Hemp & Lime Part 3
This episode recaps some of the lessons we think we learned while building with hemp and lime. See the previous episode for construction details.
After all, there are less than 10 structures in the American Northeast built with hemp & lime. In the spirit of learning from each other, it is important to not only show amazing images , but also help the next person avoid our mistakes.
Block and Spray Hempcrete Systems
In both cabins we used somewhat novel installation methods for hempcrete to save time and labor compared to the more tried and tested method of casting the hempcrete in place.
This did save time and labor during the installation process, but both methods (using prefabricated blocks & spray-applied hempcrete) came with some downsides. Taken all together, they probably didn’t save us time and definitely didn’t save money.
The Blocks
We chose to work with prefabricated blocks manufactured by Coexist in Pennsylvania. This in theory saves time -both during installation as well as drying. Instead of the typical 6–12 weeks of drying time assuming 1 inch per week, we could continue working on the interior immediately. Coexist is a small start-up that doesn’t have a warehouse full of hempcrete blocks. They did not produce the block until we placed an order, so the manufacturing and drying time for the blocks still had to be calculated in upfront. Which we didn’t — our mistake.
The blocks are very lightweight, which made it easy to install them and move them to and around the site. But we faced a number of challenges with them as well.
First, Coexists’ equipment to manufacture blocks broke down during the production process. We only had about enough blocks for one structure. Waiting for it to be fixed and then producing and drying the rest of the blocks meant losing all the advantage of a speedy process. So we didn’t use blocks on the second structure.
Coexist developed two block prototypes. The first using lime by limework.us. The second set was prepared using ereasy lime binder, the same as what we used for spraying the outer portion of the wall. Both have pros and cons and Coexist is currently experimenting with other lime mixtures to maximize the “pros”.
The ereasy blocks were good for compression, but crumbled too easily, especially when cutting them to fit corners or around window openings. We used them only at the bottom. The limework (darker in the image) blocks performed better when cutting (using woodworking tools), but were structurally less strong and provided a less than perfect surface for applying plaster. We ended up needing an additional coat of lime plaster and installed a base coat mixed with hemp hurd to create a solid substrate for the middle and final plaster coats.
Spray-applied Hempcrete
With no blocks left for the second house, we changed our design to a full 12” spray-applied hempcrete wall. Cameron McIntosh from Americhanvre was confident this would work. With the 2x6 framing on the exterior, this meant a full 7 ½ inches of continuous hempcrete with only a few horizontal battens installed in between to key the hempcrete in place.
From the outset, our construction timeline was bumping up against the limits of a spray installation season. Weather, particularly freezing weather, is a hazard on fresh hempcrete, before the lime binder has adequate time to harden and dry. We needed 2 solid weeks above frost for our installation.
We paid close attention to the weather and pushed our window of installation to allow framing and electrical work to be completed.
Once installed by late October, with impending freezes, tarps were placed around the structure and heat was used in the building to aid in drying. This worked too well, drying the hempcrete in record time. This record drying may have adversely affected the structural integrity of the hempcrete, resulting in a crumbly surface that could not hold the weight of plaster on its own. So we needed to install a plaster and lathe system against the interior face of the hempcrete. This way, the insulation was safely encased within the wall. On top of that, here too, we needed a total of three coats of plaster.
What would we do differently?
Don’t give up on blocks!
While the experience with blocks in our case presented challenges, the ambition to pre-fabricate modular units out of hempcrete still holds great promise. As mentioned above, Coexist is working to perfect its mix and a number of emerging firms have developed blocks. As the market grows these blocks will improve and can address the challenges we faced and deliver the full benefit of a versatile building module. Additional research is needed to not only create strong self-supporting blocks but units that are load bearing and replace a cement-based block in construction. In hindsight, we could have given ourselves more time researching and testing blocks or even ordering the blocks much more in advance, some of this would have challenged typical construction procedures and timelines. Who orders the blocks -client or contractor or sub-contractor? Who takes responsibility?
The only material that was purchased directly by the owner were the windows — with the foresight that they need a lot of lead time. Out of lack of experience we did not have the same foresight for the hemp blocks.
Redesign the framing to have more support for the spray-applied hempcrete
While there may have been a variety of reasons, why this wall behaved the way it did, more support for the hempcrete would have probably helped.
The speed with which the entire walls of our second micro house were completed was impressive and the surface texture and evenness between studs was amazing. So despite the obnoxious noise and carbon intensity of the machine used for spraying, it will likely continue to be increasingly used as a method to install hempcrete and replace cast-in-place. It will be good to learn from experience of how to optimize the relationship between hempcrete and structure.
Meticulously monitor the drying of hempcrete?
To simply blame it on the weather, would be too easy. But monitoring the humidity and temperature to ensure a not too slow-not too fast drying process might help to ensure the structural integrity of the sprayed hempcrete.
Better understand schedule and sequencing. Absolutely!
During our research design phase, the concern over drying time that a hemp & lime wall assembly adds to a construction schedule feels somewhat misplaced in retrospect.
While both blocks and spray installations were fast, the additional plaster coat on both the spray and block added time and labor cost. Given that the plaster crew was not local, it also added a lot of scheduling and sequencing headaches and additional periods of drying and moisture control that impacted other trades in their work flow. The “stop” — “go” shifting between plaster and other trades was probably the most stressful part for all.
With supply chains becoming more and more challenging, making and keeping to a schedule was challenging for other materials as well. Our progress was slowed several times, whether it was waiting for wood fiber insulation from Europe or for the local lumber yard to mill our floor and ceiling planks. Having two small structures to toggle back and forth was sometimes helpful. One crew could plaster in one, while the electrician or plumber was in the other. But the nature of their size (300 and 400 sf respectively) also made it hard at times to work around each other or each others material.
