Brick oven won't hold heat

[Qeharris]

I have a brick oven attached to the back side of a chimney on my house. I've dreamed of cooking pizzas in it so I built a huge fire in it the other night and had a bunch of friends over to eat. I built a fire on one half of the oven thinking I would cook pizza in the other half. I used an infrared thermometer to measure the temp of the floor of the oven and after several hours of burning a fire the temp was only up to about 300 degrees. I touched the top of the outside of the oven (like an idiot) and burned my finger bad. It's obviously not holding heat. It appears that the oven is only made of the same bricks as the house. Also, the roof seems to just be a cement slab. You could see smoke pouring through it when I had the fire going (smoke also poured out the chimney). What's the best way to insulate this thing so it will hold heat. It is a pretty large oven compared to the traditional pizza ovens I've seen on here. How long should it take to get the floor up to 700 degrees? Thanks to anyone who can help.

[UtahBeehiver]

By looking at your pics, IMHO, this set up will not yield the results that you envision for cooking pizza. There appears to be no insulation under the floor surface which looks like concrete or brick (can't tell for sure) and no insulation on the wall bricks. So the floor acts as heat sink draining away the heat .The shape of the oven with two doors is not an effective shape with substantial heat loss from this large openings. The standard house bricks will break down after continuous firing and need to solid and refractory. Sorry to be bearer of bad news.

[Fizz]

Strange looking setup, could it be for smoking?

You got some space in there, so you might get away with building with firebricks inside, isolating under and above, and adding some insulation outside, on the sides and on top, but that will likely be as much work as tearing the whole thing down and building a new oven in place ...

Another idea could be to use the top of the existing oven to build a WFO on, using the existing chimney ....

[Toomulla]

good for a wood store that's about all