Using Steel in Hempcrete


I have a few reservations about putting steel framing, particularly thin gauge steel, into hempcrete. This is not because I think it cannot work. It is because I think it creates risks of future failure brought about by corrosion that do not need to exist if timber is used. In addition to the risks, steel has a much greater carbon footprint than timber and this alone should encourage us to avoid steel framing. Building reliable buildings without defects is an important goal for the adoption and long-term success of any building material and the combination of hempcrete and timber frames is well tried and tested. There are very few examples of steel frames in hempcrete buildings and much less clarity on whether or not durability may have been compromised. I have written this essay to explore a few thoughts on using steel framing to replace timber in new-builds, particularly in relation to hot dip galvanising.

Steel itself is an amazing material but it has a relatively high carbon footprint compared to wood. According to the International Energy Agency, the iron and steel industry accounts for approximately 6.7% of total world CO2 emissions so arguably we should be using it sparingly and avoiding its unnecessary use where lower carbon options are possible.

If steel is used in a wall we run the risk of it generating interstitial condensation. This is because the steel has the potential to create a cold spot in the wall. Warm air can carry more water vapour than cold air and so as the air is cooled next to the steel the moisture condensates as it reaches its ‘dew-point’. It is the same mechanism as water condensing on a cold mirror in your bathroom. In the case of a hempcrete wall, this water may then act as a kind of electrolyte, reacting with the lime, which in turn could speed up the corrosion of a protective coating. For me, the first question regarding steel in hempcrete is always where it is in the wall. Towards the inside of the insulation layer it is unlikely to get cold enough to allow condensation. Towards the outside of the wall condensation becomes a likely result.

Even if we accept the higher carbon footprint of steel and its condensation risk there are additional considerations that should not be ignored regarding which protective coating is used. Hot dip galvanized was recently suggested on the forum and I have strong reservations about its use with hempcrete in the long term. According to the Galvernizers Association “An average coating thickness of 85 microns, can protect a steel structure for the better part of 100 years” and “One of the inherent benefits of hot dip galvanizing is that when clean steel that is greater than 6mm thick is submerged into molten zinc, it will develop a minimum mean coating of 85 microns”. “The coating thickness depends upon the gauge of the steel that is galvanized” so thinner steel generally ends up with a coating of less than 85 microns. The thinner the steel the thinner the coating.

As you read further the Galvanizers Association website goes on to say “Even in coastal environments an 85-micron coating can still deliver a life expectancy of 30 years or more”. This is where it gets interesting as it is clearly stating that life expectancy can be reduced significantly by environmental factors. Personally, when I build or renovate with hempcrete I am not aiming to have it last only 100 years. I aim to create a building that will last indefinitely and, if we are serious about carbon emissions, the building industry will have to learn to do the same. We know that timber will last for hundreds of years under the correct conditions and hempcrete provides those conditions perfectly. It does not necessarily provide perfect conditions for steel and that is why steel needs protection in the first place.

The Galvanizers Association goes on to say “The lifespan of the coating is also exceptionally long compared with other forms of corrosion protection and will weather at less than one micron per year in certain climates”. Now all things considered therefore we can assume that the coating does not last forever. In fact, when I called and spoke to a long-serving advisor for the Galvanizers Association he said that he would expect the coating to deteriorate at a rate of 6 to 8 microns a year if placed in a substance like hempcrete.

We discussed the pH and he said that hot dip galvanized was not suitable for environments with a pH lower than 5 or higher than 12. Despite the fact that binders vary, it is well known that lime binders create an extremely high alkaline environment, particularly when wet, and although alkalinity may reduce when dry(?) it is far from a benign environment.  This is particularly true in conditions where dew point is reached and repeated moist conditions may mean that water is regularly present. This effect could, of course, be reduced in very dry climates, but for many more temperate areas with poor detailing this could well become a problem.

VM Zinc goes further in their warnings of placing zinc coatings in contact with certain materials ( “Products containing mineral such as calcium carbonate, cement, gypsum will have a corrosive effect on zinc if exposed to damp conditions”. They add “Limestone dust and gypsum dust generated during cutting operations can react with zinc in the presence of water”. VM Zinc even say that a pH of below 5 or above 7 can cause problems with zinc coatings! Lime Technology were always clear on covering galvanized surfaces with paint to avoid a chemical reaction with the lime and I have not seen any evidence whilst researching for this essay to contradict this.

