Vent system

[MikeD55]

I’ve seen quite a few builds here with vents to help release steam during curing. I inquired into it on some of the Facebook groups but was quickly shot down. Seems like a good idea to me. Wanted to get some feedback.

thanks

[Giovanni Rossi]

First, curing of the refractory mortars is a chemical process that occurs over time. I believe you're referring to the drying process. The initial drying is accomplished very slowly after insulation is added and preferably before any moisture blocking render is applied. I discuss the period after the oven is complete below.

Adding a vent through the hard shell covering the insulating layers is a more recent addition to the scheme. I did it because it was recommended by builders on this site who's opinion I respect based on reading thousands of Forum posts.

As I continued my research, it became clear that any oven will accumulate and retain moisture no matter the climate unless it is routinely fired year round. Water ingress at points on the exterior is well documented. That's why we accept it may happen and take steps to provide a path for removal (weep holes in the supporting slab). While true in any climate, the fellows in the tropics are very clear about moisture entering through the oven door.

I admit, when I first read about it, I thought; wouldn't moisture in the insulation layer simply evaporate as the oven warms without building pressure against the outer render? But, as I was a novice and it made sense at some level and it just wasn't that big of a deal to do it, I added a vent. However, after a year of firing, it's pretty clear that my thought process early on was naive. The peak of the dome heats up incredibly fast, so it's not impossible that trapped moisture could form steam. If so, that rapid expansion could pose a problem if an egress is not provided.

At the end of the day... it makes sense, it's simple, it's inexpensive, there's no downside.

Then again, isn't Facebook the source of all truth?

Are you close to starting your business?

[RandyJ]

I agree with everything Giovanni Rossi said. It is a super simple thing to do and there is no down side. I did it on my new build. I also spoke with the owner of the company that made the kit that I and Giovanni built and he is very intrigued with it.

Randy

[MikeD55]

Ok thank you. My thoughts are why not?

I fabricated a weather door for my oven so water won’t enter the front. On top I went with a storm collar and a storm cap (vs a rain cap). The tile is frost proof glass pool tile and the scratch coat is pool scratch coat. The grout is prism grout which is for pools and showers and has a built in sealer. I’m trying to cover every angle regarding water entry since my town won’t allow a roof.

Getting back to the vent I’d like to try this on future builds. The only downside is the potential for water entry as it breaks the surface of the dome. If assume you just caulk the seam.

[Giovanni Rossi]

I fabricated a weather door for my oven so water won’t enter the front.

I don't know how you've constructed your opening, but a door will certainly stop gross water entry. However, fire brick will absorb a lot of water This can be from rain on your landing or simply humidity. Again, read the posts from guys in the tropics. This moisture will find its way though to your floor and dome from the landing or flue gallery.

I'm sure you read on the Forum about using a breather cap. It provides a cover for the opening while allowing gases to escape. You can render right up to the collar. I guess you could caulk the seam I you felt it was necessary. No downside to that either.

[MikeD55]

Ok. I see merit in it. Especially for post construction curing.

[david s]

When I developed my design 17 years ago, because it incorporated dry perlite as an underfloor insulator in a sunken space in the precast supporting slab, I included a drain vent for moisture to escape from. This now seems to be widely adopted and by all accounts most effective.
To adddress the issue of moisture elimination and steam pressure build up in the insulation layers I also included a system that supports the flue pipe which allows airflow between two terracotta caps that I fabricate. This was primarily a system designed to get around the problem of cracks developing between the outer rendered shell and the flue pipe. Having seen many ovens with problems here, when the much more conductive stainless pipe heats first and expands against the outer rendered shell, creating cracks and a nice entry for rain. The terra cotta caps not being a perfect fit against each other also provide a release for steam pressure in much the same way as the little hole in a saucepan lid.
Many builders have subsequently followed my lead and placed a vent at the crown of the outer dome to achieve the same effect.
On further reflection, because the top of the dome gets hotter first, moisture will be driven away from that area first, leaving the lower areas of the insulation still wet or moist.
I think that the steam pressure at the top of the dome insulation can be utilized to push moisture down, assisted by gravity, so the drains are better placed in the supporting slab and aligned with the insulating layers. I now cast four extra holes in my precast supporting slab to facilitate this function.This also has the advantage of all drain holes being out of sight.
I hope the pics tell the story.

