If the front of your oven is closed off w/ a door (when not in use) AND you have a chimney cap that overlaps the chimney opening by 2" or so… you should have a completely sealed oven that will prevent rain / moisture from entering the oven.
It is kinda crucial that you keep the oven sealed when not in use as you have 2 materials in your oven that like to hold moisture:
- The fire brick in the hearth / cooking surface
- The insulation materials under the firebrick (perlite or vermiculite)
Fire brick will absorb moisture like a sponge and retain that moisture until it is removed. This is the reason you have to soak fire brick before applying mortar as dry fire brick will suck the moisture right-out of mortar and leave you with brittle mortar.
Ditto w/ the insulation material you used to create the insulation layer under the fire brick. Whether you used Vermiculite or Perlite - both elements will retain water (Vermiculite more so than Perlite). This is the reason why you find this material in potting soil… it helps retain water.
Once your fire brick hearth is nice and dry - you want to avoid moisture (rain / snow) by sealing the oven w/ a pizza oven door and a DuraTech exhaust kit (or similar exhaust kit with a wide chimney cap). Both items will prevent moisture from entering your oven which will result in a nice, dry oven that can be used year-round.
The cure… order a door & chimney cap and then slowly (SLOWLY) repeat the 6-day curing cycle again. You don’t want to turn that water into steam as steam cracks concrete & brick.