Insulating a Pre-Cast Oven

[furry]

Hi,

I'm hoping for a bit of advice, I have read a load of information accross various forums / websites and I could do with a bit of confidence to press ahead!

Last year I bought a pre-cast pizza oven from pizza forno / wood-fired-pizza-ovens.co.uk/ (The Rustica). It is delivered as a seperate base and dome both constructed from some form of refractory concrete, each section weighing approximately 85 Kg. There are then firebricks which are placed inside on the oven floor as the cooking base.

I used the oven with a mobile stand last year and it works great for pizzas, albeit it does need recharging after each couple of pizzas. I did want to slow cook in it but I'm not sure it hold its temperature quite long enough. I think I could get better performance by insulating it and i'd be keen to branch out into some overnight cooking / baking if this is at all feasible.

I now have a permanent brick stand ready to site the oven. After reading online i'm considering putting 50 mm calsil board on the base, then the oven, covered with 50mm fire blanket, a layer of vermiculite insulating cement and a final waterproof render. Does this all sound about right? Does anyone see any major flaws or have other suggestions?

I'm having trouble finding a supplier of calsium silicate board. Kilnlinings have it but my order value isn't up at £200. Any other ideas or should I consider the ceramic fibre board from vitcas?

Thanks very much for any help.

[albacore]

There is a seller on Ebay who has calsil strips 215 x 1000, 65mm thick. Maybe you could get 4 or 5 of those? I know you really only want 50mm, but if you can stand the thickness, 65 would be even better and probably counteract the heat loss of using strips. The price is good too - £16 per strip posted and looks like you could get that down for 4 or 5 strips. Search for Ebay item 262341395029

Lance

[davenott]

I agree with Lance, I'd go for at least 6.5cm thickness.
I have 13cm thickness of insulation under my hearth. I used Foamglass which is equivilant to about 16cm of calcium silicate board. A well insulated oven, lets you do lovely slow cooking. I often start with pizza in the hot oven, do a roast or bread the next day, then a casserole overnight, and then use residual heat to dry out my next load of firewood.

[furry]

Thanks for your suggestions. I have contacted the eBay seller for a quote. I also had a look on eBay for the foamglas - would item 370688035015 be any good as an alternative to calsil? 1.62 Sq metre pack of foamglas s3 at 80mm thickness.

Thanks again

[davenott]

Calsil has been used in a lot more builds than Foamglas. I used Foamglas mainly because I wanted something waterproof. My thinking being I didn't have to worry about material getting damp and either deteriating or reducing its insulating properties, but lots of people use Calsil without problems. I used 2 layers of Foamglas and staggered the sheets,so the joins didn't overlap.

[gracie]

Hi Dave, quick question re Foamglas...given it's water resistance, does it mean there's no need for a dpc in the block work, or would it be better to prevent the rise of moisture that far up even if the Foamglas prevents it reaching the hearth?

[davenott]

I would always build a DPM in. I put 2 in mine, first above single brick level above ground level, and a second one just below the hearth, my thinking being the second dpm might allow me to lift the hearth and inner oven with a forklift, if I ever needed to move it.
Foamglas is brittle and fragile, so if used for under hearth, any exposed edges will need protecting by a layer of mortar, as otherwise they could be subject to damage if knocked. It is strong undercompression, so fine to take oven weight when covered by firebrick hearth.

[albacore]

It looks like the maximum temperature for Foamglas S3 is 430C. I'm just wondering if this is borderline for under an oven base refractory, though presumably Dave has had no problems.

Lance

[davenott]

Apr 13, 2016 19:18:26 GMT albacore said:

It looks like the maximum temperature for Foamglas S3 is 430C. I'm just wondering if this is borderline for under an oven base refractory, though presumably Dave has had no problems.

Lance

This figure put me off when I first looked into Foamglas. 
When I read other people's builds, here and abroad who had used Foamglas, they said not to worry, as although the manufacturer quoted 430'C as maximum parameter of use, the material doesn't start to soften until 300'C above this temperature (730'C)
Also they said the temperature on the underside of the firebricks used in the hearth, would be lower than the oven floor temperature. The real proof for me was that a couple of the people had been using their oven very regularly, even commercially for quite a while without problems. I did put a thermocouple above my Foamglas S3 layer and below the firebricks of the hearth, to measure what temperature it would reach, but although I've used my oven a few dozen times, I've not got round to connecting the thermocouple up and taking a reading.
When I fired my oven up to cook the pig, the hearth temperature reached above 600'C, and the only problem I had was that the pizza I tried to cook was cremated in about 30 seconds! I have however cracked my clay chimney pipes from Kiln Linings.

[nabs]

Stonebake oven company sell decent ones.

[furry]

Hello again,

So I have the calsil board, the fireblanket is on its way.

I'm now looking at what to do about a flue. The unit was supplied with a 5 inch single wall flue section and a raincap.

I'm assuming that you need to have a damper in the flue to retain heat in the oven for overnight cooking. The only units I can find with a damper seem to be single wall, or twin wall but unsealed so there is a gap between the two walls. Should I be looking at twin wall and if so, how do you seal the gap between the two walls so the insulation is not exposed? If single wall is it okay topack the fireblanket and cement right up to the flue or is it necessary to have some kind of break around the flue?

Also, for someone with very little DIY experience / know how, is it particularly easy to mix and apply the vermiculite insulating cement? I am going to get someone in to apply the final render coat as I dont want it to look terrible, but i'll do the cement if its easy enough.

Thanks again for your help on this.

[albacore]

Normally, the door goes the other side of the flue for slow cooking, thus blocking off the flue. The door sits against a rebate at the entrance to the oven dome. However, it looks as if you may not have that rebate in your oven. Nevertheless, if the entrance tunnel is straight, without obstruction, you could maybe construct a door that slides in and sits in the right place.
Your other option, as you say, is another flue with a damper - make sure it can close fully. If you go twin wall, you can get an adaptor from twin wall to vitreous single wall which might terminate your twin wall properly.
Flue pipe going through render is always a potential source of problems; I used twin wall and still got render cracks. It needs a gap and some sort of elastomeric seal. The problem must be much worse with single wall.
I'll let someone else answer the vermicrete question - I hated the stuff!

Lance