The Drying of Hempcrete Walls
June 6th marked the completion of the hempcrete walls for this home, notable as the first hempcrete house in Massachusetts. HempStone is now conducting weekly monitoring of the hempcrete drying process of these walls. This information will guide the Village Carpentry crew for when to begin plastering the hempcrete.
Hempcrete is a relatively simple material - made of hemp hurd, a lime-based binder, and water. The water acts as a catalyst to create the chemical reaction of the lime, which allows it to harden up into a monolithic form. In this home, it acts as the exterior walls. However, the desired final form of hempcrete is a solid dry material. This drying process happens naturally, yet looks can be deceiving. Is that dry-looking wall really dry? The only way to truly know is to test.
Moisture Content Tests
We test both the interior and exterior faces of the walls on each facade, as there can be a lot of variability in the drying time of each wall. Drying time can be sped up by exposure to sun, wind, and airflow. Conversely, drying time can be slowed down in areas of the building that are shielded from sun, wind, and airflow. Weather also has a big impact on drying time; wet, humid conditions will slow or stop drying altogether. Typically, the sunny South face will dry the fastest, and the North face, or in this case, the Northeast inset corner (as shown in these videos) will hold moisture the longest.
Drying Time Rule of Thumb for Cast-in-Place Hempcrete
The rule of thumb in moderate temperature and reasonably dry conditions is one week of drying time per inch of wall thickness. If the wall is exposed on both sides and allowed to dry from each side, the drying occurs from both faces. Therefore, a 12” hempcrete cast-in-place wall left exposed on both sides can be dry enough for plaster in 6 weeks, assuming optimal conditions. If the hempcrete is cast against a permanent form, then the cast-in-place drying time doubles to 12 weeks in optimal conditions.
This home is an 11” cast-in-place wall, and our summer thus far has been hot and humid. However, during the past week, we’ve experienced a great deal of rain and damp conditions.
Moving Forward
During the hempcrete drying time, there are many aspects of the project that can continue, including interior partition walls and finishes, ceiling finishes, electrical, plumbing, mechanical. Hempcrete needn’t delay the construction timeline, it simply needs to be taken into account in the planning and sequencing process.
If drying time is a serious concern, other hempcrete installation methods should be considered instead of cast-in-place: spray application using a drier mix, or pre-dried block or panel forms. Additionally, a combination of one or more of these solutions can be utilized. For example, a spray application of hempcrete can be cast along a hempcrete block, or cast-in-place could be formed against a hempcrete block or panel. Using a combination of methods allows for versatility in design and construction sequencing.
Key Moisture Metrics and Important Considerations
For this cast-in-place home, we’re looking for a consistent moisture reading of 25% or lower throughout the hempcrete wall face, measured at a depth of 1.5”. Once this is achieved, we give the green light to commence plastering. This is so that the tannin in the hemp hurd doesn’t leach through to stain the plaster. Also, plastered walls do two things:
1) they introduce more moisture at the faces of the walls, and
2) they slow the process of the hempcrete wall drying.
Best practice dictates that we let the walls dry out as much as possible, but at a minimum of 25% moisture before we introduce elements that slow down that drying process.
We test for moisture levels on whichever wall face will be be plastered. In this building, both faces will be plastered, though the construction crew aims to plaster the interior face first in order to install kitchen cabinetry and flooring to support an expedient move-in date. For this reason, windows are left out of the assembly for now to aid in drying time. Alternatively, electric fans could be used, or in the case of a structure with a chimney, a wood stove could be fired up to dry the interior air. In this case, the windows would be installed and sealed to keep warm, moist outside air from infiltrating the interior.
Test Equipment
We are testing the hempcrete moisture at a depth of 1.5” and 3” on each face of the wall to get a solid sense of the moisture content of this 11” hempcrete wall. We use a Delmhorst B2100 Moisture Meter with 4” insulated test prongs for this purpose. We also have 7” insulated test prong on a hammer slide to allow us to test to a depth of 6” into the face of the wall, which allows us to test midway into a 12” wall. With this tool, we can see precisely how much moisture the wall is holding throughout the depth of the assembly.
Data Results Thus Far
The drying process has already been considerable since we began moisture testing on June 13th. Presently, the first few feet of the first floor walls have met our critical metric of <25% on the exterior face. However, at a height of 6 foot above the bottom plate, moisture is still reading around 35% moisture content at a depth of 1.5”. This is normal, as the walls are built vertically, and the upper portion of the walls are installed later than the lower portion. Thus, higher portions of the wall will tend to dry a bit later than the lower portion. However, it’s important to note that drying time is not always a linear process depending on exposure (wind and sun) and design (overhands depths and bulk moisture controls); as the building enters the second story, airflow conditions higher on the building will speed up the drying speed.
For now, we’re testing only the first floor, since it’s been only a few weeks since the upper story of hempcrete was installed. But soon enough, we’ll need a ladder extension to conduct testing along each facade. Check back with us in a few weeks to see the moisture testing progress on this house!