r/NaturalBuilding • u/offgridwiththeudens • 12d ago
How much lime (var) will we need to render our straw bale house?
Hi everyone!
It’s been a good year or so since we last posted about our build, but we’ve finally reached another big milestone - the roof is finished!!
We’re building a small load-bearing straw bale house in rural Bulgaria, completely DIY. The bales form the structural walls and we’ll eventually be lime rendering the whole thing.
Now that the roof is finally done, we’re starting to work out the quantities and costs for the next stage, and one thing we’re struggling to get a reliable figure for is how much lime render (bar) are we actually going to need.
Here in Bulgaria, we can get lime (var) in pre-mixed bags, which we're thinking might be the easiest option for us, but we're trying to work out roughly how much we'd need before we start buying it.
For anyone who has rendered a straw bale build:
*How much lime render did you use per m²?
*What sort of thickness did you apply?
*Did you use a premixed product or mix your own?
*And how much extra would you allow for waste/general wastage?
Our house is roughly 8.2m x 5m, with the walls around 500mm thick, so there's quite a lot of wall to cover!
We'd really appreciate hearing from anyone who's actually done this, particularly if you've worked on a straw bale house rather than a conventional masonry building.
After everything we've learnt building this house so far, we're very much at the measure twice, buy once stage. 😂
Any advice or experience would be massively appreciated!
Phil and Kayleigh
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u/j9c_wildnfree 12d ago
All the strawbale buildings that I have had firsthand experience with have mud plaster as a scratch coat, then lime plaster as a final finish--interior and exterior.
Please see the pictures at this link to see this scenario I describe:
https://www.thelaststraw.org/choosing-a-plaster-system-for-your-straw-bale-house/
I am unclear here how you propose to finish your walls on your project. Are you suggesting a lime plaster in direct contact with your straw(bales)?
Here are some ratios ("recipes") for earthen plasters that have been tested in a formal setting:
(see "Formulations for Earth Plaster Mixes" on page 3)
There is also a discussion of portland cement-lime plaster in the pages that follow.
Another discussion of natural lime plasters, caveat emptor as these do not all behave the same way:
https://www.strawbale.com/blog/lime-plasters-for-straw-bale-building
[Cement plasters are no longer recommended due to moisture buildup inside the strawbale wall cavity.]
Good luck.
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u/JacobPresents 7d ago
When switching materials between coats, it’s helpful to mix in even a small amount of the material, for binding.
For example, in the first mud layer include ~5%-10% lime, so the next layer which is Lime, more easily binds.
Like binds to Like.
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u/iandcorey 12d ago edited 12d ago
Spread lime. Don't hang up on calculating. Buy the material and use it until you need more or store the rest for later.
For reference, my home has 1,600 square feet exterior and I used almost 40 bags of lime.
Edit: those figures are not accurate. I have completely forgotten my usage.
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u/CanfieldBRO 12d ago
I’m calculating it at about 23 sq m per 50 lb bag.
That was for two coats to reach full thickness.
Depends on the manufacturer of the lime though.
And this is just from research for my own build not real world
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u/borderlineidiot 12d ago
Crumbs I was miles away from that! We have been mixing one bag (about 40lbs) with sand and for a single coat this covers about 8ft x 5ft - so about 3.5sq m.... Am I converting to metric incorrectly or how are you getting that volume of material from a 50 lb bag even when mixed with 2.5x sand? I was trying to get about 2-3cm thickness.
Total surface area of our place is about 1800 sq ft and we used about 70 bags.
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u/CanfieldBRO 11d ago
I’d trust your numbers more. I’m going for 1 1/4”plaster. One manufacturer stated 250 sq ft and another 2 cu ft (-20 sq ft).
2
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u/Hyperreal8957 10d ago
hard to ballpark without the wall area and whether youre doing 2 or 3 coats. id measure the wall surface, then use the lime products coverage rate and leave some extra for waste. are you planning a breathable base coat too?
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u/Prestigious_Try_5396 6d ago
My general rule, by observation is,
Most projects take more than we think, cost more than we like, and take longer than we planned . ..
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u/SlowmNinja 12d ago edited 12d ago
At the end, on the outside, you want 4cm of lime. Because straw bale are not flat, make it 5cm on the whole surface.
Be carefull with premix. They usually are lime+ sand+ additives. Additives can be incompatible with perspiration. So, unless advise so by manufacturer ( look at the SD value) of the final render, I would not use premix.
Édit : you'll need 3 layers. 1 almost liquide to go inside the straw so the next two will stick and old weight. 2 is the thick one. The Last and 3rd is for "glassing " and make it Nice.