighten our son's bedroom, we applied two coats of an ivory latex paint to the cordwood walls. It looks great - you still see the cordwood masonry texture - and it really brightens up the room.

Q: I was unable to complete a cordwood building I started this summer. There is no roof, but I have covered the wall tops with plastic to keep rain and snow from entering. I am a bit worried about the bottom of the walls where they meet the concrete slab. If they are wet and it freezes will they be damaged? Do I need to seal the concrete and mortar?

A: This is a difficult situation, and tough to comment on without seeing it. Also, I don't know the size of the building or where you are located, but, presumably, it is a place with a snowy and freezing winter. Covering the tops of the wall is good, but it is the bare minimum of protection. I guess what worries me most is the slab you refer to. If water collects on the slab and stands against the bottom courses of cordwood masonry, this could cause a swelling of the wood, which would almost certainly result in structural damage. If I were you, I would apply a waterseal product or siliconized sealer such as Cabot Waterproofing ("An Advanced Technology Silicone Sealer") to the bottom course of cordwood, both wood and mortar. This will greatly lessen moisture absorption into the wood. If the building is small and you can set up some kind of tent arrangement to shed the water away to the outside, that would be the best solution.

Comment: As we know, wood is highly susceptible to decay, so placing wood repeatedly in direct contact with cement--which is all too happy to transfer its moisture to the wood it surrounds--and running the logs perpendicular through the building, leaving the most porous butt-ends of the logs exposed to the elements, is tempting fate indeed, particularly in wet climates.

Response: It is exactly this special method of placing log-ends next to mortar that is the reason the wood doesn't rot out, when rudimentary cautions are taken. This breathability along the longitudinal cells takes moisture away. Wood rot occurs when a constant moisture is maintained, key to the propagation of fungi.

Q: My Husband and I purchased a cordwood cottage around 7 years ago. We find in the winter you can feel the wind thorough the walls, and in August/September we get a lot of cluster flies. So to solve this problem, we have decided we would side the outside of the cottage. We are going to strap with 2 x 4 and 24" centres and spray foam in between the strapping to a thickness of 1 1/2 " to fill all the voids from the different cordwood lengths. Is the spray foam going to cause us problems with the cordwood.

A: Your plan sounds like a good one to me. No, the foam will not hurt the cordwood. You will certainly improve the infiltration issues and increase the insulation value of the walls. I assume that your overhang will adequately protect the extra thickness of the wall.

Q: On the question of whether to seal or not seal cordwood walls, I see you responded to a question about foaming and siding the exterior of a cordwood cottage. I was a little surprised at your answer, always thinking that you need both sides open for the building to breath. I have a similar problem with a cordwood addition on my home. The walls are more than 20 years old and are leaking air. When it’s windy and cold outside, we can certainly feel the cold air coming in. I’ve been trying to plug the leaks using cedar shims in the small gaps, but it’s a long process and I’m not sure how well it’s working. I really like the idea of foam insulation and siding. Can you address the issue of possible moisture building up in the house when the outside is sealed in this manner? Keep in mind that the cordwood is only an addition to the main house.

A: All of the original sap moisture has long since transpired out of your 20-year-old cordwood walls. It should be fine to insulate and sheathe the exterior if you like.  The wall can still breathe to the interior, if necessary.

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