[MikeD55]

Makes sense that the moisture would condense and fall. Pretty neat idea.

[MikeD55]

David does your system incorporate some kind of “rain screen” or drainage layer for the condensate to run down?

[david s]

"Ok. I see merit in it. Especially for post construction curing."
Yes the moisture for a new oven is better driven off before it is sealed in with an outer rendered shell. A saucepan full of water having a small hole in lid would be sufficient to relieve pressure that would otherwise lift the lid, but it is insufficient if you wanted to boil the saucepan dry in a reasonable time.

" David does your system incorporate some kind of “rain screen” or drainage layer for the condensate to run down?"
No, just some insect screen glued over the holes at the top to stop insect entry. A very lean (10:1 vermicrete) results in plenty of voids between grains that act as passageways for moisture to travel through.

About 15 years ago I was firing a new kiln and the recommendation was to ramp the temperature up to service temperature @100C/hr unloaded with kiln furniture in only, I was amazed when, at 400C internal, water began dripping from the front corner. My conclusion was that the heat had pushed water vapour through the IFBs into the insulation and it was condensing against the cooler outer steel skin. Around 200ml of water pooled on the floor before it ceased dripping. This means that the water purging goes on at temperatures much higher than one would expect and is a very good reason for progressing slowly if you want to avoid possible damage.

On the outside I slope the corners of the supporting slab away from the outer dome shell to prevent rain accumulating at that point. This is easy to do when I pre-cast the supporting slab. I only have to tilt each corner of the mould upwards before laying up. This position can often be another entry point for water and if a crack develops there it should be sealed up.

[MikeD55]

David

When I say rain screen I’m referring to a drainage plane like one finds behind some of the newer stucco or veneer stone systems that allow water and moisture to drain down and out of a building envelope.

[david s]

Yes, I thought that was what you meant.I had considered that, but discounted it because of the difficulty in creating the channels. The 10:1 vermicrete I use as an insulating layer, acting as a substrate onto which the render is applied, has the characteristic of plenty of small gaps between grains that should provide sufficient space for the travel of moisture. This lean mix is primarily designed to be as insulating as can be at the same time as still being workable. Some builders go down to as lean as 13:1, but I find that too tricky to hold up vertically. Another solution that I used in my home oven(now 17 years old) was to use aluminium foil, held in place with chickenwire. I thought that moisture would condense against it assisting in its removal to the top of the oven’s outer shell where I have a vent around the support and pipe seal. Worrying about the foil trapping moisture in, I perforated the foil in two places, about 4 sq in each. Now, if the oven has picked up moisture from rain or excessive, continued humidity, the outer shell is much hotter to touch at those two places, indicating that steam is passing through the perforated foil at those locations. I no longer use that system because I concluded it holds moisture in, as well as the chickenwire taking ages to apply. Random fibres added to the render mix ar a far faster, stronger and superior solution.
Sorry for the long winded explanation, but experimentation and observation are the best teachers in my experience.

[MikeD55]

David

There are numerous products out there sold specifically as rain screens that aid in drainage and are marketed specifically for use with stucco or stone. I’m sure they can be secured to the insulation and under the render.

As for foil there are now a couple of foil products that have holes in them. While putting them directly on the dome may be counter productive as they would conduct heat rather then block it, I have seen some professional builders put a layer or two between insulation layers to block conductive heat. My thinking here is that with the holes in them vapor can easily pass. The other benefit is that the foil could act as an added layer of protection against outside moisture from getting in.

[david s]

Yes, perforated foil is a possibility. In hindsight I wish I had perforated all of the foil rather than just two spots. My original thinking was that it would prevent the dried vermicrete from drawing moisture out of the freshly applied outer render. To counter this I now simply cover the whole oven in cling wrap, for a week which seals in the moisture in the outer render to damp cure it. I’ve been doing this for years. Works for me.
If the foil is in contact with any conductive material its insulation value is totally negated. Aluminium is very conductive and only works to reflect radiant heat.

Here's one I rendered this morning.

[mongota]

I incorporated a vent at the apex of the dome after my dome was completed.

After a long sit and then a fire, moisture does indeed escape via the vent. I was both surprised and pleased.

A couple of pics here: https://community.fornobravo.com/for...088#post438088