A question that I get asked a lot in Revit is “how do I get my walls to behave properly so I can take a party wall cavity through the external wall”. (As shown in Figure A)
This process is relatively simply when you understand how a wall is made up within Revit.
A wall is made up of something called “Layers”…
No, not those pesky things Dave in the corner keeps creating his own kind of in AutoCAD… quite literally a wall in Revit is made of several layers.
Each of these layers consists of several properties:
- Structural Material
We shall concentrate on the Function part of these walls.
Functions have the following options to choose from:
• Structure : (Highest Priority)
Layer that supports the remainder of the wall, floor, or roof.
• Substrate :
Material, such as plywood or gypsum board, which acts as a foundation for another material.
• Thermal/Air Layer :
Provides insulation and prevents air penetration.
• Membrane Layer:
A membrane that commonly prevents water vapor penetration. The membrane layer should have zero thickness.
• Finish 1 :
Finish 1 is typically the exterior layer.
• Finish 2 : (Lowest Priority)
Finish 2 is typically for the interior layer.
Rules for Layer Joins (Taken from Revit Help)
• The structure layer has the highest priority, Priority 1.
• Finish 2 has the lowest priority, Priority 5.
• Revit connects high priority layers before connecting layers with the lowest priority.
For example, suppose that you join 2 compound walls. A layer in the first wall with Priority 1 joins to a layer with Priority 1 in the second wall. That Priority 1 layer can pass through lower priority layers before joining to the other Priority 1 layer. A layer with a lower priority cannot pass through a layer of equal or higher priority.
• When layers join, the join cleans up if the 2 layers have the same material. If the 2 layers have different materials, a line appears at the join.
• Each layer must have a function assigned to it for Revit to match layers accurately.
• Layers inside the core of one wall pass through layers of higher priority that are outside the core of the joined wall. The layers in the core extend to the core of a joined wall, even if the core layers are set to Priority 5.
The above is taken straight from the Revit online Help file but it can be a little confusing when you’re first learning the software.
The biggest confusion lies in the naming of the functions (Structure, Finish, etc.) We automatically assume that the Structure function must be applied to a structural element. This however, is not true. The name is purely there for guidance and can be applied in the majority of cases.
We can change these to suit.
NOTE: The priority must descend from the core boundary as it moves away, or you will get this error:
I’ve chosen 2 simple wall build ups as shown in figure a.
A masonary external wall with an air cavity and a simple masonary party wall.
To access the layers of the wall we must edit the type settings. Simply click on the wall, in the properties you then select the “edit type” button.
Once inside the Type settings for the wall you need to select the “Structure Edit” button. This will take you in to the wall build up.
Wall types are usually set up in what seems to be a common-sense way.
The finishes are set to “Finish ”
The structural Elements are set to “Structure ”
And the Air gap is set to “Thermal/Air Layer ”
However, because the Air layer has a lower priority to the Structural element of the external wall it will not pass through it. Hence it will stop.
What we need to do is change the priorities if each affected wall to allow the layers to join in the correct way.
In this situation I have changed the Function within the wall as follows:
I’ve swapped the Function of Layers 3, 4 and 5 to give the Layer 4 (the cavity) the highest priority.
Layers 3 and 4 (the blockwork) is given a slightly lower priority of 2.
Once complete, I then do the same to the external wall. Given the Air layer a priority of 1 and the structural blockwork a priority of 2.
This will allow the walls to clean up as per figure a.
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