Envelope Diff

An assembly change looks small. One layer swapped, one thickness reduced, one membrane moved to the other side of the sheathing. The consequence is never in the section: it is at the twenty junction details drawn against the old one, and nobody re-opens those.

Workflow

Step 1. Establish the two versions and who wants the change

Ask, in one message:

1. Both versions, in order, inside face to outside face, with values.
2. Which is the earlier one? Say it explicitly.
3. Who proposed the change and what reason was given: cost, availability, programme, buildability, performance, or a manufacturer's standard detail?
4. What junction details exist against the old version? Point at design-envelope for the junction schedule.
5. Has anything been built to the old version already?

Question 3 is not a formality. A change proposed for cost and a change proposed for availability need different questions asked of them, and a change proposed because a manufacturer's standard detail was easier to draw usually loses something nobody costed.

Step 2. Report what moved, layer by layer

#	Old	New	Change	Control layer affected

Then, separately, the four questions that matter more than the layer list:

Control layer	Old position	New position	Still continuous?

A layer that stayed the same but moved relative to the insulation has changed, and a plain layer comparison hides it. That is the single most common consequential envelope change and it is invisible in a side-by-side list.

Step 3. Say what the change does to the junctions

The step that earns the exercise. For each junction detail drawn against the old version:

Junction	Still correct?	What has to change

A membrane that moved outboard changes every lap at every opening, every termination at the base and the top, and every penetration closure. Report the count of junction details now wrong, before anything else.

Where the junction list is unavailable, say so and say the change cannot be assessed properly without it.

Step 4. Say what else moved with it

• Drying direction. From where the low-perm layers now sit. A direction, never a rate.
• Thickness. Anything that changes the overall build-up changes the structure line, the window position, the parapet height, and the sill and head details.
• Weight, if it changed materially, which is the engineer's.
• Fire and acoustic, if either version was tested or listed. A tested assembly modified is not that tested assembly, and this is the finding that most often comes back at inspection.
• Buildability and sequence, which is often the real reason for the change and is worth stating out loud.

Step 5. Report

• Count of junction details now wrong, first.
• The control layer table, with any layer that moved relative to the insulation named.
• Drying direction, before and after.
• Whether any listing or test is invalidated.
• Every value missing from either version, counted.
• What has to be re-run: condensation-check, and air-barrier-trace if the air barrier plane moved.

Say plainly whether this is a change of build-up or a change of strategy. A different insulation of the same thickness in the same place is the first. A membrane on the other side of the sheathing is the second, and the second is never a like-for-like substitution however it is presented.

Step 6. Save, if asked

Ask whether to write the comparison to a file and where. Keep it where a substitution was proposed; it is the record of what was accepted and why.

Rules

• Establish which version is earlier, explicitly.
• Ask who proposed the change and why.
• Report the control layers' positions relative to the insulation, not just the layer list.
• Report the count of junctions now wrong before anything else.
• Say whether a listing or a tested assembly is invalidated.
• Say whether this is a build-up change or a strategy change.
• Never supply a value to complete a comparison.

Anti-patterns

• Comparing the two lists and calling equal thicknesses equivalent.
• Missing that a layer moved relative to the insulation.
• Reviewing the section and not the junctions drawn against it.
• Accepting a manufacturer's standard detail as a like-for-like change.
• Treating a modified tested assembly as still tested.
• Reporting a drying rate.
• Assessing a change without the junction list and not saying so.

Resources

None. This skill is one file. Output is written directly at the path you choose.