waterproofing dome

[Kevink]

Hi there, so we have been working away on our oven and are looking into the final coat for waterproofing.

I'm locking at a 3/4 to maximum 1.5 inch coat of cement plaster (1 Portland, 0.5 lime, 5 to 9 sand). in this mix I want to add a waterproofing admix of some sort, ore put it on as is, and coat the outer shell with something like glasswater.

we don't want to use a concrete waterproofing coating like you put on a driveway (at least we prefer not to make it a shiny dome)

any other options or thoughts would be welcome.

we are in Vancouver island Canada, no extreme winters but it hit sometimes -10C for a few days a year + it rains a fair bit in fall/winter.

the oven was build as flow:

1) 5 inch base with 3 layers of Re-bar (6x6 mesh)
2) 4 inches of insulating concrete (perlite (3x) portland (1x) mix 1:3
3) fire brick flat on bottom, with fire brick for dome sitting around it
4) dome inside 44 inches inside diameter x 23 inch tall from inside, fire brick was secured with DIY mix of silica sand (3x) lime (1x) fire clay (1x) portland (1x)
5) a few rows of red mason bricks, 6 inches from the fire brick
6) fill void between firebrick and red mason brick with perlite (3x) portland (1x) lime (0.5x)
7) insulating dome with perlite (3x) portland (1x) lime (0.5x) mix -- thickness of this insulating mix - side and top - 10 to 12 inches

8) waterproofing topping ? (3/4 to 1.5 inch thick)

any suggestions ??

[gastagg]

I am planning an igloo stucco finish. One of the considerations that I can't find any comments on is the amount of steam produced inside the oven and how that can negatively affect ovens that are finished as a igloo and sealed. I've seen a large portable oven that had a stucco finish and then a rubberized coating and it had huge water problems, and I don't think that the issue was from external water, but from internal steam that never escaped and then froze. Traditional exterior home finishes that use stucco also have some type of house wrap (Tyvek, etc.) that allows moisture to escape but limits water from getting in. I haven't seen anyone use Tyvek under a stucco finish, but I think it's a mistake to completely seal the dome. Enclosed domes have an "attic" with ventilation, much like a house would, where the moisture from cooking can eventually escape. Stucco is permeable, which is why you need to avoid sitting water, but to completely seal it would be a mistake in my opinion. I'd love to see some more experienced people weigh in on this. I don't have room for an enclosure and I prefer the look of the igloo, but I live in Minneapolis. So, I'm planning to use a pigmented stucco and cover during the winter and make every effort to protect against sitting water.

[david s]

I think it probably depends a lot on your local weather conditions. It is true that if the dome is totally sealed to prevent moisture getting in, then it also works to prevent it from getting out. Most proprietary renders contain a waterproofing agent that works to partially waterproof the render/stucco layer. I prefer to totally waterproof the exterior, but only after around 10 decent cooking fires (not including the curing fires) and to allow the insulation to communicate with the ambient air, I mhave designed a vent around the flue.This allows the release of damaging steam pressure as well as allowing moisture to find its way out. After our wet season(we live in the tropics) my oven needs a dry out with a few long slow fires to restore normal function.

[Alomran]

Kevink & Gasstag , George:
Very interesting 2 points...Steam & vapor. I live in wet England and I know these 2 critical issues as an architect.
Allow me to think loud here and I may be wrong.
The ventilation for vapor is always absorbed by the cement particles in the perelite. However, if there is too much water between the dome shell and the insulating shell, then a simple single plastic tube reaching to the insulation blankey/void and letting it outside as a trickle vent will do the job for few months after which all vapor will be gone, then you can fill out with cement and seal it.
As for the stem from cooking, it should evaporate with fire and I doubt it will seap thrrough the fire bricks.
As for the damp dome you have referred to, the builder must have had a plenty of water somewhere that it was so huge id did not get absorbed by the cement particles during the curing process.

[johnnybrewmeister]

So I'm at the end stages of my oven build, and like the stucco dome look, but would like to waterproof it (I think). Question: why not use Tyvek? In hind-sight, I would think a layer of Tyvek (water proof, but breathable as it passes water-vapor) over the CF blanket insulation layer sounds pretty smart. I just now have a very thin scratch coat of stucco over my CF, so can't really do this, but am considering a Tyvek layer in-between stucco coats. Is this a bad idea? I'm in So.Cal., so a fairly dry climate, but keeping things as dry as possible just sounds like a smart thing to me. I'm looking into acrylic renders, and elastomeric paints, but as most people agree - completely sealing the dome is not a good thing.
Thoughts?

