What happens if a character casts Fireball in an enclosed space? [duplicate]

Recently I had a Warlock character who woke up trapped in a wooden coffin with no knowledge of how he got there. It wouldn't move when he tried to roll around in it, and there wasn't room to try to break out manually (a la Kill Bill).

In a panic since he is claustrophobic, he quickly casts Fireball targeted on himself to try to bust out. However, the coffin happened to have been formed around where he fell asleep, in the middle of Leomund's Tiny Hut, surrounded by sleeping party members.

Which of the following should happen?

1. My character takes damage and the coffin remains in tact.
2. My character takes the damage and the coffin breaks open.
3. My character takes the damage, the coffin breaks open, and the rest of the party get to have a Dex save to take reduced damage.
4. My character takes the damage, the coffin breaks open, and the rest of the party auto-fail the Dex save since they were asleep.
• – Rubiksmoose Aug 1 '18 at 17:18

Option 1 or 2.

As has been discussed in other questions about fireball (1, 2), it can spread around corners but not go through walls. Assuming the coffin is airtight or mostly airtight, the fire cannot go through and so detonates on you and cannot explode through the coffin.

The coffin may or may not explode, depending on its size and the damage rolled for fireball. This is from the object hit points table in the DMG (p. 247):

Size                                   Fragile    Resilient
Medium (barrel, chandelier)            4 (1d8)    18 (4d8)
Large (cart, 10-ft.-by-10-ft. window)  5 (1d10)   27 (5d10)


So if you roll low, a large resilient object could force you into option 1, or if you roll really low, a medium resilient object could force you into option 1.

Note: you can also make the Dexterity save, but may have disadvantage if the space is tight enough for you to be restrained.

• Would it not go through objects even if the damage had enough to spill over past the health of the object itself? – Michael Ziluck Aug 1 '18 at 17:16
• @MichaelZiluck See the duplicated question. While I don't agree with some of the comments, I do agree with the second answer – Blake Steel Aug 1 '18 at 17:31