Base insulation: Into the void!


#1


I am building a Mattone Barile Grande, and am getting ready to start with the “oven” part of the oven. The first step in doing that is really the last step of building the base: installing insulation for the oven floor. This is the first thing you will do that actually does something oven-like.

I chose to use a Perlite/Portland Cement mix. I decided on Perlite for two reasons:

  1. It is a water-repellant mineral, rather than a water-absorbent mineral like Vermiculite. So, it will cure more quickly because it’s not trying to retain moisture.
  2. It costs 25 percent less than the same quantity of Vermiculite.

I found both Perlite and Vermiculite at my local orange big-box store, in the indoor nursery section, in 2 cubic foot bags. The materials bill calls for 4 cubic feet, and since I was there to purchase Portland Cement it meant one less shopping stop. The price (about $15 for a 2 cubic foot bag) was competitive with other sources.

If your eyes bug out when you see two big bags of either material, especially when you compare that with the apparent volume of the void you created in your slab, they should—but only a little. Volume of a space is always tricky to estimate by eye, because most of us have a very hard time with 3-dimensional space and measurement. You’ll need much more material to fill that “shallow” void than your eyes tell you you’ll need.

I found what I needed around the house to measure Perlite and cement in the proper 5:1 ratio. I have a 10 quart cleaning bucket—voila! 2 buckets equals five gallons. I also have a 1 quart measuring cup I’ve been using to measure water into my concrete. So, 2 buckets of Perlite and 4 cups of dry Portland Cement = 6 gallon dry mix. And of course, 1 bucket and 2 cups is a half batch.

The good news is that this is not back-breaking work like concrete mixing. Pour Perlite into a wheelbarrow, add Cement, then dry-mix with a hoe. When the white Perlite is coated with gray, spray lightly with water. Do this just enough for the surface Perlite to turn white again. Mix with the hoe until all the Cement is wet and the consistency is, as they say, like oatmeal. I found that when the mix of materials and water is correct, the hoe will make a shoop-shoop noise as it’s pulled through the mix.

Even when mixed, the insulation will be very lightweight and easy to transfer.


I used my material transfer shovel to transfer the mix to the slab. I spread it out to the edges, then used my tamper to compact it. Then, I repeated.

It took 3½ loads to fill the void, with just a little left on the slab when I had screeded for the last time. I troweled it smooth, then decided to float it as well.

I found that I only needed 2 cubic feet plus 2½ gallons of Perlite from the second bag. And, since Portland Cement around here comes in 94 pound bags, I still have plenty of that left over as well. The remaining Perlite (almost a full bag!) will go to good use in my spouse’s garden, and there’s plenty of call for the remaining Cement in this project.


Cleanup for this part will offer another surprise: when you rinse your tools, you’ll find that the Perlite quickly washes clean. Useless information, perhaps, but it gave me a smile.


#2

This is helpful info. Wish it came a little sooner because we’ve just used a vermiculite/Portland cement mixture and I would’ve used perlite had I know it was not absorbant. BTW, we started out with one 50lb bag of insulating castable, per the instructions, but it filled only half of the void. Since it is a special order item and the supplier is a 2 hour drive away, we opted to finish out with the vermiculite mixture obtained from a much closer orange big box home supply store. It’s turned out fine. We’ve started on the next step but talented masons we are definitely NOT. (But that’s not stopping us…)


#3

Sounds like you’re piecing it together just fine, Peter. I’m a novice mason, too. My grandfather and his sons all worked masonary, and he in particular was a legendary French Canadian plasterer. I’ve seen some of his work, now 75 years old, and no cracks. That was skill!


#4

Hope you don’t mind me butting in. Peter this is interesting. I decided to go with insulating castable as well, and like you, followed the directions, and ordered 50 lbs. I’m about two weeks behind you and Matt so I can still correct this. However, I’m curious, did you mix and “pour” the castable discover you didn’t have enough and let it dry? or race to the store and get back in time to mix it in with the castable. Also, if i decide to stay with the castable, do you think I would be safe in ordering one more 50 lb bag? thanks for any help you can lend.


#5

That’s interesting. I know that insulating castable is there as an option, but I’m wondering (completely out of curiosity) what led you to choose it over the Portland Cement/vermiculite mix?

For what it’s worth, I considered it too, and the thought that I had in its favor was that it sounded somehow more suited to the purpose—a ready-made mix that was engineered just for this kind of application.


#6

Hey Matt nice to talk to you. Yes that’s exactly right. Based on Peters description, I think i’m going to order another 50 lbs today.
If anyone else out there has any input on this subject, certainly would appreciate it.


#7

Hey Brad, and with you too. I think I should have added in my previous post, though, that I decided that the Perlite/Portland Cement mixture works, is inexpensive, and doesn’t have to be ordered—you just pick up the materials and go.

Also, if I’m understanding Peter’s post correctly, he decided to finish his insulating base with Vermiculite/Portland Cement. Even though it will take longer to cure than the Perlite, it worked fine for him and saved him from having to get more insulating castable, which involves a considerable trip.

Here’s the Brickwood post on the difference between these two classes of insulator: What is insulating castable? If you end up having to use 100 pounds of insulating castable, it ends up being way more expensive but you get a “slightly better” insulating base. I think, either way, it ends up allowing your oven to stay very, very hot.

I’m sorry if my reply misled you! I really shouldn’t post before my first cup of coffee. Sometimes the finishing sentences are still in my head when I click “Reply”!


#8

Evn’n gents! Just to weigh in, I went with the perlite setup. Worked great. Been through all the curing fires and a few cooking sessions—not a hint of warmth under that base.


#9

Ok thank you for weighing in, appreciate it.


#10

Hi Guys. I’ve been travelling so sorry for the delayed response. We put about a 1" deep vermiculite/Portland cement mixture right on top of the already dried insulating castable to finish filling the cavity. This upper layer cured in less than 2 days. It did get rained-on a little but seems to be in good shape. We hope to get to laying the fire brick sometime this week.


#11

Hi Peter, thank you for your response on this subject. Ive been away and missed this until just now.“right on top of the already dried insulating castable to finish filling the cavity” completely answered my question. again thanks for getting back to me. hope your build is going well. - Brad