36" build in coastal VA

[Giovanni Rossi]

nlinva , I added the insulation simply bc my design didn't have a heat break for the entryway floor or flue gallery. I was always going to get heat loss via conduction through the bricks supporting the landing. Does the insulation reduce heat loss via convection through the airspace between the bricks? I doubt it's significant, but it was a good way to use some leftovers. FOAMGLAS is really easy to shape to odd dimensions and it doesn't shed toxic fibers.

[nlinva]

Moving ahead with the vent arch planning. Made the form today, and laid out some bricks to get a sense of what the vent arch will look like. In the picture the form is not sitting at floor level, and it will also get some shims to make removal easier, so will be much higher than it looks in the picture.

Adding a picture of the top of the dome, with the top of the keystone. It was too dark for a good picture of the inside, but will add that tomorrow. The keystone stuck out a bit (mortar between the two pieces of the keystone added more height than expected), so I added some mortar to the outside of the last course to make the overall dome shape smoother. Probably won't matter at all, but might make it just a bit easier to fit the insulation blankets smoothly.

[UtahBeehiver]

Probably not needed to smooth keystone area out with mortar. Ceramic blanket will take care of it. At least 3/8" plus release under arch form. The vent chamber looks fairly deep. This does make it a little harder to access the inner part of the oven while cooking.

[david s]

In addition to shims or wedges to raise/ lower the form, for easy removal, consider also stapling some thin (around 0.5mm), plastic like that found in display books, onto the form. This will prevent any mortar sitting in the voids in the form. It also produces a tidier result. Coat the plastic with some WD40 as a release agent. You should aim for a rebate of 3/4” to 1” all round for the door to fit and move comfortably.

[nlinva]

Thanks for the suggestions for the wedges to raise the form and material to cover the form. I put some ram board over it already, but I like david s suggestion to put some plastic on too. Will definitely do that. I have it set to take a 1/2" wedge, so that should be fine. And the reveal against the inner arch will be 2", so that should also be fine, per David's suggestions.

Here are some pics of the interior, now that it is done. I miscalculated on the last course and needed a thin shim, but other than that I'm pretty satisfied with how it all worked out.



I was also visited by a grasshopper. I feel like maybe I need to start a series of pictures of "animals that have taken an interest in the dome" :-)

[nlinva]

In cutting the CalSil board to fit under the vent landing, it turns out I was able to cut off strips of 1.25" width, which is exactly the height of the space to be filled underneath the slate vent landing. So I can basically fill the open spaces shown in post #60 with CalSil board. One problem solved!

Meanwhile, I have a new question: do I just rest the vent arch on the CalSil board as is (see pic), or should I mortar it to the board? I'm not sure how well mortar will adhere, but it feels potentially a bit flimsy to have the arch just be freestanding without any attachment. In addition to the arch weight itself, it will need to support another 16 bricks or so for the start of the chimney, plus a bit of the weight of the metal chimney itself. The chimney is going through the roof, and will get a collar there, which will support some of the top weight.

The vent arch will cut around the entry arch, with a gasket rope between the two, so there is certainly some structural support there (couldn't easily push the arch sideways or directly down, at least on the side closest to the dome).

One possible alternative is to mortar the bottom arch bricks to the thin firebricks underneath the slate landing, which will also add a little bit of stability.

Thoughts and suggestions most welcome!

By the way, although it may look like a lot on the pic, the vent arch will be 13.5" deep, which I think is not unusual. The mounting plate for the chimney is 12" on the narrower side, so I can't make it all that much more shallow anyway.

[nlinva]

I thought some more last night about how to make the bottom of the vent landing arch more stable, and think I have found a good solution. I am planning to add 2" of insulation around the arch anyway. For the bottom several inches, I will simply use some remaining pieces of the CalSil board instead of Superwool blanket. On the other side of the CalSil, there will be a slate slab for the workspace, plus a line of bricks as the bottom of the outer coating of the oven. Those will serve nicely to prevent any tendency for the arch to shift outward under pressure. Problem solved!

