D105 Pre-Cut brick kit build in Tassie

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

Thanks Gulf - I am glad I asked as I hadn't considered it would trap the moisture.

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

When the vermicrete has dried I like to do the curing fires to be sure there is no moisture in the insulation layer, before doing the cement render outer shell. You may want to consider putting in a vent so your insulation layer can communicate with the atmosphere to relieve steam pressure build up.

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

When the vermicrete has dried I like to do the curing fires to be sure there is no moisture in the insulation layer, before doing the cement render outer shell. You may want to consider putting in a vent so your insulation layer can communicate with the atmosphere to relieve steam pressure build up.

I don't want a permanent vent, but I can understand the potential m3 of steam/gas that has to make its way out of the brickwork and insulation during the drying/curing stage.
If I drill a 8-10mm hole in the vermiculite render when I finish, and leave it open for the duration of the curing fires, I assume I could simply seal it up before applying the final coat of cement render and final seal coating?

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

Is it realistic to cut stone bench tops to shape/size myself for a landing using a cut-off wheel on an angle grinder - if I pick up some up 2nd hand from the local paper or Gumtree etc.

[Greenman]

Re: D105 Pre-Cut brick kit build in Tassie

I don't want a permanent vent, but I can understand the potential m3 of steam/gas that has to make its way out of the brickwork and insulation during the drying/curing stage.
If I drill a 8-10mm hole in the vermiculite render when I finish, and leave it open for the duration of the curing fires, I assume I could simply seal it up before applying the final coat of cement render and final seal coating?

You could do that, it would work but I would make the hole a little larger. No big deal to render it later. A permanent vent would be handy should you be unfortunate enough to get the hearth soaked in the future but that is up to your preference.

I can not help with your benchtop cutting question. One for someone who knows.

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

I don't want a permanent vent, but I can understand the potential m3 of steam/gas that has to make its way out of the brickwork and insulation during the drying/curing stage.
If I drill a 8-10mm hole in the vermiculite render when I finish, and leave it open for the duration of the curing fires, I assume I could simply seal it up before applying the final coat of cement render and final seal coating?

I can't see that a hole through your vermicrete layer will be of much benefit, as it is the moisture in the whole vermicrete layer that you are tiring to remove. When you start the fires the water moves away from the heat and the whole surface of vermicrete layer will be sweating it out. You should be able to feel the surface clamy, until it is completely dry. Try throwing some plastic over the oven to see if water is condensing under it. You need to have the vermicrete completely dry before doing the outer cement shell.

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

Is it realistic to cut stone bench tops to shape/size myself for a landing using a cut-off wheel on an angle grinder - if I pick up some up 2nd hand from the local paper or Gumtree etc.

Yes. But make sure you wear a mask, the stuff is dangerous to breathe. A 4" angle grinder with a diamond blade works ok but will not give you much depth of cut. A 5" one is much better and well worth getting if you haven't already.

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

When the vermicrete has dried I like to do the curing fires to be sure there is no moisture in the insulation layer, before doing the cement render outer shell. You may want to consider putting in a vent so your insulation layer can communicate with the atmosphere to relieve steam pressure build up.

I can't see that a hole through your vermicrete layer will be of much benefit, as it is the moisture in the whole vermicrete layer that you are tiring to remove. When you start the fires the water moves away from the heat and the whole surface of vermicrete layer will be sweating it out. You should be able to feel the surface clamy, until it is completely dry. Try throwing some plastic over the oven to see if water is condensing under it. You need to have the vermicrete completely dry before doing the outer cement shell.

I think I have misunderstood your first post.

This was my thought process;

If there is moisture in the fire brick from the build or in future from rain on the hearth etc I assumed it would escape as steam into the air void between the bricks and the vermiculite render where the ceramic fibre blanket is - and you were suggesting it would be advantageous to put a vent in to allow the atmosphere in this space to equalise with the external atmosphere. That's why I was thinking to "drill a hole"

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

Yes. But make sure you wear a mask, the stuff is dangerous to breathe. A 4" angle grinder with a diamond blade works ok but will not give you much depth of cut. A 5" one is much better and well worth getting if you haven't already.

Thanks - I have a range of grinders up to 230mm (9") - I will see what granite/stone I can find.

I work in the fibreglass construction industry and have access to all the PPE I could ever need

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

I think I have misunderstood your first post.

