While I was browsing the forum, I noticed a posting some time ago asking for information on hemp-lime insulating render.
This might come a little late to respond to the original enquiry, but with the general interest in building and renovation, I wanted to answer some of the questions and tackle some of the misconceptions regarding the use of hemp-lime insulating render in renovations compared with glass wool, stud and plasterboard.
I have a lot of personal experience of both of these solutions, the first being with glass wool and plasterboard, when I started to renovate our own house ten years ago. At that time I had very little experience in renovating cob and stone buildings and since everyone I knew seemed to be using glass wool and plasterboard, I followed suit.
I had my doubts from the beginning as to whether I was doing the right thing and began to research other possibilities.
I’ve been fascinated by bio- sourced materials in building for years and when I came across hemp and lime being used as insulation I knew I’d found what I was looking for.
At first we tried applying the render by hand which certainly taught us a lot about the potential of the material but the drying time was just too long due to the excess of lime and water needed to get it sticky enough to stay on the wall.
Three years ago we invested in a system for spraying the hemp-lime mix directly onto the wall, resulting in a much reduced drying time (can be applied during the winter period) and a superior insulation value thanks to a much lower density (less lime and water).
So why use vapor permeable materials such as hemp and lime in old buildings? The answer is quite simple.Some of the oldest buildings that survive to this day are built from natural materials, wood, stone, clay, lime for example. They have survived because their builders understood the properties of the materials they used and the need to keep them ventilated and allow humidity to pass unimpeded through those materials to avoid rot and decay.
The evidence for what happens when “modern materials” such as cement and non permeable paints are employed in these old buildings is only too apparent. In extreme cases, a failed cement render on cob can allow the wall to become saturated causing it to collapse.
So to get back to the glass wool versus hemp argument here are the pros and cons in a nut shell:
Glasswool (Laine de verre)
Easy and relatively cheap to install. Although more expensive semi-rigid panels need to be used, to prevent the whole lot collapsing and ending up in a pile between the wall and the stud.
A vapor break should also be installed to prevent water vapor entering the insulation.
It’s worth noting here that the theoretical thermal resistance of a given thickness of glasswool (R value) will diminish significantly as the level of humidity in the insulation increases.
Another disadvantage of stud and glasswool is the attractive environment it presents for rodents. We have personal experience of rats coming through from an adjoining building, nesting in the insulation and gnawing through electric cables.
Aside from the purely technical aspects of the material, there are also the considerations of health and the environment.
Formaldehyde has been used as a component of the binders in glasswool for years although the major manufacturers are now trying to play down this rather unsavory aspect of their product.
The formaldehyde will gas off for a period of time in your house after being installed, so it’s not surprising we hear a lot about “sick house syndrome” these days.
If we then look at the environmental impact of the production and disposal of glasswool (high embodied energy and co2 production), it becomes apparent that it may not be the best choice.
Hemp-lime Insulation
More expensive than glasswool and plasterboard
Provides a solid wall, with no voids.
Rounded forms can be created for arches and window openings to retain the character of a building.
The cellular structure of the hemp gives it the ability to stock excess humidity in the building that will then be released through evaporation in dryer periods. This creates a natural warming and cooling cycle.
By reducing the ambient humidity in a room it will require less energy to be heated.
Unlike plasterboard, the hemp-lime allows the walls to retain their thermal mass which has the effect of buffering temperature changes.
Both the hemp and the lime binder help to maintain high air quality in the building.
The hemp also locks up significant amounts of Co2 which justifiably gives it it’s “eco”label.