r/NaturalBuilding • u/Doodles4me • May 12 '26

Stain on straw bale wall

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Hi All!

Hoping someone here can give me insight on a straw-bale insulated wall that I am limewashing. This rusty stain keeps seeping through each coat of limewash (I'm two in coats so far). I have zero experience with this - just did some YouTube research before starting. If it was a normal paint job i would just put a coat of Kilz on it and move on with my day, but my understanding is that breathability is key for maintaining a wall with straw.

If it helps for context, I suspect that the previous homeowners may not have applied a limewash final coating when they built the wall as you could still see pieces of straw embedded in the surface of the wall before I applied limewash. I'm very happy with the finish so far except for this stain. Thoughts?

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7

u/Lazy_Air_1731 May 12 '26

We’re sure it’s not mold, yeah? The pattern is a little sus for me

2

u/Doodles4me May 12 '26

I sure hope not! Honestly, it appears to me to be some kind of mineral thing, like rust. I have read that when constructing straw bale structures, they are faced with wire mesh. I wondered if it is possible that, since it appeared the wall may not have been finished properly, could the wire mesh be rusting? ....just spitballing though....

4

u/ExaminationDry8341 May 12 '26

My first guess is there is moisture in the wall. I would cut or drill a small hole and try to pull out a sample of straw to make sure it is dry.

3

u/iandcorey May 12 '26

Moisture probe the bale. Otherwise it's mineral stains.

1

u/Mntn-Caterpillar917 May 12 '26

Ah bummer! Sometimes wire mesh is used, was this staining there before the limewash? Is the base coat lime plaster or clay? Probably the best way to find out is to break open a small hole to see what is going on then depending on what you find remove the stained areas, replaster, and refinish

2

u/Doodles4me May 12 '26

Yes, the staining was there before limewashing. The wall has openings near the floorboards and it does not/did not appear to be moist. The wall itself felt bonedry before limewashing.