[johnnybrewmeister]

Looks like Tyvek melts at 275F, I can't imagine the outer layer (after 3" of CF blanket) getting anywhere near that warm, would it?

[Gulf]

Only if there is no place to relieve the pressure. Water boils at 212F @ sea level. The steam from it should be about the same. That is, unless there is no vent to relieve the pressure. If not, then the water will boil at a much higher temperature. That means superheated steam which expands more dramatically causing even more back pressure and even higher temps until something finally gives .

That being said: If the oven and the insulation is totally dry, the outside of three inches of cf blanket should be near ambient temperature.

[johnnybrewmeister]

Thanks for the info Gulf. Since a Tyvek sheet passes water vapor, I would think there would be no build up of pressure.

Does CF blanket swell when saturated with water? I think this is my main concern: if the CF blanket swells, it wouldn't take much to crack the stucco layers on top of it. Maybe this isn't really a problem? Lots of people seem to have stucco domes, and as David mentioned, in the tropics his must get really soaked during his wet season. I'm probably over-thinking this, and worrying too much about it, but hey...I only really want to build one of these suckers! Keeping it dry (somehow) still sounds like a good thing.

[david s]

No, the blanket doesn't swell when wet, but it can hold a lot of water. It dries out ok too. At least they're the characteristics of the stuff I use (Morgan super wool), there are plenty of others.

[UtahBeehiver]

Heed Gulf's and Dave's advise. Water has a way to sneak in no matter what you do, even on fully enclosed ovens or even during the curing process. A "vent" on the dome will prevent build of pressure caused by water converting to steam. Since you only have a scratch coat on you can still put in a vent at the apex of the dome Dave's vent is in his vent stack. I would be concerned about using Tyvek in "between" stucco layers and still getting a good bond between the stucco brown and finish coat.

[johnnybrewmeister]

Thanks guys for the info. Very interesting...if the CF blanket doesn't swell when wet, then I am much less concerned about the stucco dome not being waterproof. Less worry is a good thing. Strange that this is the first I'm hearing about a vent tube into the dome, and I've been trolling this forum for years, and working on this build for almost 2 years (built a "practice" oven before this real one). Since you mention Dave's vent specifically, I will see if there is a build-thread that shows this. Do most people include a vent? (sorry if my ignorance is showing here).

Regarding Tyvek in-between stucco layers, I was basically going to ignore the existing thin scratch coat - that is, thinking of adding Tyvek layer, then adding more wire, then do another scratch coat, followed by brown and finish coats.

Is it bad if the CF blanket repeatedly goes thru wet/dry cycles?

[david s]

John,
I've pm'd you. Take a look at Gulf's thread.
Dave

[david s]

I've only just seen the OP on this thread and as it's over two years old any advice is presumably too late, but for anyone following this build, remember that perlcrete or vermicrete takes around a third of its volume in water and less than half of that will be used up in the hydration process leaving you with an enormous amount of water to eliminate. You can calculate that for a 6" layer but it's enormous. The beauty of CFB is that it's dry so you don't need to remove any water from it. If the insulation is enclosed the perlite can be added loose and dry and will be a better insulator. At 3:1 you are making the density four times greater than the loose perlite and reducing its insulation by around the same amount, so a doubly bad idea.See attached experiment on drying vermicrete.

Vermicrete insulating slab copy.doc.zip

[UtahBeehiver]

Vents in dome
Gastagg
David S
Gulf
Utahbeehiver - me
a host of others but can't recall exactly who.

But this is up to you. Since I believe you are not enclosing the oven in a structure and the dome will be seen you are probably wanting to minimize potential cracking of the stucco. Both Gulf and I and several inches of p/v-crete over our CFB so there was water to get removed.David's is vent is unique since it is built in the vent stack. Gastagg's is some copper tubing near the front of the dome.

[johnnybrewmeister]

Thanks Dave for the PM, and others for the details. So regarding the vent tube, it seems that if I did seal the dome with something like Tyvek, then a vent tube would be a smart idea. But if I don't seal it (which is where I'm leaning), since stucco passes water (and therefore water vapor), then there would be no need for the vent tube - the whole stucco dome would vent.
Sorry if I'm being thick-skulled here, but at least I'm learning alot.