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Converting latex painted drywall to lime plaster  RSS feed

 
Bill Bradbury
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Location: Richmond, Utah
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Most homes these days are built with latex painted drywall, but this is a "non-breathable" wall assembly that is well known for growing mold and creating myriad health hazards. So, how do we go about converting our non-breathable homes full of toxic gick into a proper Permies palace?

First we delaminate the paper facing on the drywall, removing the latex paint in the process. The ability of a material to allow air to move through is called permeability. Drywall is a very permeable material if it has a breathable coating, so once the outer paper is removed along with the paint, the wall becomes breathable. This permeability can actually be enhanced with the addition of a good plaster. When you spray water on bare papered drywall, the water runs down the face, but when there is lime plaster, a large amount of water can be readily absorbed.

When the drywall is delaminated, the joint compound comes off as well; so we retape the joints with a 50:50 mix of lime plaster and gypsum based chemical/latex free joint compound. There are only two companies that I know of that make such a produce such a joint compound; National Gypsum Pro Formula or Murco all purpose. These have low gypsum contents and a lot of other things that help lime plaster to spread easier and cure faster without cracks.

In the first photo we are spraying the joints with water after delamination, taping and curing, in preparation for the first coat of plaster.

The plaster mix should have a mix of 5gal wet mixed lime plaster to 1 large scoop of joint compound. Mix the lime plaster first, making a very wet mix. Then add the joint compound; it will seize the mix, making it easier to use/less runny and more plastic, so it spreads real nice.

Start on the ceiling, using a stainless steel trowel, I like the Japanese style trowels; 255mm long by 0.5mm thick.

Smooth the plaster onto the ceiling by starting in the corner with a large blob on your trowel, then press down on the back edge of the trowel and pull toward you, rotating the trowel towards the front edge and pressing the plaster in at the same time. This will give you one even streak. Do this side by side 5 or 6 times and then smooth the area and clean up the edges and corners. Keep going like this to cover everything.

Don't let the plaster dry! Keep wetting it as necessary for the next 3-5 days, to ensure proper carbonation.
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Wetting the joints
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Lime/gypsum plaster mix
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Ceiling's first
 
Zach Muller
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Bill Bradbury wrote:

First we delaminate the paper facing on the drywall, removing the latex paint in the process. The ability of a material to allow air to move through is called permeability. Drywall is a very permeable material if it has a breathable coating, so once the outer paper is removed along with the paint, the wall becomes breathable. This permeability can actually be enhanced with the addition of a good plaster. When you spray water on bare papered drywall, the water runs down the face, but when there is lime plaster, a large amount of water can be readily absorbed.


I bet it is my complete ignorance on drywall matter, but how is the best way to delaminate?
 
Mike Cantrell
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So the reason paper is included on drywall sheets is for tensile strength. Like any mineral, gypsum is exceedingly weak in tension.

I'm just thinking out loud here. Stripping the paper off the sheets is ok because they're already screwed to the wall? The tension conditions (such as lifting off of a stack with two hands) are over and done, and now each sheet is being supported with screws in many spots, so it can resist falling apart because its weight is spread out?

Are you using any kind of fiber in the lime plaster (I think I saw in your other thread that yes)? So the lime plaster will replace the paper as the tensile layer?
 
Bill Bradbury
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Zach Muller wrote:

I bet it is my complete ignorance on drywall matter, but how is the best way to delaminate?


Hi Zach,

We use a 4" putty knife to get right between layers of paper. We don't want to remove all the paper as this, like Mike said, will weaken the drywall. That's why you are seeing brown paper on the old drywall sections and some small areas of exposed gypsum where we have gone too deep.. The white paper is new drywall.
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Brown paper shows when drywall has been delaminated
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Before
 
Bill Bradbury
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Mike Cantrell wrote:So the reason paper is included on drywall sheets is for tensile strength. Like any mineral, gypsum is exceedingly weak in tension.

I'm just thinking out loud here. Stripping the paper off the sheets is ok because they're already screwed to the wall? The tension conditions (such as lifting off of a stack with two hands) are over and done, and now each sheet is being supported with screws in many spots, so it can resist falling apart because its weight is spread out?


Hi Mike,

The paper is still there, only the outermost portion is removed, in order to ensure tensile strength is maintained as this is a large amount of the holding power for the screws. After delaminating we add supplemental fasteners to flatten the surface and to ensure attachment strength as we will add several hundred pounds of plaster to a room like this. I am only delaminating to remove the paint, the new drywall is completely intact. I agree, I think it would be difficult to install drywall without the paper and I'm not sure how strong it would be.

Mike Cantrell wrote:
Are you using any kind of fiber in the lime plaster (I think I saw in your other thread that yes)? So the lime plaster will replace the paper as the tensile layer?


We use fiber for the taping mix, but not anywhere else on drywall. We only use fiber to reinforce a spot of plaster that may crack like the base coat on lath or seams between sheets. This plaster is quite strong; basically it's Roman concrete, check out their website limestrong.

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First coat complete
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Have you seen Paul's rant on CFLs?
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