Roof and Skylights

The roof is a major component of the building envelope for top-floor spaces. Roof construction significantly affects heating and cooling loads.

You work with roofs on the Roofs tab, and skylights on the Skylights tab, in Building Model → Envelope.

Roof Assignment

Auto Create Envelope

The fastest way to add roofs is Auto Create Envelope (the ✨ button in the Envelope toolbar). When you choose a Roof Type, it assigns that type — along with walls and slabs — to unassigned spaces on the highest occupied level, across All Levels or the current level. It never overrides existing assignments, so it’s safe to run again as you add spaces.

Assign roofs manually

1. On the Roofs tab, click Assign Roofs
2. Choose a Roof Type, an optional Roof Pitch, and whether to Apply roof load to ceiling only
3. Click spaces on the canvas to toggle the roof assignment
4. Click Done when finished

Edit a roof

Select a roof from the Roofs list, or click it on the canvas, to open the Roof Data panel.

Roof Properties

Selecting a roof opens Roof Data, where you can set:

• Roof Type — links to the roof type definition from Basis of Design, which supplies the U-value, mass, and surface absorptance used in the calculations.
• Roof Pitch — the roof’s slope and azimuth. A pitched roof has a larger surface area than the space footprint and changes how solar gain is calculated.
• Apply roof load to ceiling only — applies the roof load through the ceiling plane instead of directly to the space.

Roof area isn’t an editable field — HVAKR derives it from the space geometry and the roof pitch.

Surface Absorptance

The roof type’s surface absorptance sets how much solar radiation the roof absorbs:

• Dark - High absorption (0.9+)
• Medium - Moderate absorption (0.5-0.7)
• Light/Cool Roof - Low absorption (0.3 or less)

Cool roofs significantly reduce cooling loads in hot climates. Set surface absorptance on the roof type in Basis of Design → Building Data.

Skylights

Skylights are windows in the roof that admit daylight and solar heat.

Adding Skylights

1. On the Skylights tab, click Create Skylights
2. Enter the skylight Width and Height and choose a Window Type — skylights use window types for their thermal and solar properties
3. Click roofs on the canvas to place the skylights
4. Click Done when finished

Skylight Properties

Selecting a skylight opens Skylight Data, where you can adjust:

• Dimensions - Width and height
• Position and rotation - Where the skylight sits on the roof
• Window Type - Determines the U-value and SHGC

Solar Gain

Skylights receive more direct solar radiation than vertical windows:

• Higher peak solar gain
• More consistent throughout the day
• Significant cooling load impact

Shading

Skylight shading options:

• Internal shades or blinds
• External shading devices
• Light wells that limit direct sun

Ceiling Plenums and Attics

For spaces with a suspended ceiling or attic below the roof:

• Ceiling height - Set the space’s ceiling height below the floor-to-floor height; the space above it acts as a plenum
• Apply roof load to ceiling only - Applies the roof load through the ceiling plane instead of directly to the space
• Temperature overrides - Roof types support summer/winter external temperature overrides for roofs below unconditioned attic spaces

Best Practices

1. Verify top floor - Confirm which spaces have roof exposure
2. Use cool roofs - Consider high-reflectance roofing
3. Size skylights carefully - Large skylights can cause overheating
4. Account for plenums - Configure ceiling height correctly
5. Set the pitch - Sloped roofs have larger area than the footprint