This was my thought process;

If there is moisture in the fire brick from the build or in future from rain on the hearth etc I assumed it would escape as steam into the air void between the bricks and the vermiculite render where the ceramic fibre blanket is - and you were suggesting it would be advantageous to put a vent in to allow the atmosphere in this space to equalise with the external atmosphere. That's why I was thinking to "drill a hole"

Ok, that would help to get the bricks and blanket dry, but there will be way more moisture in the vermicrete layer where it will collect from being pushed out of the bricks and the large amount that you added in the mix, that needs to be also removed. I cracked a couple of outer shells from not drying the vermicrete properly first, so I'm now really careful.
Remember that one litre of water will make over 1600 litres of steam. There's lots of free water ileft in the vermicrete not used in the hydration process.

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

When the vermicrete has dried I like to do the curing fires to be sure there is no moisture in the insulation layer, before doing the cement render outer shell. You may want to consider putting in a vent so your insulation layer can communicate with the atmosphere to relieve steam pressure build up.

and an old post from 19-05-2013 #72 new build in werribee

Looks good to me Steve. I think that the problem of the outer shell cracking is caused by large amounts of steam creating pressure. A vent can go some of the way to relieving this, but steam will still have trouble finding its way through the vermicrete to the vent. I've found that once you get the oven dry any subsequent rain and moisture take up by the insulation layer is dealt with quite adequately by the vent. A hand held to the outside of the oven can be a great indicator of the dryness of the insulation layer. If it is moist it makes the outside of the oven hot, rather than just cosy warm (in my ovens at least) depending how you have done your insulation and how thick it is, this may vary.

Dave

I was having trouble understanding how a vent just through the cement render onto the outside surface of the cured vermiculite render would do anything but after some more research I found your post (david s) in Greenman's build thread that I can understand.

I have followed Ben Guilford's kit instructions for the vermiculite render (well almost) and it is made up with;

1 part Portland cement,
1 part fine washed sand
1 part lime
5 parts vermiculite - except I went 6 parts.

Is this similar to the mix others have used on here? if not will it make a difference to the vent advice?

Some of the photos of vermiculite render I have seen on here look way more porous / have a really rough surface compared to mine - maybe its just the particle size of the vermiculite - mine being very small

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

It depends on what you want the vermicrete layer to do. If only needed for insulation and just firm enough to render against 10:1 is ok. If you want some strength a richer brew like 5:1 will increase its strength a lot, but reduce its permeability a lot, sand even more so. Making the vermicrete rich also drastically reduces its insulation value. I don't know how thick your vermicrete layer is and I don't want to interfere with whatever Ben's build instructions say, but the moisture contained in the layer needs to be removed at some time. From my experience it's better removed before doing the final render.

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

Here's some info on vermicrete re strength etc.

http://www.schundler.com/vermcon.htm

[david s]

Re: D105 Pre-Cut brick kit build in Tassie

And another on some tests I made with drying a vermicrete slab. The same applies for vermicrete over the dome.

[Steady]

Re: D105 Pre-Cut brick kit build in Tassie

Thanks for the info david s. I will leave the cement render off until the vermiculite render has dried out using the plastic sheet method to test. I suspect the K value of my V-render will be lower than many others on here due to the addition of sand. Like many others when I finish my oven I expect I will have a list as long as my arm of things I would do differently next time. I can easily drop the sand out of the mix - looking at the kit instructions I downloaded in Jan 2015, the render was only 1 part cement and 5 parts vermiculite - this has since been revised. The instructions currently on the MFBC site for download has the sand and lime added to the mix.

I have a question in regards to vent location options,

Pressure caused by steam, from moisture entering via the hearth or similar (in the future after everything has been dried out) that is trapped by the eventual external waterproof membrane would be equal in the air void / blanket space - so as a pressure reliever the vent could go anywhere. A PVC capped vent closer to the bottom where it can be reached without a ladder would be more practical.

Would a low vent as opposed to one in the TDC make much difference to how much moisture can get out?

This is my rudimentary understanding thus far for after the oven is dry and cured and then maybe gets wet/damp during winter next year - if I have it wrong please correct me;

The moisture that re-enters the vermiculite render layer, wicking through from rain on the hearth or directly from cracks in the external waterproof membrane etc is going to be driven away from the heat of the next fire, towards the waterproof membrane and then be stopped by it - I have been assuming this would result in one the following happening;
a) the moisture will blister an acrylic coating or similar and eventually escape
b) the heat would build and eventually turn the moisture to steam with it expanding and making its way back into the blanket void space eventually exiting via a crack or installed vent.

The moisture will be evident by the external surface of the dome getting hot as the insulation is inefficient when wet/damp but as it re-dries the outside will cool back down. The moisture in the V-render turning to steam can/will crack it as it expands so keeping it bone dry in the first place is the ultimate goal - venting is the mitigation.