hemp-lime plaster as exterior insulation

Hello everyone! Greetings from Bulgaria.

We are planning the renovation of a brick house which we bought unfinished (just bricks and concrete). We are planning to plaster the interiors with a clay mix from our land and still wondering what to do in terms of exterior insulation. There aren't any breathable alternatives here in Bulgaria other than mineral wool, which is an imported and energy-intensive material. A friend mentioned the idea of hemp-lime or hemp-earth plaster, which if made thick (about 10 cm) would have insulation properties.

My questions are:
1. How insulative would this plaster be, is there any technical data out there?
2. Does anyone have a 'recipe' for the proportions used and are any other materials are added to the mix in addition to fibers and lime?
3. Can you recommend a better (more insulative) and inexpensive alternative?

I thought about using some kind of sheep's wool made into sheets below the plaster for an additional layer of insulation...

Thank you very much!
All the best,
Diana

Diana, I'll need more info to help steer you in the right direction.

How available is hemp there and is it expensive to source? The quality of the shiv is somewhat important as in few loose fibers, longer the better.

What limes can you get, for example NHL 2, 3.5, 5? Where are they being sourced? Or, Type S, SA, N, NA?

Where can you source mineral wool and is it as expensive as hemp? How about straw?

Do you have access to wood chips for a lower cost than hemp and mineral wool? What species?

Insulation hempcrete, limecretes, are going to need to be casted into frame work, as mineral or other wools need support structure, they are not stand alone. I'm guessing 2 x 6's min but many opt for double studded 2x4s 30-45 CMS (your brick could serve as one wall) there are lots of reasons why. @ ~ R 4/ 10 CM may not do much of anything not that r-value is everything.

Can you get portland cement and fly ash? Together with lime they reduce drying times and improve compression strengths.

Can you get magnesium oxide it dries much faster than lime (24-48 hours) and has better properties need about 1/3 less than lime cements. Needs no wet curing.

What is climate like there, your coldest and hottest temps relative to the indoor desired temp and how is the current house performing as far as electric. Is the current HVAC gas, electric?

How humid? When you plan to build too? Drying times need to be considered.

So, depending on the brick it can be very good mass as an internal or external wythe separated by an air or insulation gap is typical such as a mineral wool core with inter and outer brick wythes or casted cretes. There are reason why that is a top performer and yes there is data that gets complex. It really depends on how energy efficent you want to design and how much you want to spend for performace. We can get you to zero utility or low bills but it may not make sense to deep energy retrofit a home when the same can be had by a solar array. If a energy retrofit makes sense it is usually an outsulation wrap, not insulation, that gets expensive since it needs a foundation.

Hi Diana,

Firstly; hemp added to lime plaster will not significantly affect the thermal resistance, but will add a source of silica which will help to strengthen the plaster. Your best bet for a plaster is lime mixed with clay that has a high amorphous silica content, then add shredded hemp or other fibers. Pumice and wood ash are also good sources. You will still need a nice sharp sand to act as the aggregate.

As for insulation, what can beat sheep's wool?

Attach felted sheets of wool to wooden boards vertically attached to the concrete with a capillary break between or frame a wall 1/2" away from the concrete. Then cover this with builder's paper if you like, or not if you don't and then lath booards nailed horizontally at 3/8" gaps.

Then plaster, 3 coats, starting with a 1 part lime/clay to 2 parts sand and finishing with 1:1 with a softer, finer sand.

I hope this helps,
Bill

Terry,  Have you links to fill in recipes to the Wood ash etc commentary you've written here?

Thanks

As for insulation, hemp can beat sheep wool and it's also more eco friendly, as sheep's wool is surprisingly an insulator with a very big footprint.

And adding hemp shivs to lime is definitely a good way to enhance it's thermal properties. That's why hemplime is such a great insulator.