Feb 22, 2017
For several years now I've been working through how to build exterior walls.
First, for the base, I'm considering wood-fiber insulation as the top candidate. More on that another day.
For the finish, my top candidate is NHL.
NHL stands for Natural Hydraulic Lime. Put simply, it's a lime plaster that also sets in water — a material with properties of both lime plaster and cement.
Cement develops strength relatively quickly, but its specific gravity is high, so when used as an exterior wall finish it needs to be mixed with lightweight aggregate as lightweight mortar. There are several pre-mixed lightweight mortars on the market, but their compositions aren't disclosed.
NHL's early strength isn't very high, but it gains strength over time. Its specific gravity is around 1.4, so it doesn't need to be mixed with lightweight aggregate. You could call it a material with honest pedigree.
Also, because of its hydraulic nature, NHL can be applied in thick coats. Lime plaster, due to its early-strength issue, is hard to apply thickly.
A lime plaster that can only be applied thinly needs cement mortar or similar as a substrate. With NHL, cement isn't required.
At the same time, NHL is a "breathing" material. It easily releases moisture from inside the insulation layer to the outside.
The currently mainstream method uses vapor-permeable waterproof sheeting to keep water out while letting moisture breathe. But the moisture is released into the ventilation cavity. A typical ventilation cavity is about 18mm, and there's a chance air doesn't flow well through it.
Rather than relying on a low-reliability ventilation cavity, applying the finish directly onto the insulation — using a direct-coat method with both finish and moisture-release performance — seems better.
For these reasons, my current top candidate is wood-fiber insulation with NHL applied directly.
I want to call this method the new-era lime plaster wall (or "earthen wall"). In performance, durability, and ecology, I think it's superior in many respects.
