Wateproofing & steam vent in dome

[pizzahead]

Hi all. I am about to seal my oven dome with a sealer to keep rain out. It is a Bondcrete product and is elastic and waterproof.

How important is it to have a vent hole in the outer render to let any moisture escape as the oven heats up?

I have a hole that goes through to the oven chamber (were i insert a thermometre) about half way up the dome (about 2cm in diameter). I am wondering if I need a second hole drilled in the top that penetrates the render to let out any steam.

Thanks
Jase

[brickie in oz]

Originally Posted by pizzahead

I am wondering if I need a second hole drilled in the top that penetrates the render to let out any steam.

And that lets more water in?

Where is the steam coming from in the first place?

[david s]

Once your oven is dry and the outside waterproofed it doesn't mean that it won't get wet inside again. After months of rain and no use the refractory will gradually suck moisture into itself, let alone water that has penetrated into the oven via driving rain into the entry. Igloo ovens are more prone to picking up more moisture than enclosure types. If you have waterproofed the outer shell then you are also locking in moisture. A vent at the top is a good idea IMO but put a lid on it so water won't get in through it.

[Neil2]

My vent at the top of the dome is simply a 1 inch diameter hole in the stucco (thru to the insulation). To keep precipitation out, this is covered by a loose fitting 6 inch copper plate shimmed up slightly and glued in three spots with acrylic chalking (any construction glue will do)

You can find these decorative plates in used stores / thrift stores very cheaply. They were all the rage 40 years ago as wall hangings.

[texassourdough]

When the water turns to steam it will eventually get out whether you have a vent or not. As David points out the oven WILL get wet - cement absorbs water from the air if nothing else. The good thing about a vent is it gives the steam a more direct way out - but it is likely to simply condense in the outer areas of your insulation and that will stay damp until you fire it enough to drive it out! I see advantages and disadvantages. Not a clear topic to me.
Jay

[david s]

Having cracked a couple of outer shells in early firings (I don't use blanket because of the safety issue and use vermicrete instead which contains tons of water) I deduced that it was steam pressure build up, remember that one litre of water creates 1000 litres of steam, so I now have a vent system around my flue pipe that allows moisture or steam out of the insulating layer. I haven't cracked an outer shell since.

[Gulf]

Great thread!
I have seen all of this before but, the timing is perfect to reinforce the venting idea for me . I did use the blanket however, I intend to add about 4 more inches of vcrete. I also used vcrete to insulate my flue liner brick form the face brick. The face brick seemed to look dry soon after laying, but the firebrick liner imediately shown signs of mosture from the inside. And, still do. I have left areas for the flue to exhaust moisture, and I will definately leave some kind of vent for the apex of the dome.

[brickie in oz]

Originally Posted by Gulf

I will definately leave some kind of vent for the apex of the dome.

But isnt your oven underneath a roof? if so why vent as there will be no moisture getting in.

[david s]

Gulf, if you have a roof over your oven you would be better not to waterproof the outside shell, so that it can breathe.
If you are doing vermicrete make sure you eliminate the water before you do the outside shell.One inch layers with a week between each to dry out works pretty well to get rid of the bulk of it.

[Gulf]

Al,
I guess that I want to install a vent just to be on the safe side. That is just in case that I don't drive all the water out of the vcrete before rendering. The 90% plus humidity in my area may be a factor . I don't know if it would be necesary to have one under a roof, but it seems like a simple feature to add .

David,
As for as waterproofing goes, I don't think mine will be exactly that. I am going to eventualy cover the dome with old brick splits. In my case I think that I will cut, but not mortar, the plug brick at the top. Neil's vent cover and vent hole through to the insulation sounds like a classy way to go .