Waterproofing over vermicrete dome

[RichC]

Hi, Id like some help with this please as I cant make my mind up how best to approach it. Ive decided to start my own thread rather than continuing to hijack others(apologies Merlyn and anyone else).
My vermicrete dome currently has a roof on it to keep out rain and I have my concrete countertop done around it. Ive been working on this for a looong time and want to finish it in the next 6~8 weeks before bad weather really sets in here.
I live in a very damp climate here(Ireland), with rain all year round. I want to have a bare dome finish without a roof on it(this is a must for my wife in particular) so it needs to be well sealed. Humidity is high here(typically >80%) so my oven will contain moisture/damp after periods of non use even if the render is impervious to water.
I also want my dome to have an unpainted render finish(white sand/cement)
I'm considering the following options:
1.(simplest) Get a waterproof render or a waterproof coating for the render and render directly onto the vermicrete.
2. Create an airgap between the vermicrete and render by applying a thin layer of ceramic fibre over the vermicrete dome, held in place with mesh and rendering onto that again, a waterproof render or coating. This seems the best idea to me?this also seems to lend itself to venting the area around the vermicrete dome

I'm still not 100% sure about how to achieve the 100% waterproof render finish. Either with a product that claims to be waterproof or by using a tanking kit(rubberised paint for shower walls), or both combined?-need to investigate locally available products.

Also, Ive been convinced to vent the dome from the other posts on here that mention it. Is the best option for this simply using a threaded copper pipe section protruding from the top of the rendered dome? I'm kind of worried this may actually introduce water around its edges so Id probably need some sort of small cap that would screw onto it. Im also worried about how to anchor this in the render properly?

Please comment on the above and point out any bad ideas or improvements!

Thanks

Richard

[Greenman]

Re: Waterproofing over vermicrete dome

Hello Richard - I am unsure how you intend to configure your insulation but I finished mine with a rendered dome.

I put ceramic fibre over the brick dome, then chickenwire, then vermicrete and then rendered over that. I then used Dulux Acratex to cover the lot. That finish is waterproof acrylic and can be applied with a roller and brush. I used a heavily textured roller and got a finish that covered the minor imperfections in the render finish.

The vent - I used a 1" poly fitting with a screwed cap on it. I drilled a couple of 1/4" holes in the cap in a position where the whole thing would be sealed if the cap was screwed down and will vent if the cap is backed of a bit. I also made a small 'chinaman's hat' and arranged it so the cap would fit into that. The pics show the detail.

I fitted the poly fitting after the ceramic fibre and it is secured by the render layer.

All of this being said I have a skillion roof over mine but it only keeps vertical rain off, when the wind driven storms come through it blows straight under.

You might also consider a door that seals tight to put in place after you use your oven and a cover of some flexible waterproof material to cover the flue landing area when you are not using it and the weather is foul.

[tonybaker]

Re: Waterproofing over vermicrete dome

My oven sits outside uncovered and is just rendered with masons mortar. It seems to be dry enough, I keep the door closed though. What's the vent for? surely if you keep door closed moisture won't get in? Hard to imagine a climate that wet!

[david s]

Re: Waterproofing over vermicrete dome

The porous refractory will absorb moisture from the atmosphere in really humid weather even if the oven is undercover. This moisture will accumulate in the insulation layer. If possible also leave the door off to allow moisture to escape from the inside of the oven, but replace it if it looks like raining. I've had mould growing inside my oven during the wet season, although where we live (tropics), mould in wardrobes is also common.

[RichC]

Re: Waterproofing over vermicrete dome

My oven sits outside uncovered and is just rendered with masons mortar. It seems to be dry enough, I keep the door closed though. What's the vent for? surely if you keep door closed moisture won't get in? Hard to imagine a climate that wet!

Hi Tony, humidity where I live is typically >80%. Our rainfall in August was 128mm(summertime) although it was the wettest sugust in a few years. Our rainfall probably isn't excessive compared to some climates but it's quite damp and rarely very warm here so things dont get much opportunity to dry out.
Rich

[david s]

Re: Waterproofing over vermicrete dome

Traditional Italian ovens are out in the weather and the outer shell is not waterproof, but is rendered with a porous lime render. This allows the oven to breathe as waterproofing the outside totally also locks moisture in. This may be fine for an Italian climate, but folk who live in areas of tropical rainfall or high humidity may want to do things differently.
If you already have a roof over your oven there seems little point in waterproofing the shell that sits under it.

Many proprietary renders include chemicals in their mix that makes the render waterproof, or at least partly waterproof.

[kstronach]

Re: Waterproofing over vermicrete dome

hi rich,

not sure if you got your oven rendered but if not you could look at K rend, comes in all sorts of colours. you should be able to get it locally as its made in ireland i think! heres a link to their site anyway incase you want to have a look . . . K Rend Textured Finish External REnder System - Suitable for one coat application or use as a finish

keith

[RichC]

Re: Waterproofing over vermicrete dome

Thanks Keith, I havent rendered yet but any day now
Actually I just recently put a fibreglass shell over the vermicrete dome. I put chicken wire over the dome and then fibreglassed over it creating a small airgap around it. Itll be vented just under the chimney flashing when done. Ill be rendering over the fibreglass and I hope this airgap means I wont have to worry too much about cracking the render when I fire up after long wet idle periods. I'll post pics when complete.

Richard

[kstronach]

Re: Waterproofing over vermicrete dome

what did you fiberglass it with? on my oven i used a product called sikalastic that we use at work for flat roofs, normally its applied in two coats with fiberglass matting in the base coat then straight over with the top coat. i skipped the matting and just gave it two coats of the product its worked a treat. no vent left i made sure it was bone dry before i applied, no problems so far its been on about a year now, i applied this on top of my render.

this is what i used as my final waterproof coat
https://www.promain.co.uk/specsheets...20UK%20PDS.pdf

look forward to seeing your finished pictures!

[RichC]

Re: Waterproofing over vermicrete dome

Hi Kstronach, I bought matting and a resin that I mixed a hardener with. I applied that. Next I have to apply whats called 'flo coat' i believe which will give me 100% waterproof finish. Ill then sand this and render over it

[kstronach]

Re: Waterproofing over vermicrete dome

if it was me i'd be tempted to leave that as the finish, depending on colour and what it looks like etc, could you put a pic up? i'd be interested to see it. you're the only other person i've known to use a system like this but i did it the opposite way to you and rendered first then coated it!

[RichC]

Re: Waterproofing over vermicrete dome

Heres pics of my fibreglass covered dome. Theres a small airgap between the grp and the vermicrete to allow for circulation. Ill be rendering over this and flashing tge chimney to prevent water getting in the edges of the fibreglass. I intend drilling some small holes through the render and fibreglass under the copper flashing. That will allow for moisture and air to escape when i fire it up. Ill fibreglass some more chickenwire to the outside to hold the render. I hope the airgap also means my render will be less likely to crack when i fire it up.
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[david s]

Re: Waterproofing over vermicrete dome

That looks like a pretty good solution, although you are going to get pretty wet if you are working the oven during rain. Couldn't you slope the roof the other way?

[RichC]

Re: Waterproofing over vermicrete dome

Hi David, the roofs a temporary arrangement. That's why im trying to seal it from rain. Theres actually a big hole above the chimney as it burned right through on Christmas day. As soon as i get the flashing and render on the roof comes off.