Is there any text stating that walls or objects made of force can or cannot conduct thermal energy?

For example: Wizard is pushed off edge of volcano into magma. Let's surmise he is able to cast wall of force or some other means that allow him to be inside a sphere of force.

Sphere hits lava, starts to sink. He's protected from the lava by the force wall, but does the thermal energy from the lava "heat" up the force wall which then heats the air inside and eventually traps him inside a death oven where he cooks inside his own wall of force?

  • 9
    \$\begingroup\$ "Sphere hits lava, starts to sink." - but it doesn't. It is immovable, isn't it? \$\endgroup\$
    – enkryptor
    Jun 2, 2017 at 17:45
  • \$\begingroup\$ Are there rules in the game for thermal energy transfer through arbitrary substances that make this unclear? \$\endgroup\$
    – T.J.L.
    Jun 2, 2017 at 17:50
  • \$\begingroup\$ @T.J.L. The rules generally focus on segmenting things to make everything simpler. Lava is environmental, so the environment section has the answer. WotC didn't model every possible outcome, rather, they defined each hazard in how it affects the PC/Party. \$\endgroup\$
    – GOATNine
    Jun 2, 2017 at 18:06
  • 4
    \$\begingroup\$ @GOATNine It was tongue-in-cheek, rhetorical question intended to get the querent to think about what he was asking... The answer is "The game does not have rules for thermal energy transfer through arbitrary substances." \$\endgroup\$
    – T.J.L.
    Jun 2, 2017 at 18:10
  • 1
    \$\begingroup\$ Why would the sphere sink in lava, anyway? Lava is molten stone. It's REALLY heavy. Way heavier than water. And I'm pretty sure a 10 foot sphere full of air with a wizard inside would float in water, probably pretty high. In lava, it would float even higher. \$\endgroup\$
    – Paul Z
    Jun 2, 2017 at 22:26

4 Answers 4


Wall of Force does not transfer thermal energy.

The Player's Handbook describes the spell as:

immune to all damage

Cold and fire are both types of damage, and in your example we can assume that the lava would cause fire damage.

The question, then is "Can Wall of Force allow damage to pass through it if it does not take damage itself?" The answer still is no, and here's why:

There are three types of heat transfer: Conduction (contact between two objects, such as in the spell Heat Metal), Convection (which happens primarily in gases and liquids, of which the Wall of Force is neither), and Radiation (does not require a medium, and consists of electromagnetic waves).

We can rule out Convection and Conduction, since the Player's Handbook describes Force as:

pure magical energy

which more than likely does not behave the same as normal matter. In addition, if Force did act in a similar way, then we have a precedent for an amount of Fire damage that does not pass through a Wall of Force, namely a Fireball spell cast at 9th level. This will do the base 8d6 fire damage, plus an additional 5d6 fire damage for a maximum of 78 fire damage. Given that there is no limit to the number of creatures that can take a turn in a round, we can assume that even with even three spellcasters casting 9th level Fireball spells, the wall (and the creature being protected by the wall) will remain unharmed.

Per the Improvising Damage chart on page 249 of the Dungeon Master's Guide,

18d10 Being submerged in lava, ...

That's a maximum of 180 fire damage per round submerged in lava. With 3 9th-level Fireball spells maxing out at 234 fire damage per round, the Wall of Force has precedent for not allowing extreme temperatures to affect the enclosed creature.

As far as heating the interior by Radiation,

Nothing can physically pass through the wall.

That would include things on the electromagnetic scale, since the waves would physically need to pass through the wall, and light would be an exception (not the rule), since the wall is specified to be invisible.

  • 1
    \$\begingroup\$ But aren't things (magically) "invisible" generally invisible across the EM spectrum? That is, to all different kinds of creature "sight"? You only have to go a little bit into infrared to get heating, so if visible light passes through, I'd think enough near-infrared would get through to cook you eventually. Sunlight passes through, so would radiant damage roast a vampire protected by wall of force? \$\endgroup\$
    – JesseM
    Jun 2, 2017 at 23:05
  • \$\begingroup\$ @JesseM No because the metaphysics are different. \$\endgroup\$ Jun 3, 2017 at 8:18
  • \$\begingroup\$ "the metaphysics are different" is a totally reasonable answer here, but let me put forward this argument. If visible light were the source of a damage causing effect, and the light wasn't blocked, why can it not generate the damage effect on the other side after passing through the wall? Think of the vampire and sunlight. There is no damage for the wall to block until after sunlight reaches the vampire. Why not the same with energetic photons and heat? Wall can stop damage, but not a non_physical source of damage. Just a thought. \$\endgroup\$
    – JesseM
    Jun 3, 2017 at 14:59
  • \$\begingroup\$ By this logic, a creature within the sphere would need to hold their breath...? \$\endgroup\$ Jun 25, 2019 at 13:38
  • \$\begingroup\$ Heat transfer via radiation is heat transfer via photons (read: light, typically infrared), so I'm actually not so sure on your last point. There's nothing to specifically suggest that only certain wavelengths of light are permitted to pass through. \$\endgroup\$
    – Remilia
    Jun 26, 2019 at 14:15

