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). It assigns the Roof Type you choose to unassigned spaces on the building’s highest occupied level only — wall types apply to every exterior edge in the chosen scope instead, and slab types go to the lowest occupied level. If you scope the run to a level that isn’t the highest occupied level, it assigns no roofs. It never overrides existing assignments, so it’s safe to run again as you add spaces.

Choosing a Roof Type in the dialog also reveals Roof Pitch and Apply roof load to ceiling only, so an automatic run can set the same roof options as a manual assignment.

Assign roofs manually

1. On the Roofs tab, click Assign Roofs
2. Choose a Roof Type, then set the Options — 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. It supplies the Overall U-Value, the Roof Mass Data (the ASHRAE construction that sets the conduction time series), and the Surface Absorptance used in the calculations. The cooling load needs all three — without any one of them, the roof adds no cooling load. The heating load reads the Overall U-Value only.
• Roof Pitch — the roof’s slope and the direction it slopes toward. The button reads the two values as pitch° @ azimuth°. Direction takes a compass azimuth and defaults to 180°. Angle takes the pitch in degrees, from 0 to 89. With a roof selected, a plan of the space footprint appears under Direction — click an edge to slope the roof toward it.
• Apply roof load to ceiling only — moves the roof’s cooling load to the space’s ceiling plenum. Peak Cooling Load still includes it; Peak Cooling Load (Space) does not. The design heating load stays on the space either way.

Roof area isn’t an editable field. HVAKR derives it from the space footprint and the roof pitch — the footprint divided by the cosine of the pitch — so a pitched roof has a larger surface area than the footprint. The load calculations then deduct the skylight area from that roof area, so the same area is not counted twice.

Surface Absorptance

The roof type’s surface absorptance sets how much solar radiation the roof absorbs, as a fraction from 0 (fully reflective) to 1 (fully absorbent). A typical value is about 0.40. The Surface Absorptance field links to an ASHRAE reference table with example ranges:

• Reflective Surfaces (metal roofing, polished concrete) - 0.1-0.2
• Natural Surfaces (grass, trees) - 0.2-0.6
• Light-Colored Surfaces (white roofing, painted concrete) - 0.3-0.5
• Dark Surfaces (dark roofing materials, asphalt) - 0.8-0.9

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. Each one sits on an assigned roof and uses a window type — there is no separate skylight type.

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, or press Enter. Escape also ends placement.

Skylight Properties

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

• Window Type - Supplies the U-value and SHGC
• Width and Height - The skylight dimensions
• Rotation - The skylight’s angle on the roof

Position is not a field in the panel. Drag the skylight on the canvas, or select it and nudge it with the arrow keys. Selecting more than one skylight edits all of them together, and the panel header carries copy and paste buttons.

Solar Gain

HVAKR calculates each skylight from the roof it sits on:

• The skylight takes the roof’s Roof Pitch as its tilt and the roof’s direction as its azimuth, so a skylight on a flat roof is horizontal glazing
• The load uses the window type’s U-value and SHGC, and needs the window type’s Window Mass Data to calculate
• Skylight area is deducted from the roof area, so it is not counted twice
• The roof type’s Summer and Winter External Temperature Override also drive the skylights in that roof
• The cooling load needs a Roof Type on the space. A skylight on a space with no roof type still produces a heating load, but no cooling load

Shading

Skylights have no shading inputs. HVAKR calculates them with no internal or external shading, so shades, blinds, overhangs, and light wells do not reduce the solar gain. Windows do support External Shading and Internal Shading — see Windows and Doors. To model a shaded skylight, use a window type with a lower SHGC.

Ceiling Plenums and Attics

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

• Ceiling Height - Set it in Space Data, below the space’s Slab to Slab Height; the space above it acts as a plenum. The field does not accept a value above the slab-to-slab height.
• Apply roof load to ceiling only - Moves the roof’s cooling load to the ceiling plenum, as above
• Summer and Winter External Temperature Override - Advanced fields on the roof type in Basis of Design → Building Data. Each replaces the outdoor condition for that roof, which models a roof under an unconditioned attic. The summer override replaces the sol-air temperature, so HVAKR adds no solar gain for that roof. See Heating Loads.

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