> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/user-guide/2d-building/building/reference-building/h1-vm1.md).
# H1 / VM1
Most aspects of an H1 / VM1 reference building are automated, wherever possible, to enable a consistent modelling approach. In some cases, where the building code is unclear, the H1 / VM1 Reference Building can be interpreted in multiple ways.
To respond to the varying interpretations of the building code, a Reference Building defaults to typical industry practice but can be updated, but can be updated to allow modellers to gain greater control of the assumptions behind the H1 / VM1 Reference Building.
## Add Resistance to Unconditioned Opaque Surfaces
Add Resistance to Unconditioned Opaque Surfaces allows you to nominate if additional thermal resistance is applied to the H1 / VM1 Reference Building.
Building elements separating conditioned space from adjacent unconditioned space (for example, a garage) may be modelled under H1 / VM1 with a construction R-value of 0.5 higher than the actual construction R-value and zero solar absorptance. This adjustment to the construction R-value considers the insulation from the still air in the unconditioned space.
## Allow Custom Window SHGC
Custom Window SHGC allows you to nominate the SHGC of the H1 / VM1 Reference Building, with the default approach to accept the recommended value (consistent with the BRANZ figures for Table E.1.1.1). Where a custom value is selected, the SHGC is manually set according to the defined value.
| Window R-value | Window U-value | Window SHGC |
| -------------- | -------------- | ----------- |
| 0.37 | 1.1 | 0.54 |
| 0.46 | 1.3 | 0.57 |
| 0.50 | 1.1 | 0.54 |
{% hint style="danger" %}
For those looking for more flexibility, as H1 / VM1 does not directly stipulate window SHGCs, a custom value can now also be selected for the reference building. While flexibility is fantastic, its use can create an incentive to reduce the performance of the reference building... so apply with caution!
{% endhint %}
## Ceiling Edge Resistance
When active, determines an area-weighted ceiling R-value per zone, in accordance with the allowance note in H1 / VM1 Section 2.1.2.
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