Union or Model, Which one is the best

I researched a lot and found nothing. Which one is the best? Model or union? if I have a building with a lot of parts, which one should I use?

I have seen multiple of these topics from beginner builders. Here is the answer for any of these topics in the future…

You should never use unions for organizing.

Unions have a whole different purpose that will cause degraded performance and mess with hit-boxes if used to organize your objects. Use grouping or folders, which are meant to organize your objects.

Why you should NOT use unions for organizing:

Unions were never intended to organize objects within roblox studio.

"Solid modeling (more commonly referred to as Unions , or a UnionOperation) is a feature in Roblox Studio that combines or negates two or more Parts to form an object called a Union. – Robox Wiki

To explain, unions are meant for negating and combining multiple parts to create a different shape.
The more complex a union is, the more polygons (or triangles) it has.
The more polygons an object has will affect performance.
Unions also create un-needed poly for simple objects like cubes.

Therefore, using unions to organize parts will have a significant effect on your game’s performance.
Also, using unions for other purposes will mess with hitboxes due to it combining multiple parts into one object.

Methods to organize your objects

Folders and groups are meant for storing and organizing objects.
These two objects will not cause performance or hitbox issues.
They simply are just containers for objects.

Hope this helps.


That may be mostly true BUT, you can also edit the union’s collision box and significantly decrease its impact on the game. Although then again you can also do the same with meshes.


Still, there is no reason to use unions for organizing. Unions were never meant for grouping and will cause problems if they are used for that. The more detailed the union is, the more polygons are in it which can affect performance.


So your saying he should use a MeshPart to group his parts together? Will that not also cause collision issues?


No, I am saying he should not use unioning for organizing his objects. He should use grouping or folders to organize them. Why would he use a MeshPart to group his parts?

There is no reason to use MeshParts to organize his parts. That would require him to create a whole mesh which will probably pass the triangle limit for importing and will mess with hitboxes.


He’s asking WHICH method he should use, if you meant you shouldn’t use either then you should have voiced it.


Unions and models are two different things?

If you want organizing chunks/furniture/objects with multiple parts, models will be your go to.


If you want organizing certain areas/places/specifications, folders.

If you want a quick alternative to meshes/simple boolean parts, CSG (unions).


Unions if you want quick and simple boolean parts (nothing complex).

If you want better unions/more detailed “unions”, meshes.


I don’t see why you are confused here. I already stated my opinion on which method he should use. I NEVER said to not use either. In fact, I said to use grouping. Please read my previous comments to answer your questions.

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Really depends on builds, Grouping is just easier and doesn’t take as long but unions can lag a game and act weird sometimes (this is just a personal opinion and the problems i have).

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My suggestion is to use Models. But why?

Models are widely used to group many parts, not necessarily of the same type, and organize a structure like a house. They cause very few rendering and therefore lag problems. When used correctly.

I don’t recommend using Unions often when building, the only times you might use them is to fix the corners of the walls in case you are using textures. Otherwise to make small details. Never use a union for entire walls.
Unions often cause lag because they interfere with the rendering of the object. Contrary to the Models that instead of keep them separate but grouped together.

My suggestion for the organization is to always create a folder, then divide the entire buildings into folders. They cause little lag and are not counted as Models.


Unions corrupt, smack performance, have jank hitboxes, etc. Do not use unions to organize models. What even is this topic.

But lets say, on this entirely new topic, could you if you wanted to, use unions to organize models, by unioning a large amount of parts together? Yes. Would you want to? I think that would be like asking a Horse to be a heart surgeon. Its a bad idea.


Union is pretty bad on it’s own, let alone using it for Grouping parts
Model 100%.

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I don’t remember this being a feature, how do you do this?

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PreciseConvexDecomposition is still quite inaccurate.

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Unions are horrible for performance, so I would do model. Although, using parts can have just as bad performance. (assuming you mean model and not mesh)

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In the properties window you can set the “Collision Fidelity”.

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And even the best one there is quite inaccurate.

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In my opinion both are pretty good in their own ways. Models for organizing. Unions for fast boolean parts.

I prefer models though, because I like my workspace organized.