[rsandler]

Sounds like you have a better solution, but just to close the loop for anyone else reading, I would recommend against trying to mortar bricks to CalSil. Depending on whether you have the new water resistant Cal-Sil, or the older porous kind, mortar definitely will not stick well. Either it will immediately suck all the water out, preventing a good bond, or refused to absorb any moisture, preventing a good bond.

[nlinva]

rsandler thanks for the input on the bad idea that is mortaring bricks to CalSil board. Useful to know for future reference!

Meanwhile, I have a new question, inspired in part by UtahBeehiver 's point about not making the vent chamber too deep. It turns out that my chimney anchor plate, for the 6" double-walled Shasta Vent, is 17"x14" -- a lot bigger than what I've seen other people have (which might be Duravent, whose anchor plate seems to be 10" square).

If I don't want to make the vent chamber any deeper, I'll need to move the anchor plate back over the inner arch a little. This is structurally not a problem: I was planning to make the vent chamber bricks in the shape of an L over the inner arch anyway, so the support bricks for the chimney can still rest completely on the vent chamber bricks (which will be separated from the inner arch bricks by a compressed rope gasket.)

However, it means that I will not have my planned 4" of insulation directly above the inner arch. How much heat is likely to leak into the chimney support bricks if some are directly above the inner arch, compared to the original plan, which had them only above the vent arch, with insulation above the inner arch? I include a back-of-an-enveloped drawing (literally :-) which may or may not help illustrate.

It's likely not a big deal, but I'm just curious if anyone has a sense of how much of a difference this makes.

[david s]

That is a pretty hot part of the oven, as it’s normal for some flame to lick around the top of the inner oven arch. Have you considered cutting back the area of the anchor plate?
Because anchor plates take up so much area they do make the building of a shallow entry difficult. For this reason as well as them being expensive and necessitating drilling bricks and fitting anchors, both of which risk creating cracks, I use a simpler and cheaper method.

[nlinva]

I can probably cut the anchor plate down, for instance to 11 x 17" or so (I don't think the width is an issue, but the depth is). However, this would mean cutting off the holes for the mounting screws, leaving only the caulk to hold the plate onto the brick. I can probably try to drill some new holes, but not sure how cleanly I can do that.

The larger the anchor plate, the more stability to the chimney against lateral pressures, so in principle I like it. However, as I mentioned earlier, my chimney goes through a roof, and will get held in place there. That means the anchor plate will comparatively few sideways stresses (I suspect), compared to otherwise unsupported chimneys. So I think I'm OK with cutting down the plate, but as always, there is a chance I'm overlooking something important and I'm happy to defer to those who are more experienced.

[david s]

For drilling some new holes remember that stainless is hard to drill, it requires low speed , but lots of pressure. Maybe take it to a friend who has a drill press. Also make sure you anchors are stainless, heat accelerates corrosion.

[rsandler]

I cut down my anchor plate for much the same reason; I had a more reasonable sized anchor plate, but was trying to achieve a 9" deep entry. I also had to re-drill the holes. I used a titanium bit in a drill press on lowest speed, with 3-in-1 oil for lubrication. The drilling was easier than the cutting, as I recall, partly because my jigsaw is kinda wimpy.

[david s]

Don’t cut it with a jigsaw, use an angle grinder with a cut off blade, easy.

[david s]

I can probably cut the anchor plate down, for instance to 11 x 17" or so (I don't think the width is an issue, but the depth is). However, this would mean cutting off the holes for the mounting screws, leaving only the caulk to hold the plate onto the brick. I can probably try to drill some new holes, but not sure how cleanly I can do that.

The larger the anchor plate, the more stability to the chimney against lateral pressures, so in principle I like it. However, as I mentioned earlier, my chimney goes through a roof, and will get held in place there. That means the anchor plate will comparatively few sideways stresses (I suspect), compared to otherwise unsupported chimneys. So I think I'm OK with cutting down the plate, but as always, there is a chance I'm overlooking something important and I'm happy to defer to those who are more experienced.

Here are some solutions to this problem.

https://community.fornobravo.com/for...ery#post446435

Russell has a solution that may suit you better

https://community.fornobravo.com/for...ate#post381172