ᐅ Roof Insulation: On the Floor of the Top Story or Under the Roof?

Hello everyone,

Next year, my wife and I will be building a single-family house and I have a question about that. The house will be built with aerated concrete blocks and will have a hipped roof. There is not much space under the roof, just some storage room. You cannot stand upright there, so it will never be used as living space or anything similar. It will only be used to store boxes with decorations or similar items. Another important point is that we are building a KFW55 energy-efficient house, so energy efficiency is quite important.

Now, I am not sure what is better in our case. Should we insulate directly under the roof or on the upper floor ceiling? Are there any experiences regarding what is more comfortable in winter and summer? Will I even notice a difference on the upper floor? Which option is more cost-effective?

Thanks in advance to everyone who shares their experiences with me.

Best regards

On the floor slab, it is considered a "cold roof." Under the roof, it is a "warm roof." It is important that warm, moist air can escape. If you only have a storage room upstairs but a warm roof, windows should be planned in the gable to allow for ventilation. With a cold roof, the air should circulate sufficiently on its own, and ventilation is not necessary.

I argue that a cold roof is more cost-effective (construction, energy). However, when storing items, temperature fluctuations between summer and winter must be taken into account. The climate inside is then roughly comparable to that of an unheated garden shed.

bauenmk2020 schrieb:

The floor deck below the roof is called a "cold roof." Under the roof itself is a "warm roof." It’s important that warm, moist air can escape. If you only have a storage room upstairs but a warm roof, you should plan windows in the gable for ventilation. With a cold roof, air should circulate sufficiently on its own, so no ventilation is necessary.

I would argue that a cold roof is less expensive in terms of construction and energy costs. However, when storing items, you need to account for temperature fluctuations between summer and winter. The climate inside is roughly like that of an unheated garden shed.

Very good point! That’s basically how I see it as well.

I think it’s worth saving here. I estimate the costs for our single-family home (probably similar to yours) at about €4,000 for insulating the floor deck and around €8,000 to €10,000 for insulating the roof.

We recently insulated our top floor ceiling. First, we installed a vapor barrier underneath, sealed the overlaps, added edge insulation strips, then laid one layer of polystyrene boards horizontally and another layer vertically, and finally glued and installed tongue-and-groove OSB panels on top. The total cost was 1200 €. The area was 50 m² (540 sq ft).

You only need two sawhorses to cut the panels, a jigsaw, a pencil, and a tape measure.

Peter Silie schrieb:

We recently insulated our attic floor. Installed a vapor barrier underneath, sealed the overlaps, added edge insulation strips along the floor perimeter, then laid one layer of polystyrene boards horizontally, another layer vertically, and glued tongue-and-groove OSB panels on top. Total cost was 1,200 €. We covered 50 sqm (540 sqft).

You only need two sawhorses to cut the boards, a jigsaw, a pencil, and a tape measure.

That’s exactly how I did it too. Covered 120 sqm (1,290 sqft), including adhesive, vapor barrier, 18cm (7 inches) thick insulation boards, plus OSB panels, cost was 2,300 EUR.
In addition, I wrapped the floor joists up to the height of the insulation boards with compression felt to prevent thermal bridging through the wood. Leaving a gap between the OSB panels and the edge was intentional.

Now I still need the appropriate attic ladder.
Concrete slab thickness is 19cm (7.5 inches).
Insulation according to energy saving regulations is 18cm (7 inches).
That totals 37cm (14.5 inches) – but the attic ladders from Dolle have a maximum box height of 24cm (9.5 inches).
Do you see any issues here? Otherwise, I would need a custom size, which would increase costs.
Maybe I’m missing something.

bauenmk2020 schrieb:

I claim that a cold roof is cheaper in terms of costs

Yes. There is also more space between the rafters compared to the ceiling. If you have it done professionally, it is also cheaper to build directly on the ceiling.

What you should choose depends on what you want to store there. If it is your only storage space, I would go for the warm roof.