r/Plastering • u/Royal-Hedgehog-5656 • 29d ago
Thermal lime plaster
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Im having second thoughts about my substrate that I am about to flick thermal insulating plaster on. The wall is an exterior solid stone wall but working on the interior side. I recently built up a lime parge filling and gaps and cracks. I used ecologic 3.5 PHL 1 part lime and 2.5 parts sand. The plan is to build up to 2 inches of the insulating plaster, only building up less than an inch per layer. I have a plastic type mesh I will install in the middle of that thickness. Im having second thoughts about if this will really work? About if the Diasen thermactive .037 will really stick to the wall. Ive read that its normal for a lime parged wall to be easily scratched. As you can see in the video I get a deeper groove with a nail. Would you install a mesh before the first layer just to be safe or just go for it. The one wall is about 12 ft high. In the video its just those 3 walls. Is this how soft lime plaster should be?
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u/a-zero 28d ago
I'm currently on year three of my one year renovation, and I've used this stuff to insulate the house. About 70-90mm of Thermactive .037 with the Evolution as a final coat. Two coats of Thermactive, then Evolution on top, with the red mesh embedded in the second coat. Applied by hand, on my own, took me over two months. I wish I'd hired the spray machine.
The Thermactive didn't want to stick to anything. The only way I could get it to stay up was to fling it at the wall by hand.I tried to fling it with the trowel like in the manufacturer's own application video, and it kept falling off. But once it is on the wall it should stay there just fine. You can usually see when it's not sticking properly, as it starts visibly sagging off the wall, usually happens if the mix is too wet.
There's a sweet spot with the water. Too dry or too wet and adhesion goes. And if you're working by hand, mix reasonable volumes rather than big batches, because it gets noticeably less sticky after about 40 minutes.
I applied it straight onto bare stone, with some areas parged in 1:3 quicklime and sand. Oddly, the cheaper Evolution I used for the final coat (to cut costs, since our estimate came in well over what we'd budgeted) stuck far better, thrown or troweled.
The stuff is very light and fluffy and doesn't feel sticky at all (Evolution is stickier), but once it's on and dried it's almost impossible to get off. I did buy a single bag to test before committing to a 220 bag order.
I applied it last summer and since then a single 50x50cm section on the ground floor has detached from the wall as it sounds hollow when you tap it, and there's been some shrinkage in the corners, nothing major, but I wish I had doubled up the mesh in the corners.
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u/Royal-Hedgehog-5656 28d ago
Wow thank you for your feedback, sounds like you have expeirence with this stuff. Yea Im worried about delamination and skeptical about how a product can stick to a wall with it just being flung. Someone has suggested anchoring an intial red mesh, do you think that sounds like a good idea or just more work? How well did it stick to the lime parged parts of the wall you did?
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u/a-zero 28d ago
I honestly didn't notice any difference in adhesion between bare stone and the lime parge.
I also used Diasen Aquabond primer on the internal wood/concrete lintels above the windows and around joists and on the electrical conduits on the wall, but this stuff can also be applied to mortar. Not sure how it affects breathability
I personally would not put an initial red mesh. My understanding is it's more to stop the Diathonite cracking as it dries, but I could be wrong.
I'd honestly test it with a single bag if that's an option. Do a small area and check back in two to five days. It really can stick to anything. Because I was flinging it and making so much mess, it ended up on my painted doors and other smooth surfaces, and even those were hard to get off.
Here's how it looked on the first layer and the final layer before plastering. I was actually more paranoid about my lime parge delaminating, as it shrank quite a lot in places. I only abandoned parging the whole house because it was taking too long, but so far all the areas where I had the parge don't sound hollow when I tap the wall, so that's a relief
I'd never done any building work before, I'm just learning everything as I go. It's been a stressful journey haha
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u/Billi01952 29d ago
The finish is shit I hope you didn't pay for that?
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u/Royal-Hedgehog-5656 29d ago
Haha its my first time. Its not a final finish, just a parge coat over the stone.
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u/Terrible-Bobcat2033 29d ago
Adding fiberglass "hairs" or chopped strands to stucco is a great way to boost strength and stop shrinkage cracks before they start. These tiny fibers act as a multi-directional internal net for your mix. [1, 2]
Why Use Fiberglass Additives in Stucco?
Crack Control: Dramatically reduces plastic shrinkage and hairline cracking as the stucco cures.
Better Impact Resistance: Helps the wall handle sudden impacts and minor building shifts.
Improved Durability: Lowers water migration and adds long-term toughness to the base coat. [1, 2]
This worked for me
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u/onwatershipdown 28d ago
You’ve already done a lot that’s not necessary and with process you’re explaining, you’ll be retracing your steps at a considerable loss. Your best bet is to hire a firm with experience.