A scanned space can sit on top of a range of different floor conditions:
Enclosed crawl space or basement
Open crawl space or unconditioned garage
Slab on grade
Slab below grade (like a basement slab)
Each scenario is handled by a different Manual J construction that drives very different loads. Getting the floor settings right is essential to accurate load calculations.
This article walks through every setting on the Floor screen.
Always verify construction details and assumptions in the home. For floors over an enclosed crawl space or basement, or an open crawl, or unconditioned garage, this is typically easy to do with a visual inspection. Make note of R-value labels that might be visible on the face of fiberglass batts, or use your professional judgment based on the type of insulation and its thickness.
Note about "Is Exterior:" Loads are only calculated on floors that are considered to be "exterior" i.e. they are exposed to temperatures that are higher or lower than typical indoor living conditions. First-story floors are what we most commonly focus on since second-story floors are typically above conditioned space, so the floor is considered "interior" and therefore loads should not be considered. A common exception to that would be a second-story room over a garage.
The "Is Exterior" toggle can be found in the upper right of the sidebar when a floor is selected.
Edit floor assumptions
To make changes to a floor, simply tap on the floor in the 3D model, and the sidebar will appear where you can edit the assumptions, like whether or not it's exterior or all the details about the floor construction.
Then tap on the existing assumptions menu to make changes. In the example screenshot below, you would tap on the menu titled "Enclosed Crawl or Basement".
Construction
Construction tells Amply what is directly underneath the room. Pick the option that matches what is actually there:
Enclosed crawl or basement — the floor sits over a closed, unconditioned space (closed crawl or unconditioned basement). The space is buffered from outdoor air, so heat transfer depends on how tight it is and how much insulation is on its walls.
Open crawl or garage — the floor sits over a space that is essentially at outdoor conditions (vented crawl, cantilever over outside air, or an attached unconditioned garage). No buffer — heat transfer is driven directly by outdoor design temperature.
Slab below grade — the room's floor is a concrete slab sitting 2 or more feet below outdoor grade, typically a finished basement floor. Heat transfer is dominated by conduction into soil.
Slab on grade — the room's floor is a concrete slab less than 2 feet below outdoor grade, typically a first-floor slab. Heat transfer is driven by the exposed slab perimeter, not the floor area.
Why "slab below grade" and "slab on grade" are separate: Manual J uses one procedure for any floor 2 ft or more below grade and a different procedure for any floor less than 2 ft below grade. Basement floors use U-value × floor area, while slab on grade uses F-value × exposed perimeter — so the same slab in two different positions produces very different loads.
Radiant Floor
Set Has Radiant Floor to Yes only if there are hydronic or electric heating coils embedded in the floor itself. Manual J applies a different heat-transfer multiplier when the floor surface is actively warmer than the room, which Amply handles automatically once this is on.
For ducted, ductless, or hot-water baseboard systems — where the floor itself is not actively heated — leave this set to No.
Wall Insulation and Tightness
Has Wall Insulation, Wall Insulation, and Tightness only apply when Construction is Enclosed crawl or basement. They describe the walls and air leakage of that enclosed space below the floor — not the building's exterior walls and not whole-home envelope tightness.
Has Wall Insulation — turn on if there is any insulation on the crawl space or basement walls. If
No, Amply uses the uninsulated-wall version of the construction.Wall Insulation — if
Has Wall InsulationisYes, pick the R-value of insulation on the crawl space or basement wall. Manual J supportsR4,R11, andR19.Tightness — describes the air leakage of the space below the floor:
Sealed — rim joist, vents and penetrations are air-sealed; no intentional outdoor venting. Matches modern "sealed and insulated" crawl spaces.
Leaky — unsealed but not actively vented to outdoors. Common in older closed crawl spaces.
Vented — open foundation vents to outdoors. Crawl space temperatures track outdoor air, increasing floor heat transfer.
Together these three inputs determine how much the space below the floor buffers heat loss in winter and heat gain in summer. A sealed, R19 crawl space reduces floor losses dramatically compared to a vented, uninsulated one.
Floor Insulation
Floor Insulation is the R-value of insulation in the floor cavity itself — between the conditioned room and whatever is below. This is the field most people think of as "floor insulation": batts or rigid foam in the joists, or insulation below a basement slab.
For
Enclosed crawl or basementandOpen crawl or garage, Manual J supports values fromR0up toR38.For
Slab below grade, the choice isR0orR3 or higherinsulation below the slab.For
Slab on grade, the options reflect how the slab edge is treated (uninsulated, 3 ft vertical edge insulation, or 4 ft horizontal under-slab insulation with or without edge insulation).
If you are not sure what is there for slabs, our smart starting assumptions provide a reasonable default based on the home's year built and climate zone. we also recommend checking with your local building inspector to understand what was common for different vintages of homes in your area.
Floor Finish
Floor Finish is the finished surface — hardwood, tile, carpet, or Any if it varies room to room.
Set your changes as the default
Once the floor matches what's under the story of the home (for example, the first story), turn on Use as default 1st story floor. Amply then applies the same settings to every subsequent 1st-story room you scan. Override it for individual rooms that sit on something different (e.g., one bonus room over a garage).