Insulating an unheated attic: on the floor, not on the slope

If you don't live in the attic, the insulation goes on the floor — the attic floor — not on the roof slope. It's the only rule that matters, and it's also the one most often broken.

The reason is simple: insulation separates what you heat from what you don't. In an unused attic, that boundary runs under your feet when you climb the stairs, not over your head. Anyone who insulates the slope of an attic nobody lives in is paying to keep an empty volume warm.

Why the floor and not the slope

A pitched roof has a much larger surface area than the floor beneath it. Insulating the slope means buying material for a larger surface and additionally heating all the air in the attic, which does you no good at all. Insulating the floor means buying less and heating only the rooms.

There's something else, too. An unused attic needs to stay ventilated — the air that enters at the eaves and exits at the ridge dries out the wood and carries moisture away. If you insulate the slope and close off the ventilation, you turn the attic into a sealed box where the vapor coming up from the house has nowhere to go.

The only situation where you insulate the slope is when the attic becomes a livable loft. Then everything changes: the build-up, the ventilation rules, and the budget.

In summer, the attic is the house's oven

In winter, the problem is heat leaving upward. In summer, the problem is reversed and far more annoying. The roof covering heats up in the sun, and from there heat comes down by radiation — like from a radiator suspended above your ceiling.

A mass-based insulation laid on the floor absorbs that heat, holds it, and releases it later, indoors. That's why the rooms on the top floor are hottest in the evening, not at noon.

A reflective surface placed on top, with its shiny face toward the cold attic and an air space in front of it, doesn't absorb the heat. It sends it back before it can enter. In winter, the same surface works in reverse.

What goes down, layer by layer

From bottom to top, from the room toward the attic:

• Vapor control, on the warm side. The air in the house carries moisture upward. If it reaches the insulation unchecked and meets the cold, it condenses there. The seams must be sealed, otherwise this layer doesn't exist.
• Fiber, for insulating mass. This is where thickness matters, and the attic is the only place in the house where thickness costs nothing — you're not taking inches away from any room.
• The reflective surface, on top, with an air gap. For the summer heat, which arrives by radiation and which thickness alone doesn't stop.
• A walkway, if you go up into the attic. Boards on spacers, over the insulation, not directly on it.

Four mistakes that cost you

Walking on the insulation. Fiber trodden underfoot no longer has the thickness you paid for. Wherever it gets walked on, the insulation stops insulating.

Blocking the eave ventilation. This is the most common side effect of a carefully done insulation job: material pushed all the way to the edge blocks exactly the gap the air was entering through. The attic stays damp, and the wood pays the price.

The forgotten hatch. You end up with a square meter of unclosed hole in the middle of an insulated surface. The hatch must be insulated and sealed around its perimeter, otherwise the whole job has a permanently open window.

Spotlights and cables. Every penetration through the ceiling is a place where warm air escapes and vapor enters the insulation. Treat each one with tape, one at a time.

The attic, for a small price

Of all the insulation work you can do on a house, the attic floor is the cheapest and simplest. It doesn't need scaffolding, doesn't damage any finishes, doesn't require a permit, and can be done in a weekend, over ordinary surfaces.

That's also why it's worth doing first, before the windows and before the facade. Not because the others don't matter, but because this one costs the least for what it gives back.

Read next: which ceiling to insulate and how · how to prevent mold in the attic · products for attics and lofts