We coat steel to protect it because we recognise that without protection it does not last very long. Once the coating is gone how much longer would thin gauge metal last in a corrosive environment? And if this metal is structural what are the implications then? I am not so concerned with thick gauge steel used in a post and beam construction correctly detailed. I have done some of those myself and am confident they will not come to harm. I am far more concerned by the thin galvanized framing used by some modern housebuilders who seek to claim innovation and care little for the planet.

I should also mention at this point that we regularly use galvanized electrical socket boxes in the UK due to a lack of better alternatives. I am not concerned about using them as they are always well and truly on the inside of the thermal envelope and even if replacement is required eventually, this should be a simple enough process. It is structural steel elements in potentially moist conditions within the hempcrete wall that concern me most.

If the above arguments do not cause reflection, then consider this. Zinc is required for hot dip galvanizing and according to Wikipedia 13 million tonnes was mined in 2019 worldwide. The pollution caused by zinc mines in local rivers and on agricultural soils has been well documented and in China was observed to be a “serious pollution level” in local paddy fields. Invertebrate populations have not fared well near zinc mines in Missouri either.

For hot dip galvanizing the zinc is heated to 450 degrees Celsius and even higher temperatures are required when the steel is thick, as it too needs to heat up to receive the coating. There is therefore an additional environmental consequence in carbon terms and pollution when hot dip galvanized frames are used instead of wood. The timber industry is not an innocent in all this either, but I think it is clear to see on environmental grounds that it is difficult to justify using galvanized steel frames over timber frames unless there is a good reason.

I have not delved deeply into the environmental implications of the various paint options, but if you need to isolate steel from your hempcrete I suggest this is potentially a more effective option, with equal, but different, environmental implications. My experiences of examining painted steel encased in hempcrete for a few years are that there is little or no noticeable change. I would acknowledge that if you are renovating a steel building using hempcrete then acceptable compromises can be made, but domestic scale new-builds should use timber frames rather than thin multi-strutted steel studwork if they wish to maintain their environmental credentials and achieve longevity.

One of the issues here is how we view our buildings and over what time scale. When I was younger I remember arguing with a local builder about using cement renders on very old timber-framed buildings in Lavenham in Suffolk. His argument was that cement render was fine as he had cement rendered numerous buildings in the village over the last twenty years and he could not see a problem. He argued that cement renders were stronger and therefore lasted better. I have had similar arguments even more recently, but the error of this thinking is that we see twenty or thirty years as a long time, because for us, that is the case.

For buildings, twenty or thirty years is not a long time, indeed the buildings in question were hundreds of years old and had been protected by lime in various forms for the bulk of that time. Their more recent maintenance had been dominated by cement repairs at that time (early 1980s) and more recently we have become more aware of the problems those renders have caused with internal black moulds and rotting timbers.

The local builder that claimed cement was better was just looking at the surface of the building and in fairness sometimes the surface did look perfectly fine. He was also pleased with the lack of aftercare required. To him, putting on a cement render and painting it with a masonry paint was a solution that avoided the aftercare that lime can require and the regular coatings of limewash that had kept those buildings healthy for all those years. What he was not seeing was that the moisture trapped behind the cement render was rotting the old timber frames from the inside and although this process can be fast, it may take a generation or even two before the destruction inside the wall becomes apparent. A similar situation may well occur with thin gauge galvanised steel.

There are numerous examples of galvanized render beads that are encased in cement renders with varying degrees of success in the short term. However, for many years now lime plasterers have been encouraged not to use them and to opt for stainless steel or plastic instead. This has been good advice I feel, but some have ignored the advice and placed galvanized steel beading into lime renders. Whilst this is not something I would recommend, the poor material choice that may well limit the life of the render is, at least, at the surface and can be removed at the end of its serviceable life. Put this same material in the heart of a hempcrete wall and any future replacement would be impossible without destroying the wall.

More research is clearly needed and I would be more than happy to be proven over-cautious or just plain wrong about galvanized steel and hempcrete in the future when more solid evidence is available. It is not my intention to stifle creativity or limit experimentation with hempcrete, but I do feel that until we have greater knowledge of what really happens with thin galvanized steel in a hempcrete building it would be more sensible to experiment on a small scale for ourselves, not offer this service to paying customers.

One Comment on “Using Steel in Hempcrete

  1. This is great, and I love the clarity and repetition with which you express our commonsense goal of building structures to last centuries > decades.

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