It is reasonable to assume it is immune to thermal energy.

Per the text in the spell.

Nothing can physically pass through the wall.

Which should logically include thermal energy - or more correctly stated - fire damage. If it had said it is immune to physical damage, I would rule differently, but this does not seem to be referencing a keyword. Especially since the very next sentence states:

It is immune to all damage......

which seems to further back up the idea that any kind of damage can not pass through the wall.

  • 1
    \$\begingroup\$ The wall could transfer heat through without taking damage itself. \$\endgroup\$
    – Oblivious Sage
    Jun 2, 2017 at 17:19
  • 8
    \$\begingroup\$ Being invisible, the wall obviously lets the light pass through, so it might be transparent for thermal radiation as well. \$\endgroup\$
    – enkryptor
    Jun 2, 2017 at 17:31
  • \$\begingroup\$ Maybe. But in the D&D rules energy is not the same as damage. In this kind of situation, it's up to the DM, but its a fairly high level spell, so erring on the side of it being nigh impenetrable seems better to me. \$\endgroup\$
    – JWT
    Jun 2, 2017 at 22:26
  • \$\begingroup\$ That said, dude is still floating in a bubble in lava. Its not like all is kosher. \$\endgroup\$
    – JWT
    Jun 2, 2017 at 22:28

Thermal temperature would pass through, but would not deal damage directly.

As with everything else in DnD 5e, if the spell specifies, it does.

It is immune to all damage

emphasis mine. However, The temperatures involved do not deal damage directly, rather

Some special abilities and environmental hazards, such as starvation and the long-­term effects of freezing or scorching temperatures, can lead to a special condition called exhaustion.

emphasis mine again (SRD Here)

Seems that RAW, thermal conditions WOULD pass through a wall of force. This would be consistent with light passing through the invisible wall. (the counter to (thermal = fire) is (light = radiant)).

On the other hand, I can see it being reasonable that thermal energy be forbidden from passing through the wall. If that is the case, then it would depend on when the wall of force was cast, as any excessive thermal energy would be trapped in the sphere with the caster. This would lead to the same exhaustion inducing conditions as though the heat could transfer into the sphere from the surrounding lava (unless it was cast from a spot at a much lower temperature).

  • 1
    \$\begingroup\$ That was my thoughts. Like a ceramic pot in a high temp oven. The ceramic can easily withstand the heat, but whatever inside gets roasted. \$\endgroup\$
    – Dustin G
    Jun 2, 2017 at 18:13
  • \$\begingroup\$ I don't have my PHB handy, but in the Appendix sections, there's a brief description of Exhaustion. Additionally, there's something in either the DMG or PHB that describes how the environment starts stacking exhaustion. I know it's a CON save, but not how often or what DC.(Or I would include it in my answer) \$\endgroup\$
    – GOATNine
    Jun 2, 2017 at 18:15

RAI, no. Thermal energy is fire damage, it cannot get through or the spell loses its meaning.

Imagine this: Wall of fire right next to the wall of force, warm side facing the wall of force. Do creatures touching the wall of force take 5d8 damage? Of course not. The spell description doesn't even bother to say it blocks spell effects.

Wall of force grants full cover to those on the other side from direct attacks.

The wall of force is immune to all damage. The fire damage would have to break the barrier to get to the other side.

So all damage from any source is just absorbed/nullified by the wall of force, and nothing happens on the other side.

If you are struggling to cope the wall of force with the physics and common sense, let me give you the missing link:

A wizard did it.

  • 1
    \$\begingroup\$ The missing link is actually that the metaphysics that the OP would like to ascribe to reality, but which may not actually apply to reality, certainly do not apply to the Forgotten Realms. \$\endgroup\$ Jun 3, 2017 at 8:20

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