Why does the radiant barrier ask for a roof area?

Because it goes on the roof, not on the floor. A pitched roof is larger than the rectangle beneath it — by a factor that depends on the pitch — so using the loft's own area there would under-order it. The page asks for the roof area separately rather than deriving a number it cannot know.

Are the air-sealing gaps counted?

No. The bypasses that leak heat into a loft — a hatch, a downlight, a pipe penetration, the top of a partition — are counted by going up and looking, not derived from a rectangle. It is the work with the best return in a loft and the page will not pretend to have measured it.

Can I board over the insulation?

Only if the boarding is raised above it. Compressing quilt under boards takes most of its value away, and the depth you are going to is usually deeper than the joists. The boarding row counts the deck; the legs or battens that raise it depend on the joist depth you have.

Does the ventilation figure account for what the insulation blocks?

The vent area is the roof's requirement, and the baffle count is what keeps the eaves clear once insulation reaches them. They are separate rows because they are separate purchases, and the second is the one most often forgotten until the insulation is already in.

Plan a loft

Every trade, one set of measurements

Loft planner

A loft is planned from the floor, because the floor of the loft is the ceiling of the room below and that is what the insulation, the boarding and the vent area are all worked from. Give it a length and a width and each comes back on its own row — with the one thing that does not follow from it named rather than assumed: the roof above it is larger than the rectangle under it.

Timber-framed house under construction with ladders, sawhorses and lumber

New loftNot drawn yet

1 Measure the loft
2 Choose its finishes
3 Save it to My Project

Give the loft a length and a width, and it is drawn here with every figure beside it.

The loft

Nothing here is required. Each figure appears as soon as it has what it needs.

Length and width — the loft floor, which is the ceiling below. Everything on the list is worked from that rectangle, and anything fixed to the rafters is measured up there and asked for separately.

Add a loft

The floor of the loft, which is the ceiling of the room below — two figures, and then loose-fill against batts over the same area, the ventilation that keeps a cold roof dry, and the depth question that decides how much of either. The radiant barrier is measured on the roof and asks for that separately, because a pitched roof is larger than the rectangle under it.

LengthftWidthft

Everything the loft needs

In the order the work is done. No row has a figure yet.

Nothing in this loft is everything the loft needs yet. Choose how a surface is finished, or what else is being done, and its rows appear here.

Assumptions assumed

Everything the figures above rest on that you did not give, and what they are still waiting for.

Nothing is assumed yet: give the loft its measurements to begin.

How the planner runs

Give the loft its length and width, measured on the floor.
Say what depth of insulation you are going to, and whether there is any there already.
Read the ventilation area the roof needs and the baffles that keep the eaves from being blocked by it.
Add boarding if the loft is being used, which changes what the insulation can do under it.
Save it into your project and price it with the rest of the work.

Where the figures come from

Reading for the decisions

Questions

Why does the radiant barrier ask for a roof area?: Because it goes on the roof, not on the floor. A pitched roof is larger than the rectangle beneath it — by a factor that depends on the pitch — so using the loft's own area there would under-order it. The page asks for the roof area separately rather than deriving a number it cannot know.
Are the air-sealing gaps counted?: No. The bypasses that leak heat into a loft — a hatch, a downlight, a pipe penetration, the top of a partition — are counted by going up and looking, not derived from a rectangle. It is the work with the best return in a loft and the page will not pretend to have measured it.
Can I board over the insulation?: Only if the boarding is raised above it. Compressing quilt under boards takes most of its value away, and the depth you are going to is usually deeper than the joists. The boarding row counts the deck; the legs or battens that raise it depend on the joist depth you have.
Does the ventilation figure account for what the insulation blocks?: The vent area is the roof's requirement, and the baffle count is what keeps the eaves clear once insulation reaches them. They are separate rows because they are separate purchases, and the second is the one most often forgotten until the insulation is already in.

When the work is a job rather than a room. Plenty of it never starts from a space — a slab, a roof, a deck, a panel upgrade. Each of those is a job takeoff: give it the few figures that job is described by, and every trade in it comes back together, the way the room does here.